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CONFIDENTIAL

GUIDELINE

02 990430 Corrections C. Brechtmann ACS/SR

Radio Network Planning

L. Schnerstedt ACS/SR


01 970811 R. Klahm ACS/SR

Radio Network Plannin

T. Quick ACS/SR


ED DATE CHANGE NOTE APPRAISAL AUTHORITY ORIGINATOR

Engineering Rules

for Radio Networks

1AA000140004(9007)A4

ED

rne_gd02.docRCD ACS/SR

02

3DF 00995 0000 UAZZA 1/50

CONFIDENTIAL April 30, 1999

Abstract:

This document describes engineering rules to be used for radio network planning purposes.


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Edition 02 CONFIDENTIAL April 30, 1999

TABLE OF CONTENTS

1 SCOPE ...........................................................................................................................5

1.1 Related Documents...............................................................................................5

1.2 Referenced Documents.........................................................................................5

1.3 Abbreviations.......................................................................................................6

2 TRAFFIC CALCULATIONS ...............................................................................................8

2.1 Occurring Traffic...................................................................................................8

2.2 Traffic Capacity.....................................................................................................9

2.2.1 Loss System ........................................................................................................9

2.2.2 Queuing system................................................................................................10

2.3 Traffic Distribution..............................................................................................11

2.4 Traffic Mix and Mobility......................................................................................11

2.5 Typical Call Mix...................................................................................................11

3 COVERAGE PLANNING...............................................................................................13

3.1 Transmitter and Receiver Characteristics ...........................................................13

3.2 Antennas ............................................................................................................14

3.2.1 Directivity and Gain ..........................................................................................14

3.2.2 Antenna Diagram .............................................................................................16

3.2.3 Downtilt Angle ..................................................................................................17

3.2.4 Polarisation ......................................................................................................17

3.3 Wave Propagation .............................................................................................17

3.3.1 Free Space Propagation ....................................................................................18

3.3.2 Knife Edge Diffraction .......................................................................................19

3.3.3 COST 231 Models ............................................................................................20


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3.4 Link Budget ........................................................................................................23

3.4.1 Isolator, Combiner and Filter .............................................................................26

3.4.2 Cable and Connector........................................................................................26

3.4.3 Body Loss .........................................................................................................26

3.4.4 RF Input Power..................................................................................................26

3.4.5 Antenna Diversity ..............................................................................................26

3.4.6 Interferer Margin...............................................................................................26

3.4.7 Fading Margin..................................................................................................27

3.4.8 Antenna Pre-amplifier .......................................................................................29

3.4.9 Indoor Penetration ............................................................................................29

3.5 Cell Ranges.........................................................................................................30

4 FREQUENCY PLANNING .............................................................................................32

4.1 Frequency Reuse.................................................................................................32

4.1.1 Interferer Probability..........................................................................................35

4.1.2 Call Success Rate and Outage...........................................................................36

4.2 Group Frequency Planning ................................................................................37

4.2.1 Determination of the Reuse Cluster Size..............................................................37

4.2.2 Definition of the Frequency Groups....................................................................38

4.2.3 Assignment of the Group Numbers ....................................................................38

4.2.4 Advantages and Drawbacks of Group Frequency Planning..................................39

5 ELECTROMAGNETIC COMPATIBILITY PROBLEMS.........................................................40

6 APPENDIX: GSM SPECIFIC SUBJECTS ..........................................................................42

6.1 BSS Interconnection Planning ............................................................................42

6.1.1 BSC Architecture ...............................................................................................42

6.1.2 Submultiplexing and Line Configurations............................................................42


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6.2 Capacity and Coverage Extension ......................................................................43

6.2.1 Next Generation Mobile Capabilities..................................................................43

6.2.2 Concentric Cells................................................................................................44

6.2.3 Frequency Hopping...........................................................................................46

6.2.4 Microcells .........................................................................................................46

6.2.5 Repeater Applications........................................................................................48

6.2.6 Extended Cells ..................................................................................................49


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1 Scope

This document provides rules and formulas for radio engineering tasks. It is not containing generalmethodologies of cellular radio network planning; these are described in more detail in [1].

To allow more flexibility in the documents contents the given rules and formulas apply for the basic

methodology and can be used for initial radio network planning. In a matured network, usually ap-proaches are used which require a more specific view on the related engineering to be applied (e.g.,microcells, concentric cells etc.). These approaches are handled in more detail in separate documents,which are referred to at the appropriate sections in this document.

The described methodology is mainly related to the GSM system, since this is Alcatels main business inthe mobile communication sector. However, most of the planning guidelines and engineering rules maybe applied for other mobile radio standards as well. In this case, notions like BTS, handover, controlchannel have to be replaced by appropriate terms used in the related standard.

The main topics in this document are

Traffic calculations - how to find the number of required radio channels based on the given sub-scriber profile and based on the given traffic distribution.

Coverage planning - how to compute cell ranges based on link budget aspects and coverage re-quirements. This also includes basics on wave propagation and antennas.

Frequency planning - basic concepts on frequency reuse, interferer distances, speech quality andfrequency planning methods.

Electromagnetic compatibility - the influence of blocking, intermodulation products, compatibilitybetween different radio systems (e.g. GSM, AMPS, TACS) and suitable problem solutions.

BSS interconnection planning- basic dimensioning rules for the BTS/BSC connection and BSC con-figuration (for GSM).

GSM network expansion- possibilities of capacity or coverage expansion for GSM networks.

1.1 Related Documents

Engineering Rules for Radio Networks: Tables & FiguresAlcatel Document 3DF 00995 0001 UAZZA

BTS Sensitivities for Network Planning PurposesAlcatel Document 3DF 00995 0000 UHZZA

1.2 Referenced Documents

[1] Radio Network Planning Process DescriptionAlcatel Document 3DF 00991 0000 UAZZA

[2] Traffic Mix and Subscriber Mobility, in:MNDT 1.3 Mobile Network Design Toolbox, Users Guide

AMCF, Department MND

[3] Antenna Engineering RulesAlcatel Document 3DF 00995 0000 UCZZA

[4] Aspects on Polarisation DiversityAlcatel Document 3BK 10023 0001 DSZZA

[5] Urban Transmission Loss Models for Mobile Radio in the 900- and 1800-MHz Bands


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(Revision 2)COST 231 TD (90) 119 Rev. 2, The Hague, September 1991

[6] Description of Morpho Classes for Radio Network PlanningAlcatel Document 3DF 00993 2000 PGZZA

[7] European Digital Cellular Telecommunications System (Phase 2); Radio Transmission and

Reception (GSM 05.05)European Telecommunications Standards Institute, 1994

[8] Slow Frequency HoppingAlcatel Document 3DF 00995 0000 UDZZA

[9] Antenna Spacing for Diversity GainAlcatel Document 3DF 00995 0000 UCZZA

[10] Frequency Planning GuidelinesAlcatel Document 3DF 00995 0000 UEZZA

[11] Alcatel 900/1800 Multiband Operation GSM 900/GSM 1800, Concept Description

Alcatel Document 3DC 21037 0001 TQZZA[12] Network Planning Guidelines for Concentric Cells

Alcatel Document 3BK 10022 0001 DSZZA

[13] Activation Strategy for Microcellular NetworksAlcatel Document 3DF 00995 0000 UZZZA

[14] Indoor Coverage Solutions (position paper)Alcatel Document 3DC 21082 0001 TQZZA

[15] Repeater Solutions for Coverage Improvements (position paper)Alcatel Document 3DC 21055 0001 TQZZA

[16] Optimum Location Area PlanningAlcatel Document 3DF 00993 7000 PGZZA

[17] BSS B5 Dimensioning Rules

Alcatel Document 8BL 00712 0070 BGBRA

1.3 Abbreviations

AD Antenna Diversity

AMPS Advanced Mobile Phone Service

ARCS Average Reuse Cluster Size

BCCH Broadcasting Control CHannel (GSM)

BSC Base Station Controller

BSS Base Station System (BSC and BTS)

BTS Base Transceiver Station

COST European COoperation in Science and Technology

CPR Common Processing Unit

CSR Call Success Rate

DTC Digital Trunk Controller


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EIRP Effective Isotropic Radiated Power

EMC ElectroMagnetic Compatibility

ERP Effective Radiated Power

FDMA Frequency Division Multiple Access

FH Frequency Hopping

FR Full Rate

GoS Grade of Service

GSM Global System for Mobile Communication (900MHz/1800MHz)

HPBW Half-Power BeamWidth (of an antenna)

HR Half Rate

LAC Location Area Code

LNA Low-Noise Amplifier

LOS Line Of Sight

MCL Minimum Coupling Loss

MS Mobile Station

MSC Mobile Switching Centre

NLOS Non Line-Of-Sight

NMT Nordic Mobile Telephone System

OML Operation & Maintenance Link

PCM Pulse Code Modulation

PTMR Public Trunked Mobile Radio

QoS Quality of Service

RCS Reuse Cluster Size

RF Radio Frequency

RNP Radio Network Planning

RSL Radio Signalling Link

SM SubMultiplexer

TACS Total Access Communication SystemTC TransCoder

TCH Traffic CHannel

TCU Terminal Control Unit

TDMA Time Division Multiple Access

TMA Tower-Mounted Amplifier

TRX TRansceiver


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2 Traffic Calculations

Traffic was modelled by A. K. Erlang in the following mathematical way:

Equation 2-1: Traffic

= h

traffic, [ ] =Erlang

mean call arrival rate, [ ] =s-1

h mean holding time, [ ]h s=

Note: The unit Erlang is a pseudo unit and presents actually a ratio without any unit. A value of1 Erlang equals an amount of traffic which occupies one channel (or, in fixed networks, one line) com-pletely during the whole observation time.

Subscribers who use their mobile phone are causing traffic in the service area of a radio network. The

traffic may have different characteristics (speech, data, fax, short message etc.). The term traffic has tobe understood with care! One has to distinguish the following meanings:

Occurring traffic is the traffic that really occurs in the cell area. If the base station provides notenough traffic channels, some subscribers are not able to receive or establish a call: they areblocked.

Traffic capacityis the traffic that can be handled by one cell. It is dependent on the number of trafficchannels (regardless whether these channels are provided in form of frequency channels, timeslots,or both) which the radio base station provides in the cell area.

The calculation of resources for traffic handling is done in a similar way as for fixed networks. Obvi-ously, it is not efficient to provide one traffic channel for each subscriber, as no subscriber will use the

channel 24 hours a day. An example of a possible channel usage is in Figure 2-1.

Instead, queue statistics are applied to deter-mine the required number of channels. Inorder to provide an acceptable availability,the system is designed for a limited blockingprobability.

2.1 Occurring Traffic

Before the traffic can be calculated, a suitablemeasure must be defined. This is achieved by

dividing the mean holding time h a subscriberis occupying the channel by a certain obser-vation time t. Generally, if no other conditionsare given, this is the main busy hour. The re-

sulting value sub is measured in Erlang per

subscriber and is called traffic per subscriberorsubscriber profile, respectively:

Observation Time (1h)

subscriber 1

call setup

call release

subscriber 2

subscriber 3

subscriber 4blocked

blocked

Channel Usage

Figure 2-1: Channel Usage


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Equation 2-2: Traffic per Subscriber (Subscriber Profile)

subh

t=

sub traffic per subscriber, [ ]sub = Erlang

h mean holding time, [ ]h = st observation time (main busy hour), [ ]t = s

By multiplying this value by the total number of subscribers nsub in the cell, the total occurring traffic

in Erlangis found:

Equation 2-3: Total Occurring Traffic

= sub subn

total occurring traffic, [ ] = Erlang

sub traffic per subscriber, [ ]sub =

Erlang

subscribernsub number of subscribers, [ ]nsub = subscriber

Example: Consider 120 subscribers, every one of them occupying a traffic channel for 2 minutes(=120sec) during the main busy hour. Then,

subh

t= =

=

=

120s

1h subscriber

120s

3600s subscriber33.3

mErl

subscriberand

= = =sub subn 33.3mErl

subscriber120subscriber 4Erl .

2.2 Traffic Capacity

The traffic capacity of a cell depends on the

available number of traffic channels, respectively the number of carriers, and the

required blocking probability

The blocking probability is calculated based on the assumption of a loss systemor a queuing system.This is described in the following chapters.

Note:The blocking probability is sometimes referred to as grade of service(GoS).

2.2.1 Loss SystemA loss system is a traffic system where call attempts are rejected without any recognition in case of acomplete occupancy of all available lines (see Figure 2-2). The related blocking probability is calculatedwith the Erlang B formula:


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Equation 2-4: Erlang B Blocking Probability

P n

i

block

n

i

i

n=

=

!

!0

Pblock blocking probability

total offered traffic, [ ] = Erlangn number of traffic channels

Loss systems are e.g. GSM, NMT, and TACS. The trafficcapacity, which can be achieved for a given blocking prob-ability, is determined by an iterative procedure that solvesfor in Equation 2-4.

Table 2-1 shows the traffic capacity for different channelnumbers.

By studying these results, the following observations aremade:

The traffic capacity is distinctively smaller than the pro-vided number of channels. This is because the differentcall attempts treated in the statistics are not just occur-ring one by one with exactly the same holding time. Ac-tually, there is a statistical distribution both of the callarrival rate and the holding time. The higher the num-ber of channels, the closer the traffic value reaches thenumber of channels.

The behaviour of the traffic depending on the number of channels is non-linear and progressive.Examplefrom fixed line networks: calculating the traffic capacity of two trunks of 7 lines each, a to-tal capacity of 2 2.9 Erlang = 5.8 Erlang results. By combining the two trunks to one with 14 lines,the capacity is 8.2 Erlang and higher than the total value of the two smaller trunks. This effect isknown as trunking efficiency.

If the number of traffic channels is known, it is easy to calculate the required number of frequency re-sources. In simple cases, e.g. for analogue radio systems, one frequency channel and hence one trans-ceiver is required for each traffic channel. GSM works with TDMA (time division multiple access) and isable to handle 8 traffic channels on one transceiver. Caution: When calculating the number of trans-ceivers out of the number of traffic channels, some control or signalling channels must be considered!

The number of control or signalling channels is system dependent.

2.2.2 Queuing system

A queuing system is a system where call attempts, which cannot be handled in case the system is fullyloaded, are put into a queue. If a call attempt in a queue cannot be handled even after a certain waitingtime, the call is rejected. For the calculation of the blocking probability, the Erlang C formula is used. Inmost cases, it is assumed that the queue has an infinite number of entries.

For example, PTMR are queuing systems.

Note:For the Alcatel BSS, queuing of calls can be enabled. Queuing duration and maximum number ofcalls to be queued are settable parameter. In this case, naturally the blocking must be calculated with

Erlang C!

Figure 2-2: Loss System with Traffic Flow

Table 2-1: Traffic Capacity

n (Numberof TrafficChannels)

Traffic (Er-lang) for2% Blocking

ChannelLoad

7 2.9 41%

14 8.2 59%

22 14.9 68%

30 21.0 70%

38 28.3 74%

46 36.5 79%

53 43.1 81%

61 50.6 83%

Handled

Occurring

Rejected

LossSystem

(n slots)


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2.3 Traffic Distribution

Designing cells with the above formulas leads normally to rather coarse figures. In fact, the subscribersare mostly not distributed equally in the cell. This can be expressed with a traffic density distributiont (x, y), wherex, yare cell coordinates. The total offered traffic is then found by integrating t (x, y) overthe cell area A:

Equation 2-5: Total Offered Traffic over Traffic Density Distribution

= t x y x yA

( , )d d

total offered traffic,

t x y( , ) traffic density distribution

( , )x y cell coordinates

A cell area

For practical purposes, that means that a certain differentiation must be done when calculating the re-quired number of base stations over a complete service area. The more exact the division in several

subareas with different traffic densities, the more accurate the result.For computer-aided radio network planning tools, the formula above allows an easy calculation of theoffered traffic for each cell. However, it is most often very difficult to obtain an accurate traffic densitydatabase for a big continuous service area.

2.4 Traffic Mix and Mobility

For many cases, the traffic calculations shown in the previous chapters are accurate enough to allow arule-of-thumb dimensioning of a network. However, depending on the systems complexity, differenttypes of traffic may occur, e.g. normal calls, fax or data transmission, and short message services, wherethe subscriber profiles are different. Each of the services and the signalling during the call requires a

certain load on common or special control channels, for which the traffic must be calculated as well. Theblocking calculation for control and traffic channels may be different because in the first case, a queuingsystem and in the second case, a loss system has to be considered.

Additionally, the subscribers are not fixed in the network. Depending on the system, their mobility leadsto the occurrence of some events like handovers / hand-offs, location updating (for systems which sup-port roaming). These events are causing additional traffic on proper control channels.

Models, which allow the calculation of all these cases, are very complex and consist of some hundredparameter [2]. Their suitable application is restricted to the cases where each or at least enough pa-rameter are known by the operator, but they can also be used to study the dependence of the total trafficon the single parameter, or they can be used for answering questions like e.g.: What happens to thecell if the handover rate is doubled (is an additional signalling channel needed)?

In order to optimise SDCCH and PCH load to a minimum in GSM networks, there are approaches onhow to achieve an optimum location area planning [16].

2.5 Typical Call Mix

The average behaviour of a subscriber in using his line was measured to be able to calculate the ex-pected TCH and SDCCH usage in GSM networks. With these results, it is possible to dimension thenumber of TRXs and the configuration how many SDCCH should be used. The values in Table 2-1:Typical Call Mix show a typical call mix situation in a matured European operational network.


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Table 2-1: Typical Call Mix

Parameter Value

Busy hour attempts per subscriber

call setups 1.3

location updates (LU) 3

short message services (SMS) 0.2

inter-BSC handover 0.6

average SDCCH duration 4 s

traffic per subscriber 16 mErlang

Blocking

TCH 2 %

SDCCH 0.5 %

Example: In GSM, SDCCHs are used for signalling during call setups, location updates, short messageservices and supplementary services. At an Alcatel 2 TRX BTS in standard SDCCH configuration, eight ofsuch SDCCH are available. For standard timeslot configurations for BTS see Engineering Rules for RadioNetworks: Tables & Figures. The values given in Table 2-1: Typical Call Mix are used in this example.

How many subscribers can be handled by an Alcatel 2 TRX BTS in standard SDCCH configuration, if thetraffic per subscriber is 16 mErlang?

512Erlang016.0

Erlang2.8 ===

=

sub

BTSsub

subsubBTS

n

n

BTS BTS traffic capacity

sub traffic per subscriber

subn number of subscribers

Statistically, in the main busy hour, each subscriber causes the following transactions: 1.3 calls per sub-scriber, 3location updates, 0.2 short message services and 0.6 inter-BSC handover. The mean occupation timefor the SDCCH is 4 s. How much SDCCH traffic has to be expected?

( )

Erlang73.2Erlang3600

45128.4

Erlangs3600

s45126.02.033.1

s3600

=

=

+++=

+++=

SDCCH

SDCCH

SDCCHsubhandoverSMSLUcallsetupsSDCCH tnaaaa

SDCCH occurring SDCCH traffic

callsetupsa number of call setups during busy hour per subscriber

LUa number of location updates during busy hour per subscriber


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SMSa number of short message services during busy hour per subscriber

handovera number of inter-BSC during busy hour per subscriber

SDCCHt mean occupation time for the SDCCH

subn number of subscribers

By a given SDCCH blocking probability of 0.5 %, is there congestion? No, because eight SDCCHs canhandle 2.73 Erlang. See Engineering Rules for Radio Networks: Tables & Figures table Blocking Prob-ability Erlang B to get this value. Using Erlang B is not exact. For example, a location update or an in-ter-BSC handover would not be tried only once. It is therefore a kind of queuing system and not a totalloss system, which leads to a mix of Erlang B and Erlang C calculation for the occurring SDCCH traffic.Using Erlang B means to be more optimistic.

3 Coverage Planning

Coverage planning of cells requires far more knowledge than traffic calculations, as radio propagationmust be treated. In the following, all aspects that have an influence on the achievable cell range are

addressed.

3.1 Transmitter and Receiver Characteristics

Each radio system is characterised by a set of transmitter and receiver parameter. In mobile radio sys-tems, the parameter must be suitable designed in order to allow two-way communication. In addition,these parameter depend on the capability of the system (digital/analogue, required bit-rate for sufficientspeech quality, improvements due to signal processing etc.).

The following parameter are of interest for radio network planning:

Transmitter output powers of base station and mobile station: these parameter are optimised in orderto allow a balanced path loss for up- and downlink (see section 3.4: Link Budget). In general, the

output powers for the different systems are divided in classes.

Receiver sensitivities (base station and mobile station): the sensitivities depend on the path loss bal-ance, noise figures of the amplifiers in the radio receiver, signal processing capabilities and protec-tion against possible interference. The optimum values are specified in order to achieve understand-able speech quality.


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Equation 3-1: Directivity

D S

Si( , ) log

( , )

= 10

D( , ) directivity, [ ]D( , ) = dBi

S( , ) power density flow, [ ]S( , ) = Wm2

Si power density flow of isotropic radiator, [ ]Si =W

m2

( , ) direction in spherical coordinates, 0 22 2

< +

,

Note: Si is direction independent.

This forms a typical radiation pattern of the antenna.

A more common parameter is the antenna gain G. It is similar to the antenna directivity, but takes also

losses into account. The efficiency contains some structures that could not be considered in an antennasimulation. An Example would be fastenings, cables and connectors.

Equation 3-2:Antenna Gain

G D= +10log

G antenna gain, [ ]G = dBi efficiency

D directivity, [ ]D = dB

Both directivity and gain can be given for certain directions and as total (integral) values. An antennawith a given radiation pattern provides this total gain value in main beam direction, so that

Equation 3-3: Effective Isotropic Radiated Power

EIRP P Gin= +

EIRP Effective Isotropic Radiated Power (in main beam direction) [ ]EIRP = dBm

Pin power fed into the antenna, [ ]Pin = dBm

G antenna gain, [ ]G = dBi

In order to enable simple calculations with power values in dBm, G is also often specified in dB and re-ferred to the isotropic radiator, i.e. the isotropic radiator has an antenna gain of Gi = 0 dBi (where i of

the unit dBi stands for isotropic).Sometimes, the notion of ERP also occurs. This stands for Effective Radiated Power. The difference tothe EIRP is the reference to the half-wave dipole (i.e., a real antenna) rather than the isotropic radiator.Since the half-wave dipole has an antenna gain of 2.1 dBi,


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where the antenna radiates half of the power radiated in main beam direction, i.e. 3 dB less power. TheHPBWs are directly related to the antenna gain. If e.g. the vertical HPBW is reduced, the antenna gainvalue is rising.

The front-to-back ratio specifies the relation between the gain in main beam direction and the highestside lobe in the back region of the antenna and is an important selection criteria if an optimum interfer-

ence limitation is required.A special case is the antenna diagram of a single monopole or dipole: it has a beam in vertical directionand a circular or omni directionalpattern in horizontal direction. Appropriate antennas are called omniantennas.

The horizontal and vertical diagrams are normally measured in an anti-echoic chamber, so that no con-ductive materials are influencing upon the antenna. In reality, the antennas are installed on conductingpoles or masts, and some materials near the antenna installation are conductive, so that the antennadiagram will be distorted. Guidelines for the installation, which minimises these distortions, are de-scribed in [3].

3.2.3 Downtilt AngleSometimes, antennas are installed with a small downtilt angle, i.e. considering the vertical diagram, themain beam is lowered against the horizontal direction by a few degrees. This is done to reduce over-shoot and minimise interference in the network. Typical values are in the order of 2 to 6. Downtilt an-gles can be applied mechanically or electrically. In the latter case, the tilted beam is achieved by feedingof the different dipoles in an antenna group with different phase-shifted signals.

Electrical downtilt antennas have some advantages compared to antennas, which are mechanicallydowntilted. Due to the phase-shifted excitation of the antenna elements, the whole vertical antenna dia-gram is downtilted, the main beam and the side lobes, but the backlobe stays straight. In case of me-chanical downtilt the main beam is tilted, but the side lobes not. Furthermore, the backlobe is uptilted. Inaddition, it is possible to mount the antennas without special mounting assemblies, e.g. downtilt kits,

which exclude further installation error sources. On the other hand, antennas with fixed electrical down-tilt are not allowing flexible downtilt adjustment.

3.2.4 Polarisation

Thepolarisation direction is given by the direction of the electric field vector. For mobile radio antennas,mainly vertical polarisation, provided by vertical slab antennas, is used. For antenna diversity, however,concepts with cross-polarised antennas (antennas comprising both a vertical and a horizontal polarisedelement) are used to provide two receiver branches (see [4] for details).

Polarised antennas are specified by an uncoupling value, which describes the transmission factor fromone branch to the other. This value must be taken into account in order to avoid receiver blocking if such

an antenna is used for both transmitting and receiving.

3.3 Wave Propagation

The propagation of radio waves is one of the most difficult subjects in electromagnetic field theory. It isobvious that practical formulas, which can be effectively applied for radio engineering purposes, arerequired. Some of the formulas, as will be seen later in this chapter, are the summary of planning expe-rience rather than derivations from field theory. They are based on real measurements and have an em-pirical character.


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3.3.1 Free Space Propagation

The simplest form of wave propagation is the free-space propagation. The according path loss can becalculated with the following formula:

Equation 3-5: Path Loss in Free Space Propagation

L d

km

f

MHzfreespace= + + 32 4 20 20. log log

Lfreespace free space loss, [ ]Lfreespace = dBd distance between transmitter and receiver antenna, [ ]d = km

f operating frequency, [ ]f = MHz

This formula can only be applied if the direct line-of-sight (LOS) between transmitter and receiver is notobstructed. This is the case if a specific region around the LOS is cleared from any obstacles. This regionis called Fresnel ellipsoid (see Figure 3-2).

This ellipsoid is the set of all

points around the LOS wherethe total length of the con-necting lines to the transmitterand the receiver is longerthan the LOS length by ex-actly half a wavelength. It canbe shown that this region iscarrying the main power flowfrom transmitter to receiver.

Note:It is also possible to define higher-order regions around the LOS by requiring that the differencebetween the total length of a path via a defining point and the LOS length is generally n/2, with n theorder of the Fresnel ellipsoid.

The cross section of the Fresnel ellipsoid at any point between transmitter and receiver is a circle withradius r (see Figure 3-3, lower part).

Equation 3-6: Fresnel Ellipsoid Radius

21

21

dd

ddr

+

=

withfff

c -13-13 ms

103.17ms10300000

=

==

( )fdddd

r21

1213 ms103.17

+=

r Fresnel ellipsoid radius, [ ] m=r

1d distance between transmitter and the considered point, [ ] m1 =d

2d distance between considered point and receiver, [ ] m2 =d wavelength, [ ] m=

f frequency, [ ] 1s=f

Using this formula makes it possible to check whether an obstruction occurs or not.

Transmitter

Receiver

LOS

Figure 3-2: Fresnel ellipsoid


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3.3.2 Knife Edge Diffraction

In case of an obstruction of the LOS path, the free-space formula with an additional correction term canbe used if the obstacle is smallcompared to the distance fromtransmitter to receiver. Based onthe assumption that this obstaclecan be replaced by an ideal con-ducting half-plane which extendsto infinity in the direction perpen-dicular to the propagation pathand which is of infinitesimal thick-ness (knife-edge, see Figure 3-3), this situation refers to a fieldtheory problem which can besolved in a deterministic way. Inthe case that this knife-edge ob-stacle type enters the Fresnel re-

gion, diffraction occurs (similar tothe diffraction known from optics) and introduces some additional diffraction loss compared to the free-space propagation.

h0

r

d1 d2

LOS

h0

LOS

Figure 3-3: Knife-Edge obstacle


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Equation 3-7: Diffraction Loss

+===

21

210

0where)(dd

ddh

r

hvvFLdiff

diffL diffraction loss, [ ] dB=L

)(vF function, see Figure 3-4, [ ] dB)( =vFv represents the number of cleared Fresnel ellipsoids, without unit

0h height of the obstacle above the LOS, [ ] m0 =h

r Fresnel ellipsoid radius, [ ] m=r

1d distance between transmitter and the considered point, [ ] m1 =d

2d distance between considered point and receiver, [ ] m2 =d wavelength, [ ] m=

The diffraction loss is 6 dB ifthe obstacle is just touching

the LOS ( 000 == vh ).For v >1, some oscillation isnoted, which appears due tothe fact that the obstaclemoves over several Fresnelregions where the phase ofthe transmitted signal is al-ternating between +180and -180 phase shift. Inreality, the conductivity of the

obstacles material is notideal, and the oscillation ap-pears smoothed to an aver-age value.

3.3.3 COST 231 Models

For mobile networks, the receiver antenna is normally situated at a low height above the ground. Thus,the Fresnel ellipsoid is obstructed in the environment of the mobile station over a big distance. In thissituation (which is typically met for mobile networks), the free-space formula cannot be used any more,and it is impracticable to describe this situation with a physical model.

In such situations, empirical relations are applied. The most effective approach is found in theCOST 231 recommendation [5]. It distinguishes between macrocells and microcells (according to typicalcell ranges) and is based on well-known empirical formulas to be applied to the different scenarios, ex-tended for the usage on higher frequencies or additional propagation effects.

3.3.3.1 Hata Model

For conventional macrocells, the path loss is calculated by an empirical formula. Hata's formula wasoriginally designed to fit propagation measurements in dense urban area. However, it can be applied inother environments as well if a certain gain Gmorphois subtracted which takes better propagation condi-tions for certain land usage (morpho structure) types into account. Standard gain values are listed in [6].

Knife-edge diffraction function

-5

0

5

10

15

20

25

30

35

-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3

Clearance of Fresnel ellipsoid (v)

F(v)[dB]

Figure 3-4: Diffraction function F(v)


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Equation 3-8: Path Loss in Order of Hata

morphoBS

MSBS

hata

Gdh

hahf

aaL

+

++=

kmlog)

mlog55.69.44(

)(m

log82.13MHz

log21

with )8.0MHz

log56.1(m

)7.0MHz

log1.1()( = fhf

ha MSMS

and the values of 1a and 2a depend on the frequency range:

Frequency range(MHz)

a1 a2

150...1500 69.55 26.16

1500...2000 46.3 33.9

HataL Hata loss, [ ] dB=HataL

f frequency, [ ] MHz=f

BSh base station height, hBS=30...200m, [ ] m=BSh

MSh mobile station height, hMS=1...10m, [ ] m=MSh

d distance between BS and MS, d=1...20km, [ ] km=d

morphoG morpho correction gain, dB=morphoG

3.3.3.2Walfish-Ikegami Model

If smaller cells, consisting of a smaller cell range than 1km, or microcells (where the antennas are typi-cally installed below roof top level) have to be considered, the Walfish-Ikegami model extended byCOST is more suitable. Parameter which are taken into account are (see also Figure 3-5):

average roof level for the buildings between transmitter and receiver

building spacing and street width at the mobiles location

street orientation relative to propagation direction


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h

BTS

h

roof w

b

d

hmob

multiple screen loss

roof top to streetdiffraction loss

Street

orientation

Incident wave

Buildings

base

station

mobile

station

Figure 3-5: Parameter for COST 231 Walfish-Ikegami model

The path loss LWIis composed of the following terms:

Equation -93-10: Path loss in Order of Walfish-Ikegami

MSDRTSfreespaceWI LLLL ++=

WIL path loss in order of Walfish-Ikegami, [ ] dB=WIL

freespaceL free space loss, dB=freespaceL

RTSL roof-top-to-street diffraction and scatter loss, [ ] dB=RTSL

MSDL multi-screen diffraction loss, [ ] dB=MSDL

For a detailed description on all parameter, refer to [5]. The above formula is valid for the non-LOScase. If a LOS exists, the loss is calculated based on street canyon propagation using a single slopemodel:


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Equation 3-11: LOS Path Loss in Order of Walfish-Ikegami

MHzlog20

kmlog266.42

fdLWI ++=

WIL path loss in order of Walfish-Ikegami, [ ] dB=WIL

d distance between BS and MS, d=1...20km, [ ] km=df frequency, [ ] MHz=f

For microcells, the BTS height hbase is smaller than the average roof top height hroof, and the Walfish-Ikegami model is not very accurate. Depending on the environment, and for estimating calculations, abreakpoint model as described below should be used:

Equation 3-12: Breakpoint Model

L

dd d

d dd d

streetcanyon

bp

bp

bp

=+

+

105 26

105 14 40

log

log log

km

km km>

Lstreetcanyon path loss in street canyon, [ ]Lstreet canyon =dB

d distance between BS and MS, [ ]d =km

dbp breakpoint distance, [ ]dbp =km

The formula is valid for GSM (f = 900 MHz). For GSM 1800, the same formula may be used by adding10 dB to the loss value. This formula is obtained by applying a dual slope fit of measured values. Physi-cally, it can be interpreted as two different power laws before and after the breakpoint distance d bpwhere the Fresnel ellipsoid hits the ground (dbp = 200 m).

The application field of this breakpoint model has not been identified yet. However, it can be easily cali-brated by measurements.

3.4 Link Budget

For bi-directional communication systems, it is very important to have the same path loss in uplink anddownlink direction.

BTS is uplink limited:If the downlink has a larger radio range than the uplink, there is more interfer-ence between cells and therefore less quality than possible. Also, the mobile could receive BCCH from aBTS, but isnt able to set up a call on this BTS. The difference in transmission power between BTS and MS

is compensated by special means at the BTS to increase the sensitivity. The link budget calculation resultsin a required EIRP to be radiated by the antenna. The output power of the BTS is adjusted in order toachieve the required EIRP.

BTS is downlink limited:If the uplink has a larger radio range than the downlink, this means, the BTSreceives signals from the mobile station, which arent reachable from the BTS. One could try to savemoney putting up the BTS without TMA and without diversity gain configuration.

To equalise the downlink and uplink path loss, it is necessary to set up a link budget for each cell, whichconsiders the RF transmission power and sensitivities of BTS and mobile station.

The link budget depends on the following data:

base station (internal power, combiner loss, sensitivity)


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mobile station (output power, antenna gain, sensitivity, power tolerance)

antenna gain of transmit and receive antennas

cable and connector loss

system options (antenna diversity, antenna pre-amplifier)

body loss (handheld used)

indoor usage

Below, a typical balanced GSM900 link budget is shown. A sketch showing the occurring gains andlosses is presented in Figure 3-6 for the downlink direction and Figure 3-7 for the uplink direction.

The link budget is based on the usage of 2 W mobiles. An omni cell is treated, using omnidirectionalantennas and diversity. The antennas have a gain of 11 dB. A cable loss of 3 dB was assumed which isin conjunction with a cable length of ca. 50...70 m. The link budget is unbalanced, therefore the lowervalue has to be taken. This leads to a maximum allowable path loss of 135.4 dB. The sensitivity of theG3 is very high (-111 dBm). This is the reason why in normal cases the maximum output power of theBTS has not to be reduced. For more examples see Engineering Rules for Radio Networks: Tables & Fig-ures Document as referred to in 1.1 Related Documents.

Link Budget for GSM900

TX Units MS to BTS BTS to MS

RF Output Power dBm 33 (2 W) 45.4 (34.7 W)

Isolator + Combiner + Filter dB 0 -5.0

Cable + Connector dB 0 -3

Antenna Gain dBi 0 +11

Body Loss dB -4 0

EIRP dBm 29 48.4

RX Units MS to BTS BTS to MS

RF Input Power (Min.) dBm -111 -102

Cable + Connector dB +3 0

Antenna Gain dBi -11 0

Antenna Diversity dB -3 0

Interferer Margin dB +3 +3

Fading Margin 50% 90% dB +8 +8

Body Loss dB 0 +4

Min. Required Received IsotropicPower

dBm -111 -87

Path Loss dB 140 135.4


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RX

RXD

RXin

TXout

SU FU

RX

RXD

RXin

TXout

SU FU

WBC

Pout

Lcomb

Lcable

Pant,in

EIRP

Pathloss(Lpath)

FadingInterferenceBase station

Mobile

Antenna

Gant,BS

Prec,isotrop

Prec,MS

Gant,MSLMS

Pout,BS

Figure 3-6: Losses and gains in downlink direction

RX

RXD

RXin

TXout

SU FU

RX

RXD

RXin

TXout

SU FU

WBC

Lcable

EIRP

Pathloss(Lpath)

FadingInterference

Base station

Mobile

Antennas

Gant,BS

Prec,isotrop

Pout,MS

Gant,MSLMS

Prec,BS

Figure 3-7: Losses and gains in uplink direction

Some notions, which occur in this path loss, need some further comments given in the following.


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3.4.1 Isolator, Combiner and Filter

These are all kind of elements which are connected to the transmitter output port of the transceivers andwhich are needed to combine the signals from several transceivers, to reduce spurious emissions etc. Allthese units introduce certain insertion losses, which are summed up here. If duplexers are used, they canbe treated in this field. For mobile stations, output power tolerances may be considered here if worst-case calculations are required.

3.4.2 Cable and Connector

These losses occur in feeder and jumper cables and connectors of the actual antenna installation. Thisvalue is treated both in up- and downlink direction.

Note:This value may vary from site to site.

3.4.3 Body Loss

This loss is especially experienced if handheld mobiles are used. It is occurring due to partial field ab-sorption in the human body. A typical value is 4 dB.

3.4.4 RF Input Power

This is the specified sensitivity of the receiver. For some systems, on one hand care has to be taken whatis specified together with this value, and on the other hand, what kind of reception quality has to beachieved. In the example above, the given sensitivities are specified for a maximum bit error rate whichprovides marginal speech quality, while a minimum signal-to-noise ratio and a minimum carrier-to-interferer ratio is maintained. Further prerequisites are the propagation environment, which specifies astatistical distribution of the original signal and a couple of multipath signals which are delayed by dif-ferent bit periods, and which also treats a Doppler spectrum due to the movement of the mobile. Indigital systems like GSM, some propagation environments are simulated with a radio channel simulationsoftware, and related maximum bit error rates are defined. The link budget shown above is valid for the

TU50environment, which indicates the model typical urban and a mobile speed of 50km/h. Other defi-nitions can be found in [7].

GSM uses a frame correction system, which works with checksum coding and convolutional codes. Un-der defined conditions, this frame correction works successfully and copes even with fast fading types asRayleigh or Rician fading. For lower mobile speed or stationary use, the fading has a bigger influenceon the bit error rate and hence the speech quality is reduced. In such a case, a degradation margin mustbe applied. The margin depends on the mobile speed and the usage of slow frequency hopping, whichcan improve the situation for slow mobiles again. For the application of suitable margins, refer to [8].The margin can reach values up to 8 dB and higher.

3.4.5Antenna DiversityThis designates the optional usage of a second receiver antenna. The second antenna is placed in away, which provides some decorrelation of the received signals. In a suitable combiner, the signals areprocessed in order to achieve a sum signal with a smaller fading variation range. Depending on thereceiver type, the signal correlation, and the antenna orientation, a diversity gain from 26 dB is possi-ble. More detailed investigations are found in [4] and [9].

3.4.6 Interferer Margin

In GSM, the defined minimum carrier-to-interferer ration (C/I) threshold of 9 dB is only valid if the re-ceived server signal is not too weak. In the case that e.g. the defined system threshold for the BTS of-104 dBm is approached, a higher value of C/I is required in order to maintain the speech quality. Ac-


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cording to GSM, this is done by taking into account a correction of 3 dB. Depending on the requiredspeech quality, a higher value may be used as well.

3.4.7 Fading Margin

Due to fading effects, the minimum isotropic power is only received with a certain probability. Figure 3-8

shows a measured varying signal, which is received by a moving mobile. The shown signal is filtered byan averaging window. Three classes of fading may be distinguished:

Fast fading: This fading is characterised by phase summation and cancellation of signal compo-nents, which travel on multiple paths. The variation is in the order of the considered wavelength. Themeasured signal in Figure 3-8 is not detailed enough to show this effect. Their statistical behaviour isdescribed by the Rayleigh distribution (for non-LOS signals) and the Rice distribution (for LOS sig-nals), respectively. In GSM, it is alreadyconsidered by the sensitivity values,which take the error correction capabil-ity into account (see also 3.4.4).

Mid-term fading: mid-term fieldstrength variations caused by objects inthe size of 10...100m (trees, buildings).Referring to Figure 3-8, one can seethat these variations are yielded. Thesevariations are lognormally distributed.

Long-term fading: long-term variationscaused by large objects like largebuildings, forests, hills (> 100m). Likethe mid-term field strength variations,these variations are lognormally dis-

tributed.The lognormal distribution, described by a mean field strength Fmedand a standard deviation , is shownin Figure 3-9 in the diagrams on the left side. A coverage probability Pcovcan be calculated, which de-fines the chance that a certain field strength threshold F thris reached or exceeded by the calculated (orpredicted) mean field strength level Fmed. This probability is represented by the area enclosed by thegraph of the probability density function and the vertical line at F=F thrin the left diagram. Fthris a staticvariable, whereas Fmed is movable. Compare the upper and the lower diagrams in Figure 3-9, to seehow Fmedmoves. The variation of the probability in dependence on Fmed is shown in diagrams on theright side. The required difference between Fmed and Fthr in order to achieve a required probability iscalled the fading margin.

Lognormal fading (typical 20 dB

loss by entering a village)

Fading hole

Lognormal fading (entering

a tunnel)

Figure 3-8: Variations of a received signal


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Without any margin, the probability is 50%, which is not a sufficient value in order to provide a good callsuccess rate. A typical design goal should be a coverage probability of 90...95%. The following normal-ised table can be applied to find fading margins for different values of . The fading margin is calcu-lated by multiplying the value of k (in the table) with the standard deviation (Fading Margin = k).

k - -0.5 0 1 1.3 1.65 2 2.33 +

CoverageProbability

0% 30% 50% 84% 90% 95% 97.7% 99% 100%

The value of the standard deviation depends on the clutter type (morpho structure) and the variation ofthe terrain height (flat or hilly). Typical values for not too dense urban areas are 7 dB (flat terrain) and14 dB (hilly terrain). For more values, see [6].

Probability Density Function (PDF)

0%

2%

4%

6%

8%

Fthr FmedReceived Power F /dBm

Margin

Coverage Probability

0%

20%

40%

60%

80%

100%

-20 -10 0 10 20

F = (Fmed- Fthr) /dB

Margin

95,2%

50% probabilityfor Fmed=Fthr

Coverage Probability

0%

20%

40%

60%

80%

100%

-20 -10 0 10 20

F = (Fmed- Fthr) /dB

Margin

15,9%

50% probabilityfor Fmed=Fthr

Probability Density Function (PDF)

0%

2%

4%

6%

8%

FthrFmedReceived Power F /dBm

Margin

Figure 3-9: Pcov(curves for = 6 dB)


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3.4.8Antenna Pre-amplifier

This option is not considered in the link budget above. Also known as LNA (low noise amplifier) or TMA(tower mounted amplifier). A low-noise amplifier can be installed directly behind the receiving antennasto compensate the cable and connector loss. Overcompensation leads to a higher noise figure at thereceiver front-end.

A more cost-effective alternative to this option, which especially reduces the antenna installation effortdistinctively, is the design of a more sensitive receiver front-end (by ca. 3 dB).

3.4.9 Indoor Penetration

This parameter is also not shown in the exemplary link budget. An often used value is 15 dB and refersto a scenario where the subscriber is standing at a maximum distance of 4m away from a first wall witha window. The first wall is understood as the wall, which separates the subscriber from the outdoorservice area, in direction of the base station. Naturally, this value can vary depending on the wall mate-rial. Also, there is a variation at high buildings consisting of several floors. Up to the 10 thfloor, the in-

door penetration value is reduced by 2.7 dB/floor, and above the 10th

floor, the reduction is

0.3 dB/floor.

See Figure 3-10 for further typical values.

For deep indoor applications, the typical values may be doubled (second wall scenario). For the stan-dard value as described above, this yields 30 dB indoor penetration.

Incident wave

Incident wave

Lindoor= 3 ... 15 dB


Lindoor= dB (deep basement)


-2.7 dB / floor(1st ... 10th floor)

-0.3 dB / floor(11th ... 100th floor)

Lindoor= 7 ... 18 dB(ground floor)

Figure 3-10: Typical values of indoor loss; residential building (left) and commercial building (right)


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3.5 Cell Ranges

If the maximum allowable path loss has been determined with the link budget, the corresponding maxi-mum range of the cell can be calculated using the propagation formulas. This further allows the calcula-tion of coverage areas, which are required to estimate the quantity of base station sites required to covera continuous service area.

To ease dimensioning work, the parts of the service area, which are characterised by a typical land us-age type, are determined. This allows considering different values of the morpho correction gain in thepath loss calculations and yields a more accurate result for the network dimensioning.

The coverage ranges are not only dependent on the value of the morpho correction gains. As mentionedearlier, a fading margin has to be added in order to achieve a higher probability for receiving the cal-culated field strength. This standard deviation depends on the considered morpho structure type and onpossible terrain variations as well. When rises, it is causing a higher fading margin and therefore hasalso a direct influence on the maximum path loss value.

The described method yields the given coverage values at the cell fringe. This is a quite tough coverageconstraint. It is more reasonable to specify the required coverage probability as an average value over

the cell area. In this case, the coverage probability can be treated as the fraction of locations where thecalculated field strength level is achieved or even exceeded.

The average value is found by integration of the local coverage probabilities over the whole cell areaand normalisation on the area value:

Equation 3-13: Coverage Probability

< >= P A P x y dx dy A

cov cov ( , )1

Pcov coverage probability, Pcov

A area, [ ]A =m2

x running variable , [ ]x =m

y running variable, [ ] m=y


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The calculation of this value is very difficult andshould be performed by a computer tool. As arule-of-thumb, there is approximately a factor of1.2 between the average coverage probability andthe coverage probability at the cell fringe, e.g. fora coverage probability of 75% at the cell border,

an average coverage probability value of 90% isachieved (= 7 dB). This factor is slightly depend-ent on the base station height, whereas the stan-dard deviation has a stronger influence.

Table 3-2 shows some coverage ranges for differ-ent clutter types. For the calculations, the maximumpath loss from the link budget shown in chapter3.4 and Hata's formula has been used. The mor-pho correction gains and standard deviations foreach clutter type are also shown in the table.

If the coverage ranges are known, it is possible tocompute the related cell areas and determine thenumber of required sites for a given service area.

Also, the ranges can be used to prepare transpar-ent circles or hexagons for performing a planning-

by-hand with a map showing the service area and the regions of different morpho structure or sub-scriber profiles, important locations etc.

Clutter type Gmorpho/ dB / dB d / km

urban, flat 3 7 2.78

urban, hilly 3 14 1.66

suburban, flat 6 6 3.62

suburban, hilly 6 12 2.35

forest, flat 10 6 4.70

forest, hilly 10 10 3.54

open area, flat 25 6 12.53

open area, hilly 25 10 9.44

Used parameter:

LHata=135.4 dB + 8 dB (interferer margin) = 143.4 dBf=900 MHz

hMS= 1.5 mhBS= 30 mPcov= 90% (area coverage)

Table 3-2: Some coverage ranges


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4 Frequency Planning

For an optimum system performance, an effective frequency planning strategy has to be followed.

On one hand, the strategy should be flexible to support the build-up phase of a network where thesystem design changes rapidly within few months or even weeks. This requires a suitable method of

group frequency planning which can be applied manually.

On the other hand, for mature radio networks, automatic frequency planning methods are impor-tant, so as to plan the available frequency resources in order to achieve the optimum system per-formance.

Detailed planning methods which support the concepts of super reuseplanning, frequency hopping andother options which maximises the usage of the available spectrum are in detail described in [10]. In thissection, a planning methodology with frequency groups is shown. At first, however, some interdepend-encies between frequency reuse, interference, and speech quality are addressed.

4.1 Frequency Reuse

In a cellular radio network, frequencies,which are assigned to a specific cell, can bereused by neighbouring cells if a certainreuse distance is kept (see Figure 4-1). Inthe region between two cells A and B usingthe same frequency, interference occursand makes a communication on that fre-quency scrambled or impossible. However,in this critical area, other cells, which aresituated between the interfering cell pair,are having a higher receive field strengthlevel and have assigned non-critical fre-quencies. Thus, the system is possible tomaintain a call of a mobile which movesfrom A to B by consecutive handoveringfrom cell to cell. Figure 4-2 shows thevariation of the received power along theconnecting line A-B. Within the region des-ignated by the dotted circle in Figure 4-1, the minimum C/I value is no longer kept.

re-usedistancecell A

cell B

interfererregion

Figure 4-1:Reuse distance


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Frequency reuseschemes are represented in form of clusters. Those clusters can be described by treating

a simplified representation of the cell areas in shape of hexagons. Figure 4-3 shows a hexagonal ar-

rangement of omnidirectional cells, having the same cell radius R. The cells are forming a 7 reuse clustersize (RCS). Each relative cell position in that cluster has been assigned a single frequency or, in case oftwo or more carriers per cell, a group of frequencies. Figure 4-4 shows an example with three-sectorisedcells (RCS = 12).

distance DR

Received PowerP rec

C/I

Prec, A Prec, B

0

Figure 4-2: Variation of the received power P rec


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There is a basic relation between the reusecluster size RCS, the cell radius r and the reusedistance d:

Equation 4-1: Reuse Distance

d a r RCS = 3 with a=

12

3

omnidirectional cells

three - sectorized cells

d reuse distance, [ ]d =m

a cell factor

r cell radius, [ ]r =m

RCS reuse cluster size

For example, a distance d r=6 between sites with identical frequencies in an omni-cell design corre-sponds to a frequency reuse cluster size of RCS = 12. Frequencies can now be planned by dividing the

Figure 4-3: Omni cell design with a 7 reusecluster

Figure 4-4: Three-sectorised design with a 4x3 re-use cluster


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Example: consider a lognormal distribution of field strength with = 6 dB. Then,dB58dB622 .!!i === , and a margin of 14 dB must be added to achieve a maximum interferer

probability of 5,0 %. This leads to a quality threshold of 23 dB, which is distinctively higher than thespecified value of 9 dB for GSM.

In opposite to the local interference probability described above, an average interferer probability overthe cell area must be taken into account for the network design. This value can be interpreted as thefraction of all locations in the service area where the actualvalue of C/I is below the threshold (C/I)system.

If a smaller interference probability is needed, the averagereuse cluster size must be increased. In general, there is a

functional dependence between the interferer probability andthe cluster size Pint = f(ARCS). It is not suitable to provide adeterministic function since the achievable values stronglydepend on factors like terrain and clutter variance or trafficdistribution. Table 4-1 shows some simulated values fromsimulations of existing radio networks and gives an orienta-tion.

In reality, the conditions are often leading to a worse situa-tion. As an advice, an initial ARCS up to 16 should beplanned if the frequency resources are sufficient.

4.1.2 Call Success Rate and Outage

Interference probability is degrading the system performance in addition to lacks of coverage. Thus, ameasure of the call success rate (CSR) is found by requiring the following conditions, which can be givenin form of a logical equation:

CSR = Coverage AND (NOT Interference)

This can be written in terms of probabilities:

Probability density function / %

0,0%

1,0%

2,0%

3,0%

4,0%

5,0%

(C/I) /dBC/ImedC/Ithr

Margin

Interferer probability / %

0%

20%

40%

60%

80%

100%

-20 -15 -10 -5 0 5 10 15 20

(C/Imed - C/Ithr) /dB

5,0% at Margin = 14 dB

31,9% at Margin = 4 dB

Figure 4-5: Pint(= 6 dB, i= 8.5 dB)

ARCS Pint[%]

6.5 ... 9.0 10

7.0 ... 9.5 7.5

8.5 ... 11.0 5

12.0 ... 16.0 2.5

Table 4-1: ARCS and Pint


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Equation 4-4: Call Success Rate CSR

CSR P Pint= cov ( )1

CSR call success rate, 0 1 CSR

Pcov coverage probability, 0 1 Pcov

Pint interference probability, 0 1 Pint

The complementary value of the CSR is called outage probability:

Equation 4-5: Outage Probability Pout

P CSR out = 1

Pout outage probability, 0 1 Pout

CSR call success rate, 0 1 CSR

For Pcov> 90% and Pint< 10%, CSR !Pcov- Pint. Then, the call success rate equals the coverage prob-ability, degraded by the interferer probability. A good quality design, therefore, requires a high coveragevalue from the beginning. For example, a CSR of 90% is reached if the design is made for 95% cover-age probability and a maximum of 5% interferer probability. For higher values of the CSR, the designvalues must be fixed at very tough requirements.

4.2 Group Frequency Planning

Based on ideal cell clusters as shown in the hexagonal structure, a frequency planning can be performedaccording to the following steps:

4.2.1 Determination of the Reuse Cluster Size

First, the reusecluster size has to be determined based on the available bandwidth and the number ofcarriers. Before the planning is started, it should be checked whether the RCS is in accordance with thequality requirements.

If the traffic requirements result in a network design with different transceiver configurations, it should betried to reserve carriers for later extension if enough frequencies are available. Example: If the designresults in a typical three-sector site configuration with base stations having 4+2+2 transceivers,4+4+4 frequencies should be reserved, which gives space for later network extension and simplifies thedefinition of the frequency groups. If the available bandwidth is not allowing this, a possible approach isthe definition of a subgroup, which allows the assignment of 2+2+2 carriers. The remaining frequen-cies are placed in a pool for the assignment of frequencies to base stations with more carriers.


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4.2.2 Definition of the Frequency Groups

With the known RCS value andthe reserved frequencies perBTS, the groups are created andthe available frequencies as-signed properly. As an example,a three-sector design is treatedwith a RCS of 12. Thus, 12 fre-quency groups are neededwhich are labelled with letters A,B, C, D (4 sites) and numbers1,2,3 (3 sectors). The group la-bels are shown in Figure 4-6.

With four reserved frequenciesfor each group, 4x12 = 48 fre-quencies are needed in total.

The frequencies must be as-signed to the groups in a waythat the widest frequency spac-ing is provided between adjacentcells. A possible assignment isshown in Table 4-2.

Note: The assignment is also restricted by requirements for electromagnetic compatibility (minimumfrequency spacing between carriers in one base station / between base stations at one site) and re-quirements for the used radio system itself (e.g. the adjacent channel interference for GSM). Sometimesthese requirements may cause even more frequency assignment problems than the interference condi-tions in the network!

Table 4-2: Frequency groups for RCS=12 and 4 carriers, GSM channels 50...97

Sector

Site

A B C D

1 50 54 58 62 51 55 59 63 52 56 60 64 53 57 61 65

2 66 70 74 78 67 71 75 79 68 72 76 80 69 73 77 81

3 82 86 90 94 83 87 91 95 84 88 92 96 85 89 93 97

4.2.3Assignment of the Group Numbers

The assignment of the initial network design is done by mapping the existing network topology as closeas possible to the idealised hexagonal structure. This is, of course, not always possible due to regions ofdifferent cell density. The proposed way is to start in the centre of the network with the highest cell den-sity and proceed towards the outer cells.

If the terrain irregularity and the traffic distribution are having a big influence on the network design, thecomparison with the ideal hexagonal structure is impossible. In this case, the group numbers are as-signed in order to achieve the best possible frequency spacing between the cells. It is useful to work withtransparencies pinned to big topographical maps of the service area in order to take natural obstacleslike hills into account. Hills may cause further restrictions of frequency reusesince they are well illumi-nated from a valley site. On the other hand, if they are not part of the service area because they arecovered with dense forest (or no important street is running over them), they may also allow an assign-

A2

A1 A3

B2

B1 B3

C2

C1 C3

D2

D1 D3

A2

A1 A3

B2

B1 B3

C2

C1 C3

D2

D1 D3

A2

A1 A3

B2

B1 B3

C2

C1 C3

D2

D1 D3

A2

A1 A3

B2

B1 B3

C2

C1 C3

D2

D1 D3

A2

A1 A3

B2

B1 B3

C2

C1 C3

D2

D1 D3

A2

A1 A3

B2

B1 B3

C2

C1 C3

D2

D1 D3

A2

A1 A3

B2

B1 B3

C2

C1 C3

D2

D1 D3

A2

A1 A3

B2

B1 B3

C2

C1 C3

D2

D1 D3

A2

A1 A3

B2

B1 B3

C2

C1 C3

D2

D1 D3

Figure 4-6: 4x3 sector design (RCS=12)


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ment of the same frequency in topological adjacent cells which have the hill as a natural boundary. Ofcourse, a handover is then not possible between these cells.

The hexagonal structure in the example fixes each sector to a certain orientation. In reality, the sectordirections are varying, so that the first step of the group assignment is to assign the letter (for the wholesite) first and then to assign the numbers (sectors) afterwards to optimise the frequency spacing. In criti-

cal cases, it is sometimes useful to swap the frequencies within one base station.Example:In GSM, the first carrier is the BCCH, which is permanently on air. The other carriers are onlyactive if traffic occurs on the related timeslots. Thus, for two BTS', where the first uses a frequency on theBCCH and the second uses the same frequency on a normal carrier, interference only appears in caseof high traffic at the second BTS.

4.2.4Advantages and Drawbacks of Group Frequency Planning

Group frequency planning allows a flexible handling of the frequency plan, which is useful in the begin-ning of a network. As shown, a group can be completely assigned to one site, this also simplifies theidentification in case of measurements. Because, during the network deployment, changes in the net-

work layout are appearing very often, it supports a fast and effective handling.Problems are occurring under very unfavourable terrain and traffic conditions. Also, the further the net-work evolution proceeds, the harder it is to find a group number for a new cell without violating fre-quency spacing requirements.

In case of cell densification, the frequency plan has to be redesigned. To some extent, the frequencyassignment of a couple of new cells can be done without any change of the existing frequency plan.Later, this method leads to quality degradation, which is no longer acceptable. Then, the complete planhas to be changed. However, in case of a huge network, the planning method by hand becomes inef-fective because it is too difficult to keep the global view.

This is the point where the group planning method should be replaced by automatic frequency planning

methods as provided by computer-aided frequency planning. Conceptual points are described in [10].


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5 Electromagnetic Compatibility Problems

Radio network planning can become a challenging task if existing services must be considered. Prob-lems are encountered in countries where an analogue network as AMPS, TACS or NMT 900 is alreadyoperating in the 900 MHz band. These networks are more harmful to GSM than a foreign GSM network

by two reasons: analogue networks use one frequency per voice channel, i.e. typically about 20 frequencies per cell

each transmitting with a power of 10 - 25 W

in particular the AMPS system downlink is immediately adjacent to the critical uplink of GSM

Even if the spectrum allocated for the GSM service is not violated directly, e.g. by spurious emissions, theGSM performance can be degraded significantly by intermodulation or blocking. These mechanismshave been investigated in theory, in laboratory and in the field.

The mechanisms of inter-system interference and corresponding countermeasures are summarised inTable 5-1 and Table 5-2. However, these measures are to be used as a global orientation only.

Combination Typical Symptom EMC Problem

TACS-GSM GSM coverage holes in the envi-ronment of TACS base stations upto 600m

intermodulation and blocking inGSM MS and BS receiver

NMT900-GSM see TACS-GSM see TACS-GSM

AMPS-GSM 50% reduction of GSM range ifAMPS base station is collocatedor near to GSM BTS

intermodulation and blocking inthe GSM BS receiver, spuriousemissions in the GSM uplinkband

Table 5-1: EMC problems with other networks


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Combination Recommendation

TACS-GSM For downlink integrity:

- collocation of TACS and GSM BS similar antenna footprints

- high GSM downlink transmission power.

For uplink integrity in spite of collocation:

- antenna uncoupling > 40 dB

- avoid harmful intermodulation products by suitable use of frequen-cies

NMT900-GSM see TACS

AMPS-GSM - collocation of AMPS and GSM base stations should be avoided

- if AMPS and GSM are collocated, provision of wide guard band anduse of additional receive filters to avoid intermodulation and receiverblocking

Table 5-2: Countermeasures for EMC problems


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6 Appendix: GSM Specific Subjects

The appendix contains subjects, which are related to GSM and especially to Alcatels A900/A1800 sys-tem.

6.1 BSS Interconnection Planning

The planning activities of a radio network are usually split in the radio network planning and the fixednetwork planning tasks, which are performed by different engineering teams. The fixed network plan-ning comprises the dimensioning and layout of fixed lines, which interconnect the different subsystems(base stations, switches, subscriber databases etc.).

In GSM, the radio planning engineer must be aware of the BSS planning to some extent, i.e. the dimen-sioning of transmission lines on Abis (BTS to BSC) and A (BSC to MSC) interfaces and the configurationof the BSC. This section gives an overview on the items, which have to be taken into account.

For detailed information and dimensioning rules read BSS B5 Dimensioning Rules [17].

6.1.1 BSC Architecture

Figure 6-1 shows, in brief, the architecture of a BSC. The BSC consists of interface controllers, commonprocessing units, and a switching entity. These are:

TCU (Terminal Control Unit): These are Abis-interface controllers. Each TCU can handle a certainnumber of traffic and signalling channels.

DTC (Digital Trunk Controller): Proper con-trollers for the A-interface.

CPR (Common Processing Unit): used for ap-

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