Chapter 1mpu.edu.iq/images/forth_class_comp/mobile_part1.pdfThe original cellular telephone networks...

Chapter 1 Introduction to Wireless Comm. System

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Chapter 1

1.1 History of wireless communications

Guglielmo Marconi invented the wireless telegraph in 1896. In 1901, he sent telegraphic signals

across the Atlantic Ocean (about 3200 km). His invention allowed two parties to communicate

by sending each other alphanumeric characters encoded in an analog signal. Over the last

century, advances in wireless technologies have led to the radio, the television, the mobile

telephone, and communications satellites. All types of information can now be sent to almost

every corner of the world. Recently, a great deal of attention has been focused on satellite

communications, wireless networking, and cellular technology. Wireless networking is allowing

businesses to develop WANs, MANs, and LANs without a cable plant. The cellular or mobile

telephone is the modern equivalent of Marconi's wireless telegraph, offering two-party, two-way

communication. The first-generation wireless phones used analog technology. These devices

were heavy and coverage was patchy, but they successfully demonstrated the inherent

convenience of mobile communications. The current generation of wireless devices is built using

digital technology. Digital networks carry much more traffic and provide better reception and

security than analog networks.

Fig. (1) Some milestones in Wireless Communications


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1.2 Evolution of mobile communications

The first version of a mobile radio telephone being used in 1924.

In 1926 telephone service in trains on the route between Hamburg and Berlin was approved

and offered to 1st class travelers.

In 1935, Edwin Armstrong demonstrated FM and it has been the primary modulation

technique used for mobile communication systems throughout the world.

In 1946, the first public mobile telephone service was launched in 25 cities across the United

States (543 users) by BELL Laboratories in USA. Early mobile systems used single high

power transmitters with AM Modulation techniques to give coverage up to about 50 miles

(only limited customers) could get the service. This inefficient use of the radio spectrum

coupled with the state of radio technology at that time severely limited the system capacity.

A solution to this capacity problem emerged during the 50’s and 60’s when researchers at

Bell Laboratories developed the cellular concept.

In 1973, Martin Cooper (a Motorola researcher and executive) made the first mobile

telephone call from handheld subscriber equipment.

In 1983, the first analog cellular system deployed in Chicago, USA.

1.3 Mobile Radio Systems around the World

Many mobile radio standards have been developed for wireless systems throughout the world.

Tables below list the most common paging, cordless, cellular, and personal communications

standards used in North America, Europe, and Japan.

1- Post Office Code Standard Advisory Group (POCSAG)

- The world’s most common paging standard.

- Developed by the British Post Office in the late 1970s.

- Supports binary FSK signaling at 512 bps, 1200 bps, and 2400 bps.

2- Cordless telephony (CT2)

- Most popular cordless telephone standards throughout Europe and Asia.

- Developed in Europe.


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- Makes use of microcells which cover small distances, usually less than 100 m, using base

stations with antennas mounted on street lights or on sides of buildings.

- High quality voice transmission: uses battery efficient FSK along with a 32 kbps adaptive

differential pulse code modulation (ADPCM) speech coder.

- Handoffs between base stations are not supported, as it is intended to provide short range

access to the PSTN.

1.4 Classification of mobile radio transmission system

Mobile radio transmission systems may be classified as simplex, half-duplex or full-duplex.

a. Simplex systems: communication is possible in only one direction.

b. Half-duplex: radio systems allow two-way communication using the same radio channel

for both transmission and reception.

c. Full duplex: systems allow simultaneous radio transmission and reception between a

subscriber and a base station, by providing two simultaneous but separate channels

(frequency division duplex, or FDD) or adjacent time slots on a single radio channel

(time division duplex, or TDD) for communication to and from the user.

1.4.1 Frequency division & Time division

Provides simultaneous radio transmission channels for the subscriber and the base station, so that

they both may constantly transmit while simultaneously receiving signals from one another.

At the base station, separate transmit and receive antennas are used to accommodate the two

separate channels.

At the subscriber unit a single antenna is used for both transmission to and reception from the

base station, and a device called a duplexer is used inside the subscriber unit to enable the same

antenna to be used for simultaneous transmission and reception.


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(A) Frequency division duplexing (FDD)

A pair of simplex channels with a fixed and known frequency separation is used to define a

specific radio channel in the system.

- The channel used to convey traffic to the mobile user from a base station is called the

forward channel.

- The channel used to carry traffic from the mobile user to a base station is called the

reverse channel. FDD is used exclusively in analog mobile radio systems.

(B) Time division duplexing (TDD)

Uses the fact that it is possible to share a single radio channel in time, so that a portion of the

time is used to transmit from the base station to the mobile, and the remaining time is used to

transmit from the mobile to the base station. TDD is only possible with digital transmission

formats and digital modulation, and is very sensitive to timing. It is for this reason that TDD has

only recently been used, and only for indoor or small area wireless applications.

1.4.2 Transmission types

•Unicast (point-to-point) transmission is made from one device to a single other device. It means

that the packet is addressed to one receiver.

•Broadcast transmission is made from one device to all other devices. In this case there is just

one sender, but the information is sent to all connected receivers.

•Multicast transmission is made from one device to a subset of the other available devices. In

this case there is just one sender, but the information is sent to a group of receivers.

1.5 Types of wireless communication systems

The major types of wireless communication systems are:

1- Paging Systems

2- Cordless Telephone Systems

3- Satellite communication systems

4- Wireless LAN systems

5- Cellular Telephone Systems


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The cost, complexity, performance, and types of services offered by each of these mobile

systems are different.

1- Paging Systems

Paging systems are communication systems that send brief messages to a subscriber. Depending

on the type of service, the message may be either text or voice messages. In modern paging

systems, news headlines, stock quotations, and faxes may be sent. The issued message is called a

page. The paging system then transmits the page throughout the service area using base stations

which broadcast the page on a radio carrier.

- Paging systems vary widely in their complexity and coverage area.

- Paging systems are designed to provide reliable communication to subscribers. This

necessitates large transmitter powers and low data rates for maximum coverage from

each base station.

Simple paging systems may cover a limited range of 2 to 5 km, or may even be confined to

within individual buildings,

Wide area paging systems can provide worldwide coverage. Wide area paging systems consist

of a network of telephone lines, many base station transmitters, and large radio towers that

simultaneously broadcast a page from each base station (this is called simulcasting)

2- Cordless Telephone Systems

• Provide wireless extension to the telephone network within a limited area

• Two-way (duplex) communications

• Consists of a portable handset, connected to dedicated base station, which is connected to the

telephone network

• 1st generation: household environment

• 2nd generation: allow mobility in workplace and public use with limited coverage in urban

areas


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3- Satellite communication Systems

The main feature of the satellite communication systems

Very wide range and coverage

Very useful in sparsely populated areas: rural areas, sea, mountains, etc.

Target: Vehicles and/or other stationary/mobile uses

Expensive base station (satellites) systems

4- Wireless LAN (WLAN)

Characterized by

Low mobility (not for vehicular use)

High speed data transmission

Confined regions – buildings and campuses

Coverage: 100m – 300m per base station

Uses the following bands (902-928 MHz, 2400-2483.5 MHz, 5725-5850 MHz)

5- Cellular Telephone Systems

The basic cellular system consists of: Mobile station (MS), Base stations (BS) and Mobile

switching center (MSC) or called Mobile telecommunications switching office (MTSO).

- Cellular systems accommodate a large number of users over a large geographic area,

within a limited frequency spectrum.

- Cellular radio systems provide high quality service that is often comparable to that of the

landline telephone systems.

- High capacity is achieved by limiting the coverage of each base station transmitter to a

small geographic area called a cell so that the same radio channels may be reused by

another base station located some distance away.

- Handoff: is a switching technique that enables a call to proceed uninterrupted when the

user moves from one cell to another.


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1.6 Comparison of Common Wireless Communication Systems

Tables below illustrate the types of service, level of infrastructure, cost, and complexity required

for the subscriber segment and base station segment of each of the mobile radio systems

discussed earlier.

Mobile stations:

Base stations:


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1.7 First Generation (1G) systems

The original cellular telephone networks provided analog traffic channels; these are now referred

to as first-generation systems. Since the early 1980s the most common first-generation system in

North America has been the Advanced Mobile Phone Service (AMPS). This approach is also

common in South America, Australia, and China.

Two 25-MHz bands are allocated to AMPS.

Each of these bands is split in two to encourage competition (i.e., so that in each market

two operators can be accommodated). An operator is allocated only 12.5 MHz in each

direction for its system.

The channels are spaced 30 kHz apart, which allows a total of 416 channels per operator.

21 channels are allocated for control, leaving 395 to carry calls.

The control channels are data channels operating at 10 kbps.

The conversation channels carry the conversations in analog using FM.

European Total Access Communication Systems (TACS):

- Deployed in 1985.

- Almost identical to AMPS except that the channel bandwidth is scaled to 25 kHz instead of

30 kHz as in AMPS.

Drawbacks

• Poor voice quality

• Poor battery life

• Limited capacity (number of subscribers)

• No security

• Poor handoff reliability (transition between cells)

• Large phone size


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1.8 Second Generation (2G) systems

Second-generation systems have been developed in early 90s to provide higher quality signals,

higher data rates for support of digital services, and greater capacity. Moreover, The 2G systems

provide:

- Digital voice coding and modulation

- Security (Encryption).

- Error detection and correction.

- Multiple channels per cell.

The key differences between the two generations are:

First Generation (1G) systems Second Generation (2G) systems

Analog systems Digital systems

Designed to support voice channels

using FM, digital traffic is supported

only by the use of a modem that converts

the digital data into analog form.

Provide digital traffic channels, these

readily support digital data; voice traffic is

first encoded in digital form before

transmitting.

No security.

Capable to encrypt the traffic to prevent

eavesdropping (since the user and control

traffics are digitized).

Poor voice quality.

Very clear voice reception, because The

digital traffic stream of lends itself to the

use of error detection and correction

techniques.

Each cell supports a number of channels.

At any given time a channel is allocated

to only one user.

Provide multiple channels per cell, but

each channel is dynamically shared by a

number of users using TDMA or CDMA.

No message service

Enable message service such as short

message service (SMS) and Multimedia

message service (MMS)


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Beginning around 1990, a number of different second-generation systems have been deployed,

such as Global system for mobile communications (GSM), and Interim Standard (IS-95) scheme.

The table below lists some key characteristics of three of the most important of these systems.

GSM IS-95

Year introduced 1990 1993

Developed in Europe North America

BS transmission band 935~960 MHz 869~894 MHz

MS transmission band 890~915 MHz 824~849 MHz

Spacing between forward

and reverse channels 45 MHz 45 MHz

Channel bandwidth 200 KHz 1250 KHz

No. of duplex channels 125 20

Mobile unit maximum power 20 W 0.2 W

Users per channel 8 35

Modulation GMSK QPSK

Carrier bit rate 270.8 kbps 9.6 kbps

The GSM standard has gained worldwide acceptance as the first universal digital cellular system

with modern network features extended to each mobile user

1.10 2.5 G systems

2.5G is a technology between the 2G and 3G; it is sometimes described as 2G technology

combined with GPRS. 2.5G is an interim solution designed to allow for improved data rates

before 3G implementation. A variety of 2.5G techniques are being employed to improve the

speed of data for enhanced e-mail and Internet access.


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The main features of the 2.5G are:

- Ability to send and receive Email messages.

- Web browsing,

- Speed: 64-144 kbps

Moreover, Built-in camera can be included in the mobile station (Camera phones).

General Packet Radio Service (GPRS)

GPRS is packet switched technology which based on existing GSM cellular network

infrastructure and adds new packet-switching network equipment.

The GPRS gives GSM subscribers access to data communication application such as e-

mail and internet using their mobile phone.

Speed: up to 114 kbps

The GPRS allows multiple slots of a GSM radio channel be dedicated to an individual

user (Physical channel can be shared between different mobile users).

Physical channel is only assigned when data needs to be transmitted or received.

1.11 Third Generation (3G) systems

The objective of the third generation (3G) of wireless communication is to provide fairly high-

speed wireless communications to support multimedia, data, and video in addition to voice. 3G

developed in the early 2000s, the main features of the 3G systems are:

High transmission rate and the support of multimedia services: Multiple-megabit

internet services, video calls, and mobile TV using a single mobile device.

Data rate: around 2Mbps. Bandwidth: in the order of MHz

The ITU’s International Mobile Telecommunications for the year 2000 (IMT-2000) initiative has

defined the ITU’s view of third-generation capabilities as


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• Voice quality comparable to the public switched telephone network.

• 144-kbps data rate available to users in high-speed motor vehicles over large areas.

• 384 kbps available to pedestrians standing or moving slowly over small areas.

• Support for 2.048 Mbps for office use.

• Support for both packet-switched and circuit-switched data services.

• An adaptive interface to the Internet.

Generally, the technology planned is digital using TDMA or CDMA to provide efficient use of

the spectrum and high capacity.

3G enhancements

3G has the following enhancements over previous networks:

1. Enhanced audio and video streaming

2. Several times higher data speed

3. Video-conferencing support

4. Web and WAP browsing at higher speeds

5. IPTV (TV through the Internet) support

Universal Mobile Telecommunications System (UMTS)

UMTS is a third generation mobile cellular system for networks based on the GSM standard,

developed and maintained by the 3GPP (3rd Generation Partnership Project).

Aims to provide services for both mobile and fixed subscribers by common call-

processing procedures.

Uses W-CDMA radio access technology to offer greater spectral efficiency and

bandwidth to mobile network operators.

The UMTS network architecture is split into 3 parts:

1. Core (switching) networks: The core network is responsible for performing switching and

transmission functions. Also known as Mobile Application Part (MAP).


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2. Radio Access networks: provides channel access to mobile users and performs radio

resource management and signaling UMTS will include both terrestrial and global satellite

components. Also known as UMTS Terrestrial Radio Access Network (UTRAN).

3. User Equipment (UE): authentication of users via SIM (subscriber identity module) cards.

High Speed Packet Access (HSPA)

HSPA extends and improves the performance of existing 3G mobile telecommunication

networks utilizing the WCDMA protocols (also referred to as 3.5G). The HSPA specifications:

increased peak data rates of up to

• 14 Mbit/s in the downlink

• 5.76 Mbit/s in the uplink.

reduced latency

Up to five times more system capacity in the downlink up to twice as much system

capacity in the uplink compared with WCDMA.

1.12 Pre-4G technology (3.9G)

The ITU specified a set of requirements for 4G standards, named the International Mobile

Telecommunications Advanced (IMT-Advanced) specification, setting peak speed requirements

for 4G service as follows:

300 Mbps for high mobility communication (such as from trains and cars).

1 Gbps for low mobility communication (such as pedestrians and stationary users).

Two 3.9G (Also known as 4G technology) standards are commercially deployed:

Mobile WiMAX standard (first used in South Korea in 2007),

Long Term Evolution (LTE) standard (first released in Oslo, Norway and Stockholm,

Sweden since 2009).


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Since the first-release versions of Mobile WiMAx and LTE support much less than 1 Gbps peak

bit rate, they are not fully IMT-Advanced compliant, but are often called 4G by service

providers.

Evolved HSPA (HSPA+)

A further improved 3GPP standard was released late in 2008 with subsequent worldwide

adoption beginning in 2010. The newer standard allows bit-rates to reach as high as 337 Mbit/s

in the downlink and 34 Mbit/s in the uplink. HSPA+ supported with multiple input, multiple

output (2x2 MIMO) technologies and higher order modulation (64 QAM).

Long Term Evolution (LTE)

LTE is a standard for wireless communication of high-speed data for mobile phones and data

terminals. It is based on the GSM/EDGE and UMTS network technologies, increasing the

capacity and speed using a different radio interface together with core network improvements.

The standard is developed by the 3GPP. The LTE features are:

downlink peak rates of 300 Mbit/s,

uplink peak rates of 75 Mbit/s

Quality of Service (QoS) provisions permitting a transfer latency of less than 5 ms in the

radio access network (RAN).

Has the ability to manage fast-moving mobiles

Supports scalable carrier bandwidths, from 1.4 MHz to 20 MHz.

Orthogonal frequency-division multiple access (OFDMA) for the downlink, Single-

carrier FDMA for the uplink to conserve power.

Supports both FDD and TDD.


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1.13 Fourth Generation (4G) systems

4G provides mobile broadband Internet access, with higher data rate and expanded multimedia

services. The main features of the 4G systems are

Higher speed 0.1~1 Gbps.

More security.

Higher capacity.

Lower cost than previous generations.

Provides digital system with voice over-IP (VOIP) technology.

IPv6 Core.

Orthogonal frequency-division multiplexing (OFDM) is used instead of CDMA.

One of the main differences between 3G and 4G technology is the elimination of circuit

switching, instead employing an all-IP network. Other differences are:

The 4G system are able to provide a comprehensive IP solution where voice, data and streamed

multimedia can be given to users on an "Anytime, Anywhere" basis.

LTE Advanced (LTE-A)

LTE Advanced is a mobile communication standard and a major enhancement of the Long Term

Evolution (LTE) standard. It was formally submitted as a candidate 4G system to ITU in late


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2009 as meeting the requirements of the IMT-Advanced standard, and was standardized by the

3GPP in March 2011.

The main features of the LTE-Advanced are:

higher capacity,

increased peak data rate,

- Downlink 3 Gbps,

- Uplink 1.5 Gbps

Higher spectral efficiency (30 bps/Hz)

Increased number of simultaneously active subscribers.

LTE-Advanced can use up to 8x8 MIMO and 128 QAM in downlink direction.

LTE-Advanced is now available in 31 countries (including South Korea, Australia, France,

Germany, U.K. and the U.S.).


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1.14 Fifth Generation (5G) systems

5G denotes the next major phase of mobile telecommunications standards beyond 4G. 5G can be

a complete wireless communication without limitation, which bring us perfect real world

wireless, supportable to the World Wide Wireless Web (WWWW). The Next Generation Mobile

Networks Alliance defines the following requirements for 5G networks:

Data rates of several tens of megabits per second should be supported for tens of

thousands of users.

High speed, high capacity.

Spectral efficiency should be significantly enhanced compared to 4G.

Coverage should be improved.

Signaling efficiency should be enhanced.

Latency should be reduced significantly compared to LTE.

The Next Generation Mobile Networks Alliance feels that 5G should be rolled out by 2020 to

meet business and consumer demands. In addition to providing simply faster speeds, they predict

that 5G networks also will need to meet the needs of new use cases, such as:

Internet of Things (IoT).

Broadcast-like services and lifeline communication in times of natural disaster.

1.15 Evolution parameters of cellular systems

In order to offer higher quality of service to subscribers, the following parameters have

been developed in cellular systems:

1. Higher spectrum utilization efficiency

2. Introduction of new frequencies (radio bands) for use in cellular systems

3. Increased capacity to support more subscribers (users)

4. Higher data transfer rates

5. Offering new services such as Internet access and video calls

6. Improving coverage and hand-off while reducing interference


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7. Improved roaming capability to solve the fragmentation of standards

8. Enhanced security

9. Improved energy efficiency to provide longer talking time

10. Lower service cost

Chapter 2 The Cellular Concept-System Design

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Chapter 2 :

The Cellular Concept-System Design

كلية التقنيات الهندسية الكهربائية والالكترونية

قسم هندسة تقنيات الحاسوب

مدرس المادة : م. الحمزة طاهر محمد


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2.1 Cellular Mobile Systems

Cellular radio is a technique that was developed to increase the capacity available for mobile

radio telephone service.

Prior to the introduction of cellular radio, mobile radio telephone service was only provided by a

high-power transmitter/receiver. A typical system would support about 25 channels with an

effective radius of about 80 km. The principal elements of a cellular system include:

1- The Base Station (BS)

2- Mobile telecommunications switching office (MTSO) or MSC

3- mobile unit (or mobile station MS)

1- Base Station (BS)

A fixed or non-moving station serves as a bridge between all mobile users in the cell

Connects the simultaneous mobile calls via telephone lines or microwave links to the MTSO.

Consists of several transmitters and receivers which simultaneously handle full duplex

communications

Generally have towers which support several transmitting and receiving antennas.

The BS includes:

1- Antennas. Hanged in towers which support several transmitting and receiving antennas

2- Controller: used to handle the call process between the mobile unit and the rest of the

network.

3- Number of transceivers: used for communicating on the channels assigned to that cell.


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2- Mobile telecommunications switching office (MTSO)

The center which is set up for coordinating the routing of calls, also called a mobile switching

center (MSC). The MTSO coordinates the activities of all of the base stations and connects the

entire cellular system to the PSTN.

The link between an MTSO and a BS is by a wire line, although a wireless link is also

possible.

The MTSO connects calls between mobile units.

The MTSO is also connected to the public telephone or telecommunications network and

can make a connection between a fixed subscriber to the public network and a mobile

subscriber to the cellular network.

The main functions of the MTSO are

1- Assigns the voice channel to each call.

2- Performs handoffs.

3- Monitors the call for billing information.

Communication between the base station and the mobiles is defined by a standard common air

interface (CAI) that specifies four different channels (2) voice channels, 2 control channels.

a. Voice channels (Traffic channels):

The channels used for voice transmission from the base station to mobiles are called

forward voice channels (FVC), and the channels used for voice transmission from

mobiles to the base station are called reverse voice channels (RVC).

b. Control channels (setup channels)

The two channels responsible for initiating mobile calls are the forward control channels

(FCC) and reverse control channels (RCC). Control channels transmit and receive data

messages that carry call initiation and service requests, and are monitored by mobiles

when they do not have a call in progress.


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3- Mobile station (MS)

A station intended for use while in motion at unspecified locations. Each mobile communicates

via radio with one of the base stations and may be handed-off to any number of base stations

throughout the duration of a call. The mobile station contains a transceiver, an antenna, and

control circuitry, and may be mounted in a vehicle or used as a portable hand-held unit.

Two types of channels are available between the mobile unit and the base station (BS): control

channels and traffic channels.

• Control channels are used to exchange information having to do with setting up and

maintaining calls and with establishing a relationship between a mobile unit and the

nearest BS.

• Traffic channels carry a voice or data connection between users.


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2.2 Cellular system operation

The steps in a typical call between two mobile users within an area controlled by a single MTSO

a. Mobile unit initialization: When the mobile unit is turned on, it scans and selects the

strongest setup control channel used for this system.

- Cells with different frequency bands repetitively broadcast on different setup channels.

The receiver selects the strongest setup channel and monitors that channel. The effect

of this procedure is that the mobile unit has automatically selected the BS antenna of

the cell within which it will operate.

- Then a handshake takes place between the mobile unit and the MTSO controlling this

cell, through the BS in this cell. The handshake is used to identify the user and register

its location.

- As long as the mobile unit is on, this scanning procedure is repeated periodically to

account for the motion of the unit. If the unit enters a new cell, then a new BS is

selected.

b. Request for connection: A mobile unit originates a call by sending the number of the

called unit on the preselected setup channel.


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The receiver at the mobile unit first checks that the setup channel is idle by examining

information in the forward (from the BS) channel. When an idle is detected, the mobile

may transmit on the corresponding reverse (to BS) channel. The BS sends the request to

the MTSO.

c. Paging: The MTSO then attempts to complete the connection to the called unit. The

MTSO sends a paging message to certain BSs depending on the called mobile number.

Each BS transmits the paging signal on its own assigned setup channel.

d. Call accepted: The called mobile unit recognizes its number on the setup channel being

monitored and responds to that BS, which sends the response to the MTSO.

- The MTSO sets up a circuit between the calling and called BSs.

- At the same time, the MTSO selects an available traffic channel within each BS’s

cell and notifies each BS, which in turn notifies its mobile unit.


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- The two mobile units tune to their respective assigned channels.

e. Ongoing call: While the connection is maintained, the two mobile units exchange voice

or data signals, going through their respective BSs and the MTSO.

f. Handoff (Handover): If a mobile unit moves out of range of one cell and into the range

of another during a connection, the traffic channel has to change to one assigned to the

BS in the new cell. The system makes this change without either interrupting the call or

alerting the user.


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Other functions performed by the system include the following:

Call blocking: During the mobile-initiated call stage, if all the traffic channels assigned

to the nearest BS are busy, then the mobile unit makes a preconfigured number of

repeated attempts. After a certain number of failed tries, a busy tone is returned to the

user.

Call termination: When one of the two users hangs up, the MTSO is informed and the

traffic channels at the two BSs are released.

Call drop: During a connection, because of interference or weak signal spots in certain

areas, if the BS cannot maintain the minimum required signal strength for a certain period

of time, the traffic channel to the user is dropped and the MTSO is informed.

Calls to/from fixed and remote mobile subscriber: The MTSO connects to the public

switched telephone network (PSTN). Thus, the MTSO can set up a connection between a

mobile user in its area and a fixed subscriber via the telephone network.

2.3 RF Planning

RF Planning is the process of assigning frequencies, transmitter locations and parameters of a

wireless communications system to provide sufficient coverage and capacity for the services

required. The RF plan of a cellular communication system has two objectives: coverage and

capacity.


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a. Coverage relates to the geographical footprint within the system that has sufficient RF

signal strength to provide for a call/data session.

b. Capacity relates to the capability of the system to sustain a given number of subscribers.

Capacity and coverage are interrelated.

To improve coverage, capacity has to be sacrificed, while to improve capacity, coverage will

have to be sacrificed. It is necessary to restructure radiotelephone system to achieve high

capacity with limited spectrum.

1- Increase the capacity of the system: by using lower-power systems with shorter

radius and to use numerous transmitters/receivers (Base stations). Thereby

providing additional radio capacity with no additional increase in radio spectrum.

2- Distributing the available channels throughout geographic region: by

systematically spacing base stations and their channel groups. The available

channels can be reused as long as the interference between co-channel stations is

kept below acceptable level.

2.4 Cell types

• Macro cell – their coverage is large (aprox. 6 miles in diameter); used in remote areas,

high-power transmitters and receivers are used

• Micro cell – their coverage is small (half a mile in diameter) and are used in urban zones;

low-powered transmitters and receivers are used to avoid interference with cells in

another clusters

• Pico cell –is a small cellular system typically covering a small area, such as in-building

(offices, shopping malls, train stations) . In cellular networks, picocells are typically used

to extend coverage to indoor areas where outdoor signals do not reach well.

• Selective cells . located at the entrances of tunnels where a coverage of 360 degrees is not

needed this case, a selective cell with a coverage of 120 degrees is used.


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Decreasing the cell size gives:

Increased user capacity

Increased number of handovers per call

Increased complexity in locating the subscriber

Lower power consumption in mobile terminal: so it gives longer talk time, safer

operation

2.5 Cellular Network Coverage

The essence of a cellular network is the use of multiple low-power transmitters, on the order of

100 W or less. Because the range of such a transmitter is small, an area can be divided into cells,

each one served by its own antenna.

A- Each cell is allocated a band of frequencies and is served by a base station (consisting of

transmitter, receiver, and control unit).

B- Adjacent cells are assigned different frequencies to avoid interference or crosstalk.

However, cells sufficiently distant from each other can use the same frequency band.

While it might seem natural to choose a circle to represent the coverage area of a base station,

adjacent circles cannot be overlaid upon a map without

leaving gaps or creating overlapping regions.

The hexagon has:

No gaps or overlapping

The largest area compared with square and triangle.

Fewest number of cells can cover a geographic region,

Closely approximates a circular radiation pattern

which would occur for an omnidirectional base station

antenna and free space propagation.

A hexagonal pattern provides for equidistant antennas.


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When using hexagons to model coverage areas, base station transmitters are depicted as

either:

- In the center of the cell (center-excited cells): omnidirectional antennas are used

in center-excited cells.

- On three of the six cell vertices (edge-excited cells): sectored directional antennas

are used in corner-excited cells.

The radius of a hexagon is defined to be the radius of the circle that circumscribes it

(equivalently, the distance from the center to each vertex; also equal to the length of a side of a

hexagon).

For a cell radius R, the distance between the cell center and each adjacent cell center is

d = R3

Therefore the area of the hexagon is

2

2

33RArea

In practice, a precise hexagonal pattern is not used. Variations from the ideal are due to:

- Topographical limitations.

- Local signal propagation conditions.

- Practical limitation on siting antennas.


12

2.6 Frequency Reuse

In a cellular system, each cell has a base transceiver. The transmission power is carefully

controlled

1-To allow communication within the cell using a given frequency

2-To limit the power at that frequency that escapes the cell into adjacent ones.

The objective is to use the same frequency in other nearby cells, thus allowing the

frequency to be used for multiple simultaneous conversations.

Generally, 10 to 50 frequencies are assigned to each cell, depending on the traffic

expected.

The essential issue is to determine how many cells must intervene between two cells

using the same frequency so that the two cells do not interfere with each other. Various

patterns of frequency reuse are possible.

Frequency reuse (frequency planning): is the design process of selecting and allocating

channel groups for all of the cellular base stations within a system.

If the pattern consists of N cells and each cell is assigned the same number of frequencies, each

cell can have K/N frequencies, where K is the total number of frequencies allotted to the system.

- For Advanced Mobile Phone System (AMPS), K = 395, and N = 7 is the smallest

pattern that can provide sufficient isolation between two uses of the same frequency.

This implies that there can be at most 57 frequencies per cell on average.

In characterizing frequency reuse, the following parameters are commonly used:

D = minimum distance between centers of cells that use the same frequency band

(called co-channels)

R = radius of a cell

d = distance between centers of adjacent cells d = R3

N = number of cells in a pattern (Cluster size)


13

(Each cell in the pattern uses a unique set of frequency bands), termed the reuse factor

In a hexagonal cell pattern: in order to tessellate (to connect without gaps between adjacent

cells), only the following values of N are possible:

N = I2 + J2 + (I × J) I, J = 0, 1, 2, 3, …

Hence, possible values of N are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, and so on.

Choice of N (assuming constant cell size)

Small N:

•More cluster are required to cover the service area

•More capacity

•Higher probability of co-channel interference

Large N:

•Less cluster are required to cover the service area

•Less capacity

•Less probability of co-channel interference

The following relationship holds:

Nq

R

D3

Where q is the reuse ratio.

This can also be expressed as

N

d

D

Consider a cellular system which has a total of K duplex channels available for use. If each cell

is allocated a group of C channels (C < K), and if the K channels are divided among N cells into


14

channel groups which each have the same number of channels, the total number of available

radio channels can be expressed as

K = C N_

where

bandwidthChannel

bandwidthTotalorbandwidthSpectrumK

)(

The N cells which collectively use the complete set of available frequencies is called a cluster. If

a cluster is replicated M times within the system, the total number of duplex channels, can be

used as a measure of capacity and is given

Capacity = MCN = MK_

The capacity of a cellular system is directly proportional to the number of times a cluster is

replicated in a fixed service area.

The cluster size (N) is typically equal to 4, 7, or 12.

If N is reduced while the cell size is kept constant, more clusters are required to cover a given

area and hence more capacity is achieved.

- A large cluster size indicates that the ratio between the cell radius and the distance

between co-channel cells is large.

- A small cluster size indicates that co-channel cells are located much closer together.

From a design viewpoint, the smallest possible value of N is

desirable in order to maximize capacity over a given

coverage area.

To find the nearest co-channel neighbors of a particular cell,

one must do the following:

i. Move i cells along any chain of hexagons and then


15

ii. Turn 60 degrees counter-clockwise and move j cells. This is illustrated in Figure below for

i = 3 and j = 2 (example, N = 19).

Example 1

Assume a system of 32 cells with a cell radius of 1.6 km, a total of 32 cells, a total frequency

bandwidth that supports 336 traffic channels, and a reuse factor of N = 7.

(a) If there are 32 total cells, what geographic area is covered, how many channels

are there per cell, and what is the total number of concurrent calls that can be

handled?

(b) Repeat for a cell radius of 0.8 km and 128 cells.

Solution:

(a)

The area of a hexagon of radius R is

222 km 6.65)6.1(2

33

2

33 RAreaa

The total area covered is 6.65 × 32 = 213 km2.


16

For N = 7, the number of channels per cell is K/N = 336/7 = 48,

Total number of concurrent calls that can be handled is

Capacity = 48 × 32 = 1536 channels

(b)

The area of a hexagon of radius R is

222 km 1.66)8.0(2

33

2

33 RAreab

The area covered is 1.66 × 128 = 213 km2.

The number of channels per cell is K/N = 336/7 = 48,

Total number of concurrent calls is

Capacity = 48 × 128 = 6144 calls

Example 2

Consider a cellular system in which total available voice channels to handle the traffic are 960.

The area of each cell is 6 km2 and the total coverage area of the system is 2000 km2. Calculate:

(a) The system capacity if the cluster size N is 4

(b) The system capacity if the cluster size is 7.

• How many times would a cluster of size 4 have to be replicated to cover the entire cellular

area? Does decreasing N increase the system capacity? Explain.

Solution

Total available channels =960 , Cell area = 6 km2

Total coverage area = 2000 km2

(a) N = 4

Area of a cluster = 4 × 6 = 24 km2


17

Number of clusters for covering total area = 2000/24 = 83.33 ~ 83

Number of channels per cell = 960/4 = 240

System capacity = 83 × 960 = 79, 680 channels

(b) N = 7

Area of cluster = 7 × 6 = 42 km2

Number of clusters for covering total area = 2000/42 = 47.62 ~ 48

Number of channels per cell = 960/7 = 137.15 ~ 137

System capacity = 48 × 960 = 46,080 channels

It is evident when we decrease the value of N from 7 to 4, we increase the system

capacity from 46,080 to 79,680 channels. Thus, decreasing N increases the system

capacity.

2.7 Channel Assignment Strategies

For efficient utilization of the radio spectrum, a frequency reuse scheme that is consistent with

the objectives of increasing capacity and minimizing interference is required. A variety of

channel assignment strategies have been developed to achieve these objectives.

Channel assignment strategies can be classified as either fixed or dynamic. The choice of

channel assignment strategy impacts the performance of the system, particularly as to how calls

are managed when a mobile user is handed off from one cell to another.

a) Fixed channel assignment strategy: each cell is allocated a predetermined set of voice

channels.

- Any call attempt within the cell can only be served by the unused channels in that

particular cell.

- If all the channels in that cell are occupied, the call is blocked and the subscriber

does not receive service.


18

- Borrowing strategy: a cell is allowed to borrow channels from a neighboring cell if

all of its own channels are already occupied. The mobile switching center (MSC)

supervises such borrowing procedures and ensures that the borrowing of a channel

does not disrupt or interfere with any of the calls in progress in the donor cell.

b) Dynamic channel assignment strategy: voice channels are not allocated to different cells

permanently. Instead,

- Each time a call request is made, the serving base station requests a channel from the

MSC.

- The switch then allocates a channel to the requested cell following an algorithm that

takes into account the likelihood of

o Future blocking within the cell,

o The frequency of use of the candidate channel,

o The reuse distance of the channel,

o Other cost functions.

- Accordingly, the MSC only allocates a given frequency if that frequency is not

presently in use in the cell or any other cell which falls within the minimum

restricted distance of frequency reuse to avoid co-channel interference.

Advantage:

- Dynamic channel assignment reduces the likelihood of blocking, which increases the

trunking capacity of the system, since all the available channels in a market are

accessible to all of the cells.

- Increases the channel utilization and decreases probability of a blocked call.

Disadvantage:

- Require the MSC to collect real-time data on channel occupancy, traffic distribution,

and radio signal strength indications (RSSI) of all channels on a continuous basis.

This increases the storage and computational load on the system.


19

2.8 Co-channel Interference

The S/I ratio at the desired mobile receiver is given as:

where:

Ik = the interference due to the kth interferer

NI = the number of interfering cells in the first tier.

In a fully equipped hexagonal-shaped cellular system, there are always six co-channel interfering

cells in the first tier (i.e., NI = 6).

Most of the co-channel interference results from the first tier.

Interference from second and higher tiers amounts to less than 1% of the total

interference (ignored).

Co-channel interference can be experienced both at the cell site and the mobile stations in

the center cell.

In a small cell system, interference will be the dominating factor and thermal noise can be

neglected. Thus the S/I ratio can be given as:

where:

2 ≤ γ ≤ 5 : the propagation path loss, and γ depends upon the terrain environment.

Dk : the distance between mobile and kth interfering cell

R : the cell radius

If we assume Dk is the same for the six interfering cells for simplification, or D = Dk, then

Equation above becomes:


20

Therefore

Since Nq 3 , therefore

2

63

1

I

SN

Example 3

Consider the advanced mobile phone system (AMPS) in which an S/I ratio of 18 dB is required

for the accepted voice quality. Assume γ = 4.

(a) What should be the reuse factor for the system?

(b) What will be the reuse factor of the Global System of Mobile (GSM) system in which an

S/I of 12 dB is required?

Solution

2

63

1

I

SN

7486.61063

1 4

2

10

18

AMPSN

(b)

4251.31063

1 4

2

10

12

GSMN


21

Example 4

Consider a cellular system with 395 total allocated voice channel frequencies. Calculate the

mean S/I ratio for cell reuse factor equal to 4, 7, and 12. Assume omnidirectional antennas with

six interferers in the first tier and a slope for path loss of 40 dB/decade (γ = 4). Discuss the

results.

Solution

For a reuse factor N = 4, the number of voice channels per cell site = K/N = 395/4 = 99.

2

63

1

I

SN

4

2

63

14

I

S

)8.13(24 dBI

S

The results for N = 7 and N = 12 are given in Table below.

N Voice channels per cell Mean S/I (dB)

4 99 13.8

7 56 18.7

12 33 23.3

It is evident from the results that, by increasing the reuse factor from N = 4 to N = 12, the mean

S/I ratio is improved from 13.8 to 23.3 dB.


22

2.9 Co-channel Interference Reduction

In the case of increased call traffic, the frequency spectrum should be used efficiently. We should

avoid increasing the number of cells N in a frequency reuse pattern. As N increases, the number

of frequency channels assigned to a cell is reduced, thereby decreasing the call capacity of the

cell.

Instead of increasing N, we either

a. Perform cell splitting to subdivide a congested cell into smaller cells.

Or

b. Use a directional antenna arrangement (sectorization) to reduce co-channel interference.

In this case, each cell is divided into three or six sectors and uses three or six directional

antennas at the base station to reduce the number of co-channel interferers

Co-channel

interference with 120°

sectorized cells.


23

Each sector is assigned a set of channels (frequencies) (either 1/3 or 1/6 of the frequencies of the

omnidirectional cell).

2.10 Adjacent Channel Interference (ACI)

Signals which are adjacent in frequency to the desired signal cause adjacent channel interference.

ACI is brought about primarily because of imperfect receiver filters which allow nearby

frequencies to move into the pass band, and nonlinearity of the amplifiers.

The ACI can be reduced by:

(1) Using modulation schemes which have low out-of-band radiation.

(2) Carefully designing the band-pass filter (BPF) at the receiver front end.

(3) Assigning adjacent channels to different cells in order to keep the frequency

separation between each channel in a given cell as large as possible.

The effects of ACI can also be reduced using advanced signal processing techniques that employ

equalizers.

Co-channel

interference with 60°

sectorized cells.


24

2.11 Handoff (Handover) Strategies

When a mobile moves into a different cell while a conversation is in progress, the MSC

automatically transfers the call to a new channel belonging to the new base station.

The Handoff decision is made depending on:

a. Power

b. Traffic

c. Channel quality

d. Distance

e. Administration

The handoff operation involves:

1. Identifying a new base station,

Review

We developed a relationship between the reuse ratio (q) and cell cluster size (N) for the

hexagonal geometry. Co-channel interference ratios for the omnidirectional and

sectorized cell were derived. A numerical example was given to demonstrate that, for a

given cluster size, sectorization yields a higher S/I ratio, but reduces spectral efficiency.

However, it is possible to achieve a higher spectral efficiency by reducing the cluster size

in a sectorized system without lowering the S/I ratio below the minimum requirement.

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