cdma2

401-614-012Issue 6August, 1999

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AUTOPLEX Cellular Telecommunications SystemSystem 1000CDMA RF EngineeringGuidelines (Volume 2)

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Contents

Chapter 1 - About This Document

Purpose 1-1

Technical Support Coverage 1-1

Scope 1-1

Intended Audience 1-2

Prerequisite Skills and Knowledge 1-2

Reason for Reissue 1-2

How to Use This Document 1-3

How to Find Information on Related CustomerDocumentation 1-5

How to Find Information on Related Customer Training 1-5

How to Comment on This Document 1-6

Chapter 2 - CDMA Overview

Concept 2-1

■ Attributes 2-2

Capacity 2-2

Power Control 2-4

Soft Handoff 2-5

Voice Activity 2-5

■ Deployment/Implementation Issues 2-6

Spectrum Clearance 2-6

Cell Site Locations 2-7

Boundaries 2-7

Issue 6, August 1999 v


Contents

Chapter 3 - Cell Site Architecture

Introduction 3-1

■ Operation 3-2

■ Series II CDMA Cell Site Equipment 3-2

Overview 3-2

Cell Site Components 3-4

Directional Cell Site Configuration 3-7

■ CDMA System Capacities 3-9

Chapter 4 - Call Processing

Mobile Access 4-1

■ Introduction 4-1

■ Description of IS-95A Mobile Access Protocol 4-1

■ Average Persistence Delay for Access Request Attempt 4-9

■ Traffic, Throughput, and Delay Performance for IS-95AMobile Access Protocol 4-15

■ Recommended Value of Access Parameters 4-23

Handoff 4-25

■ Hard Handoff 4-25

■ Soft and Softer Handoff 4-26

Definition 4-26

Procedure 4-26

Comparisons 4-27

Performance 4-29

Parameters 4-38

vi Issue 6, August 1999


Contents

Chapter 5 - Packet Pipe Engineering

Introduction 5-1

■ Simulation Model and Numerical Results 5-2

Assumptions 5-2

Packet Dropping Rate and Criterion 5-3

Numerical Results 5-3

Appendix A: CDMA Cellular Antenna Guideline

Introduction A-1

Antenna Concepts A-1

Antenna System with Interference and Cell Coverage A-3

■ Directional Antenna and Sectorization Gain A-4

■ Coverage with Antenna Height and Gain A-6

■ Reducing Interference Using Antenna Downtilt A-7

Diversity Antenna Systems A-10

■ Space Diversity Antenna A-10

■ Polarization Diversity Antenna A-11

Appendix B: Antenna Isolation Guidelines for Collocated RF Stations

Introduction B-1

Mathematical Models for Mutual Interference Evaluation B-1

■ Receiver Desensitization B-3

■ Intermodulation Product Interference B-4

■ Receiver Overload B-5

Antenna Isolation Criteria and Safe Antenna Isolation B-6

Issue 6, August 1999 vii


Contents

Example of Antenna Isolation Calculation B-7

Antenna Separation Between Two CollocatedRF Stations B-11

Mutual Interference Between Multiple CollocatedRF Stations B-12

Site Survey B-15

Appendix C: References and Acronyms

References C-1

List Of Acronyms C-2

viii Issue 6, August 1999


401-614-012

Contents

Lucent Technologies - ProprietUse Pursuant to Company Instruc

1
About This Document
Purpose 1

Technical Support Coverage 1

Scope 1

Intended Audience 2

Prerequisite Skills and Knowledge 2

Reason for Reissue 2

How to Use This Document 3

How to Find Information on Related CustomerDocumentation 5

How to Find Information on Related Customer Training 5

How to Comment on This Document 6

Issue 6, August 1999 1-i

arytions

Contents

1-ii Issue 6, August 1999

Lucent Technologies - ProprietaryUse Pursuant to Company Instructions

Lucent Technologies - ProprietaryUse Pursuant to Company Instructio

1
About This Document
Purpose

This document provides basic radio frequency engineering guidelines and recommendations for use in the planning and design of a Code Division Multiple Access (CDMA) cellular telecommunications system. These guidelines are generic. Specific implementations will vary from system to system. All information is consistent with the CDMA common air interface (IS-95A), minimum performance standards for base stations (IS-97), and minimum performance standards for mobile stations (IS-98).

Technical Support Coverage

Warranty and non-warranty support may be provided by several Lucent Technologies organizations. The technical support may vary as outlined in specific customer contracts and agreements. Your Lucent Technologies Account Executive can provide details regarding the extent of your technical support coverage.

Scope

This material comprises guidance, recommendations, and insights necessary to optimally engineer a CDMA service system. Particular emphasis is given to concepts such as power control that are key to successful system operation.

Issue 6, August 1999 1-1

ns

401-614-012

All information is consistent with the CDMA common air interface (IS-95A), minimum performance standards for base stations (IS-97), and minimum performance standards for mobile stations (IS-98). These standards are listed in the Chapter footnotes.

Intended Audience

This document is intended for system-level CDMA RF Engineers working in the field who are planning new CDMA installations or adding to existing installations.

Prerequisite Skills and Knowledge

To use this document effectively, the user should have an in-depth understanding of past analog and digital cellular telephone technologies with regard to RF engineering.

Reason for Reissue

This is the sixth release of this document. CDMA technology is in an evolving technology that is undergoing change on a day-by-day basis. Issue 6 brings the document up to date with current applications being used in the field.

The size of the original document has increased over time making it necessary to split this document into 2 documents. This document has been split into 2 documents, Volume 1 and Volume 2.

"Appendix E: Microcell RF Engineering Guidelines" has been removed from this document (original document). A new document, the "FlexentTM PCS/Cellular CDMA Microcell RF Engineering Guidelines" - 401-703-349, has been written to replace Appendix E.

Chapter 9, contained a section on the Multiple-Carrier CDMA System. This section has been removed from this document (original document). A new document, the "Multi-carrier CDMA Systems RF Engineering Guidelines" 401-614-014, replaces the information in Chapter 9 of this document.

1-2 Issue 6, August 1999


How to Use This Document

This document consists of Chapters 1 through 5 and Appendix A through C. You are presently reading Chapter 1, ‘‘About this Document’’ .

This document is organized as follows:

■ Chapter 1 ‘‘About this Document’’ Provides general information about this document. It includes information regarding the intended audience, organization and contents, how to order, and how to make comments.

■ Chapter 2 ‘‘CDMA Overview’’ An overview of CDMA features is presented here. This chapter summarizes CDMA concepts and operations, and provides a basis for understanding the sections that follow.

■ Chapter 3 ‘‘Cell Site Architecture’’ The CDMA cell site is based on the existing Series II platform. Components such as RF combiners, linear amplifiers, and antenna interface equipment can be shared by Advanced Mobile Phone Service (AMPS) and CDMA. The CDMA specific components must be housed in CDMA exclusive frames. The CDMA components required for CDMA cell site implementation and configuration are described in this chapter.

■ Chapter 4 ‘‘Call Processing’’ The elements of call processing related to mobile access and handoff are addressed.

In IS-95A, CDMA mobiles transmit on the access channels according to a random access protocol. The packet throughput and delay performance of the mobile access protocol are analyzed in terms of various mobile access parameters. Based on the obtained numerical results from the analysis, values of various parameters of the mobile access channels are recommended.CDMA system supports several types of handoff. These include hard handoff, soft handoff, and softer handoff. A mobile in hard handoff switches from one cell site to another cell site by a brief interruption of the traffic channel. Examples of hard handoff include handdown from CDMA system to an analog system, and handoff from one CDMA carrier to another CDMA carrier. A CDMA-to-CDMA hard handoff can also occur at the boundary between different mobile switching centers.

Soft handoff is a technique whereby a mobile in moving between one cell and its neighboring cells transmits and receives the same signal from several cell sites simultaneously. On the forward link, the mobile in soft handoff can combine the signals using appropriate diversity techniques. On the reverse link, the Mobile Switching Center (MSC) can decide which cell site is receiving stronger. In softer handoff, the mobile’s call is supported by neighboring sectors of the same cell. Proper use of soft and softer handoff can enhance call quality, improving cell coverage and capacity.



401-614-012

In soft handoff, mobile continuously scans for pilots, and establishes communication with any cell (up to three) whose pilot exceeds a given threshold. Similarly, communication with cells whose pilot drops below a threshold is terminated. The operation of soft handoff is addressed in this chapter. The values of various soft handoff parameters are also recommended.

■ Chapter 5, ‘‘Packet Pipe Engineering’’ In this chapter, the number of traffic channels that a packet pipe with a given bit rate can support is determined.

■ Appendix A, ‘‘ CDMA Cellular Antenna Guideline ’’ In wireless communication systems, the antenna is one of the most critical components that can either enhance or constrain system performance. This Appendix addresses the antenna as a subsystem, including antenna and feed, and how the antenna is designed to transmit or receive radio waves. The basic function of an antenna is to couple electromagnetic (EM) energy between free space and a guiding device such as a transmission line, coaxial line, or waveguide. The orientation of the antenna plays a role in improving capacity with a directional cell site. The directional antenna, as a particular direction served to a sectored cell, can be used to increase system capacity due to reducing the cochannel interference in CDMA cellular communication systems. Antenna diversity is an important issue in wireless communication systems. Multipath propagation due to many paths (reflection, diffraction, and scattering) causes fading which results in rapid variations in the received signal. The antenna diversity, such as space diversity and polarization diversity at the base station or mobile, is received by two separated antennas or an orthogonal polarized antenna to reduce the severity of fading and to provide significant link improvement of the reception.

■ Appendix B, ‘‘ Antenna Isolation Guidelines for Collocated RF Stations ’’ Due to deployment constraints, estate acquisition difficulties and other reasons, sometimes it is highly desired that CDMA cellular cell sites (CSs) can be collocated with RF stations of other communications systems such as TDMA PCS, CDMA PCS, Cellular TDMA, AMPS, AM, SMR, etc. When they are collocated, mutual interference between stations always exist that may cause receiver desensitization, overload and/or Intermodulation Product (IMP) pollution, thereby degrading their system performances. Therefore, if the service provider wants to collocate a CDMA cellular CS with other RF station(s), precaution should be taken to avoid/minimize those harmful mutual interferences. This Appendix addresses these issues.

■ Appendix C, ‘‘ References and Acronyms ’’ A list of references and acronyms that are used within the document is provided.



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401-614-012

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Lucent Technologies — ProprietarSee notice on first page

2
CDMA Overview
Concept 1

■ Attributes 2

Capacity 2

Power Control 4

Soft Handoff 5

Voice Activity 5

■ Deployment/Implementation Issues 6

Spectrum Clearance 6

Cell Site Locations 7

Boundaries 7


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Contents



Lucent Technologies — PrSee notice on first pa

22
CDMA Overview
Concept 2

CDMA is a multiple access concept based on the use of wideband spread spectrum techniques that enable the separation of signals that are coincident in time and frequency. All signals share the same wideband spectrum. The energy in each user's signal is spread over the entire bandwidth and coded so as to appear like broadband noise to every other user. Individual signals are identified and demodulated at the receiver by applying replicas of the coding used for each signal. This process enhances the signal of interest, while dismissing all others as broadband interference. This level of interference rises with the number of users. Since a minimum signal-to-interference ratio is required to ensure call quality, the total level of background interference ultimately limits system capacity; consequently, all transmissions are carefully controlled in order to operate with the least necessary power.

The CDMA concept can be contrasted with other multiple access techniques. In Frequency Division Multiple Access (FDMA), each user has full-time use of part of the spectral allocation. The allocation is divided into a number of narrowband portions (channels). Each user's signal energy is confined to a channel. Signals coincident in time are distinguished by using frequency-selective filters. In Time Division Multiple Access (TDMA), each user has part-time use of all the spectral allocation. The allocation is broken down into a number of time slots. Each user's signal energy is confined to a slot. Signals coincident in frequency are distinguished through time gating. In CDMA, each user has full-time use of the entire spectral allocation. Each user's signal energy is spread over the entire bandwidth. Signals coincident in time and frequency are distinguished through the use of coding unique to each signal.


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CDMA Overview 401-614-012

Attributes 2

The chief rationale behind the deployment of a CDMA system is its potential for high spectral efficiency; that is, its ability to support significantly more users within a given bandwidth. Key system components such as power control and soft handoff are designed to realize and enhance this potential while maintaining acceptable call quality. In addition, the modulation concept permits the offering of such desirable system attributes as dynamic capacity and voice privacy.

In the following, key attributes of a CDMA system are summarized. More detailed explanations can be found in the sections that follow.

Capacity 2

Capacity considerations are fundamental to CDMA planning and operation. For the purpose of this discussion, capacity will be defined simply as the number of users that can be simultaneously supported. Forward and reverse link capacity will be addressed separately.

In each link, CDMA signals share the same wideband spectrum (carrier). Each user's signal is coded so as to appear as broadband interference to every other user. Power control minimizes the impact of this interference by adjusting each signal level to the minimum necessary to achieve call quality. In the following, these principles are applied to describe the dynamics of CDMA capacity.

Reverse Link 2

In order to place a call, a CDMA mobile must have sufficient power to overcome the interference generated by all other CDMA mobiles within the band; that is, the received signal at the cell site must achieve a required signal-to-interference ratio. The required mobile transmit power will thus depend on the distance of the mobile from the cell site as well as on the total level of interference (that is, cell loading).

The establishment of each additional call raises the interference levels seen by all users. Accordingly, to maintain call integrity, each user appropriately increments its transmit power. These adjustments, in turn, raise the level of interference that must be overcome by the next user. This process repeats itself until a new user cannot achieve acceptable voice quality at the cell site. At this point, system capacity has been reached.

The capacity limit occurs because the mobile stations eventually have insufficient transmit power to overcome interference levels. The limit thus depends upon factors that influence the level of interference seen at the cell site, for example, traffic distributions within and outside the cell. Since the mobile restricts output power when the user is not speaking, the limit will also depend upon the average level of reverse link voice activity.




The many factors influencing CDMA capacity give rise to a desirable flexibility in system operation. Since capacity is dependent upon interference levels, a cell's capacity is inherently dynamic, that is, a cell can naturally absorb more users if neighboring cells are lightly loaded. In addition, the system can naturally exploit the reduced levels of interference generated by low voice activity. Finally, capacity limits are soft rather than hard because system capacity can be increased by lowering voice quality requirements. In this procedure, more users are supported at the expense of slightly degrading the call quality of all users.

The flexibility described above makes it difficult to exactly assess CDMA capacity in a manner that will be applicable to all situations. As discussed in Chapter 4, “Reverse Link Capacity”, a useful reference point can be obtained by assessing the number of allowed users in an embedded cell when the power control is ideal and the mobile transmit powers are not limited. The maximum number of users that can be supported under these circumstances is the "pole point" or "power pole".

The values used in assessing pole point are tabulated in Chapter 4, “Reverse Link Capacity”. For these figures, the pole point is approximately 27. This value applies to a single sector of a directional cell. The pole point is obtained using 1.23 MHz or 1/10 of the current cellular spectral allocation. In contrast, a single sector of an FM system with frequency reuse of 7 will support about 2 channels within this bandwidth. Thus, a CDMA system with significant capacity can be obtained by implementing CDMA within a modest portion of the cellular band.

Forward Link 2

Upper limits on forward link capacity are fundamentally determined by restrictions on cell site radiated power. The forward link signal comprises message traffic for subscribers, a sector-specific signal (pilot) used by all mobiles, and miscellaneous signals (for example, sync and paging). Total power is allocated among these functions. Additional users cannot be supported when the sum of the allocations required exceeds the available transmit power.

Required allocations are governed by the need for a minimum signal-to-interference ratio at each mobile. The power allocated to other users within the cell, as well as the received power from neighbor cell sites, contribute to this interference. Interference from same-cell users is partly mitigated by the use of orthogonal codes which allow the receiver to suppress these signals; however, multipath effects limit the extent to which this interference can be screened out.




Forward link power distributions are further restricted by the requirement that a generous fraction of power must be allocated to the sector pilot. The sector pilot is important because it is used by all mobiles in site acquisition and tracking. Capacity limits are therefore reached when the remaining power, distributed among all users, is not enough to meet mobile signal-to-interference requirements.

Power Control 2

As discussed above, capacity can be maximized by minimizing the total level of system interference, that is, by controlling all CDMA signals to be at the lowest level necessary to meet signal-to-interference requirements. Power control ensures that each signal meets minimum requirements for communication while not causing undue levels of interference to other signals.

Control is accomplished via a closed-loop algorithm on the forward link, and open- and closed-loop algorithms on the reverse link. The open-loop mechanisms are based on measurement of parameters known to influence the desired output, whereas the closed-loop mechanisms are based on direct measurements of the output itself.

The objective of reverse link control is to ensure the minimum necessary signal-to-interference ratio at the cell site for each mobile. In the open-loop path, the mobile makes power adjustments based on its estimate of path loss from cell site to mobile. This estimate is based on the mobile's measurement of received total power. These adjustments compensate for path loss variations that are correlated between the forward and reverse links. In the closed loop-path, the cell site compensates for uncorrelated path loss variations (for example, multipath fading) and additional sources of interference by measuring the received signal-to-interference from the mobile and transmitting appropriate power adjustment commands. The final value of mobile transmit power is jointly determined by these two control paths.

The objective of the forward link control is to ensure the minimum necessary signal-to-interference ratio at each mobile from the cell site. In this closed-loop mechanism, the mobile requests forward link power adjustments based on its received FER.




Soft Handoff 2

In CDMA, various mechanisms are provided to ensure a robust handoff; that is, to ensure call support when a mobile crosses the boundary from one cell to another. The chief mechanism employed is "soft" handoff, where the mobile's call is simultaneously supported by up to three sectors. This process enables the mobile to establish contact with the sectors that it is likely to proceed into well before it leaves its serving (host) site. In addition, the simultaneous support provides a diversity gain that improves link quality in "fringe" areas. The application of power control from neighbor sites also ensures that a progressively distant mobile will not unduly boost its transmit strength and become a primary source of interference to a nearby cell site.

The CDMA soft handoff differs from the more familiar analog "hard" handoff in several ways. In an analog system, cochannel interference is controlled by not reusing the same channels in nearby cells. Accordingly, a mobile proceeding out of one cell into another must switch its call to an available channel in the new cell. This hard handoff requires a brief interruption of the traffic channel. In CDMA soft handoff, channel switching per se is not required because the same channel (carrier) is reused in every cell. Moreover, the acquisition of new sites is accomplished before contact with the old (serving) site is broken. No interruption of the traffic channel occurs. This handoff procedure is robust because the connection with the new host(s) is made before the connection with the old host(s) is broken. This process is often referred to as a make-before-break connection, as opposed to the analog break-before-make.

Voice Activity 2

Capacity may be enhanced through exploitation of voice activity. On the average, each link in a two-way voice conversation is active about half the time. If transmitters vary output power with voice activity, the total interference power from a large number of users will therefore be reduced by about a factor of two*. This reduction in interference translates naturally into a direct increase in system capacity. Such use of voice activity is possible because the same channel is reused by all subscribers; in contrast, an analog FM system could exploit voice activity only through the difficult task of reassigning the channel resource whenever the speaker pauses.

* The actual reduction depends upon the level of channel activity, which includes both voice activity as well as message activity (for example, power control). These considerations are detailed further in Chapter 4.




Deployment/Implementation Issues 2

Implementation of a CDMA system requires careful planning, particularly if the system is to be deployed within an area already possessing analog service. Deployment issues include clearance of spectrum, location of cell sites, boundary handoff (CDMA-to-analog) procedures, and cost. Several of these issues are summarized below.

Spectrum Clearance 2

The use of CDMA requires a block of RF spectrum. In an isolated service area with no pre-existing analog service, the spectrum required for each link is simply the CDMA bandwidth (1.23 MHz). The spectrum requirement in an area with analog capability within and outside its boundaries is more complex because CDMA must coexist and interact with these other systems. In this situation, a guard band is required at carrier edges, and a guard zone is required in the surrounding area.

The objective of the guard band is to ensure that CDMA and spectrally adjacent services do not interfere with one another. The spectrum requirement in the service area thus comprises the CDMA bandwidth plus guard band. Spectrum is required outside the service area as well. The CDMA bandwidth must be cleared within the surrounding area (guard zone) in order to ensure that CDMA and geographically adjacent services that employ the same carrier do not interfere with one another. A modest guard band is also recommended within the guard zone.

The use of the CDMA bandwidth by the analog system is forbidden within the guard zone. This restriction results in a loss of analog capacity within this region. This loss might be tolerated (as, for example, in a region of typically low traffic) or compensated for by such means as microcells or TDMA.

Computations based on minimum performance standards indicate that a guard band of 0.27 MHz on either side of the carrier is sufficient within the CDMA service area. Accordingly, a total of 1.77 MHz must be set aside within the service area for a single CDMA carrier. Some relaxation of this requirement may be possible based on the specific performance of Lucent hardware. A reduced guard band is required in the guard zone. Geographical guard zone requirements range from 1 to 3 tiers of cells.




Cell Site Locations 2

In general, cell site locations must be chosen so as to meet coverage and capacity objectives. Considerable flexibility in choice may be available in isolated areas where analog capacity does not pre-exist. In contrast, cell site candidates in areas where analog capacity is already available should generally be restricted to analog sites in order to reduce costs.

In areas with pre-existing analog capability, the CDMA system may be "overlaid" on the existing analog system. Overlays may be 1:1 (that is, a CDMA cell collocated with each analog cell) or n:1 (i.e., 1 CDMA cell for every n analog cells, where n is greater than 1). In the latter process, a single CDMA cell encompasses the equivalent area and traffic of more than one analog cell; however, many-to-1 overlays are not desirable due to the fact that the CDMA traffic capacity per underlying analog cell is decreased. In addition, many:1 overlays could compromise CDMA coverage as well as impair the ability of the CDMA system to combat sources of external interference (see for example “Mobile Receiver Intermodulation”, Chapter 2). In general, n:1 overlays with modest n (for example, between 1 and 2) are preferred, and 1:1 overlays are required in order to realize the full benefit of CDMA capacity within every cell.

Deployment choices for a specific area entail a number of trade-offs. Collocating CDMA sites with existing cell sites is generally preferred because it obviates the need to acquire new sites and allows reuse of certain cell site equipment. A n-to-1 overlay needs less cell sites and entails less start-up cost, but requires careful engineering to address issues of receiver overload (for example, a CDMA mobile passing an analog site). A 1-to-1 overlay obviates these concerns but is more expensive; however, the one-to-one correspondence facilitates CDMA-to-analog handoff and may better accommodate future growth. Actual deployments could be mixed, with n-to-1 in the interior of the service area and 1-to-1 at the boundary. The latter choice might be made to facilitate boundary handoffs (see below).

Boundaries 2

Particular attention must be given to handoffs at a geographic boundary between CDMA and an analog system. A mobile proceeding out of the CDMA service area must hand-off to the analog system in order to maintain its call. Analog locate radios do not detect CDMA mobiles; accordingly, mechanisms must be devised to engineer the (hard) CDMA-to-analog handoff without the use of locate information.

Such handoffs can be accomplished through associating analog channels with each pilot in a CDMA border cell. A mobile served by that pilot may then hand-off to an analog channel before crossing the CDMA-analog border. Handoff across the border is then analog-to-analog.




An unambiguous mapping between CDMA pilot and analog channels permits the CDMA-analog handoff without the use of locate information. Such mappings may be established through 1-to-1 overlays, dualization, or CDMA beacons. The last automatically hands off a CDMA call to preset analog channels.

The strategy for facilitating analog handoffs must entail careful cost trade-offs. In particular, the cost of local infrastructure required to raise the probability of success must be weighed against the cost of dropping some fraction of calls in a boundary area that may serve little traffic. For example, overall CDMA cell count might be minimized by adding infrastructure (for example, additional CDMA cells or beacons) only in local boundary areas where a substantial amount of CDMA traffic is anticipated to exit the CDMA service area.




3
Cell Site Architecture
Introduction 1

■ Operation 2

■ Series II CDMA Cell Site Equipment 2

Overview 2

Cell Site Components 4

Directional Cell Site Configuration 7

■ CDMA System Capacities 9


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33
Cell Site Architecture
Introduction 3

A functional view of the Series II CDMA application is shown in Figure 3-1.

Figure 3-1. CDMA System Architecture

Control (RCC)

DS1

DFU

Analog

TDMA

CDMA

rf/ampl/dist

Control (RCC)

DS1

DFU

Analog

TDMA

CDMA

Control (RCC)

DS1

DFU

Analog

TDMA

CDMA

Series II


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Cell Site Architecture 401-614-012

Operation 3

In the AUTOPLEX CDMA system, digital voice frames from mobiles are relayed by the cell sites to the Speech Handlers (SHs) at the MSC for speech processing and frame selection. The speech frames are in a packetized voice format using a standard protocol. A Digital Facilities Unit (DFU) provides interface between the cell site and the DS1 facility linking the site to the MSC. The frame relay function is performed by the 5ESS DCS switch.

The architecture of a CDMA system is similar to that of a traditional analog and/or TDMA digital system except that the CDMA system requires speech processing equipment at the MSC and new CDMA radio equipment at the cell site. In addition, CDMA utilizes an embedded switching platform for switching CDMA voice information.

The MSC will continue to support an Executive Cellular Processor (ECP) complex and up to 16 DCSs. These DCSs can be in any combinations of 5ESS or G2 DCSs. There is no change in ECP hardware.

The 5ESS DCS performs the following:

■ Speech processing

■ Frame selection (for soft/hard hand-offs)

■ Echo cancellation.

Series II CDMA Cell Site Equipment 3

Overview 3

This section describes and lists all new circuit packs and components necessary to support CDMA in a given cell site configuration.

The CDMA cell site is based on existing Series II (SII) platform. The CDMA system can easily be integrated with cell site also supporting AMPS, or TDMA, or both AMPS and TDMA. The cell site is fully backward-compatible with the existing analog (AMPS) and new digital (TDMA) systems.

While the Series II Primary Radio Channel Frame can contain the cell site components for AMPS or TDMA, or both AMPS and TDMA, the CDMA specific components must be housed in a CDMA-exclusive frame, the CDMA Growth Radio Channel Frame. Once a frame is dedicated for CDMA support, neither analog AMPS nor digital TDMA components can be housed in that frame. A block diagram to illustrate the frames of a SII and CDMA cell site is shown in Figure 3-2. Also, as shown in Figure 3-3, components, such as, RF combiners, linear amplifier, and RF equipment can be shared by the analog and CDMA systems.

R




Figure 3-2. SII /CDMA Cell Site Frames Block Diagram

Figure 3-3. SII Cell Site with CDMA and Analog Components

A SII cell site (SII-CS) can be implemented to support either an omni or up to six sectors. In the case of microcells, each cell supports an omni or up to six microcells. The CDMA Growth Frame consists of six shelves. Up to three CDMA shelves can be interconnected to support combinations of three sector cells and/or microcells. When two or three adjacent shelves are interconnected, they will transmit and receive at the same CDMA carrier frequency. In CDMA Release 1, only three sectors and one carrier are supported. In CDMA Release 2.0 (and beyond), full six sectors (one carrier) configuration will be supported.

LinearAmplifierFrame 0

LinearAmplifierFrame 1

AntennaInterfaceFrame 1

CDMAGrowthRadioChannelFrame 1

CDMAGrowthRadioChannelFrame 2

AntennaInterfaceFrame 2(optional)

RF RF

RFDS1 InputsData Links &Voice Trunks

GPS

Series IIPrimaryRadioChannelFrame 0




Major hardware required to support CDMA are:

■ CDMA Digital Radio Frame(s)

■ Global Positioning System (GPS) receiver

■ CDMA Radio Test Unit (CRTU).

Cell Site Components 3

Shelf 3

A typical shelf layout for a CDMA application is shown in Figure 3-4.

Figure 3-4. CDMA SII-CS Shelf Layout

The CDMA specific components follows.

CDMA Channel Unit (CCU) 3

A CDMA Channel Unit (CCU) supports two identical channel elements. Each channel element, in turn, provides the baseband spread spectrum signal processing for a given channel.

Note that in rate set 2 applications the CCU becomes a TCU (See Appendix D, “CDMA Channel Unit (CCU)”).

CDMA Cluster Controller CCC) 3

A CDMA Cluster Controller (CCC) supports call processing for each of the channel elements and also provides interface between the CCU and the Radio Control Complex (RCC). Packet pipe is terminated at the CCC.




Analog Conversion Unit (ACU) 3

In the transmit direction, the Analog Conversion Unit (ACU) digitally combines signals from the CCUs, performs D/A conversion, and limits the signal with a low-pass filter. Each ACU has six analog outputs which represent the I and Q signals to each of three sectors.

In the receive direction, the baseband signals from up to three Baseband Combiner and Radios (BCRs) are sampled and sent to each channel element.

Baseband Combiner and Radio (BCR) 3

In the transmit direction, the Baseband Combiner and Radio (BCR) combines the I and Q signals from each of the ACUs and converts the signals to RF with an up-converter.

In the reverse direction, it receives RF signals and down-converts them to baseband levels.

Bus Interface Unit (BIU) 3

A Bus Interface Unit (BIU) provides the interface between the BCR, ACU, and the TDM bus. It also provides power conversion and alarm control functions.

Synchronized Clock and Tone (SCT) 3

Synchronized Clock and Tone (SCT) board provides the timing signals for the CDMA system operation.

Digital Facilities Unit (DFU) 3

A Digital Facilities Unit (DFU) provides interface between the cell site and the DS1 facility linking the site to the MSC.

CDMA Radio Test Unit (CRTU) 3

A CDMA Radio Test Unit (CRTU) is used to perform the functional and diagnostic tests either on a scheduled basis or on demand basis. This radio is placed in the RCC shelf as shown in Figure 3-5.




Figure 3-5. RCC Shelf and Boards

The Linear Amplifier Frame (LAF) and Antenna Interface Frame (AIF) are the same as the Release 5.0 cell site.

CDMA Radio Shelf Backplane 3

A new backplane is required for the radio shelves that support CDMA components.

Cabling Specifications 3

In a CDMA Radio Channel Frame unit, when multiple shelves are used to support sectorized cells, coaxial cables are used to connect ACUs and BCRs across shelves. In such cases, the lengths of connecting cables must be matched to maintain equal propagation delays on various connections.

Redundancy and Reliability 3

To improve reliability of cell sites and to avoid a "single point" failure, two ACUs, two BCRs, and two BIUs are maintained in each shelf. Similarly, there will be two SCTs equipped for each TDM bus.

Global Positioning System (GPS) Receiver 3

The rubidium oscillator is in the AIF. The GPS receiver and its antenna are mounted externally to the cell site frames (RCF, LAF, and AIF).




TDM Bus Configuration 3

In a CDMA cell, the configuration of the TDM Bus is similar to any TDMA SII-CS configuration. The timing for the TDM bus is provided by the Synchronized Clock and Tone (SCT) board. When needed for reliability and redundancy purposes, two SCTs are placed in each even and odd addresses of a shelf of a cabinet.

Directional Cell Site Configuration 3

CDMA components include CDMA Channel Unit (CCU), CDMA Cluster Controller (CCC), Analog Conversion Unit (ACU), Baseband Combiner and Radio (BCR), and Bus Interface Unit (BIU). The location and interconnection of these components of a 3-sector configuration are indicated in Figure 3-6. There are two CCC's per shelf, each controlling up to 7 CCU's. The CCC also provides an interface to the Time Division Multiplex (TDM) bus. A CCU comprises two Channel Elements (CEs). Each CE supports a CDMA channel performing baseband processing. The BIU controls TDM bus interface for the ACU and BCR.

In the forward link, the ACU (one per sector) digitally combines CCU outputs and performs digital-to-analog (D/A) conversion to intermediate frequency (IF). Each ACU can connect to all BCRs in order to support softer handoff. The BCR (one per sector) combines the IF signals and up-converts to RF for transmission.




In the reverse link, the BCR down-converts RF to IF and distributes the signals to the appropriate ACUs. The ACU A/D converts the IF to digital and passes the signals to the CCUs.

Figure 3-6. 3-Sector Configuration

CDMA

CCCACU

BCR

CCU

CCUCCU

CDMA

CCC

D

F

U

CCU

LAF

AIF

&ACU BCR

CDMA

CCC

CCU

CCUCCU

CDMA

CCC

CCU

BIU

BIU

ACU BCR

CDMA

CCC

CCU

CCUCCU

CDMA

CCC

CCU

BIU

CRTU

GPSSCT




CDMA System Capacities 3

This section summarizes major system capacities.

■ The CDMA BHCA capacity is a percentage of the total BHCA capacity of AUTOPLEX System 1000 product.

■ The CDMA system handles up to 10 CDMA carriers (1.23 MHz each). CDMA carriers can be re-used in every sector and every cell.

■ A SII /CDMA cell site with two CDMA Growth frames support up to 336 channel elements (CEs).

■ The MSC supports:

— 222 cell sites

— 17,000 trunks

— 30 packet pipes per cell site

■ The 5ESS DCS supports:

— The Packet Switching Unit (PSU) and the Switching Modules (SMs) support at least 10000 BHCA.

— The SM handles up to 108,000 BHCA

— The PSU requires 35 speech handler cards to handle 10,000 BHCA (assuming call holding time of 100 seconds).




4
Call Processing
Mobile Access 1

■ Introduction 1

■ Description of IS-95A Mobile Access Protocol 1

■ Average Persistence Delay for Access Request Attempt 9

■ Traffic, Throughput, and Delay Performance for IS-95A MobileAccess Protocol 15

■ Recommended Value of Access Parameters 23

Handoff 25

■ Hard Handoff 25

■ Soft and Softer Handoff 26

Definition 26

Procedure 26

Comparisons 27

Performance 29

Parameters 38


y

Contents




44
Call Processing
Mobile Access 4

Introduction 4

In IS-95A [1], CDMA mobiles transmit on the access channels according to a random access protocol. Detailed procedures of this random access protocol and ranges of various access parameters can be found in TIA IS-95A Standard. In references [4] and [5], protocol performance has been analyzed and appropriate settings of mobile access parameters have been determined. “Mobile Access” consists of five subsections. In “Description of IS-95A Mobile Access Protocol”, the operation of the mobile access protocol and its associated parameters are described. In “Average Persistence Delay for Access Request Attempt”, based on reference [4], persistence delays for access request attempt are presented. In “Traffic, Throughput, and Delay Performance for IS-95A Mobile Access Protocol”, based on reference [5], traffic, throughput, and delay performance of the mobile access protocol are presented. In “Recommended Value of Access Parameters”, settings of the various parameters associated with the mobile access channel are recommended.

Description of IS-95A Mobile Access Protocol 4

As described in IS-95A, the mobile transmits on the access channel using a random access procedure. A flow chart of the CDMA mobile access protocol is shown in Figure 4-1. The entire process of sending one message and receiving (or failing to receive) an acknowledgment for that message is called an access attempt. Each transmission in the access attempt is called an access probe (see Figure 4-2). The mobile transmits the same message in each access probe in an access attempt. Each access probe consists of an access channel preamble and an access channel message capsule.


oprietaryge

Call Processing 401-614-012

The length of the preamble as well as the length of message capsule are expressed with the number of 20 millisecond frames. Thus,

the duration of an access probe (access channel slot) is frames.

Within an access attempt, access probes are grouped into access probe sequence. There are two types of messages sent on the access channel: a response message (one that is a response to a base station message, see Figure 4-3), or a request message (one that is sent autonomously by the mobile, see Figure 4-4). Each access attempt consists of up to max_req_seq (for a request access) or max_rsp_seq (for a response access) access probe sequences.

The timing of the start of each access probe sequence is determined pseudo randomly. For every access probe sequence, a backoff delay, RS, from 1 to

slots is generated pseudo randomly.

In the case for request access probe sequences, for each slot after the backoff delay RS, the mobile performs a pseudo random persistence test. If the persistence test passes, the first probe of the sequence begins in that slot. If the persistence test fails, the access probe sequence is deferred until at least the next slot. Thus, an additional delay, PD, is imposed by the use of a persistence test. For each access channel slot, the persistence test generates a random number and compares it with a pre-determined threshold. The pre-computed threshold is different depending on the nature of the request, the access overload class n and its persistence value psist(n) as well as its persistence modifier msg_psist for message transmission or reg_psist for registrations.

Each access probe sequence consists of up to access probes, all transmitted on the same access channel. The access channel number, RA, used for each access probe sequence is chosen pseudo randomly from 0 to acc_chan among all the access channels associated with the current paging channel. The mobile will use this access channel number for all access probes in that access probe sequence. The first access probe of each access probe sequence is transmitted at a specified power level relative to the nominal open-loop power level. Each subsequent access probe is transmitted at a power level a specified amount PI (determined from pwr_step) higher than the previous access probe until an acknowledgment response is obtained or the sequence ends. Between access probes, the mobile will disable its transmitter.

The mobile transmits the first probe in each access probe sequence at a mean output power level (referenced to the nominal CDMA channel bandwidth of 1.23 MHz) depending on open-loop power estimate, the initial power offset for access init_pwr and the nominal transmit power offset nom_pwr.

1+ pam sz_3+ max cap sz_ _

4+ +pam sz max cap sz_ _ _

1+ bkoff

1+ num step_




The timing of access probes and access probe sequences is expressed in terms of access channel slots. The transmission of an access probe begins at the start of an access channel slot. The precise timing of the access channel transmissions in an access attempt is determined by a procedure called PN randomization. For the duration of each access attempt, the mobile computes a delay, RN, from 0 to

PN chips using a (non-random) hash function that depends on its electronic serial number ESN. The mobile delays its transmit timing by RN PN chips. This transmit timing adjustment includes delay of the direct sequence spreading long code and of the quadrature spreading I and Q pilot PN sequence, so it effectively increases the apparent range from the mobile to the base station. This increases the probability that the base station will be able to separately demodulate transmissions from multiple mobiles in the same access channel slot, especially when many mobiles are at a similar range from the base station.

Timing between access probes of an access probe sequence is also generated pseudo randomly. After transmitting each access probe, the mobile waits a specified period, milliseconds, from the end of the slot to receive an acknowledgment from the base station. If an acknowledgment is received, the access attempt ends. If no acknowledgment is received, the next access probe is transmitted after an additional backoff delay RT, from 1 to

slots.

2 probe pn ran_ _

( )TA acc tmo= × +80 2 _

1+ probe bkoff_




Figure 4-1. CDMA Mobile Access Protocol




Figure 4-2. Mobile Access Probe

��




Figure 4-3. Mobile Access Response Attempt




Figure 4-4. Mobile Access Request Attempt




Average Persistence Delay for Access Request Attempt 4

As discussed in “Description of IS-95A Mobile Access Protocol”, for access due to mobile request, TIA IS-95A requires that persistence test be performed prior to initiating the access probe sequence in order to control the rate at which the mobile transmits request. To assess the appropriate range of persistence values to be assigned to the mobiles, the amount of delays due to persistence tests are needed. In this section, based on reference [4], average persistence delays as a function of persistence values are calculated for various types of request and access overload classes.

For each access channel slot, the persistence test generates a random number and compares it with a pre-determined threshold . If the generated random number RP is smaller than the pre-determined threshold P, access probe sequence is initiated. Since the random number is generated from uniform distribution over the unity interval, thus

In other words, the larger implies the higher probability to initiate the access probe sequence.

The pre-computed threshold , in general, is different depending on the nature of the request, the access overload class, and its persistence value , as well as its persistence modifier. As an example, for registration request of access overload classes 0 through 9, if , then , thus the persistence test fails, and no access probe sequence is initiated; if is not equal to 63, for a given persistence modifier , is monotonic, decreasing function of

; the largest , the smaller , thus the smaller probability to initiate the access probe sequence. Table 4-1 summarizes the persistence test thresholds for various types of requests and access overload classes.

From Table 4-1, it is noted that maximum persistent value is 63 for access overload classes 0 through 9, and is 7 for access overload classes 10 through 15. If the maximum persistent value is assigned to the mobile, the access attempts fails, and the mobile enters the system determination substate of the mobile initialization state [1].

( )0 1< <RPRP PP

P

{ }Pr RP P P< =

P

P( )psist n

( )psist n = 63 P = 0( )psist n

reg psist_ P( )psist n ( )psist n P




For persistence value not equal to the maximum, random persistence delays can be incurred to control the transmissions of mobile requests. The persistence delay PD is the number of times to perform the tests before the condition is satisfied. Thus, persistence delay PD is a random variable, and its discrete probability density is geometric with parameter P, that is,

One performance measure to characterize the persistence test is the average persistence delay. Let be the expectation operator. The average persistence delay is:

Using the values of persistence test thresholds P in Table 4-1 for various types of requests and access overload classes, Table 4-2 summarizes their average persistence delay .

Table 4-1. Persistence Test Thresholds for Registration, Message, and Other Requests

Persistence Test Threshold

access overload classes access overload classes

Registration Request 0 0

Message Request 0 0

Other Request 0 0

P

n= 0 1 9, , , � n=10 11 15, , , �

( )psist n ≠ 63 ( )psist n = 63 ( )psist n ≠ 7 ( )psist n = 7( )

2 4− −

psist nreg psist_ ( )2− −psist n reg psist_

( )

2 4− −

psist nmsg psist_ ( )2− −psist n msg psist_

( )

2 4−

psist n( )2− psist n

RP P<

{ } ( )Pr , , ,PD k P P kk= = − = slots for ,1 0 1 2 �

{}E ⋅{ }E PD

{ } { }E PrPD k PD kk

= ==

∞

∑ slots0

= −1 P

P

{ }E PD




The derived expressions of the average persistence delay in Table 4-3 are numerically evaluated. For registration requests with persistence modifiers ( ) 0 through 7, average persistence delay as function of persistence value are plotted for access overload classes 0 through 9 in Figure 4-5, and for access overload classes 10 through 15 in Figure 4-6, respectively. For message requests with persistence modifiers ( ) through 7, average persistence delay as function of persistence value are plotted for access overload classes 0 through 9 in Figure 4-5, and for access overload classes 10 through 15 in Figure 4-8, respectively. For other requests, average persistence delay as function of persistence value are plotted for access overload classes 0 through 9 in Figure 4-9, and for access overload classes 10 through 15 in Figure 4-10.

In general, for each type of request and access overload class, a maximum allowable persistence delay is expected not to be exceeded. Thus, given the maximum allowable persistence delay for each type of request and access overload class, the appropriate range of persistence values can be assessed.

Table 4-2. Average Persistence Delay for Registration, Message and Other Requests

Average Persistence Delay

access overload classes access overload classes

Registration Request

Message Request

Other Request

{ }E PD

n= 0 1 9, , , � n=10 11 15, , , �

( )psist n ≠ 63 ( )psist n = 63 ( )psist n ≠ 7 ( )psist n = 7( )

2 14

psist nreg psist+

−_ ∞ ( )2 1psist n reg psist+ −_ ∞

( )

2 14

psist nmsg psist+

−_ ∞ ( )2 1psist n msg psist+ −_ ∞

( )

2 14

psist n

− ∞ ( )2 1psist n − ∞

reg psist_

msg psist_




Figure 4-5. Average Persistence Delay for Registration Request of Access Overload Classes 0 through 9

Figure 4-6. Average Persistence Delay for Registration Request of Access Overload Classes 10 through 15

Persistence Value

Ave

rag

e P

ers

iste

nc

e D

ela

y (s

lots

)

0.1

1

10

100

1000

10000

100000

1000000

10000000

0 5 10 15 20 25 30 35 40 45 50 55 60 65

reg_psist = 0 reg_psist = 1 reg_psist = 2 reg_psist = 3


Persistence Value

Ave

rag

e P

ers

iste

nc

e D

ela

y (s

lots

)

1

10

100

1000

10000

0 1 2 3 4 5 6






Figure 4-7. Average Persistence Delay for Message Request of Access Overload Classes 0 through 9

Figure 4-8. Average Persistence Delay for Message Request of Access Overload Classes 10 through 15

Persistence Value

Ave

rag

e P

ers

iste

nc

e D

ela

y (s

lots

)

0.1

1

10

100

1000

10000

100000

1000000

10000000

0 5 10 15 20 25 30 35 40 45 50 55 60 65

msg_psist = 0 msg_psist = 1 msg_psist = 2 msg_psist = 3


Persistence Value

Ave

rag

e P

ers

iste

nc

e D

ela

y (s

lots

)

1

10

100

1000

10000

0 1 2 3 4 5 6






Figure 4-9. Average Persistence Delay for Other Requests of Access Overload Classes 0 through 9

Figure 4-10. Average Persistence Delay for Other Requests of Access Overload Classes 10 through 15

Persistence Value

Ave

rag

e P

ers

iste

nc

e D

ela

y (s

lots

)

0.1

1

10

100

1000

10000

100000

0 5 10 15 20 25 30 35 40 45 50 55 60 65

Persistence Value

Ave

rag

e P

ers

iste

nc

e D

ela

y (s

lots

)

1

10

100

0 1 2 3 4 5 6




Traffic, Throughput, and Delay Performance for IS-95A Mobile Access Protocol 4

As described in “Description of IS-95A Mobile Access Protocol”, CDMA mobiles transmit on the access channel according to a random access protocol. The access channel performance is characterized by its packet throughput and average delay under certain offered traffic load. In reference [5], analytical expressions to relate offered traffic, throughput, and average packet delay in terms of various system parameters of the IS-95A mobile access protocol have been derived. It is shown that traffic-throughput and throughput-delay characteristic of a CDMA access channel depends on the number of access channels, number of probe sequence, number of probes per sequence, sequence backoff, probe backoff, acknowledgment time-out, power increment, cell radius, spread spectrum chip rate, and resolution time window due to random access. Since access transmissions are bursty, large numbers of active accesses can be supported. Maximum number of supportable accesses depends on throughput-delay trade-off, preamble length, message capsule length and user burst rate. maximum number of accesses that can be supported are increased as the throughput rate increases. For performance trade-off, extensive numerical results for various values of system parameters have also been obtained in reference [5].

In this section, throughput as a function of offered traffic is plotted in Figure 4-11 and Figure 4-12; average packet delay as a function of throughput is plotted in Figure 4-13 through Figure 4-18.

It is noted that maximum achievable throughput can be increased by using a larger power increment. The use of power increment 1 dB can reduce the maximum achievable throughput by 40% of that obtained from the use of power increment 7 dB. Also, for a given throughput, average packet delay can be decreased by using larger power increment.

Acknowledgment time-out is determined by round-trip propagation delay, queuing delay, as well as the amount of time required to process the access message and to generate the acknowledgment; message. Acknowledgment time-out does not affect the system throughput, however, it affects the average packet delay. Shorter acknowledgment time-out can result in lower average packet delay.

Average packet delay and maximum achievable throughput are functions of probe backoff. The use of smaller probe backoff can increase the probability of collision for retransmitted packets, thus reduces the maximum achievable throughput. On the other hand, the use of larger probe backoff can increase the average packet delay. Numerical results indicate that the use of probe backoff equal to 8 slots can provide lower delay when operating at throughputs 80 to 90% of the ultimate capacity.

In the next section, recommended settings of access parameters of the IS-95A CDMA mobile access protocol are summarized.




Figure 4-11. Throughput versus Offered Traffic for Power Increment from 1 dB to 7 dB: 1 access channel; 15 probe sequences; 16 probes per sequence; sequence backoff 16 slots; probe backoff 16 slots.

Offered Traffic (packets/slot)

Thro

ughp

ut (

pa

cke

ts/s

lot)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10 12 14 16 18 20

power increment = 1 dB

2 dB

3 dB

4 dB

5 dB

6 dB

7 dB




Figure 4-12. Throughput versus Offered Traffic for Probe Backoff from 2 slots to 16 slots: 1 access channel; 15 probe sequences; 16 probes per sequence; sequence backoff 16 slots; power increment 7 dB.

Offered Traffic (packets/slot)

Thro

ughp

ut (

pa

cke

ts/s

lot)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10 12 14 16 18 20

probe backoff = 2 slots

4 slots

8 slots

16 slots




Figure 4-13. Average Packet Delay versus Throughput for Power Increment from 1 dB to 7 dB: 1 access channel; 15 probe sequences; 16 probes per sequence; sequence backoff 16 slots; probe backoff 16 slots; acknowledgment time-out 3 slots.

Throughput (packets/slot)

Ave

rag

e P

ac

ket D

ela

y (s

lots

)

10

100

1000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1


2 dB

3 dB

4 dB

5 dB

6 dB

7 dB




Figure 4-14. Average Packet Delay versus Throughput for Power Increment from 1 dB to 7 dB: 1 access channel; 15 probe sequences; 16 probes per sequence; sequence backoff 16 slots; probe backoff 16 slots; acknowledgment time-out 6 slots.


Ave

rag

e P

ac

ket D

ela

y (s

lots

)

10

100

1000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1


2 dB

3 dB

4 dB

5 dB

6 dB

7 dB




Figure 4-15. Average Packet Delay versus Throughput for Probe Backoff from 2 slot to 16 slots: 1 access channel; 15 probe sequences; 16 probes per sequence; sequence backoff 16 slots; acknowledgment time-out 3 slots; power increment 7 dB.


Ave

rag

e P

ac

ket D

ela

y (s

lots

)

1

10

100

1000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1


4 slots

8 slots

16 slots




Figure 4-16. Average Packet Delay versus Throughput for Probe Backoff from 2 slots to 16 slots: 1 access channel; 15 probe sequences; 16 probes per sequence; sequence backoff 16 slots; acknowledgment time-out 6 slots; power increment 7 dB.

1

10

100

1000

0 0.2 0.4 0.6 0.8 1 1.2



4 slots

8 slots

16 slots




Figure 4-17. Average Packet Delay versus Throughput for Acknowledgment Time-out from 3 slots to 15 slots: 1 access channel; 15 probe sequences; 16 probes per sequence; sequence backoff 16 slots; probe backoff 4 slots; power increment 7 dB.

1

10

100

1000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

T hroughput (packets/slot)

ack time-out = 3 s lots

6 s lots

9 s lots

12 slots

15 slots




Figure 4-18. Average Packet Delay versus Throughput for Acknowledgment Time-out from 3 slots to 15 slots: 1 access channel; 15 probe sequences; 16 probes per sequence; sequence backoff 16 slots; probe backoff 8 slots; power increment 7 dB.

Recommended Value of Access Parameters 4

In this section, recommended settings of access parameters of the IS-95A CDMA mobile access protocol are presented. The recommendations are based on the results from the analysis of the performance of IS-95A CDMA mobile access protocol in references [4] and [5], which have been summarized in previous sections, “Average Persistence Delay for Access Request Attempt” and “Traffic, Throughput, and Delay Performance for IS-95A Mobile Access Protocol”.


Ave

rag

e P

ac

ket D

ela

y (s

lots

)

1

10

100

1000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

ack time-out = 3 slots

6 slots

9 slots

12 slots

15 slots




m

m

.

ac

pw

n

p

m

b

p

a

p

Table 4-3. Recommended Access Parameters

Recommended

Binary Field Value

ax_req_seq 1111 For an access channel request, maximum number of access probe sequences max_req_seq is 15.

ax_rsp_seq 1111 For an access channel response, maximum number of access probe sequences max_rsp_seq is 15.

1111 In each access probe sequence, maximum number of access probes is 16.

0001 The length of the access channel preamble is 2frames.

111 The message length is 10 frames. Since the access channel slot length is equal to the sum of preamble length and message length, the access channel slot length (packet length) is 12 frames, that is, 240 ms long

c_chan 00000 for

low traffic;

00001 for

high traffic

Number of access channels available for personal station access is . Under low-traffic conditions, oneaccess channel is recommended. Under high-traffic conditions, two access channels are recommended. If acc_chan = 00000, then access channel number RA is equal to 00000. If acc_chan = 00001, then access channelnumber RA is selected randomly from 00000 and 00001.

0111 Maximum number of slots that personal station is to delay due to random backoff between successive access probe sequences, , is 8. Sequence backoff RS is selected randomly from 1 to 8 slots.

0111 Maximum number of slots that personal station is to delay due to random backoff between the consecutive access probes, , is 8. Probe backoff RT is selected randomly from 1 to 8 slots.

1000 Acknowledgment time-out TA is:

ms = 800 ms.

r_step 0110 Power increment PI = pwr_step, equal to 4 dB.

0111 Time randomization range is from 0 to 127 chips

um step_1+num step_

am sz_ 1+ pam sz_

ax cap sz_ _ 3+max cap sz_ _

1+ acc chan_

koff

1+ bkoff

ro b e b k o f f_

1+ probe bkoff_

cc tmo_( )80 2× + acc tmo_

robe pn ran_ _




Handoff 4

The CDMA system supports several types of handoff. These include: hard handoff, soft handoff, and softer handoff. Handoff procedures and parameters are tailored to maintain call integrity while enhancing CDMA operation.

"Hard" handoff requires a brief interruption of the traffic channel; "soft" handoff does not. For example, a handoff from CDMA to an analog system is hard because the link must be interrupted while the mobile station switches from the CDMA carrier to an analog channel. As another example, if inter-MSC soft handoff is not supported, a handoff from CDMA to CDMA between cells belonging to different MSCs is hard. In soft handoff, the mobile's call is simultaneously supported by multiple sectors. No interruption of the link occurs.

In the following, handoff types are defined further. Considerations relevant to parameter settings for soft and softer handoff are then described.

Hard Handoff 4

Hard handoffs are characterized by a brief interruption of the communication link. Examples of hard handoff include handoff from CDMA to an analog system, a change to a different frame offset, and handoff from one CDMA carrier to another. (Handoff from analog to CDMA is not supported.) In CDMA-to-analog, the link is interrupted while the mobile switches from the CDMA carrier to an available analog channel. Similarly, the link must be interrupted if the mobile switches from one CDMA carrier to another. This type of handoff is applicable only in situations where two CDMA carriers are available. In a frame offset change, the link must be interrupted while the mobile changes the offset of its frame transmissions with respect to system time.

A CDMA-to-analog handoff can occur at the boundary between CDMA and analog systems. It can also occur within the CDMA service area if the system is deployed (overlaid) in an area already having analog service. These types of handoffs must be "blind" because the analog system cannot track or communicate with the mobile while it is on the CDMA carrier. Accordingly, the mobile must hand off to an analog channel that is unambiguously associated with the CDMA serving sector. Handoff to a CDMA beacon which then facilitates handoff to an analog channel is also possible. Strategies for accomplishing CDMA-to-analog handoff are considered further in Chapter 5, “Methodology For Developing a CDMA Deployment Plan”.




In early deployment of CDMA, soft handoff may not be supported between cells that belong to different MSCs. Accordingly, CDMA-to-CDMA hard handoff will occur across the cells on the MSC’s boundary. The absence of inter-MSC soft handoff can potentially impact system performance along the MSC’s boundary; the impacts are considered further in Chapter 4, “Coverage Probability for Pilot Channel”.

Soft and Softer Handoff 4

Definition 4

In soft handoff, the mobile's call is simultaneously supported by multiple cells. In softer handoff, the mobile's call is simultaneously supported by multiple sectors of the same cell. The mobile continuously scans for the pilot signals transmitted by each cell/sector (site), and establishes communication with any site (up to 3) whose pilot power exceeds a given threshold. Since the same wideband channel is reused by every cell, these types of handoff do not require an interruption of the communication link.

Procedure 4

The soft and softer handoff procedures dictate the way in which a call is maintained as a mobile crosses boundaries between CDMA cells.

In soft handoff, the mobile's call is simultaneously supported by multiple cells; in softer handoff, the call is simultaneously supported by multiple sectors of the same cell. Each sector transmits a pilot signal of sufficient power to be detected by mobiles within its vicinity. The mobile continuously scans for pilots, and establishes communication with any sector (up to three) whose pilot exceeds a given threshold. Similarly, communication with sectors whose pilot drops below a threshold is terminated.

The identification of distinct pilot signals by the mobile relies on the fact that each pilot exhibits a different time offset within the same PN code. The mobile's search for pilots is facilitated by the fact that these offsets are in integer multiples of a known time delay. The pilots identified by the mobile, as well as other pilots specified by the serving sectors(s), are categorized by the mobile as follows:

■ The Active Set consists of those pilots whose sites are currently supporting the mobile's call.

■ The Candidate Set consists of those pilots whose sites, based on the received strength of their pilots, could also support the mobile's call.




■ The Neighbor Set consists of those pilots whose sites are not in the active set or the candidate set but are nevertheless likely candidates for soft handoff (for example, these sites may be in known geographic proximity).

■ The Remaining Set consists of those pilots within the CDMA system but not within the other three sets.

Movement of pilots among the sets is determined by the mobile's assessment of pilot signal strength and a set of (adjustable) thresholds. This movement is coordinated with the serving sector. The mobile assesses pilots by comparing pilot strengths to one another, and by comparing each pilot's power to the total received forward link power. The latter comparison (normalized pilot strength) is the ratio of the pilot energy in a time chip to the spectral density of total received forward link power:

(Equation 4-1)

where:

= Thermal noise power.

Pilots in the neighbor and/or remaining set whose Ec/Io exceeds a threshold T_ADD are associated with sites that can support the call; accordingly, these pilots are moved to the active or candidate set. Similarly, pilots in the active and/or candidate set whose Ec/Io drops below a threshold T_DROP for a period of time exceeding the parameter T_T_DROP are moved to the neighbor or remaining set. Finally, a candidate set pilot whose strength exceeds an active set pilot by at least T_COMP will be moved to the active set, possibly displacing that pilot.

Comparisons 4

Further insight into soft handoff operation can be gained by contrasting this process with the conventional handoff process used in an analog system.

E

I

P W

N P Wc

i

i

th j0

=

+ ∑µ /

/all j

Ec = time chip energy received from ith sector

I0 = spectral density of total received interference

µ = fraction of sector power allocated to pilot signal

Pi = received power from ith sector

W = system bandwidth

Nth




In an analog system, each cell is assigned a set of narrowband channels for use in communication links. Cochannel interference is controlled by not reusing the same channels in adjacent cells. A mobile proceeding out of one cell into another must switch to an available channel in the new cell. This switch requires a brief interruption of the communication link. In a CDMA system, the same wideband channel is reused in every cell. Cochannel interference is accepted but controlled so as to achieve greater capacity. Accordingly, soft/softer handoff does not require channel switching and its associated link interruption. Moreover, with proper threshold settings, the acquisition of new sites is accomplished before the old (serving) sites are too far away to be useful. The soft handoff procedure is more robust because the connection with the new host(s) is made before the connection with the old is broken. This process is often referred to as a make-before-break connection, as opposed to the analog break-before-make.

The use of a break-before-make handoff procedure (that is, no soft handoff) in a CDMA system could have adverse consequences for system operation. The inability to exploit the diversity gain inherent in multi-site support can cause the areas of cell boundary overlap (the locations furthest from cell sites) to be regions of poor link quality. The mobiles in these fringe areas would also be more susceptible to cell site interference (see Chapter 4, “Coverage”). These effects increase the probability that a call will be dropped, since the handoff procedure would typically not be initiated until a mobile reached this area (that is, until the mobile noted a drop in signal strength from its host cell). In addition, the use of power control without soft handoff could create a situation where a mobile generates considerable amounts of interference to neighbor cells. This interference reduces capacity.

The last situation arises because the mobile would detect a drop in received signal strength before it requested a handoff. Since cell boundaries overlap, this reporting point could be well into the boundary of the neighbor cell. Within this area, power control would boost the mobile's transmit strength in an attempt to maintain the link with the (distant) serving cell. This call-dragging phenomena reduces the capacity of the neighbor cell because the mobile's transmissions increase the level of interference at the neighbor cell. In contrast, if the mobile were in soft handoff, power control commands from both cells would ensure that the mobile did not produce undue interference; in fact, the reverse link could be maintained at a lower level of mobile transmit power due to the gain involved in combining the signals received at the two cell sites.




Performance 4

Judicious use of the soft/softer handoff process can enhance service by raising call quality, improving coverage, and enhancing capacity. In the following, we discuss the considerations and trade-offs involved in realizing these benefits. Example of soft handoff performance follow.

Considerations 4

In the following, we consider the impact of the parameter T_ADD. Considerations for other parameters (for example, T_DROP) are analogous.

The setting of T_ADD for each cell site will impact site coverage. This value, coupled with the site pilot strengths and area propagation characteristics, will determine those regions in which a mobile will add the site pilot to its active/candidate set. (Similarly, T_DROP will determine regions in which the pilot will be dropped). These regions, together with typical traffic distributions, will determine the handoff populations, that is, the number of mobiles in handoff with 2 or 3 sites, and the number of mobiles in softer versus soft handoff. The number and location of mobiles in handoff will impact call quality as well as site capacity. For example, a setting that places mobiles in fringe areas (areas where the link is poor due to distance/propagation) into soft handoff will improve call quality via diversity gain. Similarly, a setting which places mobiles in soft handoff before they get too close to neighbor sites will lower the mobile's required transmit strength, reducing interference and improving capacity. Finally, the setting will impact the probability that a mobile detects a pilot, as well as the probability that it mistakes noise/interference for a pilot.

Some insight into the above considerations can be obtained by examining the results of generic simulations using randomly distributed mobiles and simple path loss laws. The following results were obtained with cells of 8 mile radius and a 38.4 dB/decade path loss (110.7 dB loss at one mile intercept). Further methodology for each study is as indicated.

Coverage Contour 4

Coverage contour surrounding each cell is the connected set of locations where the Ec/Io relative to that cell is equal to the value T_ADD:

(Equation 4-2)100

logE

ITADDc

=




Values of Ec/Io within the contour will be greater than T_ADD; values outside the contour will be less. Accordingly, a mobile crossing the boundary into the cell will add that cell's pilot to its active set. (A mobile crossing the boundary out of the cell will not necessarily drop the pilot, as this function depends on the values of T_DROP and T_T_DROP.) Coverage areas also change with varying T_ADD.

Populations 4

The impact of T_ADD on handoff populations is shown in Figure 4-19 and Figure 4-20 for omnidirectional and sectorized cells, respectively. These curves were obtained from a two-tier service area in which mobiles were randomly distributed. This study is described in greater detail in reference 10.

The fraction of mobiles in handoff with n cells (n-cell soft handoff) is shown in Figure 4-19 for omnidirectional cells. Note that, in this nomenclature, 1-cell soft handoff is equivalent to no soft handoff. The fraction of mobiles in soft and/or softer handoff with n cells (n-cell soft/softer handoff), as well as in softer handoff with a single cell (softer only), is shown for sectorized cells in Figure 4-20. Both curves also indicate the total fraction of mobiles in a soft handoff and/or softer handoff condition.

The figures indicate how the handoff populations change as the threshold T_ADD is varied. As expected, the total fraction of mobiles in a soft and/or softer handoff state drops as the threshold is raised. Handoff populations for thresholds above T_ADD=-11 dB are seen to be negligible. The 2-cell soft handoff curve in Figure 4-19 exhibits a definite maximum across the T_ADD range: as the threshold is lowered, more mobiles go into handoff and the curve rises; as the threshold is lowered further, more and more of these mobiles go into 3-cell soft handoff and the population in the 2-cell soft handoff state must drop. These considerations apply to any n-cell soft or soft/softer curve. In contrast, the fraction of mobiles in softer only handoff (Figure 4-20) remains approximately constant over the T_ADD range. As the threshold is lowered, mobiles not previously in a handoff state enter into softer handoff with an adjacent sector. At the same time, mobiles already in this state enter into handoff with a sector from another cell. The curve is flat because the rate of mobiles entering the softer only category roughly balances the rate at which they leave.




Figure 4-19. Handoff Populations for Omnidirectional Cells

Figure 4-20. Handoff Populations for Sectorized Cells

F raction in S oft Handoff

T ADD (dB )

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

total

2-cell

3-cell

more-cell

F raction in S oft/S ofter Handoff

T ADD (dB )

00.10.20.30.40.50.60.70.80.9

total soft/softer

tot soft

2-cell

3-cell

softer only

more-cell




Pilot Detection 4

The impact of the T_ADD setting on the probability of detecting a pilot and on the probability of mistaking noise/interference for a pilot is shown in Figure 4-21 and Figure 4-22, respectively. These plots are based on a simplified mobile searcher element algorithm and on a forward link modulation model. The latter comprises weighted message bit streams coded by Walsh functions and in-phase and quadrature PN sequences:

(Equation 4-3)

where:

.

At the mobile, the signal is heterodyned and decoded. A pilot from a particular sector is sought by using that sector's PN sequences and the zero Walsh function in the decoding. If the pilot is present, the receiver output consists of pilot plus interference (noise) and can be related to a non-central chi-square distribution. If the pilot is absent, the receiver output consists of noise (interference) alone and can be related to a central chi-square distributions. The probability of detecting the pilot if present and the probability of mistaking noise for pilot when pilot is absent can be computed from these distributions. The primary source of interference in each case is the total broadband power received on the forward link from all sectors.

s t A t p t c m t w t A t q t c m t w tb b b bk bk bk b b b bk bk bk( ) cos( ) ( ) ( ) ( ) sin( ) ( ) ( ) ( )= + + +∑ ∑ ∑ ∑ω φ ω φal l b al l k al l b al l k

s t( ) = received signal

Ab = carrier amplitude for bth sector

φb = phase for bth sector

p tb ( ) = PN code for bth sector (in - phase)

q tb( ) = PN code for bth sector (quadrature )

m t m tbk bk( ) ( ( ) )= message (bit ) stream 2 1=

w tbk( ) = Walsh function for kth signal in bth sectorc cbk bk= allocation coefficient for kth signal in bth sector (

k

2 1∑ = )




The probability of detection and the probability of false alarm are shown in Figure 4-21 and Figure 4-22, respectively. These probabilities are plotted against Ec/Io, which represents a measure of the received pilot strength at the mobile. The setting T_ADD functions as a detection threshold since it represents the point at which the decision to add the pilot to the mobile's active/candidate set is made. The threshold T_ADD should be chosen low enough so that the probability of detecting a pilot if present is very high; however, too low a setting will result in an unacceptable number of false alarms.

.

Figure 4-21. Probability of Detecting a Pilot Signal When Present

Probability of Detecting Pilot

0

0.2

0.4

0.6

0.8

1

1.2

-20 -19 -18 -17 -16 -15 -14 -13 -12 11

received Ec/Io

pro

ba

bili

ty TADD=-14 dB

TADD=-15 dB

TADD=-16 dB

TADD=-17 dB




Figure 4-22. Probability of Mistaking Noise/Interference for Pilot When Absent

Mobile Transmitter Strength 4

Reduced mobile transmit strength in soft/softer handoff is possible because of the gain inherent in the use of multiple sector receivers. In soft handoff, this gain is realized at the switch by selecting the best signal (on a frame-to-frame basis) of those received. In softer handoff, this gain is realized at the cell site channel element by combining the signals received from multiple sectors.

The impact of soft handoff T_ADD on mobile transmit strength is shown in Figure 4-23. This curve was obtained from a two-tier service area of omnidirectional cells in which mobiles were randomly distributed. The figure shows the average difference in transmit strength for center cell mobiles that go into soft handoff. The diversity gain from multiple cell sites is modeled by coherently combining the signals received at the cell sites.

The average difference in transmit strength is plotted as a function of varying handoff threshold T_ADD. At high thresholds, only 2-cell soft handoff is possible. The average transmit reduction is high because only mobiles advantageously placed with respect to two cells (for example, mobiles equidistant between the two) go into soft handoff. At lower thresholds, more mobiles go into 2-cell soft handoff with distant cells. The average reduction is less because one of the signals is slightly weaker, and therefore more frequently ignored. The average reduction improves for lower thresholds because some of the mobiles go into 3-cell soft handoff, with commensurately greater diversity gain.

Probability of False Alarm

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

-20 -19 -18 -17 -16 -15 -14

TADD (dB)

log

10(p

rob

ab

ility

)




The transmitter strengths accordingly fluctuate with T_ADD but consistently show a net drop for all thresholds considered.

Figure 4-23. Impact of Soft Handoff T_ADD on Mobile Transmit Power

Interference 4

The drop in mobile transmitter strength with varying T_ADD was considered in “Mobile Transmitter Strength”. This drop was obtained by enabling soft handoff for center cell users in a two-tier service area. In this section, the corresponding impact on center cell site received power is considered. This impact is of some interest as the level of received cell site power is the interference that a mobile must overcome in order to establish and/or maintain a call.

The net impact on the center cell received power is shown in Figure 4-24. The power shows a general decrease as T_ADD is lowered. The rate of decrease is not constant because it depends on how many mobiles are in soft handoff and on what transmit strength reductions transmit strength reductions these mobiles are able to achieve (see Figure 4-23). Since the received power is the interference that a new mobile must overcome to establish a call, the net decrease indicates that judicious use of soft handoff can improve capacity.

Soft handoff requires the use of a channel element at all cell sites supporting the call. The benefit of reduced interference must therefore be balanced against the resource cost of putting a mobile into soft handoff. Marginal benefit may result if the threshold is low enough to place mobiles close to their serving cell sites into soft handoff with distant cells. In this case, the signal frames received at the distant cells would frequently be ignored (that is, not selected) in favor of those arriving at the serving site.

Change in Mobile T ransmit Power

T ADD (dB )

-2.7

-2.5

-2.3

-2.1

-1.9

-1.7

-1.5

-18 -17 -16 -15 -14 -13 -12




Accordingly, little to no interference reduction could accrue. In contrast, selecting handoff thresholds that place mobiles roughly equidistant between two cells into soft handoff with these cells is fully cost-effective. Neither received signal is favored at the switch, and the mobiles realize the full effect of diversity gain.

Figure 4-24. Impact of Soft Handoff on Received Power at Center Cell Site

Change in Cell Received Power

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

-18 -17 -16 -15 -14 -13 -12

TADD (dB)

dB

cha

nge




Figure 4-25. Soft Handoff Impact on Center Cell Site Received Power (nonuniform mobile distribution)

Some insight into this comparison can be obtained by examining the decrease in center cell site interference for a non-random distribution of mobiles in a one-tier service area. In order to accentuate the effect, center cell mobiles are placed with increased density near the center cell site and interference from outer cells is neglected. The center cell mobiles are put into soft handoff with the outer cells progressively, proceeding from the outer mobiles inward. Only 2-cell soft handoff is allowed.

The drop in cell site interference as a function of the percentage of mobiles in soft handoff is shown in Figure 4-25. The cell site interference drops significantly at first but levels out around the 30% point. Placing further layers into soft handoff beyond this point only marginally lowers the cell site interference and is less cost-effective.

Capacity 4

Separate studies have shown that a drop in cell site interference is accompanied by a corresponding increase in cell capacity. This effect becomes negligible when the decrease in interference is marginal. The increase in capacity can be measured by computing the cell's new pole point when soft handoff is enabled (see reference 11).

Change in Cell Site Received Power

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

0 15 30 45 60 75

% mobiles in 2-cell soft handoff

dB

cha

nge




Parameters 4

This section summarizes the use and impact of soft handoff parameters. These parameters are specified in IS-95A. A brief description of each parameter is given, followed by a tabulation of recommended parameter ranges. The definitions of active, candidate, neighbor, and remaining sets are given in “Procedure”.

The parameter T_ADD controls movement of pilots from the neighbor/remaining sets to the active/candidate sets. A neighbor or remaining set pilot with strength Ec/Io exceeding T_ADD is moved to either the candidate or active set (the decision is based on serving site direction). This parameter is set per sector.

The parameters T_DROP and T_TDROP controls movement of pilots out of the active/candidate sets. A timer is started when the strength Ec/Io of an active or candidate set pilot falls below T_DROP. An active set pilot that falls below T_DROP for a period exceeding T_TDROP is moved to either the candidate or neighbor set (the decision is based on serving site direction). A candidate set pilot that falls below T_DROP for a period exceeding T_TDROP is moved to the neighbor set. T_DROP is measured in dB (see Table 4-4), and T_TDROP is measured in units that map into seconds (see Table 4-5). These parameters are set per sector.

The parameter T_COMP controls movement of pilots from the candidate set to the active set. A candidate set pilot with strength Ec/Io exceeding that of an active set pilot by T_COMPx0.5dB is moved to the active set, replacing that pilot. T_COMP is measured in units of 0.5 dB (see Table 4-4). This parameter is set per sector.

The parameter NEIGH_MAX_AGE controls movement of pilots out of the neighbor set. The mobile maintains an AGE counter for each pilot in the neighbor set. This counter is updated under direction of the serving site. A pilot with AGE count exceeding NEIGH_MAX_AGE is moved to the remaining set. This parameter is set per sector.

The parameters SRCH_WIN_A, SRCH_WIN_N, and SRCH_WIN_R govern the mobile's search for pilots in the active/candidate, neighbor, and remaining sets, respectively. These parameters specify the size of the search window used in detecting the pilot. The search window for pilots in the active and candidate sets (SRCH_WIN_A) is centered around the earliest arriving multipath component. The search window for pilots in neighbor (SRCH_WIN_N) and remaining (SRCH_WIN_R) sets is centered around the pilot's PN sequence offset (see “Procedure”). Search window values are measured in units that map into window size in PN chips (see Table 4-6).




Table 4-4. Handoff Parameter

parameter recommended range impact

T_ADD -13 to -17 dB impacts site coverage, pilot detection, capacity, handoff populations

T_DROP -13 to -17 dB impacts site coverage, capacity, handoff populations

Table 4-5. T_TDROP Conversions

T_TDROP Timer Expiration (seconds)

T_TDROP Timer Expiration (seconds)

0 <0.1 8 27

1 1 9 39

2 2 10 55

3 4 11 79

4 6 12 112

5 9 13 159

6 13 14 225

7 16 15 319




Table 4-6. Search Window Conversions

SRCH_WIN_A

SRCH_WIN_N

SRCH_WIN_R

Window Size

(PN chips)

SRCH_WIN_A

SRCH_WIN_N

SRCH_WIN_R

Window Size

(PN chips)

0 4 8 60

1 6 9 80

2 8 10 100

3 10 11 130

4 14 12 160

5 20 13 226

6 28 14 320

7 40 15 452




5
Packet Pipe Engineering
Introduction 1

■ Simulation Model and Numerical Results 2

Assumptions 2

Packet Dropping Rate and Criterion 3

Numerical Results 3


y

Contents




55
Packet Pipe Engineering
Introduction 5

In CDMA R1.0, Packet Pipes are defined by associating 4 DS0s on a T1 line going from the DCS to the cell-site. This association is performed on the pptg (Packet Pipe Trunk Group) form of the AUTOPLEX® RC/V system. Each Packet Pipe is given a Packet Pipe Trunk Group Member (PPTGM) number. This number corresponds to an associated CDMA Cluster Controller (CCC) on the cell site. Therefore, there is a one-to-one mapping between PPTGM number and CCC number. When PPTGM #n is equipped, it is associated with CCC #n at the cell site.

Since Packet Pipes are assigned to CDMA clusters (via the PPTGM/CCC Number), and a Packet Pipe with bit rate 256 kbps (4 DS0s) is capable of backhauling 14 CDMA Traffic Channels, all of the CEs that are controlled by the CCC in the cluster are backhauled by the assigned Packet Pipe. In CDMA R1.0, up to 7 CCUs (14 CEs) may be equipped in a cluster, so a fully equipped Cluster can still be backhauled on the Packet Pipe.

There have been discussions on specifying packet pipes with variable bit rate in order to support variable traffic density more efficiently. For CDMA of future release, the size of the Packet Pipe (in terms of DS0s) can be configured at cell initialization via RC/V. In this section, the number of traffic channels that a packet pipe with given bit rate can support is examined.


oprietaryge

Packet Pipe Engineering 401-614-012

Simulation Model and Numerical Results 5

Assumptions 5

To facilitate the simulation, the following assumptions are made:

1. No skew group is assumed. It should be noted that using skew groups with packet pipes will reduce the average packet delay but will increase the packet dropping rate slightly. This effect is similar to the reduction in trunking efficiency when a channel set is divided into three to be used in sectored cells.

2. During soft handoff, the serving CDMA cell site and secondary cell site need to transmit the same forward link voice and signaling to a mobile at the same time. If the serving cell site decides to send forward signaling, it will send this forward signaling with the reverse link packet to an MSC first, and the MSC will transmit the same voice and signal information to both cell sites. Therefore, the mobile can receive the same forward signaling from both cell sites.

3. Data rate of a frame is determined based on a number produced by a uniform random generator. For each CE, two independent random numbers are mapped into the data rates of the reverse link packet and forward link signaling such that the reverse link packet (voice + signaling) pattern are:

■ full rate packet with probability 0.6

■ half rate with probability 0.05

■ quarter rate with probability 0.05

■ one-eighth rate with probability 0.3.

and the forward signaling information addition to the reverse link packet are:

■ full rate signaling with probability 0.1 (from field test data, the signaling rate is usually less than 1%, or probability of 0.01)

■ half rate signaling with probability 0.1.

4. The layers 2 and 3 overhead frames (such as XID, UA for layer 2 and FS_CONNECT, FS_ACK for layer 3) are ignored for simplicity. These overhead frames may not have impact on the overall delay and/or the packet dropping rate since most of these overhead frames are transmitted on the packet pipe while the packet pipe is not fully loaded.

5. The propagation delay from a cell site to a MSC is negligible compared to the queuing delay.




Packet Dropping Rate and Criterion 5

Consider a packet pipe which is supporting n traffic channels, for the ith packet, if its queueing time (that is, the time required to transmit the 1st, 2nd,..., and (i-1)th packets) is longer than 20ms, this ith and all the remaining packets in the queue will be dropped. This kind of packet dropping due to "overflow" will degrade the frame error rate on top of the over-the-air frame error rate. To minimize such effect, the packet dropping rate criterion is set to be less than 0.02%, which is significantly less than the end-to-end frame error rate objective of 1%.

Numerical Results 5

To summarize the numerical results, we depict in Table 5-1 and Figure 5-1 the number of channel elements supported versus the packet pipe width (in terms of the number of DS0s) with an objective of less than 0.02% of packet dropping rate for 8-kbps vocoder.




Table 5-1. Packet Pipe Width as a function of Number of Channel Elements Supported for 8-kbps Vocoder and Packet Dropping Rate < 0.02%

Packet Pipe Width

Number of Channel Element Supported

(No. of DS0s) 56 kbps DS0 64 kbps Clear

1 2 2

2 5 6

3 9 10

4 12 14

5 16 18

6 19 22

7 22 26

8 26 30

9 29 34

10 33 38




Figure 5-1. Packet Pipe Width as a function of Number of Channel Elements Supported for 8-kbps Vocoder and Packet Dropping Rate < 0.02%

0

5

10

15

20

25

30

35

40

0 1 2 3 4 5 6 7 8 9 10

Packet Pipe Width (No. of DS0s)

No.

of C

hann

el E

lem

ents

Sup

port

ed

56 kbps DS0

64 kbps Clear




A1
Introduction 1

In wireless communication systems, the antenna is one of the most critical components that can either enhance or constrain system performance. The antenna as a subsystem, including antenna and feed, is designed to transmit or receive radio waves. The basic function of an antenna is to couple electromagnetic (EM) energy between free space and a guiding device such as a transmission line, coaxial line or waveguide.

The orientation of the antenna plays a role in improving capacity with a directional cell site. The directional antenna, as a particular direction served to a sectored cell, can be used to increase system capacity due to reduction in cochannel interference in CDMA cellular communication systems.

Antenna diversity is an important issue in wireless communication systems. Multipath propagation due to many paths (reflection, diffraction, and scattering) causes fading which results in rapid variations in the received signal. The antenna diversity, such as space diversity and polarization diversity at the base station or mobile, is received by two separated antennas or orthogonal polarized antennas to reduce severity of fading and to provide significant link improvement of the reception.

Antenna Concepts 1

Some important antenna concepts, such as antenna radiation pattern, directivity, gain, efficiency and polarization, suffice to illustrate the characteristics of the type of antennas used in modern wireless communication systems.

Issue 6, August 1999 A-1

oprietaryge


Antenna Radiation Pattern : A graphical representation of the radiation properties of an antenna as function of space coordinates. Radiation properties include radiation intensity, field strength, phase, and polarization. In most cases, the radiation pattern is determined in the far-field region (d≥2D2/λ, d is a distance from the antenna, D is a maximum overall dimension) and is represented as a function of the directional coordinates. An antenna radiation pattern includes a main lobe and side lobe. The main lobe is the radiation lobe containing the direction of maximum radiation. The side-lobe is a radiation lobe in any direction other than that of the main lobe. The amplitude level of a side lobe relative to the main lobe is referred to as side-lobe level.

In wireless communications, two kinds of antenna patterns are used, one is an omnidirectional antenna which has an essentially nondirectional pattern in azimuth and a directional pattern in elevation, another is directional antenna which has the directional pattern in both azimuth and elevation.

Antenna Radiation Beamwidth : The angular separation between two directions in which the radiation intensity is identical, with no other intermediate points of the same value. When the intensity is one-half of the maximum, it is referred to as half-power beamwidth.

Antenna Directivity D : The ratio of the maximum radiation intensity in a given direction (usually 00 or foresight) from an antenna to the radiation intensity averaged over all directions. In mathematical form, directivity can be written as:

(Equation A-1)

where Umax is maximum radiation intensity, P is total radiated power, and S is normalized radiation pattern. The directivity is an indicator of the relative directional properties of the antenna.

In wireless communications, the higher the antenna directivity, the lower the received signal power for a given performance averaged over fading.

Antenna Gain G( θ,ϕ): The ratio of the radiation intensity in a given direction to the radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically. The gain of an antenna is related to the directivity, and also takes into account the efficiency of the antenna. The antenna power gain is defined as 4π times the ratio of the radiation intensity in that direction to the net power accepted by the antenna from a connected transmitter.

DU

PS d d

= =

∫ ∫4 4

0

2

0

π π

θ φ θ θ φπ π

max

( , )sin

A-2 Issue 6, August 1999



It is clear that the antenna gain is related to the antenna directivity by the antenna efficiency, η, for example,

(Equation A-2)

The D in Equation A-2 is the directivity Equation A-1 measured in the specific direction (θ,ϕ). The antenna efficiency is used to take into account losses at the input terminals and within the structure of the antenna. The antenna gain is expressed with respect to the ideal isotropic radiation pattern, in which case, it is measured in dBi.

Antenna Polarization : The polarization of an antenna is the polarization of the wave radiated by the antenna in a given direction, when the antenna is excited. In general, the polarization of an antenna is classified as linear, circular, or elliptical. In linear polarization, if the electric lines of force are parallel to the surface of the earth, the wave is called a horizontally polarized wave. Similarly, if the electric lines of force are perpendicular to the surface of the earth, the wave is called a vertically polarized wave.

Input impedance : The ratio of the voltage to current at a pair of terminals which are the input terminals of the antenna. The value of the antenna input impedance is dependent on the shape of the antenna, wavelength, and surrounding antennas or objects. Antenna input impedance is important to the transfer of power from a transmitter to the antenna or from an antenna to a receiver. Maximum power can be transferred from the antenna if the impedance match is good between the antenna, transmitter and transmission line.

The characteristics of some typical antennas and antenna arrays have been reported in [15,16,17].

Antenna System with Interference and Cell Coverage 1

Antenna pattern, antenna gain, antenna height, and antenna tilting all affect the wireless system design. The antenna pattern can be omnidirectional or directional in both the vertical and the horizon planes. Antenna gain compensates for the transmitted and received power. The antenna height of cell site can affect the area and shape of the coverage in the system. The antenna downtilt can reduce the interference to the neighboring cells and enhance the weak spots in the cell.

G D( , ) ( , )θ φ η θ φ=




Directional Antenna and Sectorization Gain 1

Since CDMA cellular systems typically employ universal frequency reuse, cochannel interference will occur. The directional antennas in a sectored cell can reduce cochannel interference so it enhances the system capacity. In a base station of 1200 cell sectors, the antenna system includes three transmitt directional antennas and six receive directional antennas. An example of this kind of antenna configuration is shown in Figure A-1 (a). There is one transmitter and two receivers in each sector. The gain for each of antennas, for example, is 8 to 12 dBi with about 1200 half power radiation pattern at azimuthal plane and about 120 beamwidth at elevational plane. In an omnicell, for example, the antenna system includes three transmitt omnidirection antennas and two receive omnidirection antennas, as shown in Figure A-1 (b), with antenna gain of 6 to12 dBi and 3600 azimuth beamwidth and about 120 elevation beamwidth.

Figure A-1. Base Station Antennas: (a) Sector Cells; (b) Omnicells

In three sector cells, the ideal 1200 antenna pattern, as shown in Figure A-2 (a), will only receive signals from 1/3 of the cell, reducing the interference by 2/3, so the sector gain is 3, on the other hand, increasing the CDMA channel capacity by a factor of 3. However, the actual antenna (see Figure A-2 (b)) does not have an ideal pattern that only accepts energy from a 1200 sector, and rejects all the energy from the rest of the cell.

Base Station Antenna Configurations

T1

T2

T3

R1 R2( b )

R11 T1 R12

T2R21

R22 R32

T3R31

( a )




Figure A-2. Directional Antenna Pattern in a 1200 Sector Site

There is always some overlap of the sector antenna patterns, so the interference is not reduced by a factor of three. The sector gain can be expressed as:

(Equation A-3)

where:

βomni is the interference factor for omnidirectional cells

β3 sector is the interference factor for three-sector cells

Tests show the upper bound on the interference is, βomni =0.6 and β3 sector =0.85. Therefore, the three sector gain is:

Ideal Pattern Real Pattern

1200

( a ) ( b )

θ3dB=1200

χ ββ

= ×+

+

numberof tors omni

tor

secsec

1

1 3

χ = × ++

=31 0 6

1 0 852 6

.

..




The sectorization efficiency, κ, is defined as:

(Equation A-4)

For three sectors, the sectorization efficiency is 86.5%. The capacity of a single sector RF channel in terms of the omnidirectional capacity and the sector efficiency is:

(Equation A-5)

where:

Msector is capacity of a single-sector RF channel in a sectorizedbase station

Momni is capacity of an omnidirectional base station.

Coverage with Antenna Height and Gain 1

The radio propagation shown in Figure A-3 is based on geometric optics and geometrical theory of diffraction (GTD), and considers the direct path, a ground reflected propagation path, and a diffracted propagation path from a building edge between the transmitter and receiver.

Figure A-3. The Radio Propagation from Transmitter to Receiver

κ χ=Numberof torssec

M Mtor omnisec = ×κ

ht

hr

D

diffractedwave

direct wave

reflectedwave




The coverage distance D from the transmitter to receiver can be expressed as:

(Equation A-6)

where:

Pr is the received power

Pt is the transmitted power

hr is the height of the receiver

ht is the height of the transmitter

Gr is the receiver antenna gain

Gt is the transmitter antenna gain

La is a correction factor which includes the diffracted path lossand others

As seen from Equation A-6, the coverage is proportional to the height of the antenna and antenna gain. In a cellular system, the base station antenna height is in a range from 30 to 200 meters, which depends on the different environments. For example, the antenna height, in an urban area is about 30 m, in a suburban area is about 50 m, and in a rural area is about 80 m. Also the requirement of the antenna gain is dependent on the environment, in rural highway sites, higher gain (for example, 12 dBi) may be used to extend coverage along the highway. However, in urban sites, lower gain (for example 8, dBi) has to be used to reduce the interference.

Reducing Interference Using Antenna Downtilt 1

As discussed earlier, antenna downtilt can enhance weak areas of coverage which reduces interference to distant sites. Accordingly, downtilt adjustments can be an important parameters for optimization; however, in many cases (for example, series 2 CDMA overlay employing growth frames) downtilt adjustment is not viable since CDMA will reuse antennas already carefully optimized for analog coverage. In the following, some general insight into downtilt is offered for those cases (for example, CDMA minicell employing a separate antenna) where downtilt adjustment are allowed.

The principle idea of the antenna beam downtilt is a certain angle technique that is used to tilt the main beam in order to suppress the direction level toward the reuse cell site and to reduce cochannel interference. This downtilt can be accomplished by mechanical or electrical means. When the antenna vertical beam pattern is tilted to a certain angle, the interference from other cell sites is reduced because the received field intensity is weak in these cell sites.

( ) [ ] ( )DP

Ph h G G Lt

rr t r t a≈

1 41 2 1 4 1 4

// / /




This is an advantage from the viewpoint of system design. A typical antenna beam pattern downtilt is shown in Figure A-4.

Figure A-4. The Antenna Beam Downtilt for Illuminating the Service Area

In cellular communications, the antenna downtilt angle, θ, is function of the antenna height, coverage radius, and antenna vertical beamwidth. In general, when the coverage radius of the service area is set to a specified value, the higher the antenna, the larger the downtilt angle, and the more effect for reducing cochannel interference is achieved. On the other hand, when the height of antenna in a base station is set, the smaller the coverage radius, the larger the downtilt angle. For different engineering considerations, two formulations of antenna downtilt can be expressed as:

(Equation A-7)

(Equation A-8)

h

R

downtilt angle(θ)

interfering

desired

θ =

+−tan 1

2 2

h

R

Vertical Beamwidth

θ π= −

−2 1tanR

h




Equation A-7 represents that the angle of antenna downtilt reduces the interference at the base of the neighbor cell (r = 2R) by 3 dB. Equation A-8 represents that the angle of antenna downtilt can preserve the coverage in the fringe of the cell (r = R).

Figure A-5 shows the angle of antenna downtilt as a function of R/h (coverage radius/antenna height) for 120 of vertical beamwidth. In the plot, the solid line is the first downtilt formula (Equation A-7) and the dashed line is the second formula (Equation A-8). As the plots show, The first formula for a downtilt angle prediction shows less dependence on the R/h ratio. For small R/h (small cell radius and/or high antenna), the second formula predicts a larger downtilt angle; while for a large R/h (large cell radius and/or low antenna), the second formula predicts a smaller downtilt angle.

Figure A-5. The angle of antenna downtilt as a function of R/h (coverage radius/antenna height) for 120 of vertical beamwidth. The solid line is the first downtilt formula (Equation A-7) and the dashed line is the second formula (Equation A-8).

Antenna Downtilt Angle

0

1

2

3

4

5

6

7

8

20 30 40 50 60 70 80 90 100

110

120

130

140

150

R/h

Dow

ntilt

Ang

le (

deg)




Diversity Antenna Systems 1

Since signal fading in the wireless radio environment causes severe reception problems, diversity antenna techniques are used to reduce fading effects. Usually, diversity is applied at the base station.Space diversity and polarization diversity are commonly used in cellular systems.

Space Diversity Antenna 1

Multiple antennas separated by finite distance can achieve space diversity. The spatial separation between multiple antennas are chosen so that the diversity branches experience uncorrelated fading. A typical sector base station configuration with space diversity consists of three antennas per sector: one transmit and two space separated receive antennas, as illustrated in Figure A-6.

Figure A-6. Space Diversity in Multipath Environment

The cross-correlation between two received signals at a base station is often used to measure this independence and determines the degree to which the rate and depth of fading may be reduced. The cross-correlation coefficient versus antenna spacing separation and different beamwidth of incoming signals all affect cell site reception. It is generally accepted that reasonable improvement in received signal statistics can be achieved with a cross-correlation coefficient of 0.7.

mobilebase station




The required spacing at the base station differs for spatial separation in the horizontal and vertical planes. For horizontal spacing, the separation has been found to depend, not only on the angle between the line joining the base station and mobile and the line joining the base station, but also on the height of the antennas above the ground as well as the presence of local scatters in the immediate vicinity of the base station antennas. It has been quoted that the space diversity at a base station requires antenna spacing of up to about 20λ [18]. For vertical space diversity, the required separation is about 15λ, which also depends on the effective scattering radius of the area in which the mobile is located. Besides, when the base station antenna height is increased, the cross correlation is decreased, provided the base station antenna spacing remain unchanged.

The space diversity gain is 3-5 dB for horizontal separation, and 2-4 dB for vertical separation. Horizontal space diversity gives better performance than that of the vertical separation, because the decorrelation of the received signals increase faster with the horizontal rather than with the vertical separation of the antennas.

When a transmit antenna (mobile or handset antenna) is tilted from a vertical direction, the received signal level reduces on both antenna branches, because the vertically polarized component of the transmitted signal reduces as the tilt angle increases. Although there is loss in signal level with tilt, the diversity gain in the space diversity receive antennas does not change, which means that the fading depth will still be reduced with diversity as the transmitted antenna is tilted.

Polarization Diversity Antenna 1

Polarization diversity in a base station, with orthogonal polarized antennas, provides a means of realizing two independently fading signals without the need for physical separation of antennas like space diversity [18,19,20]. In cellular communication systems, polarization diversity is likely to become more important, because it employs small cells and antenna heights comparable to or lower than the surroundings where it may be difficult to mount two antennas with the appropriate spacing for space diversity. Figure A-7 shows the polarization diversity antenna system coordinates. As shown in Figure A-7 (a), when the polarization angle, α, is zero, the antenna system is V-H polarization diversity, however, when α=450, a slant 450 polarization diversity; can be obtained. Figure A-7 (b) shows that a mobile user is located in the direction of offset angle β from the main beam direction of the antennas.




Figure A-7. Base Station Polarization Diversity Antenna

For polarization diversity antenna as shown in Figure A-7, the cross-correlation coefficient ρp, can be expressed theoretically as:

(Equation A-9)

and the average value of signal loss, L, relative to that received using vertical polarization is given by:

(Equation A-10)

where Γ=<r2>/<r1> is the cross-polarization discrimination of the propagation path between a user and a base station, r1 and r2 are two independent Raleigh distribution variables. From Equation A-9 and Equation A-10 we know that both cross-correlation coefficient and signal loss are determined by three factors: polarization angle α, offset angle β, and cross-polarization discrimination Γ.

Figure A-8 shows the three dimension plots of the calculated cross correlation coefficient ρ and received signal level decrease L as function of α and Γ. The range of α is from 00 to 450. In Figure A-8 (a) and (b), ρ generally becomes higher as both polarization angle α and cross-polarization discrimination Γ become larger. ρ increases when offset angle β increases, and there is no diversity gain when β = 900 (ρ = 1.0, from Equation A-9). This is because the received signal on two branches is only a vertical polarization component at β = 900. In Figure A-8 (c) and (d), L becomes larger as α increases or as Γ decreases.

α

Y

Xβ

X

Z

(a) x-y plane (b) x-z plane

Main Beam

ρ α β α βα β α β

( , , )tan ( )[ cos ( )]

[tan ( )cos ( ) ][ tan ( ) cos ( )]Γ Γ

Γ Γ= −

+ +

2 2 2

2 2 2 2

L( , , )tan ( )cos ( )

tan ( ) cos ( )α β α β

α βΓ Γ

Γ= +

+

2 2

2 2




Additionally, L increases as increasing β. This is because the horizontally polarized component becomes smaller as β increases.

Published measurements [18-20] have shown that cross-correlation coefficient, with H-V and 450 slant polarization diversity, is lower than 0.7 in urban, suburban and rural areas on the cellular band. The cross-polarization discrimination is typically between 6-20 dB which corresponds to different environments. Comparisons between the performance of space diversity and polarization diversity show that:

1. There can be a power loss (for example, 3 dB) in signal for polarization diversity in the forward link, since the transmitter power is split into the two polarizations.

2. In the case of a slanted mobile transmitting antenna, the polarization diversity antenna system at the base station can show better correlation statistics than the space diversity antenna system in reverse link. The diversity gain of the polarization diversity antenna is about 2 dB when the mobile antenna is vertical. However, the gain may increase to 3-5 dB when the mobile antenna is tilted.

The specific performance of the polarized diversity antenna relative to space diversity is a function of the amount of reflection and/or scattering in the local environment. Field measurements suggest that a performance loss of up to 2 dB may be encountered when polarized diversity antennas are used in certain morphologies.




Figure A-8. Cross correlation coefficient and received signal level decrease as function of polarization angle and cross polarization discrimination for two kinds of offset angle.

-15

-12

-9

-6

-3

0

010

2030

40

4

8

12

4

8

12

-15

-12

-9

-6

-3

0

010

2030

40Γ (dB)α (deg)

β = 100

( c )

L (

dB)

-16

-12

-8

-4

0

010

2030

40

4

812

4

812

-16

-12

-8

-4

0

010

2030

40Γ (dB)α (deg)

( d )

β = 400

L (

dB)

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40

510

15

510

15

0 10 20 30 400.0

0.2

0.4

0.6

0.8

1.0β = 100

Γ (dB) α (deg)

ρ

( a )

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40

510

15

510

15

0 10 20 30 400.0

0.2

0.4

0.6

0.8

1.0

Γ (dB)

β = 400

ρ

α (deg)

( b )




B2

Appendix B: Antenna Isolation Guidelines for Collocated RF Stations

Introduction 2

Due to deployment constraints, estate acquisition difficulties and other reasons, sometimes it is highly desired that CDMA Cellular Cell Site (CS) can be collocated with RF stations of other communications systems such as TDMA PCS, CDMA PCS, TDMA Cellular, AMPS, AM, SMR, etc. When they are collocated, mutual interference between stations always exist that may cause receiver desensitization, overload and/or intermodulation product (IMP) interference, thereby degrading their system performances. Therefore, if the service provider wants to collocate a CDMA Cellular CS with other RF station(s), precaution should be taken to avoid/minimize these harmful mutual interferences.

The degree of degradation is dependent on the strength of interfering signals which is determined by the transmit/receive (TX/RX) unit performance, spectrum spacing, and antenna separation between the collocated stations. In this section, a set of mathematical models necessary for the evaluation of the mutual interference between two (2) collocated RF communications stations will be introduced first. The criteria for determining the required antenna isolations between the collocated stations will be specified. A typical example showing the calculation of required antenna isolations between a CDMA PCS cell site and a TDMA PCS cell site will be presented. The models will then be generalized for the cases in which more than two RF stations are collocated (called multiple collocated RF stations hereafter). Finally, Site Survey (SS) which is an important step in the process of collocating a CDMA Cellular cell site with any other RF station(s) will then be explained.

Mathematical Models for Mutual Interference Evaluation 2

Figure B-1 is a schematic diagram showing the mutual interference between two collocated RF stations.

Issue 6, August 1999 B-1

oprietaryge

Appendix B: Antenna Isolation Guidelines for Collocated

Figure B-1. Schematics Diagram Showing Mutual Interference between two Collocated RF Stations

The RF components which are very important in evaluating the mutual interferences between two collocated stations are (i) The TX amplifier and TX filter of the interfering station, and (ii) RX filter and receiver (and a preamplifier) of the interfered station, as indicated in Figure B-1. Note that the term antenna isolation (called isolation hereafter) refers to the path loss between the J4 terminals (such as, input/output port of the RX/TX unit) of the collocated stations, that includes the propagation loss through the air and the effective antenna gains (such as, antenna gain plus cable loss) of both stations.

There are three (3) kinds of degradation need to be considered: (a) receiver desensitization, (b) IMP interference, and (c) receiver overload. The receiver desensitization is caused by strong spurious emission received from the interfering stations. The IMP interference is generated by combination of carriers received from the collocated stations. Receiver overload occurs when the total RF power being received is too strong for the front end of the receiver. To minimize this degradation, without modifying the existing TX/RX units, appropriate isolation will be maintained between the collocated stations. The required isolation will be determined using the criteria set forth in “Antenna Isolation Criteria and Safe Antenna Isolation”.

��

��

Antenna Isolation

Received Interference Level

@ J4

INTERFERED STATION

Receiver

Received Interference Level

@ Rcvr Input

J4RX Filter

RECEIVE UNIT

INTERFERING STATION

J4 TX Filter

TX Carrier &Interference Levels

@ AMP Output

TRANSMIT UNIT

TX Carrier LevelSpecified @ J4

TX AMP

B-2 Issue 6, August 1999



Receiver Desensitization 2

Receiver desensitization is defined as the degradation in receiver sensitivity due to an increase in the receiver noise floor. In this discussion, we assume that the TX band of the interfering station is adjacent to the RX band of the interfered station. Therefore, a substantial amount of spurious emissions are generated by the interfering station. If the isolation between two stations is not sufficient and/or the interfering station’s TX filter does not provide enough out-of-band attenuation (such as, rejection), the spurious emissions which are falling in the RX band of the interfered station may be too strong, and result in an increase in the receiver noise floor.

It can be seen from Figure B-1 that the interference power (such as, spurious emissions) generated at the TX amplifier output of the interfering station is filtered by the TX filter, attenuated by the isolation between two stations, and then received by the RX unit at the interfered station. Therefore, the affected interference power received at the J4 terminal of the interfered station can be expressed as:

(Equation B-1)

where: Iaff_J4 = affected interference level at interfered station’sJ4 (dBm)

CTX_amp = nominal maximum carrier power level at TX amplifieroutput (dBm)

ICRTX_amp = interference-to-carrier ratio of TX amplifier (dBc)

LTX_rej = TX filter rejection of “signal” in interfered station’s RXband (dB)

= isolation between J4 terminals of co-located stations(dB)

Winterfered = interfered system channel bandwidth (kHz)

Winterfering = interference level measurement bandwidth (kHz)

Swapping parameters I aff_J4 and , we can express (Equation B-1) in an alternate form as:

= CTX_amp + ICRTX amp - LTX_rej - Iaff_J4 + BWAF (Equation B-2)

Iaff_J4 = CTX_amp + ICRTX_amp - LTX_rej - L isolationspurious+ 10 log10(Winterfered/Winterfering)

Lisolationspurious

Lisolationspurious

Lisolationspurious




Note that the BWAF is the Bandwidth Adjustment Factor which is defined as:

BWAF = 10 log10(Winterfered/Winterfering) (Equation B-3)

If the level of TX carrier power is specified at J4 (denoted as CTX_J4, in dBm), the CTX_amp can be calculated by:

CTX_amp = CTX_J4 + LTX_pass (Equation B-4)

where LTX_pass is the TX filter passband loss.

Substituting (Equation B-4) into (Equation B-2), we have:

= CTX_J4 + ICRTX_amp + (LTX_pass - LTX_rej) - Iaff_J4 + BWAF (Equation B-5)

Intermodulation Product Interference 2

Due to nonlinearity of the receiver gain transfer function, if the strengths of received interfering carriers are higher than a certain level, the IMPs will be generated in the receiver (or the preamplifier) and presented at its output. Sometimes, these IMPs may be too strong. If falling in the RX band of the interfered system, they may cause interference and degrade the receiver performance.

The nature of IMPs have been studied by many scientists. It has been pointed out in these reports that the third-order IMPs (IMP3) are the stronger ones, and they may have adverse effects on the receiver performance. Therefore, in the following discussions, we will focus our attentions on the IMP3.

Mathematically, the IMP3 (in dBm) which is generated by two (2) carriers of equal strength (so-called 2-Tone IMP3) can be calculated by:

IMP3 = 3 x CRX_rcvr - 2 x TOI (Equation B-6)

where:

TOI = Third-Order Intercept point specified at receiver input(dBm)

CRX _rcvr = interference carrier level at interfered station’s receiverinput (dBm)

Note that, to minimize receiver degradation caused by IMP3 interference, the strengths of IMP3s will be lower than an acceptable level, which will be specified in “Antenna Isolation Criteria and Safe Antenna Isolation”.

L isolationspurious




From Equation B-6, the CRX_rcvr for achieving a specified IMP3 level can be obtained by:

CRX_rcvr = (Equation B-7)

Referring to Figure B-1, we can also express the CRX_rcvr as:

CRX_rcvr = CTX_J4 -L - LRX_cxr_rej (Equation B-8)

where:

CTX_J4 = nominal maximum TX carrier power at interferingstation’s J4 (dBm)

L = isolation between J4 terminals for achieving a specified CRX_rcvr (dB)

LRX_cxr_rej = RX filter rejection of carrier received from interfering station (dB)

Swapping L and CRX_rcvr, we have

L = CTX_J4 - LRX_cxr_rej - CRX_rcvr (Equation B-9)

Note that the CRX_rcvr is determined by the specified IMP3 level as indicated in Equation B-7.

Receiver Overload 2

Receiver overload is caused by a received signal power level at the receiver that is too strong. When a receiver is driven into overload, its amplification gain is decreased (such as, depressed). To prevent the receiver from being overloaded, the level of total carrier power received from the interfering station will be well below its 1-dB compression point (P1dB). The acceptable level will be specified in “Antenna Isolation Criteria and Safe Antenna Isolation”.

Similarly, referring to Figure B-1, the isolation (denoted as L ) between two collocated stations to suppress the level of total affected carrier power received at the interfered station’s receiver to an acceptable level can be expressed as:

L = CTX _tot_J4 - LRX_cxr_rej - CRX_tot_rcvr (Equation B-10)

IMP TOI3 2

3

+ ×

isolationIMP3

isolationIMP3

isolationIMP 3

isolationIMP3

isolationover load−





where:

CTX_tot_J4 = total carrier power transmitted at J4 terminal ofinterfering station (dBm)

CRX_tot_rcvr = total carrier power received at J4 terminal of interferedstation (dBm)

L = isolation between J4 terminals for achieving aspecified CRX_tot_rcvr (dB)

LRX_cxr_rej = RX filter rejection of carrier power received frominterfering station (dB)

Note that every receiver (and preamplifier) is designed to operate properly within a specific bandwidth which is the so-called operating band. If the received signal is falling in the operating band, its strength will be amplified, otherwise it will be attenuated. Therefore, the receiver behaves as an active band-pass filter in a manner that it has uniform gain (for example, 18 dB) within its operation band and has high attenuation (such as, loss) outside the band. The degree of attenuation is dependent on the receiver design and how far the carrier is in frequency away from the operating band. In some cases, the receiver may become inactive to the incoming carriers whose frequencies are only a few tens of MHz outside its operating band; therefore, it seems that these carriers are not received by the receiver. In the other words, the power of these incoming carriers are totally rejected by the receiver. Due to this fact, we will point out that the Equation B-10 given above is applied only under the condition that the collocated stations are operating in the adjacent bands (for example, Cellular at A-Band and Cellular at B-Band) in which the receiver of the interfered station is active to the carriers from the interfering stations.

Antenna Isolation Criteria and Safe Antenna Isolation 2

To ensure proper system performance, the three types of degradation mentioned above will be avoided and/or minimized. To achieve this goal, the antenna isolation between the collocated stations will be maintained to meet the criteria given below:

1. At the interfered station, the strength of affected spurious emissions received from the interfering station(s) should be 10 dB below its receiver noise floor.

2. At the interfered station, the level of IMP3 products generated should be 10 dB below its receiver noise floor.





3. At the interfered station, the strength of the total affected carrier power received from the interfering station(s) should be 5 dB below its receiver 1-dB compression point (P1dB).

Note that, in order to meet all the criteria listed above, the largest antenna isolation will be chosen. In most of cases, the largest antenna isolation is determined by Criteria 1. Using this antenna isolation, the RX sensitivity of the interfered system is degraded by only 0.5 dB, which is considered acceptable for most communications systems.

As mentioned at the beginning of this section, mutual interference always exists between collocated stations. For example, considering two (2) collocated RF stations, say Station A and Station B, the TX unit of Station A will affect the RX unit of Station B, and vice versa. Therefore, two sets of required antenna isolation is needed, one for the interference from Station A to Station B and the other for interference from Station B to Station A, need to be calculated. Similarly, to avoid/minimize the degradation to both stations, the largest antenna isolation (called safe antenna isolation) obtained in these two sets will be utilized. Using the safe antenna isolation (SAI), the maximum receiver degradation of both stations is approximately 0.5 dB.

Example of Antenna Isolation Calculation 2

To illustrate the application of models and criteria previously given, a typical example showing the calculation of required antenna isolation between a D-Block CDMA PCS CS and A-Block TDMA PCS CS is presented. To obtain the SAI, two cases listed below will be considered:

Case 1: CDMA PCS CS transmits in D-Block, adjacent TDMA PCS CS receives in A-Block

Case 2: TDMA PCS CS transmits in A-Block, adjacent CDMA PCS CS receives in D-Block

a. TX and RX Unit Specifications

The equations used for calculation of the required isolation meeting the Criteria set forth in “Antenna Isolation Criteria and Safe Antenna Isolation” are given in Equation B-5, Equation B-9, and Equation B-10, respectively. To perform these calculations, one will know the specifications (such as, performances) of both TX unit and RX unit of all the collocated stations under study. In this example, we assume that the TX and RX unit performance of the collocated D-Block CDMA PCS and A-Block TDMA PCS stations are given in Table B-1 and Table B-2, respectively.




.

Table B-1. TX Unit Performance Assumptions

TX Station CTX J4_one No. ICRTX_amp LTX_pass LTX_rej

Carrier

CDMA PCS 39.0 dBm 1 -60 dBc/12.5 kHz

1.2 dB 93 dB

(D Block, TX) (A Block, RX) (A Block, RX)

TDMA PCS 42.1 dBm 3 -88 dBc/30 kHz 0.8 dB 69 dB

(A Block, TX) (D Block, RX) (D Block, RX)

Table B-2. RX Unit Performance Assumptions

RX Station RCVR LRX_rej_cxr TOI P1dB

Noise Floor

CDMA PCS -108 dBm/1.25 MHz 110 dB -6.0 dBm -18.0 dBm

(D Block, RX) (A Block, TX)

TDMA PCS -124 dBm/30 kHz 80 dB -5.8 dBm -17.8 dBm

(A Block, RX) (D Block, TX)




In

S

CDPC

(D

TDPC

(A

b. Required Isolations for Criteria 1

Using Equation B-5 and assumptions given in Table B-1 and Table B-2, the required isolation, , for minimizing the receiver desensitization and meeting the Criteria 1 are calculated as shown in Table B-3.

Note that, according to Criteria 1, the acceptable Iaff_J4 is set 10 dB below the RX unit noise floor as given in Table B-2. The BWAF defined in Equation B-3 is expressed here as:

BWAF (dB) = 10 log10 (Bandwidth of Iaff _J4 / Bandwidth of ICRTX_amp (Equation B-11)

Table B-3. Calculation of Required Isolations for Meeting“Criterion 1”

terfering Interfered I aff_J4 CTX_J4 LTX_pass CTX_amp ICRTX_amp LTX_rej BWAF Isolation

ystem System Acceptable Required

MA S

TDMA PCS

-134 dBm/30 kHz

39.0 dBm

1.2 dB 40.2 dBm

-60 dBc/1.25 kHz

93 dB 3.8 dB 25.0 dB

Block) (A Block)

MA S

CDMA PCS

-118 dBm/1.25 MHz

42.1 dBm

0.8 dB 42.9 dBm

-80 dBc/30 kHz

69 dB 16.2 dB

28.1 dB

Block) (D Block)

L isolationspurious




In-

re

CD

(

TD

(

I

C

T

c. Required Isolations for Criteria 2

Using Equation B-9 and assumptions given in Table B-1 and Table B-2, the required isolation, L , for minimizing the IM interference and meeting Criteria 2 are calculated as shown in Table B-4.

d. Required Isolations for Criteria 3

Using Equation B-10 and assumptions given in Table B-1 and Table B-2, the required isolation, L , for avoiding receiver overload and meeting Criteria 3 are calculated and shown in Table B-5.

Table B-4. Calculation of Required Isolation for Meeting “Criteria 2”

terfering Interfered IMP3 TOI CRX_rcvr CTX_J4 LTX_cxr_rej

Isolation

System System Acceptable @ Rcvr Input AcceptableRequi

d

MA PCS TDMA PCS -134 dBm -5.8 dB -48.53 dBm 39.03 dBm 80 dB 7.6 dB

D Block) (A Block)

MA PCS CDMA PCS -118 dBm -6.0 dB -43.33 dBm 42.05 dBm 110 dB 0 dB

A Block) (D Block) (-24.6dB)

Table B-5. Calculation of Required Isolation for Meeting“Criterion 3”

nterfering Interfered P1dB CRX_tot_rcvr CTX_one_J4 No. CTX_tot_J4 LRX_cxr_rej Isolation

System System Acceptable Carrier Required

DMA PCS TDMA PCS -17.8 dBm

-22.8 dBm 39.0 dBm 1 39.0 dBm 80 dB 0 dB

(D Block) (A Block) (-18.2 dB)

DMA PCS CDMA PCS -18.0 dBm

-23.0 dBm 42.1 dBm 3 46.9 dBm 110 dB 0 dB

(A Block) (D Block) (-40.1 dB)

isolationIMP3





e. Safe Antenna Isolation ( SAI)

The SAI is 28.1 dB, which is the largest of required isolation obtained in Table B-3 through Table B-5. Therefore, to minimize the system degradation of the D-Block CDMA PCS CS and A-Block TDMA PCS CS to less than 0.5 dB, the isolation between the J4 terminals of these two collocated cell sites will be at least 28.1 dB.

Antenna Separation Between Two Collocated RF Stations 2

Antenna isolation, Lisolation (in dB), between the J4 terminals of two co-located stations can be expressed as:

Lisolation = Lprop (d) - G - G (Equation B-12)

with

G = G - L

G = G - L

where:

Lprop = propagation loss between co-located stations (dB)

G = interfering station TX antenna gain in interfered station RXband and in the direction pointing to interfered station (dBi)

G = interfered station RX antenna gain in its RX band and in thedirection pointing to interfering station (dBi)

L = interfering station’s TX path cable loss (dB)

L = interfered station’s RX path cable loss (dB)

Note that the G and G are the actual antenna TX/RX gains which takes into account all the reflection, diffraction and/or scattering caused by the support structures which are close to the stations. Therefore, the G and G may not be the same as that specified by the antenna manufacturer. The antenna gain is conventionally specified in dBi, which is a relative quantity used to expressed the difference in gain between the antenna under test and an isotropic radiator.

If there is line-of-sight (LOS) between the collocated stations and the free space propagation loss model is applied, Equation B-12 can be expressed explicitly as:

int erferringeffect

int erferredeffect

int erferringeffect

int_errferring

TX antennaint

_erferring

TX cable

int erferredeffect

int_errferred

RX antennaint

_erferred

RX cable

int_errferring

TX antenna

int_errferred

RX antenna

int_erferring

TX cable

int_erferred

RX cable

int_errferring

TX antennaint

_errferred

RX antenna

int_errferring

TX antenna

int_errferred

RX antenna




Lisolation = [20 log10(f) + 20 log10(d) - 37.93] - G - G (Equation B-13)

where:

f = frequency of affecting interference (spurious emission or carrier) (MHz)

d = antenna separation between two co-located stations (feet)

Therefore, if f, G and G are known, the antenna separation d to achieve the required isolation Lisolation can be determined by the following equation:

d = 10 SUM/20 (Equation B-14)

with:

SUM = Lisolation + G + G + 37.93 - 20 log10(f)

Note that Equation B-14 is useful only under the condition that the free-space propagation loss model is valid (for example, LOS), and G and G can be accurately estimated. However, in reality, due to the effects caused by nearby antenna support structures as mentioned above, it is very difficult to generate a mathematical model to accurately predict the antenna pattern and its gain. Therefore, it is recommended that the required antenna separation will be determined experimentally.

Mutual Interference Between Multiple Collocated RF Stations 2

Having sufficient knowledge about how to deal with two collocated stations, now we are ready to expand the scope of discussion to include the interference between multiple (such as, more than 2) collocated RF stations. For illustration purposes, we only consider five collocated RF stations as shown in Equation B-2. However, due to a generalized description/explanation, the concepts and conclusions provided herein can be extended and applied to the cases having any number of collocated stations.

int erferringeffect

int erferredeffect

int erferringeffect

int erferredeffect

int erferringeffect

int erferredeffect

int erferringeffect

int erferredeffect




Figure B-2. Mutual Interference between Five Collocated RF Stations

As indicated in Figure B-2, there are four (4) existing stations (such as, Station A, Station B, Station C, and Station D) in the site area, and a service provider is planning to install a new station (such as, Station E) at the location as specified by the dashed circle. The solid lines represent the mutual interference between the existing stations, whereas the dashed lines indicate the mutual interference between the new station and the existing stations. To simplify the discussion, we assume that, before installing the new station, all the existing stations were functioning normally. It means that the interference received at each existing station was at an acceptable level.

The decision of whether to allow the new station to collocate with the existing stations is mainly determined by two factors (i) inclusion of new station will not degrade the RF performance of all the existing stations, and (ii) the levels of total interference received from all the existing stations will meet the three criteria as set forth in “Antenna Isolation Criteria and Safe Antenna Isolation” for the new station.

Station B

Station C

Station D

Station E

Station A

(existing)

(new)

(existing)

(existing)

(existing)




Due to difficulties in gathering the required TX/RX unit specifications for all of the existing stations, it is a cumbersome and time-consuming task to deal with the multiple stations collocation issue. The degree of difficulty depends on the number of collocated stations considered. In this section, based on the drawing given in Figure B-2, we will briefly describe the approaches for determining a location for the new station (such as, Station E) in the collocate area.

a. Interference from a New Station to Existing Stations

As explained at the beginning of this section, to evaluate the interference from the new station (such as, Station E) to each existing station, we need to have the following information:

■ New Station - TX Unit Performance Specifications

■ All Existing Stations - RX Unit Performance Specifications.

Allowing the new station to be installed in the collocated area is under the condition that all the existing stations have sufficient sensitivity margins to tolerate/accept additional interference coming from the new station without causing any undesired degradation. In order to limit the additional interference to an acceptable level, appropriate antenna isolation between the new station and each existing station will be maintained.

b. Interference from Existing Stations to New Station

To determine the interference from all the existing stations to the new station, we need to have the following information:

■ New Station - RX Unit Performance Specifications

■ All Existing Stations - TX Unit Performance Specifications

To evaluate the degradation at the new station, the total interference coming from all the existing stations will be considered. In order to meet the criteria set forth in “Antenna Isolation Criteria and Safe Antenna Isolation”, another set of required antenna isolation between the new station and each existing station will be maintained. Note that the antenna isolation may be different from that obtained in (A). To ensure the interference received at the new station and all the existing stations are at their acceptable levels, the safe antenna isolation, that is the largest of antenna isolations obtained in (A) and (B), between the new station and each existing station will be chosen.

c. Antenna Isolation/Separation Estimations

Due to difficulties in obtaining all the TX/RX unit performance specifications and accurately predicting all the antenna gains/patterns in the field, it is impossible to derive a mathematical equation to estimate the required antenna isolation/separation between the new and all the existing stations.




Due to a lack of sufficient information, to ensure proper operation for all the collocated stations under study, we usually add and/or make a lot of assumptions to include relatively large margins on the TX/RX unit performance (for example, TX/RX filter rejection factors etc.). These margins may be too conservative thereby resulting in an over-estimated antenna isolation results which are much larger than are actually required. Due to this reason, we recommend that, in order to achieve more accurate and reasonable results, the antenna isolation/separation will be determined experimentally.

Site Survey 2

As explained above, accurate and meaningful results can only be achieved through direct measurement. The Site Survey (SS) is an approach to fulfill the measurement. It is a very important step in the process of setting up a new station (for example, CDMA Cellular cell site), and therefore will be performed after the location is selected and before the equipment is installed for the new station, which is either stand-alone or collocated with other stations. The tasks needed to be performed in the SS will be briefly described. In the following discussion, we assume that the new station is a CDMA Cellular CS.

The major tasks needed to be completed in the SS are listed below:

1. Measure the frequencies and strengths of all the existing carriers, and perform IMP3 study to evaluate the IMP3 interference before and after the new station is installed.

2. Measure the strengths of spurious emissions falling in the new station’s RX band.

3. Perform a receiver overload study based on the frequencies and strengths of carriers measured in (1).

4. Measure the electrical field (E-field) intensity at the equipment frame (option for high power interference case).

Note that all the measurements given above will be performed right at the location selected for the new cell site. To achieve more accurate results, we suggest that the same type of RX/TX antenna chosen for the CDMA Cellular CS will be utilized for the measurements (1) and (2) listed above. To closely simulate the real situation, the antenna will be lifted as high as possible to the height proposed for the antenna, and its mainbeam will be pointing to the desired direction.

The reason we suggest using the exact RX/TX antenna chosen for the new CDMA Cellular CS in the measurements is due to the fact that the antenna is a frequency sensitive device; its gain, radiation pattern and terminal impedance (so-called antenna impedance) change with frequency. The performance of these quantities (so-called antenna specifications) are provided by the antenna manufacturer.




These specifications are valid only within the bandwidth (so-called operating band) designed for the antenna. Normally it is very narrow, about 5% to 10% of its central frequency. Within the operating band, antenna performance meets the nominal specifications. However, when operating outside the band, antenna performance deteriorates very rapidly; the antenna’s gain and radiation pattern are degraded in an unpredictably way, and its terminal impedance become mismatched to the feeding system. If mismatch becomes very high, which usually occurs at frequencies much higher or lower than the operating band, the signal is reflected and no power can be transmitted and received by the antenna. The out-of-band performance information is not important for the normal use of an antenna; therefore, it is not included in the specifications provided to the customers.

Based on the reasons described above, it is obvious that, when the collocated stations are operating in different bands whose frequencies are far apart (for example, PCS at 1900 MHz, Cellular at 850 MHz, AM at 1 MHz, etc.), it is meaningless and also is a mistake using the “in-band” antenna gain/pattern information (such as, specifications) provided by the antenna manufacturer to evaluate the “out-of-band” interference effect. The antenna specifications may be useful only when dealing with the collocated stations which are operating at the same and/or adjacent bands.

The tasks of SS outlined at the beginning of this section are the easy, correct and straightforward approache for evaluation of interference received from the known and/or unknown stations. The “known stations” are the stations whose locations are explicitly known; whereas the “unknown stations” are the hidden stations whose locations are not known. Note that sometimes the interference and jamming coming from the hidden station(s) may be the strongest one which requires more attention.




a. Task 1 - IMP3 Interference

The purpose of Task 1 is to evaluate the difference in the IMP3 interference in the collocated site before and after the new station is installed. To determine the IMP3 interference, we will first measure the frequencies of all the existing carriers at the location chosen for the new station. Using the frequency data obtained in the measurement, the frequencies of all the IMP3 are calculated, and the IMP3 interference can then be evaluated.

As mentioned earlier, the IMPs are generated due to nonlinearity of the gain transfer function. The general form of the nonlinear transfer function for a receiver can be expressed by a infinite power series as:

= (Equation B-15)

where are coefficients, and and are the carriers at input and output of the receiver, respectively.

Here we assume that the transfer function is quasi-linear which implies that the coefficient in Equation B-15 is very much larger than the coefficients through . Furthermore, if is sufficiently small, the infinite series in Equation B-15 may be approximated by the first three terms as:

= + + (Equation B-16)

The IMP3s are generated by the third term in Equation B-16. For any three-carrier frequencies (for example, α, β and γ), a set of IMP3s will be generated and their frequencies are given below:

(3α), (3β), (3γ)

(2α + β), (2α - β), (2α + γ), (2α - γ),(2β + α), (2β - α), (2β + γ), (2β - γ),(2γ + α), (2γ - α), (2γ + β), (2γ - β),

(α + β + γ), (α + β - γ), (α - β + γ), (α - β - γ).

oute ann=

∞

∑1

in

ne

na ine oute

1a 2a∞a ine

oute 1a ine 2a ine23a ine3




Since all the receivers have a similar transfer function as that given in Equation B-16 and every three carriers will generate a set of IMP3 frequencies as listed above, all the IMP3 frequencies existing at the collocated site before the new station is installed can be determined using the measured carrier frequencies obtained in Task 1 of the site survey. Since having been harmonically collocated, it is a reasonable assumption that these IMP3 frequencies do not fall in any of existing station’s RX band.

If the new station is installed, the combination of existing carrier(s) and new carrier(s) from the new station will generate additional IMP3s and increase the number of IMP3s in an exponentially way. Note that the more IMP3s are generated, the higher the chance that the collocated stations will be polluted by the IMP3s. The ideal situation is that the new IMP3 frequencies do not fall in the RX band of any collocated station, including the existing stations and new station. If this is true, the work of Task 1 is completed.

However, if some of IMP3 frequencies are falling in the new station’s RX band, the causes (such as, affecting carriers) of these damaging IMP3 products will be identified and their strengths should be determined. Since it is very difficult to generate a mathematical model to accurately estimate the strength of IMP3 products, the IMP3 strength should be determined through direct measurement.

The measurement is performed by simply attaching the actual RX filter and receiver at the other end of antenna cable (such as, J4). If the affecting carriers are existing outside the RX band, due to extra rejection provided by the RX filter, their strengths may be suppressed dramatically, thereby substantially reducing the levels of damaging IMP3s. Note that, in general, a 1-dB reduction in the carrier levels results in a 3-dB drop in IMP3 power. Therefore, with the RX filter, all the affecting IMP3s should be suppressed to the acceptable level.

b. Task 2 - Spurious Emissions

The purpose of Task 2 is to measure the strength of total spurious emissions received from all the existing stations. This measurement is performed simply by attaching a spectrum analyzer at the J4 terminal, setting an appropriate resolution bandwidth (for example, 1 kHz) and tuning the scanning bandwidth to cover the entire new station’s RX band. Note that the level of total spurious emissions measured will meet the Criteria 1 set forth in “Antenna Isolation Criteria and Safe Antenna Isolation”.




c. Task 3 - Receiver Overload

Receiver overload is determined by the total power of all the carriers received at the receiver’s input. The power strength of each carrier can be accurately determined by attaching the exact RX filter between the J4 terminal and spectrum analyzer. Note that the total power, which is the summation of all the carrier power measured, will meet the Criteria 3 set forth in “Antenna Isolation Criteria and Safe Antenna Isolation”.

d. Task 4 - Electrical Field Intensity Measurement

The purpose of this task is to determine the intensity of electrical field (E-field, in volt/meter) generated by the strong interfering carrier. The E-field intensity is an important quantity in evaluating the capability/possibility of installing the equipment frame close to a station which is transmitting a very high power (for example, 1000 watts). Therefore, this measurement is necessary when planning to locate the CDMA PCS cell site close to an active AM station/tower.

In order to achieve a large coverage (for example, R = 40 miles), the AM station normally transmits at a very high power level (for example, 1000 watts or more). The AM station usually consists of an array of monopoles to create a specific radiation pattern. Those monopoles use the earth surface as their ground plane. Most of the electromagnetic (EM) waves (such as, energy/power) launched by these monopoles are supported by the earth surface (so-called ground waves), and propagate away in the horizontal direction. Therefore, the strongest field intensity is concentrated at a level/height very close to the earth surface where the CDMA cell site equipment frames (cabinets) are normally installed. To avoid equipment from being damaged by a high intensity-field, the E-field measured at the equipment frames surface will be below the limits as specified in Table B-6.

If the E-field intensity is stronger than that specified in Table B-6, we will either relocate the new station or cover the equipment frame with extra shielding materials.

Table B-6. E-Field Intensity Specifications

Standard E-Field Intensity Frequency Range

V/m

IEC 1000-4-3 3 27 MHz - 1000 MHz

Bellcore GR-1089-CORE

10 10 kHz - 10 GHz




C3
References 3

1. C. A. Balanis, Antenna Theory Analysis and Design, Harper & Row, Publication Inc., 1982.

2. W. L. Stuzman and G. A. Thiele, Antenna Theory and Design, John Willey & Sons, Inc., 1981.

3. E. C. Jordon, Reference Data for Engineering: Radio, Electronics, Computer, and Communications, H. W. Sams & Co., Macmillan Inc., 1992.

4. W. C. Y. Lee, Mobile Communications Engineering, McGraw-Hill Company, 1982.

5. R. G. Vauhan, ``Polarization Diversity in Mobile Communications,” IEEE Trans. Vehic. Technol., vol. 39, no. 3, pp. 177-186, 1990.

6. S. Kozono, T. Tsuruhara, and M. Sakamoto, ``Base Station Polarization Diversity Reception for Mobile Radio,” IEEE Trans. Vehic. Technol., vol. 33, no. 4, pp.301-306, 1984.

Issue 6, August 1999 C-1

oprietaryge


List Of Acronyms 3

ACU Analog Conversion Unit

AIF Antenna Interface Frame

ASIC Application Specific Integrated Circuit

AMPS Advanced Mobile Phone Standard

AWGN Additive White Gaussian Noise

BCR Baseband Combiner and Radio

BHCA Busy Hour Call Attempts

BIU Bus Interface Unit

CAF Channel Activity Factor

CCC CDMA Cluster Controller

CDMA Code Division Multiple Access

CE Channel Element

C/I ratio of carrier power to impairment power

CRTU CDMA Radio Test Unit

D/A Digital to Analog

dBm decibel with respect to a milliwatt

dBi decibel gain with respect to an isotropic antenna

dBd decibel gain with respect to an ideal dipole

DCS Digital Cellular Switch

DFU Digital Facilities Unit

ECP Executive Cellular Processor

Eb/No ratio of bit energy to spectral density of noise

Ec/Io ratio of chip energy to spectral density of interference

C-2 Issue 6, August 1999



ERP Effective Radiated Power

FDMA Frequency Division Multiple Access

GPS Global Positioning System

I In-phase

IF Intermediate Frequency

LAC Linear Amplifier Circuit

LAF Linear Amplifier Frame

MSC Mobile Switching Center

PCS Personal Communication Service

PN Pseudo Noise

PSU Packet Switching Unit

Q Quadrature

RCC Radio Control Complex

RCF Radio Channel Frame

RF Radio Frequency

RTU Radio Test Unit

SH Speech Handler

SM Switching Module

SCT Synchronized Clock and Tone

SII-CS Series II Base Station

TDM Time Division Multiplexed

TDMA Time Division Multiple Access

VAF Voice Activity Factor

Issue 6, August 1999 C-3



C-4 Issue 6, August 1999


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