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Mobile Communications

Dr.-Ing. Ibrahim Ismail Al-Kebsi

2011

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Chapter One

Introduction to Mobile Radio Communications

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In 1897, Marconi demonstrated radios ability to provide

continuous contact with ships sailing the English channel.

By 1934, 194 municipal police radio systems and 58 state

police stations had adopted Amplitude Modulation (AM) for

public safety in the US.

5000 radios were installed in mobiles.

Vehicles ignition noise was the major problem.

In 1935, Edwin Armstrong demonstrated Frequency

Modulation (FM) for the first time.

FM has been the primary modulation technique for mobile

communication systems throughout the world.

Evolution of Mobile Radio Communications

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The Metric SystemPrefix Symbol 10n Since

yotta Y 1024 1991

zetta Z 1021

1991exa E 1018 1975

peta P 1015 1975

tera T 1012 1960

giga G 109 1960

mega M 106 1960

kilo k 10

3

1795hecto h 102 1795

deca 101

1 1 1795

deci d 101 1795

centi c 102 1795

milli m 103 1795

micro 106 1960

nano n 109 1960

pico p 1012 1960

femto f 1015 1964

atto a 1018 1964

zepto z 1021 1991

yocto y 1024 1991

The metric system

was introduced in

1795 with six

prefixes. The other

dates relate to

recognition by a

resolution ofthe General

Conference on

Weights and

Measures (GCWM).

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The ELECTROMAGNETIC SPECTRUM

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ISM Band

The industrial, scientific and medical (ISM) radio bandswere originally reserved internationally for noncommercialuse of RF electromagnetic fields for industrial, scientific andmedical purposes.

Free of license.

The ISM bands are defined by the ITU-R. 900 MHz, 1.8 GHz, 2.4 GHz, 5.8 GHz Bands.

Most microwave ovens use 2.45 GHz.

Individual countries' use of the bands designated in thesesections may differ due to variations in national radioregulations.

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Evolution of Mobile Radio Communications (Cont.)

World war II accelerated the

improvements of the worldsmanufacturing and miniaturizationcapabilities.

The number of US mobile users climbed

from several thousand in 1940 to: 86,000 by 1948.

695,000 by 1958.

1.4 million users in 1962.

The vast majority of mobile users in the1960s were not connected to the PublicSwitched Telephone Network (PSTN).

They were not able to directly dialtelephone numbers from their vehicles.

One of the first mobile radio

telephony system used for

police application purpose

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THE FIRST CAR radiotelephone (1946-53)

Motorola "Deluxe Urban

radiotelephone installed in a taxicab in

Delaware in 1948

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Radiotelephone after 1964

Mobile Phone from 1964

VHF High Band

ChannelBase

Frequency(MHz)

MobileFrequency

(MHz)

JL 152.51 157.77

YL 152.54 157.80

JP 152.57 157.83

YP 152.60 157.86

YJ 152.63 157.89

YK 152.66 157.92

JS 152.69 157.95

YS 152.72 157.98YA 152.75 158.01

JK 152.78 158.04

JA 152.81 158.07

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With the boom in Citizens Band (CB) radio and cordless appliances.

The number of users of mobile and portable radio in 1995 was about 100million (37% of US population).

Research in 1991 estimated between 25 and 40 million cordlesstelephone were in use in the US.

This number was 100 million as of late 2001.

The number of worldwide cellular telephone users grew from25000 in 1984 to about 25 million in 1993.

Customer growth rates in excess of 50% per year.

The worldwide wireless subscriber base of cellular and PCSsubscriber is approximately 630 million as of late 2001 comparedwith 1 billion wired telephone lines.

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In the first few years of 21st century Over 1% of the

worldwide wireless subscriber population had already

abandoned wired telephone service for home use.

They had begun to rely solely on their cellular serviceprovider for telephone access.

Consumer are expected to increasingly use wireless service

as their sole telephone access method in the years tocome.

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Mobile radiotelephony in the US

The first public mobile telephone service was introduced in 25 major citiesin 1946. Single high-powered transmitter.

Large tower.

Cover distance of over 50 km.

Called FM push-to-talk telephone system.

The actual telephone-grade speech occupies only 3 kHz.

Used 120 kHz of RF bandwidth. Why?

Half duplex mode. Call placement was manual operation.

In 1950 Federal Communications Commission (FCC) doubled the numberof mobile telephone channels per market. No new spectrum allocation.

Channel bandwidth is 60 kHz.

The FM bandwidth of voice transmitter was cut to 30 kHz by the mid1960s.

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Mobile radiotelephony in the US (cont.)

In the 1950s and 1960s automatic channel truncking wasintroduced and implemented under the label IMTS(Improved Mobile Telephone Service). Full duplex.

Auto trunking phone systems.

Become saturated in major markets.

Still in use in the US.

Very spectrally inefficient compared to todays US cellular system.

By 1976 the Bell Mobile Phone service for the NY citymarket had only twelve channels Could serve only 543 paying customers.

There was waiting list of over 3,700 people.

Service was poor

suffered from call blocking . why?

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Mobile radiotelephony in the US (cont.) During the 1950s and 1960s, AT&T and Bell laboratories developed the

theory and techniques of cellular radiotelephony.

Break a coverage zone into small cells. Reuse portions of the spectrum usage.

Greater system infrastructure.

Channels are only reused when there is sufficient distance between transmitters toprevent interference.

AT&T proposed the concept of a cellular mobile system to the FCC in

1968.

In 1983 the FCC finally allocated 666 duplex channels for the USAdvanced Mobile Phone System (AMPS). In 800 MHz band.

40 MHz of spectrum.

Duplex channel with bandwidth of 60 kHz.

The first US cellular telephone system.

In 1989 FCC granted an additional 166 channels to US cellular serviceto accommodate the rapid growth and demand.

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In the late 1991 the first US Digital Cellular (USDC)system hardware was installed in major US cities.

The USDC Interim Standard (IS-54) and later (IS-136)allowed cellular operators to replace gracefully somesingle-user analog channels with digital channels. Digital channel can support 3 users in the same 30 kHz

bandwidth.

US carriers gradually phased out AMPS as more usersaccepted digital phones.

Capacity improvement offered by USDC is 3 times that ofAMPS.

Using the digital modulation increases the capacity to 6users/channel in the same 30 kHz bandwidth.

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A cellular system based on code division multipleAccess (CDMA) has been developed byQualcomm and standardized by theTelecommunications Industry Association (TIA) asan Interim Standard (IS-95). Variable number of users in 1.25 MHz.

Using direct sequence spread spectrum (DSSS).

Has inherent interference resistance properties.

Can operate with much smaller signal-to-noise ratio(SNR).

Can use the same set of frequencies in every cell.

High users capacity.

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In the early 1990s a new Specialized Mobile Radio service (SMR)was developed to compete with US cellular radio carriers. Small groups of radio system licenses from a large number of independent

private radio service providers have been purchased.

Nextel and Motorola formed an extended SMR (E-SMR) network in the 800MHz band.

By using Motorolas Integrated Radio System (MIRS), SMR integrates voice

dispatch, cellular phone service, messaging, and data transmission capabilitieson the same network.

In 1995 Motorola replaced MIRS with the integrated Digital EnhancedNetwork (iDEN).

In early 1995 the Personal Communication Service (PCS) licenses in

1800/1900 MHz band were auctioned by the US government towireless providers. It was new wireless service that complement as well as compete with cellular

and SMR.

One of the stipulations of PCS license was the majority of the coverage areabe operational before the year 2000.

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Major Mobile Radio Standards in North

AmericaStandard Type Year of

Introduction

Multiple

Access

Frequency

Band

Modulation Channel

BW

AMPS Cellular 1983 FDMA 824-894

MHz

FM 30 kHz

NAMPS Cellular 1992 FDMA 824-894

MHz

FM 10 kHz

USDC Cellular 1991 TDMA 824-894

MHz

/4- DQPSK 30 kHz

CDPD Cellular 1993 FH/Packet 824-894

MHz

GMSK 30 kHz

IS-95 Cellular/

PCs

1993 CDMA 824-894

MHz

1.8-2.0

GHz

QPSK/ BPSK 1.25 MHz

GSC Paging 1970s Simplex Several FSK 12.5 kHz

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Major Mobile Radio Standards in North America (cont.)

Standard Type Year of

Introduction

Multiple

Access

Frequency

Band

Modulation Channel

BW

POCSAG Paging 1970s Simplex Several FSK 12.5 kHz

FLEX Paging 1993 Simplex Several 4-FSK 15 kHz

DCS-1900(GSM) PCS 1994 TDMA 1.85-1.99GHz GMSK 200 kHz

PACS Cordless

/PCS

1994 TDMA/

FDMA

1.85-1.99

GHz

/4- DQPSK 300 kHz

MIRS SMR/

PCS

1994 TDMA Several 16-QAM 25 kHz

iDEN SMR/

PCS

1994 TDMA Several 16-QAM 25 kHz

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Major Mobile Radio Standards in EuropeStandard Type Year of

Introduction

Multiple

Access

Frequency

Band

Modulation Channel

BW

ETACS Cellular 1985 FDMA 900 MHz FM 25 kHzNMT-450 Cellular 1981 FDMA 450-470

MHz

FM 25 kHz

NMT-900 Cellular 1986 FDMA 890-960

MHz

FM 12.5 kHz

GSM Cellular/PCS

1990 TDMA 890-960MHz

GMSK 200 kHz

C-450 Cellular 1985 FDMA 450-465

MHz

FM 20 kHz/

10 kHz

ERMES Paging 1993 FDMA Several 4-FSK 25 kHz

CT2 Cordless 1989 FDMA 864-868MHz

GFSK 100 kHz

DECT Cordless 1993 TDMA 1880-

1900 MHz

GFSK 1.728

MHz

DCS-

1800

Cordless/

PCS

1993 TDMA 1710-

1880 MHz

GMSK 200 kHz

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Major Mobile Radio Standards in Japan

Standard Type Year of

Introduction

Multiple

Access

Frequency

Band

Modulation Channel

BW

JTACS Cellular 1988 FDMA 860-925

MHz

FM 25 kHz

PDC Cellular 1993 TDMA 810-1501

MHz

/4- DQPSK 25 kHz

NTT Cellular 1979 FDMA 400/800

MHz

FM 25 kHz

NTACS Cellular 1993 FDMA 843-925

MHz

FM 12.5 kHz

NTT Paging 1979 FDMA 280 MHz FSK 12.5 kHz

NEC Paging 1979 FDMA Several FSK 10 kHz

PHS Cordless 1993 TDMA 1895-1907

MHz

/4- DQPSK 300 kHz

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Abbreviations

CDPD: Cellular Digital Packet Data.

DCS: Digital Communication System. FLEX: 4-level FSK based paging standard developed by Motorola.

GSC: Golay Sequential Coding.

MIRS: Motorola Integrated Radio System (for SMR use).

NADC: North American Digital Cellular.

NEC: Nippon Denki Kabushiki Gaisha. NMT: Nordic Mobile Telephone.

NTACS: Narrowband Total Access Communication System.

NTT : Nippon Telephone and Telegraph.

POCSAG: Post Office Code Standard Advisory Group. JTACS: Japanese Total Access Communication System.

PACS: Personal Access Communication System.

PDC: Pacific Digital Cellular.

PHS: Personal Handyphone System.

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Wireless communications System Definitions

Base Transceiver station (BTS)

A fixed station in a mobile radio system used for radio communicationwith mobile stations.

Base stations are located at the center or on the edge of a coverage

region and consist of radio channels, transmitter and receiver

antennas mounted on a tower.

Control channel

Radio channel used for transmission of call setup, call request, and call

initiation.

Forward channel

Radio channel used for transmission of information from the base

station to the mobile.

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Wireless communications System Definitions (cont.)

Full Duplex Systems Communication systems which allow simultaneous two-way

communication.

Transmission and reception is typically on two different channels (FDD)although new cordless/PCS systems are using TDD.

Half Duplex Systems

Communication systems which allow two-way communication by usingthe same radio channel for both transmission and reception.

At any given time the user can only either transmit or receive information.

handoff The process of transferring a mobile station from one channel or base

station to another.

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Mobile station (MS) A station in the cellular radio service intended for use while in motion at

unspecified locations. Mobile stations may be hand-held personal units (portables) or installed in

vehicles (mobiles).

Mobile Switching Center (MSC) Switching center which coordinates the routing of calls in a large service area.

In a cellular radio system the MSC connects the cellular base stations and themobile to the PSTN.

An MSC is also called a Mobile Telephone Switching Office (MTSO).

Base Station Controller (BSC) Typically a BSC has tens or even hundreds of BTSs under its control.

The BSC handles allocation of radio channels, receives measurements from themobile phones, and controls handovers from BTS to BTS

Page A brief message which is broadcast over the entire service area, usually in a

simulcast fashion by many base stations at the same time.

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Reverse channel Radio channel used for transmission of information from the mobile to basestation.

Roamer A mobile station which operates in a service area other than that from which

service has been subscribed.

Simplex systems Communication systems which provide only one-way communication.

Subscriber A user who pays subscription charges for using a mobile communications

system.

Transceiver A device capable of simultaneously transmitting and receiving radio signals.

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A Cellular System

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Examples of wireless communication systems

Paging systems

Communication systems.

Send brief messages to subscriber.

Numeric message.

Alphanumeric message.

Voice message.

The issued message is called a page.

The paging system transmits the page throughout the service

area using base station.

Simple paging system may cover a limited range of 2 to 5 km.

Wide area paging systems can provide worldwide coverage.

Paging receivers are simple and inexpensive whereas the

transmission systems is quite sophisticated.

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Paging systems

Wide area paging systems consist of:

A network of telephone lines.

Many base station transmitters.

Large radio towers.

Broadcast a page from each base stationsimultaneously. (This is called simulcasting).

Simulcast transmitters may be located within

the same service area or in different cities orcountries.

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Paging systems

A wide area Paging system

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Cordless telephone systems Full duplex communication systems.

Use radio to connect a portable handset to a dedicated base station whichis then connected to a dedicated telephone line with specific telephonenumber on PSTN.

Provide the user with limited range and mobility.

Can not maintain a call if the user travels outside the range of the basestation.

First generation: Manufactured in the 1980s.

Can cover distance of a few tens of meters.

For in-home use.

Second generation: Have recently been introduced.

Allow subscribers to user their handset at many outdoor locations within urban centers.

Sometimes combined with paging receivers.

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Cordless telephone systems

A Cordless telephone system

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Cellular telephone systems Provides a wireless connection to the PSTN for any user location

within the radio range of the system.

Accommodate a large number of users over a large geographic areawithin a limited frequency spectrum.

Provide high quality service that is comparable to that of the

landline telephone systems.

Can achieve high capacity by limiting the coverage of each basestation transmitter to a small geographic area called a cell.

The same radio channels may be reused by another base stationlocated some distance away.

A sophisticated switching technique called handoff enables a call toproceed uninterrupted when the user moves from one cell toanother.

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Cellular telephone systems

A basic cellular system consists of Mobile stations (MS).

Base stations (BS).

Mobile Switching Center (MSC) or Mobile Telephone Switching Office(MTSO).

Mobile station Contains a transceiver, an antenna, and control circuitry.

Mounted in a vehicle or used as a portable hand-held unit.

Base station Consists of several transmitters and receivers.

Handles full duplex communications simultaneously.

Have tower that support several transmitting and receiving antennas.

Serves as a bridge between all mobile users in the cell.

Connects the simultaneous mobile calls via telephone lines ormicrowave links to the MSC.

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Cellular telephone systems

A Cellular telephone system. The towers represent base stations which

provide radio access between mobile users and

the mobile switching center (MSC)

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Traditional Cellular Networks

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Cellular telephone systems MSC or MTSO

It is responsible for connecting all mobiles to the PSTN.

Coordinates the activities of all of the base stations. A typical MSC handles 100,000 cellular subscribers and 5000 simultaneous conversations at a

time.

Accommodates all billing and system maintenance functions.

In large cities, several MSCs are used by a single carrier.

Communications between the base station and the mobiles is defined by a

standard Common Air Interface (CAI) that specifies 4 different channels.

Forward Voice Channels (FVC) The channels used for voice transmission from the base station to mobiles.

Reverse Voice Channels (RVC) The channels used for voice transmission from mobiles to the base station.

Forward Control Channels (FCC) and Reverse Control Channels (RCC) The two channels responsible for initiating mobile calls.

Control Channels are only involved in setting up a call and moving it to an unusedvoice channel.

Control channels are often called Setup Channels.

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How a cellular telephone call is made

When a cellular phone is turned on, but is not yet engaged in a call. It scans the group of FCCs.

Determines the one with strongest signal. Monitors that control channel until the signal drops below a usable level.

It again scans the control channels in search of the strongest BS signal.

Control channels make up about 5% of the total number of channelsavailable in the system. The other 95% are dedicated to voice and data traffic.

When a telephone call is placed to a mobile user. MSC dispatches the request to all base stations in the cellular system.

The Mobile Identification Number (MIN), which is the subscriberstelephone number is broadcast as a paging message over all of the FCCsthroughout the cellular system.

Figures 1.1 and 1.2 depicts the timing diagram of how a call to or frommobile user.

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Station Class Mark The SCM (Station Class Mark) is a 4 bit number which holds

three different pieces of information.

The cellular telephone transmits this information (and more) tothe cell tower. Bit 1 of the SCM tells the cell tower whether your cell phone uses the older

666 channel cellular system, or the newer 832 channel cellular system.

The expansion to 832 channels occurred in 1988. Bit 2 tells the cellular system whether your cellular telephone is a mobile unit

or a voice activated cellular telephone.

Bit 3 and 4 tell the cell tower what power your cellular telephone should betransmitting.

Bit 1:0 == 666 channels

1 == 832 channels Bit 2:

0 == Mobile cellular telephone1 == Voice activated cellular telephone

Bit 3/4:00 == 3.0 watts (Mobiles)01 == 1.2 watts (Transportables)10 == 0.6 watts (Portables)

11 == Reserved for future use

Receives call Verifies that the Requests BS to Connects the

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Timing diagram illustrated how a call to a mobile user initiated by a landline subscriber isestablished

MSC

Receives call

from PSTN.

Sends the

requested MIN

to all BSs.

Verifies that the

mobile has a valid

MIN&ESN.

Requests BS to

move mobile to

unused voice

channel pair.

Connects the

mobile with

the calling

party on the

PSTN.

BS

F

CC

Transmits page

MIN for

specified user.

Transmits data

message for

mobile to moveto specific VC.

RCC

Receives

MIN,ESN, SCM

&passes to MSC.

FVC Begin voice

transmission.

RV

CBegin voicereception.

Mobile

FCC

Receives page

&matches the

MIN with its

own MIN.

Receives data

message to

move to

specified VC.

R

CC

Acknowledges

receipt of MIN.

Sends ESN&SCM.

FVC Begin voice

reception.

RVC Begin voice

transmission.

time

Receives call Instructs FCC of Connects the

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MSC

Receives call

initiation request

from BS. Verifies

that the mobile

has a valid MIN&

ESN.

Instructs FCC of

originating BS to

move mobile to a

pair of voice

channels.

Connects the

mobile with

the called

party on the

PSTN.

BS

FCC

Page for called mobile,

instructing the mobile tomove to VC.

RCC

Receives call initiation

request and MIN, ESN,

SCM.

FVC Begin voice

transmission.

RVC Begin voice

reception.

Mob

ile

FCC

Receives page and

matches the MIN with its

own MIN. receives

instruction to move to VC.

RC

C

Sends call initiation

request along with

subscribe MIN and

number of called party.

FVC Begin voice

reception.

RVC Begin voice

transmission.

Timing diagram illustrated how a call initiated by a mobile is established

time

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Comparison of mobile communication system (Mobile Station)

Service Coveragerange Requiredinfrastructure complexity Hardwarecost Carrierfrequency functionality

TV

remote

control

Low Low Low Low Infrared Transmitter

Garage

door

opener

Low Low Low Low < 100

MHz

Transmitter

Paging

system

High High Low Low

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Comparison of mobile communication system (Base Station)

Service Coverage

range

Required

infrastructure

Complexity Hardware

cost

Carrier

frequency

functionality

TV remote

Control

Low Low Low Low Infrared Receiver

Garage

dooropener

Low Low Low Low

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Modern Wireless Communication Systems Second generation (2G) cellular networks

cdmaONE, IS-95, ANSI J-STD-

008

GSM, DCS-1900, ANSI J-STD-

007

NADC, IS-54/IS-136, ANSI J-STD-

011, PDC

Uplinkfrequencies

824-849 MHz1850-1910 MHz (US PCS)

890-915 MHz (Europe) 1850-1910 MHz (US PCS)

800 MHz, 1500 MHz, (Japan)1850-1910 MHz (US PCS)

Downlink

Frequencies

869-894 MHz (US Cellular)

1930-1990 MHz (US PCS)

935-960 MHz (Europe)

1930-1990 MHz (US PCS)

869-894 MHz (US Cellular)

1930-1990 MHz (US PCS)

800 MHz, 1500 MHz (Japan)

Duplexing FDD FDD FDD

Multiple accesstechnology

CDMA TDMA TDMA

Modulation BPSK with quadrature

spreading

GMSK with BT=0.3 /4 DQPSK

Carrier

separation

1.25 MHz 200 kHz 30 kHz (IS-136)

Channel datarate 1.2288 Mchips/sec 270.833 kbps (48.6 kbps for IS-136)(42 kbps for PDC)

Voice channel

per carrier

64 8 3

Speech coding Code excited linear prediction

(CELP) @ 13 kbps, Enhanced

variable rate codec (EVRC) @ 8

kbps

Residual pulse excited long

term prediction (RPE-LTP) @

13 kbps

Vector sum Excited Linear

Predictive Coder (VSELP) @ 7.95

kbps

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Current and emerging 2.5G and 3G data communication standards

Wireless data

technologies

Channel

BW

Duplex Infra-

structure

change

Requires

new

spectrum

Requires new handsets

HSCSD

(High Speed

Circuit

Switched

Data)

200 kHz FDD Requires

software

upgrade

at base

station

No Yes

New HSCSD handsets provide

57.6 kbps on HSCSD networks,

and 9.6 kbps on GSM networks

with dual mode phones. GSM

only phones will not work in

HSCSD networks.

GPRS

(General

Packet Radio

Service)


new

packet

overlayincluding

routers

and

gateways

No Yes

New GPRS handsets work on

GPRS networks at 171.2 kbps, 9.6

kbps on GSM networks with dualmode phones. GSM only phone

will not work in GPRS networks.

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Current and emerging 2.5G and 3G data communication standards (2)

Wireless

data

technologies

Channel

BW

Duplex Infra-

structure

change

Requires

new

spectrum


EDGE

(Enhanced

Data Rates

for GSM

Evolution)


new

transceiver

at base

station.

Also,

software

upgrades

to the

base

station

controller

and base

station.

No Yes

New handsets work on EDGE

networks at 384 kbps, GPRS

networks at 144 kbps, and

GSM networks at 9.6 kbps with

tri-mode phones.

GSM and GPRS only phones

will not work in EDGE

networks.

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Wireless data

technologies

Channel

BW

Duplex Infra-

structure

change

Requires

new

spectrum


W-CDMA 5 MHz FDD Requires

completely

new base

station.

Yes Yes

New W-CDMA handsets will

work on W-CDMA at 2 Mbps,

EDGE networks at 384 kbps,

GPRS networks at 144 kbps,GSM networks at 9.6 kbps.

Older handsets will not work in

W-CDMA.

IS-95B 1.25

MHz

FDD Requires

new

softwarein base

station

controller.

No Yes

New handsets will work on IS-

95B at 64 kbps and IS-95A at14.4 kbps. Older handsets can

work in IS-95B at 14.4 kbps.

C t d i 2 5G d 3G d t i ti t d d (4)

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Wireless data

technologies

Channel

BW

Duplex Infra-structure

change

Requires

new

spectrum


cdma2000

1XRTT

(Radio

Transmission

Technology)

1.25

MHz

FDD Requires new

software in

backbone and new

channel cards at

base station. Also

need to build a

new packet

service node.

No Yes

New handsets will

work on 1XRTT at 144

kbps, IS-95B at 64 kbps,

IS-95A at 14.4 kbps.

Older handsets can

work in 1XRTT but at

lower speeds.

Cdma2000

1XEV (DO

and DV)(Evolution,

Data Only),

(Evolution

Data and

Voice)

1.25

MHz

FDD Requires software

and digital card

upgrade on 1XRTTnetworks.

No Yes

New handsets will

work on 1XEV at 2.4Mbps, 1XRTT at 144

kbps, IS-95B at 64 kbps,

IS-95A at 14.4 kbps,

older handsets can

work in 1XEV but at

lower speeds.

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Current and emerging 2.5G and 3G data communication

standards (5)

Wireless data

technologies

Channel

BW

Duplex Infra-structure

change

Requires

new

spectrum


cdma2000

3XRTT

3.75

MHz

FDD Requires

backbone

modifications

and new

channel cards

at base

station.

Maybe Yes

New handsets will work

on 95A at 14.4 kbps, 95B

at 64 kbps, 1XRTT at 144

kbps, 3XRTT at 2 Mbps.

Older handsets can work

in 3X but at lower speeds.

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Various upgrade paths for 2G technologies

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Leading IMT-2000 candidate standards

Air interface Mode of Operation Duplexing Method Key Features

Cdma2000US TIA

TR45.5

Multi-carrier andDirect spreading

DS-CDMA at

N1.2288 Mcps

With N=1,3,6,9,12

FDD and TDD Modes Backwardcompatibility with IS-

95A and IS-95B.

Downlink can be

implemented using

either Multi-carrier

or Direct Spreading.Uplink can support a

simultaneous

combination of

multi-carrier or

Direct spreading.

Auxiliary carriers tohelp with downlink

channel estimation

in forward link

beam-forming.

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Abbreviations

ANSI: American National Standards Institute.

DECT: Digital Enhanced Cordless Telecommunications.

ETSI: European Telecommunications Standards Institute.

FOMA: Freedom of Mobile Multimedia Access.

GPRS: General packet radio service.

IMT2000: International Mobile Telecommunications 2000.

UWC: Universal Wireless Communications.

TIA: Telecommunications Industry Association.

UMTS: Universal Mobile Telecommunications System.UTRA: UMTS Terrestrial Radio Access.WIMS: Wireless Integrated services digital network

Multimedia Services.

Frequency Division Duplexing (FDD)

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Frequency Division Duplexing (FDD)

Provides simultaneous radio transmission channels forthe users and the base station.

At the base station separate transmit and receiveantennas are used to accommodate the two separatechannels.

At the subscriber unit a single antenna is used for bothtransmission to and reception from the base station. A device called duplexer is used to enable the same

antenna to a be used for simultaneous transmission andreception.

It is necessary to separate the transmit and receivefrequencies The duplexer can provide sufficient isolation while being

inexpensively manufactured.

FDD is used exclusively in analog mobile radio systems.

Time Division Duplexing (TDD)

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Time Division Duplexing (TDD)

TDD uses the fact that it is possible to share a singleradio channel in time

a portion of the time is used to transmit from the basestation to the mobile.

The remaining time is used to transmit from the mobile tothe base station.

If the data transmission rate of the channel is muchgreater than the end-users data rate. it is possible to store information bursts

provide the appearance of full duplex operation to a user,even though there are not two simultaneous radiotransmissions at any instant of time.

TDD is only possible with digital transmission formatsand digital modulation, and is very sensitive to timing .

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Leading IMT-2000 candidate standard (cont.)

Air interface Mode of Operation Duplexing

Method

Key Features

UTRA (UMTS

Terrestrial Radio

Access) ETSI SMG2

DS-CDMA at rates

of N0.960 Mcps

With N=4, 8, 16

FDD and TDD

Modes

Wideband DS-CDMA

system.

Backward compatibility

with GSM/DCS1900.

Up to 2.048 Mbps on

downlink in FDD Mode.

Minimum forward

channel bandwidth of 5

GHz.

W-CDMA/NA

(Wideband CDMA

North America)USA T1P1-ATIS

W-CDMA/Japan

ARIB

CDMA II

South Korea TTAWIMS/W-CDMA

USA

TIA TR46.1

L di IMT 2000 did t t d d ( t )

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Leading IMT-2000 candidate standard (cont.)

Air interface Mode of

Operation

Duplexing

Method

Key Features

CDMA ISouth Korea TTA

DS-CDMA atN0.9216 Mcps

With N= 1,4,16

FDD andTDD

Modes

Up to 512 kbps per spreading code,code aggregation up to 2.048 Mbps.

UWC-136

(Universal

Wireless

Communications

Consortium)

USA TIA TR45.3

TDMA Up to

722.2 kbps

(Outdoor/

vehicular) Up to

5.2 Mbps

(indoor office)

FDD

(outdoor/

Vehicular)

TDD

(indoor

office)

Backward compatibility and upgrade

path for both IS-136 and GSM.

Fits into existing IS-136 and GSM.

Explicit plans to support adaptive

antenna technology.

TD-SCDMA

China Academy

of Telecomm.Technology

(CATT)

DS-CDMA

1.1136 Mcps

TDD RF channel bit rate up to 2.227 Mbps.

Use of smart antenna technology is

fundamental (but not strictly required)in TDSCMA.

DECT (ETSI

Project (EP)

DECT)

1150-3456 kbps

TDMA

TDD Enhanced version of 2G DECT

technology.

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Toward 4G

GSM

(TDMA)

PDC

(TDMA)

iDEN

(TDMA)

IS-136

(TDMA)

IS-95A

(CDMA)

GPRS EDGE

WCDMA

(UMTS)

HSPA

LTE LTE-A

IS-95B

(CDMA)

1X

(CDMA2000)

EV-DO

(CDMA2000)

2G 2.5G 3G 4G

world

Japan

U.S

U.S

U.S

Asia

L T E l ti (LTE)

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Long Term Evolution (LTE) LTE is a preliminary mobile communication standard

formally submitted as a candidate 4G system to ITU-T in

late 2009, and expected to be finalized in 2011.

An attempt to bring 4G technology into 3G spectrum.

LTE challenges other advanced technologies (like WiMAX)and will try to meet high data rate service demands.

LTE Advanced also introduces multicarrier to be able to useUltra Wide Bandwidth (UWB), up to 100 MHz of spectrum

supporting very high data rates.

LTE system advantages Wide deployment

Mobility Support

Motivation for LTE

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Motivation for LTE

Need for higher data rates and greater spectral efficiency

Can be achieved with High Speed Downlink Packet Access(HSDPA) and High Speed Uplink Packet Access (HSUPA).

New air interface defined by LTE 3GPP.

Need for Packet Switched optimized system.

Evolve UMTS toward Packet only system.

Need for high quality of services. Use of licensed frequencies to guarantee quality of services.

Always on-experience (reduce control plane latency significantly).

Reduce round trip.

Need for cheaper infrastructure. Simplify architecture reduce number of network elements.

LTE f i

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LTE performance requirements

Channel Bandwidth Scalable system bandwidth in LTE-Advancedexceeding 20 MHz, Up to 100 MHz.

also LTE supports 1.25 MHz, 5 MHz, 10 MHz, 15 MHzand 20 MHz.

LTE utilizes hybrid OFDMA and SC-FDMA. LTE radio access should be based on OFDMA in the

downlink (DL).

and Single-Carrier Frequency Division MultipleAccess (SC-FDMA) in the uplink (UL).

http://e/OFDM.pptxhttp://e/OFDM.pptx

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Data rate Instantaneous downlink peak data rate of 100 Mbps in a 20 MHz Downlink

spectrum (i.e 5 bps/Hz).

Instantaneous uplink peak data rate of 50 Mbps in a 20 MHz uplink spectrum(i.e 2.5 bps/Hz).

LTE-Advanced provides almost 3.3 Gbit peak download rates per sector of thebase station under ideal conditions with utilizing 8x8 MIMO and 100 MHzbandwidth.

Cell range. 5 km optimal size.

30 km with reasonable performance.

100 km with acceptable performance.

Cell capacity Up to 200 active users per cell. (5 MHz)


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Mobility

Optimised performance (0-15 km/h), high performance (15-120 km/h),

service maintained (120-350 km/h).

Latency

User Plane Latency < 5 ms.

Control Plane Latency < 50 ms.

Improved spectrum efficiency.

Improved broadcasting.

Co-existence with legacy standards.

Migration to LTE

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Migration to LTE

PCF: packet Control Function

BSC: Base station Controller

MME: mobile management entity

PCU: packet control unit

PDN: packet Data Network

PDSN:Packet Data Serving node

SGSN: Serving GPRS Support Node

HRPD: High Rate Packet Data

HSGW: HRPD Serving Gateway

RNC: Radio Network Controller

HLR: Home Location Register

HSS: Home Subscriber Server

PCRF

USN: unified service node

UGW: unified gateway

Definitions

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Definitions

Mobile Management Entity (MME) is a processing element within the UMTS LTEcan be used to find a route, then transfer communication connections to WiMAX

wireless devices.

Packet Control Unit (PCU) is a late addition to the GSM standard. It performs someof the processing tasks of the BSC, but for packet data. The allocation of channelsbetween voice and data is controlled by the base station, but once a channel isallocated to the PCU, the PCU takes full control over that channel.

Serving GPRS Support Node (SGSN) is responsible for the delivery of data packetsfrom and to the mobile stations within its geographical service area.

Home Location Register / Home Subscriber Server (HLR / HSS) provides acomprehensive, centralized subscriber database management solution across

current and next-generation wireless technologies.

UnifiedService Node (USN) and Unified GateWay (UGW) developed by Huaweiand had been used by a mature core network solution to build logical nodes fordifferent access networks.

WiMAX

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WiMAX

WiMAX stands for Worldwide Interoperability for Microwave Access.

WiMAX is a wireless digital communications system, also known as IEEE802.16.

The availability of802.16e and 802.16d as two different and incompatibleiterations of the WiMAX standards has in some cases added confusion to theoperators investment decisions. 802.16d has often been considered the (fixed standard). and 802.16e as the (mobile standard).

802.16e supports the full spectrum of fixed, nomadic, portable and mobile solutions.

WiMAX can provide Broadband Wireless Access (BWA) up to 30 miles (50 km) for fixed stations, and

3- 10 miles (5 - 15 km) for mobile stations.

WiMAX operates on both licensed and non-licensed frequencies.

Soon, WiMAX will be a very well recognized term to describe wireless Internetaccess throughout the world.

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WiMAX system

Mobile WiMAX

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Mobile WiMAX In February 2002, the South Korea allocated 100 MHz of electromagnetic

spectrum in the 2.32.4 GHz band.

In late 2004 WiBro Phase 1 was standardized by the TTA of Korea.

In late 2005 ITU reflected WiBro as IEEE 802.16e (mobile WiMAX).

Mobile WiMAX is a broadband wireless solution that enables convergenceof mobile and fixed broadband networks through a common wide area.

The Mobile WiMAX Air Interface adopts Orthogonal Frequency DivisionMultiple Access (OFDMA) for improved multi-path performance in non-

line-of-sight environments.

Scalable OFDMA (SOFDMA) is introduced in the IEEE 802.16e Amendmentto support scalable channel bandwidths from 1.25 to 20 MHz.

DVB-T

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DVB T

ATSC : Advanced Television Systems Committee.

DMB-T/H: Digital Multimedia Broadcast-Terrestrial/Handheld.

DVB-T : Digital Video Broadcasting- Terrestrial.

ISDB-T : Integrated Services Digital Broadcasting-Terrestrial.

ISDB-T : Integrated Services Digital Broadcasting Terrestrial.

DVB T (2)

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DVB-T (2)

W ld bil h

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World mobile phone usage

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DVB-S

DSS : Digital Satellite Service.

ISDB-S : Integrated Services Digital Broadcasting-Satellite.

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Assignment 1Aspects of Research

Services and Features of Cellular System.

The Architecture of Cellular System.

Radio Interface of Cellular System. System Channel Types.

Frame Structure of Cellular System.

Signal Processing in Cellular System.

Mobile phone standards

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Mobile phone standards

3GPP Family

GSM (2G)

GPRS CSD

HSCSD

EDGE

(evolutionary 3G, Pre3G)

EDGE (EGPRS)

EvolvedEDGE (EGPRS2B)

UMTS/UTRA(revolutionary 3G)

UTRA-FDD (W-CDMA)

FOMA UTRA-TDD

UTRA-TDD HCR(TD-CDMA)

UTRA-TDD LCR(TD-SCDMA)

HSPA

HSDPA

HSUPA

DC-HSDPA

3GPPRel.7 and 8 (Pre-4G)

EvolvedHSPA (HSPA+; Rel.7)

Long TermEvolution (Rel. 8)

LTE Advanced (4G)

Other

GAN (UMA)

3GPP2 Family

cdmaOne (2G)CDMA2000 (evolutionary 3G)

EV-DO

UMB (Pre-4G)

AMPS Family

AMPS (1G)

TACS / ETACS

D-AMPS (2G)

Other Technologies

Pre Cellular PTT

MTS

IMTS

AMTS

OLT

MTD

Autotel / PALM

ARP

1G

NMT

Hicap

CDPD Mobitex

DataTAC

2G

iDEN

PDC

CSD

PHS

WiDENPre-4G

iBurst

HIPERMAN

Flash-OFDM

WiMAX

WiBAS (LMDS)

WiBro

Channel AccessMethods

FDMA OFDMA

TDMA STDMA

SSMA CDMA

SDMA

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Chapter Two

The Cellular Concept and

System Design Fundamentals

The Cellular concept

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The Cellular concept

Some of the important concepts will be

discussed in detail:

a) Frequency reuse.

b) Channel assignment.

c) Handoff.

d) Interference and system capacity.

e) Tracking and Grade of Service (GoS)

f) Improving coverage and capacity.

Frequency reuse

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Frequency reuse

Each cellular BS is allocated a group of radio channels to be used

within a small geographic area called a Cell.

BSs in adjacent cells are assigned channel groups which contain

completely different channels than neighbouring cells.

The BS antennas are designed to achieve the desired coverage

within the particular cell.

By limiting the coverage area to within the boundaries of a cell

The same group of channels may be used to cover different cells.

Those cells are separated from one another by distances large enough to

keep interference levels within tolerable limits.

The design process of selecting and allocating channel groups for

all of the cellular BSs within a system is called frequency reuse or

frequency planning.

Frequency reuse (2)

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Frequency reuse (2)

Cells labelled with the

same letter use the sameset of frequencies.

A cell cluster is outlined

in bold and replicatedover the coverage area.

In this figure the cluster

size (N) is equal to 7.

The frequency reuse isfactor is (1/7).

Fig. 2.1 Cellular frequency reuse concept

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Frequency reuse (4)

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Frequency reuse (4)

There are three sensible choices asquare, an equilateral triangle, and a

hexagon. For a given distance between the

centre of a polygon and its farthestperimeter points, hexagon has thelargest area of the three.

By using the hexagon geometry, the fewest number of cells can cover a

geographic region.

The hexagon closely approximates acircular radiation pattern which wouldoccur for an omni-directional BSantenna and free space propagation.

The actual cellular footprint isdetermined by the contour in which agiven transmitter serves the mobilessuccessfully.

Fig. 2.2 Cellular BS footprint

Frequency reuse (5)
http://localhost/var/www/apps/conversion/tmp/scratch_3/SC_2010.pptxhttp://localhost/var/www/apps/conversion/tmp/scratch_3/SC_2010.pptxhttp://localhost/var/www/apps/conversion/tmp/scratch_3/SC_2010.pptx

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Frequency reuse (5)

When using hexagons to model coverage

areas BS transmitters are depicted as eitherbeing in the

centre of the cell (centre-excited cells) or

on three of the six cell vertices (edge-excited

cells).

Omni-directional antennas are used in

centre-excited cells.

Sectored directional antennas are used in

corner excited cells.

Fig. 2.3 centre-excited cells

Fig. 2.4 corner excited cells

Frequency reuse (6)

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Frequency reuse (6)

An omni-directional antenna isan antenna which radiates power

uniformly in one plane, with theradiated power decreasing withelevation angle above or below theplane, dropping to zero on theantenna's axis. This radiation pattern isoften described as "donut shaped".

A sector antenna is a typeof directional microwave antenna witha sector-shaped radiation pattern. Thelargest use of these antennas is asantennas for cell phone base-stationsites. They are also used for othertypes of mobile communications, forexample in WiFi networks. They areused for limited-range distances ofaround 4 to 5 km.

Fig. 2.5 Omni-directional radiation pattern

Fig. 2.6 A directional antenna radiation pattern

Frequency Reuse (7)

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Frequency Reuse (7)

A cellular system has a total ofSduplex channels.

Each cell is allocated a group ofkchannels (k< S).

The Schannels are divided amongNcells into unique and disjoint channelgroups. Each group has the same number of channels.

The total number of available radio channels can be expressed as

S=kN (2.1)

TheNcells which collectively use the complete set of available frequenciesis called a cluster.

If cluster is replicatedMtimes within the system The total number of duplex channels Ccan be used as a measure of capacity and is

given byC = MkN = MS (2.2)

The capacity of a cellular system is directly proportional to the number oftimes a cluster is replicated in a fixed service area.

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Frequency Reuse (9)

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In order to connect without gapsbetween adjacent cells The geometry of hexagons is such

that the number of cells per cluster,can have values which satisfy:

N= i2+ij+j2 (2.3)

Where i andj are non-negative integers

To find the nearst co-channelneighbour cell one must do thefollowing:1. Move i cells along any chain of

hexagons.

2. Then turn 60 counter-clockwise.

3. Move j cells.

As illustrated in the figure 2.7

i = 3, andj = 2

N= 9+6+4

= 19

Fig. 2.7 Method of locating co-channel

cells in a cellular system

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Frequency Reuse (11)

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q y ( )

ExampleIf a total of33 MHz of bandwidth is allocated to a particular FDD cellular telephone

system which uses two25 kHz simplex channels to provide full duplex voice andcontrol channels, compute the number of channels available per cell if a systemuses

(a) 4 cell reuse, (b) 7 cell reuse (c) 12 cell reuse.

If1 MHz of the allocated spectrum is dedicated to control channels, determine anequitable distribution of control channels and voice channels in each cell for eachof the three systems.

SolutionGiven total bandwidth = 33 MHz

Channel bandwidth = 25 kHz 2 simplex channels = 50 kHz/duplex channel

Total available channels = 33,000/50 = 660 channels

(a) For N = 4,

Total number of channels available per cell = 660/4 = 165 channels.

(b) For N=7,

Total number of channels available per cell = 660/7 95 channels.

(c) For N=12,

Total number of channels available per cell = 660/12 = 55 channels.


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q y ( )

Solution (cont.)

A 1 MHz spectrum for control channels implies that there are1000/50 = 20 control channels out of the 660 channels available.

To evenly distribute the control and voice channels.

Simply allocate the same number of channels in each cellwherever possible.

Here 660 channels must be evenly distributed to each cell withinthe cluster.

In practice only the 640 voice channels would be allocated, sincethe control channels are allocated separately as 1 per cell.

(a) For N = 4,

1 cell with 5 control channels and 160 voice channels.

In practice, however each cell only needs a single control channel.

Thus 1 control channel and 160 voice channels would be assigned toeach cell.


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(b) For N = 7,

4 cells with 3 control channels and 92 voice channels (12 CCs), (368 VCs)

2 cells with 3 control channels and 90 voice channels (06 CCs), (180 VCs)1 cell with 2 control channels and 92 voice channels.(02 CCs), (092 VCs).

In practice, however each cell would have 1 control channel,

4 cells would have 91 voice channels (364 VCs), and

3 cells would have 92 voice channels (276 VCs).

(c) For N=12,

8 cells with 2 control channels and 53 voice channels (16 CCs), (424 VCs)

4 cells with 1 control channel and 54 voice channels (04 CCs), (216 VCs)

In practice, however each cell would have 1 control channel,8 cells would have 53 voice channels (424 VCs), and

4 cells would have 54 voice channels (216 VCs).

Channel Assignment Strategies

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Channel assignment strategies can be classified as either fixed or

dynamic.

The choice of channel assignment strategy impacts the performance ofthe system. How calls are managed when a mobile user is handed off from one cell to

another.

In 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

The subscriber does not receive service.

Several variations of the fixed assignment strategy exist.

In one approach called borrowing strategy A cell is allowed to borrow channels from a neighbouring cell if all of its own

channels are already occupied.

Channel Assignment Strategies (2)

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In a dynamic channel assignment strategy. Voice channels are not allocated to different cells

permanently.

Instead each time a cell request is made the serving BSrequests a channel from the MSC.

The switch then allocates a channel to the requestedcell following an algorithm that takes into account: The likelihood of future blocking within the cell.

The frequency of use of the candidate channel.

The reuse distance of the channel and other cost functions.

MSC allocates a given frequency if that frequency is notpresently in use in the cell or any other cell which fallswithin the minimum restricted distance of frequencyreuse to avoid co-channel interference.


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Dynamic channel assignment reduce the likelihood ofblocking. All available channels in a market are accessible to all of

the cells.

Dynamic channel assignment strategies require MSC tocollect real time data on channel occupancy, trafficdistribution, and Radio Signal Strength Indications(RSSI) of all channels on a continuous basis. This increases the storage and computational load on the

system. But provides the advantage of

Increased channel utilization.

Decreased probability of a blocked call.

Handoff Strategies

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When a mobile moves into a different cell while aconversation is in progress.

MSC automatically transfers the call to a new channel belongingto the new BS.

This handoff operation not only involves identifying a newBS. It requires that the voice and control signals be allocated to

channels associated with the new BS.

Many handoff prioritize handoff requests over call initiationrequests when allocating unused channels in a cell site.

Handoffs must be performed successfully and asinfrequently as possible and be imperceptible to the users.

System designer must specify an optimum signal level atwhich to initiate a handoff.

Handoff Strategies (2)

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Once a particular signal level is specified as the minimumusable signal for acceptable voice quality at the BS receiver Normally taken as between -90 dBm and -100 dBm.

A slightly stronger signal level is used as a threshold at which a handoff ismade.

The threshold is given by

=Pr handoff - Pr minimum usable

If is too large Unnecessary handoffs which burden the MSC may occur.

If is too small There may be insufficient time to complete a handoff before a call is lost

due to weak signal conditions.

Therefore is chosen carefully to meet these conflictingrequirements.


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The figure demonstrates two casesof handoff situation.

In case (a) where a handoff is notmade The signal drops below the minimum

acceptable level to keep the channelactive.

This dropped call event can happen

when there is an excessive delay bythe MSC in assigning a handoff, or

When the threshold is set too smallfor the handoff time in the system.

Excessive delays may occur During high traffic conditions due to

computational loading at the MSC, or Due to the fact to that no channels are

available on any of the nearby BSs.

Thus forcing the MSC to wait until achannel in a nearby cell becomes free.

Fig. 2.9 Illustration of handoff scenario at

cell boundary

Illustration of handoff scenario at cell boundary

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In deciding when to handoff:

It is important to ensure that the drop in the measuredsignal level is not due to momentary fading and that themobile is actually moving away from the serving BS.

In order to ensure this

BS monitors the signal level for a certain period of timebefore a hand-off is initiated.

This running average measurement of signal strengthshould be optimized.

Unnecessary handoffs are avoided

Ensuring that necessary handoffs are completed before acall is terminated due to poor signal level.


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The length of time needed to decide if a handoff is necessarydepends on the speed at which the vehicle is moving.

If the slope of the short-term average received signal level in agiven time interval is steep.

The handoff should be made quickly.

Information about the vehicle speed

which can be useful in handoff decisions. Can also be computed from the statistics of the received short-term

fading signal at the BS.

The time over which a call may be maintained within a cell withouthandoff is called the dwell time.

The dwell time of a particular user is governed by a number offactors which include:

Propagation

Interference

Distance between the subscriber and the BS.


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In 1G analog cellular systems

Signal strength measurements are made by the base stations.

Supervised by the MSC.

Each BS constantly monitors the signal strengths of all of its reversevoice channels.

To determine the relative location of each mobile user with respect tothe BS tower.

In addition to measuring the RSSI of calls in progress within the cell A spare receiver in each BS called the locator receiver is used to

determine signal strengths of mobile users which are in neighbouringcells.

The locator receiver is controlled by the MSC and is used to monitor

the signal strength of users in neighbouring cells which appear tobe in need of handoff and reports all RSSI values to the MSC.

Based on the locator receiver signal strength information from eachBS The MSC decides if a handoff is necessary or not.


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In 2G systems that use digital TDMA technology. Handoff decisions are mobile assisted.

In mobile assisted handoff (MAHO) Every mobile station measures the received power from surrounding

BSs and continually reports the results of these measurements to theserving BS.

A handoff is initiated when the power received from the BS of aneighbouring cell begins to exceed the power received from thecurrent BS by certain level or for a certain period of time.

The MAHO method enables the call to be handed over between BSs

at a much faster rate than in 1G analog systems since the handoffmeasurements are made by each mobile. MSC no longer constantly monitors signal strengths.

MAHO is particularly suited for microcellular environments where

handoffs are more frequent.


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During the course of a call If a mobile moves from onecellular system to a different cellular system controlled by a

different MSC. An intersystem handoff becomes necessary.

An MSC engages in an intersystem handoff when A mobile signal becomes weak in a given cell and

The MSC cannot find another cell within its system to which itcan transfer the call in progress.

There are many issues that must be addressed whenimplementing an intersystem handoff. A local call may become a long-distance call as the mobile

moves out of its home system and becomes roamer in aneighbouring system.

Also compatibility between the two MSCs must be determinedbefore implementing an intersystem handoff.

Different systems have different policies and methods for

managing handoff requests.


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g ( )

Some systems handle handoff requests in the same

way they handle originating calls In such systems the probability that a handoff request will

not be served by a new BS is equal to the blockingprobability of incoming calls.

From the users point of view: Having a call abruptly terminated while in the middle of a

conversation is more annoying than being blockedoccasionally on a new call attempt.

To improve QoS as perceived by the users various methods have been devised to prioritize handoff

requests over call initiation requests when allocating voicechannels.

Prioritizing Handoffs

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One method for giving priority to handoffs is called theguard channel concept. A fraction of the total available channels in a cell is

reserved exclusively for handoff requests from ongoingcalls.

Disadvantage

Reducing the total carried traffic As fewer channels are allocated to originating calls.

Guard channels offer efficient spectrum utilizationwhen Dynamic Channel Assignment (DCA) strategiesare used. Because DCA minimize the number of required guard

channels by efficient demand based allocation.

Prioritizing Handoffs (2)

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Queuing of handoff requests is another method to decreasethe probability of forced termination of a call due to lack ofavailable channels.

There is trade-off between the decrease in probability offorced termination and total carried traffic.

Queuing of handoffs is possible

due to the fact that there is a finite time interval between thetime the received signal level drops below the handoff thresholdand the time the call is terminated due to insufficient signal level.

The delay time and size of the queue is determined from thetraffic pattern of the particular service area.

It should be noted that queuing does not guarantee a zeroprobability of forced termination. Large delays will cause the received signal level to drop below the

minimum required level to maintain communication and hencelead to forced termination.

Practical handoff considerations

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In practical cellular systems several problems arise whenattempting to design for a wide range of mobile velocities.

High speed vehicles pass through the coverage region of acell within a matter of seconds. Whereas pedestrian users may never need a handoff during a

call.

Particularly with the addition of microcells to providecapacity. MSC can quickly become burdened if high speed users are

constantly being passed between very small cells.

Several schemes have been devised to handle thesimultaneous traffic of high speed and low speed userswhile minimizing the handoff intervention from MSC.

Practical handoff considerations (2)

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Another practical limitation is the ability to obtain new cellsites.

Zoning laws, ordinances, and other nontechnical barriersoften make it more attractive for a cellular provider toinstall additional channels and BSs at the same physicallocation of an existing cell rather than find new sitelocations.

By using different antenna heights often on the samebuilding or tower and different power levels.

It is possible to provide large and small cells which are co-located at a single location.

This technique is called the umbrella cell approach and isused to provide large area coverage to high speed userswhile providing small area coverage to users travelling atlow speeds.

Practical handoff Figure 2.10 illustrates an umbrella cellwhich is co-located with some smaller

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considerations (3)which is co located with some smallermicrocells.

The umbrella cell approach ensuresthat the number of handoffs isminimized for high speed users andprovides additional microcell channelsfor pedestrian users.

The speed of each user may beestimated by the BS or MSC byevaluating how rapidly the short-termaverage signal strength on the RVCchanges over time or moresophisticated algorithm may be used

to evaluated and partition users.

If a high velocity is rapidly decreasing

BS may decide to hand the user intothe co-located microcell without MSCintervention.

Fig. 2.10 The umbrella cell approach


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Another practical handoff problem in microcell systems is known ascell dragging.

Cell dragging results from pedestrian users that provide a verystrong signal to the BS.

Such a situation occurs in an urban environment when there is aline of sight (LOS) radio path between the subscriber and the BS.

As the user travels away from the BS at a very slow speed. The average signal strength does not decay rapidly.

Even when the user has travelled well beyond the designed rangeof the cell. The received signal at the BS may be above the handoff threshold.

Thus handoff may not be made.

Since the user has meanwhile travelled deep within a neighbouringcell. This creates potential interference and traffic management problem.

Handoff thresholds and radio coverage parameters must be adjusted

carefully.


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In 1G analog cellular systems

The typical time to make a handoff once the signal isdeemed to be below the handoff threshold is about10 seconds.

This requires that the value for be on the order of 6dB to 12 dB.

In new digital cellular systems such as GSM

The mobile assists with the handoff procedure bydetermining the best handoff candidates

The handoff once the decision is made typicallyrequires only 1 or 2 seconds.

Consequently is usually between 0 dB and 6 dB.


Th IS 95 C d Di i i M lti l A (CDMA) t l

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The IS-95 Code Division Multiple Access (CDMA) spectralspectrum cellular system provides a unique handoffcapability that can not be provided with other wirelesssystems.

Channelized wireless systems that assign different radiochannels during a handoff called a hard handoff.

Spread spectrum mobiles share the same channel inevery cell. Thus the term handoff does not mean a physical change in the

assigned channel.

Rather that a different BS handles the radio communication task.

By simultaneously evaluating the received signals from a singlesubscriber at several neighbouring BSs.

MSC may actually decide which version of the users signal is best atany moment in time.

The ability to select between the instantaneous receivedsignals from a variety of BSs is called Soft Handoff.

Interference and system capacity Interference is the major limiting factor in the performance of

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Interference is the major limiting factor in the performance ofcellular radio systems.

Sources of interference include: Another mobile in the same cell.

A call in progress in a neighbouring cell.

Other BSs operating in the same frequency band, or

Any noncellular system which inadvertently leaks energy into thecellular frequency band.

Interference on voice channels causes crosstalk The subscriber hears interference in the background due to an

undesired transmission.

On control channels

Interference leads to missed and blocked calls due to errors in thedigital signalling.

Interference is more severe in urban areas due to

the greater FR noise floor and

large number of BSs and mobiles.

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Co-channel interference and system capacity

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Frequency reuse implies that in a given coverage area thereare several cells that use the same set of frequencies.

These cells are called co-channel cells. The interference between signals from these cells is called co-

channel interference.

Unlike thermal noise which can be overcome by increasing

the signal-to-noise ratio (SNR).

Co-channel interference cannot be combated by simplyincreasing the carrier power of a transmitter. This is because an increase in carrier transmit power increases

the interference to neighbouring co-channel cells.

To reduce co-channel interference Co-channel cells must be physically separated by a minimum

distance to provide sufficient isolation due to propagation.

Co-channel interference and system capacity (2)

When the size of each cell is approximately the

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When the size of each cell is approximately thesame and the BSs transmit the same power.

The co-channel interference ratio is independent ofthe transmitted power and becomes a function of

The radius of the cell (R ), and

The distance between centers of the nearset co-channel

cells (D ).

By increasing the ratio of (D/R) The spatial separation between co-channel cells

relative to the coverage distance of a cell is increased.

Thus interference is reduced from improved isolationof RF energy from the co-channel cell.

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Six effective interfering cells in tier 1 of cell 1

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Relationship between Q and N

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NR

DQ 3

Co-channel interference and system capacity (4) Let iobe the number of co-channel interfering cells.

Th i l i f i (S/I SIR) f bil i hi h

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The signal-to-interference ratio (S/I or SIR) for a mobile receiver whichmonitors a forward channel can be expressed as

where S is the desired signal power from the desired BS, and

Ii is the interference power caused by the ith interfering co-channel cell BS. Propagation measurements in a mobile radio channel show that the

average received signal strength at any point decays as a power law of thedistance of separation between a transmitter and receiver.

The average received power Prat a distance d from the transmittingantenna is approximated by

WhereP0 is the power received at a close-in reference point in the far region of theantenna at a small distance d0 from the transmitting antenna, and n is the path loss

exponent.

(2.5)0

1

i

i

iI

S

I

S

(2.6)or,0

0

n

rd

d

PP

(2.7))log(10-dBm)(dBm0

0

d

dnPPr


N id th f d li k h th d i d i l i th

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Now consider the forward link where the desired signal is the

serving BS and where the interference is due to co-channel BSs.

IfDi is the distance of the i th interferer from the mobile, the

received power at a given mobile due to the i th interfering cell

will be proportional to (Di)-n .

The path loss exponent typically ranges between 2 and 4 in

urban cellular systems.

When the transmit power of each BS is equal and the path loss

exponent is the same throughout the coverage area.

S/Ifor a mobile can be approximated as

(2.8)

0

1

i

i

n

i

n

D

R

I

S


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Considering only the first layer of interfering cells.

If all the interfering BSs are equidistant from the desiredBS and if this distance is equal to the distance D between

cell centers.

Then equation (2.8) simplifies to

Eq. (2.9) relates S/Ito the cluster size Nwhich in turn

determines the overall capacity of the system from eq.

(2.2).

It should be noted that eq. (2.9) is based on the hexagonal

cell geometry where all the interfering cells are equidistant

from the BS receiver.

(2.9)300

nn

i

N

i

RD

I

S

Co-channel interference and system capacity (7) From figure 2.11

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It can be seen for a 7-cell cluster with the

mobile unit is at the cell boundary.

The mobile is a distance D-R from the twonearest co-channel interfering cells and,

approximately D+R/2, D, D-R/2, and

D+R from the other interfering cells in the

first tier.

Using eq. (2.9) and assuming n equals 4.

The SIR for the worst case can closely

approximated as

Eq. (2.10) can be rewritten in terms of the

co-channel reuse ratio Q as

(2.10)

2)(2)(2444

4

DRDRD

R

I

S

Fig. 2.11 Illustration of the firsttier of co-channel cells forN=7

(2.11)2)1(2)1(2

1444

QQQI

S

Illustration of the first tier of co-channel cells forN=7

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Example 2 2

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Example 2.2

If SIR of 15 dB is required for satisfactory forward channel performance of

a cellular system, what is the frequency reuse factor and cluster sizethat should be used for maximum capacity if the path loss exponent is

(a) n=4, (b) n=3?

Assume that there are 6 co-channels cells in the first tier, and all of them

are at same distance from the mobile. Use suitable approximations.

Solution

(a) n = 4

First, let us consider a 7-cell reuse pattern.

Using eq. (2.4), the co-channel reuse ratio D/R = 4.583.Using eq. (2.9), the SIR is given by

S/I= (1/6) (4.583)4 = 75.3 = 18.66 dB.

Since this is greater than the minimum required S/I, N=7 can be used.


(b) n = 3

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(b) n = 3

First let us consider a 7-cell reuse pattern.

Using equation (2.9), the SIR is given by:

S/I = (1/6) (4.583)3 = 16.04 = 12.05 dB.

Since this is less than the minimum required S/I, larger Nshould be used.

Using eq. (2.3), the next possible value ofNis 12, (i = j = 2).

The corresponding co-channel ratio is given by eq. (2.4) as

D/R = 6.0

Using eq. (2.3) the SIR is given by

S/I= (1/6) (6)3 = 36 = 15.56 dB.

Since this is greater than minimum required S/I, N= 12 canbe used.

Adjacent Channel Interference

Interference resulting from signals which are adjacent in

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Interference resulting from signals which are adjacent infrequency to the desired signal is called adjacent channel

interference. Adjacent channel interference results from imperfect receiverfilters which allow nearby frequencies to leak into the passband.

The problem can be particularly serious if An adjacent channel user is transmitting in very close range to a

subscribers receiver. While the receiver attempts to receive a BS on the desired

channel.

This is referred as the near-far effect.

Alternatively the near far effect occurs when a mobileclose to a BS transmits on a channel close to one beingused by a weak mobile. The BS may have difficulty in discriminating the desired

mobile user from the bleedover caused by the closeadjacent channel mobile.

Adjacent Channel Interference (2) Adjacent channel interference can be minimized through careful

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Adjacent channel interference can be minimized through carefulfiltering and channel assignments.

Since each cell is given only a fraction of the available channels. A cell need not be assigned channels which are all adjacent in

frequency.

By keeping the frequency separation between each channel in agiven cell as large as possible

The adjacent channel interference may be reduced considerably.

Thus instead of assigning channels which form a contiguous band offrequencies within a particular cell Channels are allocated such that the frequency separation between

channels in a given cell is maximized.

By sequentially assigning successive channels in the frequency bandto different cells Many channels allocation schemes are able to separate adjacent

channels in a cell by as many as Nchannels bandwidths.

Where Nis the cluster size.

Adjacent Channel Interference (3)

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If the frequency reuse factor is small

The separation between adjacent channels may not besufficient to keep the adjacent channel interference

level within tolerable limits.

If a mobile is 20 times as close to the base station as

another mobile and energy spill out of its passband.

SIR for the weak mobile is approximately

For a path loss exponent n = 4, this is equal to -52 dB.

(2.12))20(n

I

S

Adjacent Channel Interference (4)Example

Thi l ill t t h h l di id d i t b t

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This example illustrates how channels are divided into subsetsand allocated to different cells so that adjacent channelinterference is minimized.

The US AMPS system initially operated with 666 duplexchannels.

In 1989 the FCC allocated an additional 10 MHz of spectrum forcellular services.

This allowed 166 new channels to be added to the AMPSsystem.

There are now 832 channels used in AMPS.

The forward channel (870.030 MHz) along with thecorresponding reverse channel (825.030 MHz) in numbered as

channel 1.

Similarly the forward channel 889.98 MHz along with thereverse channel 844.98 MHz is numbered as 666.

The extended band has channels numbered as 667 through

799, and 990 through 1023.

Example (cont.)

I d t titi

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In order to encourage competition

FCC licensed the channels to two competingoperators in every service area.

Each operator received half of the total channels.

The channels used by the two operators aredistinguished as block A and block B channels.

Block B is operated by companies which havetraditionally provided telephone services called

(wireline operators), and Block A is operated by companies that have not

traditionally provided telephone services called(nonwireline operators).

Example (cont.)

O t f th 416 h l d b h t

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Out of the 416 channels used by each operator 395 are voice channels

The remaining 21 are control channels.

In block A Channels 1 through 312 (voice channels). (312 CHs)

Channels 313 through 333 (control channels). (21 CHs)

Extended block A voice channels

Channels 667 through 716 (voice channels). (50 CHs)


In block B Channels 355 through 666 (voice channels). (312 CHs)

Channels 334 through 354 (control channels). (21 CHs)

Extended block B voice channels


Example (cont.)

E h f h 395 i h l di id d i

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Each of the 395 voice channels are divided into21 subsets.

Each containing about 19 channels.

In each subset

The closet adjacent channel is 21 channels away.

In a 7-cell reuse system

Each cell uses 3 subsets of channels.

The 3 subsets are assigned such that every channel inthe cell is assured of being separated from every otherchannel by at least 7 channels spacing.

AMPS Frequency Allocation

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AMPS Channel Allocation for A and B Side Carriers

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Power Control for Reducing Interference

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In practical cellular radio and PCS the power levelstransmitted by every subscriber unit are under constant

control by the serving base stations.

This is done to ensure that each mobile transmits thesmallest power necessary to maintain a good quality link onthe reverse channel.

Power control not only helps prolong battery life for thesubscriber unit. It also dramatically reduces the reverse channel S/I in the

system.

power control is especially important for emerging CDMAspread spectrum systems that allow every user in every cellto share the same radio channel.

Trunking and Grade of Service

Cellular radio systems rely on trunking to

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y y gaccommodate a large number of users in a limitedradio spectrum.

The concept of trunking allows a large number of usersto share the relatively small number of channels in acell

By providing access to each user on demand from a pool ofavailable channels.

In a trunked radio system each user is allocated a channelon a per call basis.

Upon termination of the call the previously occupied

channel is immediately returned to the pool of availablechannels.

Trunking exploits the statistical behavior of users so that afixed number of channels or circuits may accommodate alarge, random user community.

Trunking and Grade of Service (2)

The telephone company uses trunking theory to determine

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The telephone company uses trunking theory to determinethe number of telephone circuits that need to be allocatedfor office buildings with hundreds of telephones. The same principle is used in designing cellular radio systems.

There is a trade-off between the number of availabletelephone circuits and the likelihood of a particular userfinding that no circuits are available during the peak calling

time. As the number of phone lines decreases, it becomes more likely

that all circuits will be busy for a particular user.

In a trunked mobile radio system when a particular userrequests service and all of the radio channels are already inuse, the user is blocked, or

denied access to the system.

In some systems a queue may be used to hold the requestingusers until a channel becomes available.


The fundamentals of trunking theory were developed

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The fundamentals of trunking theory were developedby Erlang a Danish mathematician,

In the late 19th century he embarked on the study of howa large population could be accommodated by a limitednumber of servers.

Today, the measure of traffic intensity bears his name.

One Erlang represents the amount of traffic intensitycarried by a channel that is completely occupied.

one call-hour per hour or one call-minute per minute.

For example a radio channel that is occupied for thirty

minutes during an hour carries 0.5 Erlangs of traffic.

The grade of service (GOS) is a measure of the ability ofa user to access a trunked system during the busiesthour.


The busy hour is based upon customer demand

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The busy hour is based upon customer demandat the busiest hour during a week, month, or

year. The busy hours for cellular radio systems typically

occur during rush hours, between 4 p.m. and 6 p.m.on a Thursday or Friday evening.

It is the wireless designers job to Estimate the maximum required capacity, and

Allocate the proper number of channels in order tomeet the GOS.

GOS is typically given as the likelihood that a callis blocked, or the likelihood of a call experiencinga delay greater than a certain queuing time.

Definitions of Common Terms Used in Trunking Theory

Set-up Time: The time required to allocate a trunked radio channel toa requesting user

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a requesting user.

Blocked Call: Call which cannot be completed at time of request, due

to congestion. Also referred to as a lost call.

Holding Time: Average duration of a typical call. Denoted by H (inseconds).

Traffic Intensity: Measure of channel time utilization, which is the

average channel occupancy measured in Erlangs. This is adimensionless quantity and may be used to measure the timeutilization of single or multiple channels. Denoted by A.

Load: Traffic intensity across the entire trunked radio system,measured in Erlangs.

Grade of Service (GOS): A measure of congestion which is specified asthe probability of a call being blocked (for Erlang B), or the probabilityof a call being delayed beyond a certain amount of time (for Erlang C).

Request Rate: The average number of call requests per unit time.Denoted by seconds1.


The traffic intensity offered

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