Data and Computer Data and Computer CommunicationsCommunications

Eighth EditionEighth Editionby William Stallingsby William Stallings

Lecture slides by Lawrie BrownLecture slides by Lawrie Brown

Chapter 14 – Chapter 14 – Cellular Wireless NetworksCellular Wireless Networks

Cellular Wireless NetworksCellular Wireless Networks

After the fire of 1805, Judge Woodward was the central After the fire of 1805, Judge Woodward was the central figure involved in reestablishing the town. Influenced figure involved in reestablishing the town. Influenced by Major Pierre L’Enfant’s plans for Washington, DC, by Major Pierre L’Enfant’s plans for Washington, DC, Judge Woodward envisioned a modern series of Judge Woodward envisioned a modern series of hexagons with major diagonal avenues centered on hexagons with major diagonal avenues centered on circular parks, or circuses, in the center of the circular parks, or circuses, in the center of the hexagons. Frederick Law Olmstead said, "nearly all of hexagons. Frederick Law Olmstead said, "nearly all of the most serious mistakes of Detroit's past have arisen the most serious mistakes of Detroit's past have arisen from a disregard of the spirit of Woodward's plan."from a disregard of the spirit of Woodward's plan."—Endangered Detroit, Friends of the Book-Cadillac Hotel

Cellular Wireless NetworksCellular Wireless Networks key technology for mobiles, wireless nets etckey technology for mobiles, wireless nets etc developed to increase mobile phone capacitydeveloped to increase mobile phone capacity based on multiple low power transmittersbased on multiple low power transmitters area divided into cellsarea divided into cells

in a tiling pattern to provide full coveragein a tiling pattern to provide full coverage each with own antennaeach with own antenna each with own range of frequencieseach with own range of frequencies served by base stationserved by base station adjacent cells use different frequencies to avoid adjacent cells use different frequencies to avoid

crosstalkcrosstalk

Cellular GeometriesCellular Geometries

Frequency ReuseFrequency Reuse must manage reuse of frequenciesmust manage reuse of frequencies power of base transceiver controlledpower of base transceiver controlled

allow communications within cell on given frequencyallow communications within cell on given frequency limit escaping power to adjacent cellslimit escaping power to adjacent cells allow re-use of frequencies in nearby cellsallow re-use of frequencies in nearby cells typically 10 – 50 frequencies per celltypically 10 – 50 frequencies per cell example for Advanced Mobile Phone Service (AMPS)example for Advanced Mobile Phone Service (AMPS)

• N N cells all using same number of frequenciescells all using same number of frequencies• K K total number of frequencies used in systemstotal number of frequencies used in systems• each cell has each cell has K/NK/N frequencies frequencies• KK=395, =395, NN=7 giving 57 frequencies per cell on average=7 giving 57 frequencies per cell on average

FrequencyFrequencyReuseReuse

PatternsPatterns

Increasing CapacityIncreasing Capacity add new channelsadd new channels

not all channels used to start withnot all channels used to start with frequency borrowingfrequency borrowing

taken from adjacent cells by congested cellstaken from adjacent cells by congested cells or assign frequencies dynamicallyor assign frequencies dynamically

cell splittingcell splitting non-uniform topography and traffic distribution non-uniform topography and traffic distribution use smaller cells in high use areasuse smaller cells in high use areas

Cell SplittingCell Splitting

Increasing CapacityIncreasing Capacity cell sectoringcell sectoring

cell divided into wedge shaped sectors (3–6 per cell)cell divided into wedge shaped sectors (3–6 per cell) each with own channel seteach with own channel set directional antennasdirectional antennas

microcellsmicrocells move antennas from tops of hills and large buildings move antennas from tops of hills and large buildings

to tops of small buildings and sides of large buildingsto tops of small buildings and sides of large buildings use reduced power to cover a much smaller areause reduced power to cover a much smaller area good for city streets, roads, inside large buildingsgood for city streets, roads, inside large buildings

Frequency Reuse ExampleFrequency Reuse Example

Overview of Cellular SystemOverview of Cellular System

Cellular System ChannelsCellular System Channels system is fully automatedsystem is fully automated see two types of channels between mobile see two types of channels between mobile

and base station (BS)and base station (BS) control channelscontrol channels

set up and maintain callsset up and maintain calls establish relationship between mobile unit and establish relationship between mobile unit and

nearest BSnearest BS traffic channelstraffic channels

carry voice and datacarry voice and data

Call Call StagesStages

Other FunctionsOther Functions call blockingcall blocking

if all traffic channels busyif all traffic channels busy call terminationcall termination

when user hangs upwhen user hangs up call dropcall drop

when BS cannot maintain required signal strengthwhen BS cannot maintain required signal strength calls to/from fixed and remote mobile subscribercalls to/from fixed and remote mobile subscriber

MTSO connects mobile user and fixed line via PSTNMTSO connects mobile user and fixed line via PSTN MTSO connects to remote MTSO via PSTN or MTSO connects to remote MTSO via PSTN or

dedicated linesdedicated lines

Mobile Radio Mobile Radio Propagation EffectsPropagation Effects

signal strengthsignal strength strength of signal between BS and mobile unit strong strength of signal between BS and mobile unit strong

enough to maintain signal quality at the receiverenough to maintain signal quality at the receiver not too strong to create cochannel interference not too strong to create cochannel interference must handle variations in noisemust handle variations in noise

fadingfading time variation of received signaltime variation of received signal caused by changes in transmission path(s)caused by changes in transmission path(s) even if signal strength in effective range, signal even if signal strength in effective range, signal

propagation effects may disrupt the signalpropagation effects may disrupt the signal

Design FactorsDesign Factors propagation effectspropagation effects max transmit power level at BS and mobile unitsmax transmit power level at BS and mobile units typical height of mobile unit antennatypical height of mobile unit antenna available height of the BS antennaavailable height of the BS antenna these factors determine size of individual cellthese factors determine size of individual cell use model based on empirical datause model based on empirical data eg. model by Okumura et al & refined by Hataeg. model by Okumura et al & refined by Hata

detailed analysis of tokyo areadetailed analysis of tokyo area produced path loss info for an urban environmentproduced path loss info for an urban environment Hata's model is an empirical formulationHata's model is an empirical formulation

Multipath PropagationMultipath Propagation

Effects of Multipath Effects of Multipath PropagationPropagation

Types of FadingTypes of Fading fast fadingfast fading

rapid changes in strength over half wavelength distances rapid changes in strength over half wavelength distances • eg. 900MHz wavelength is 0.33m see 20-30dBeg. 900MHz wavelength is 0.33m see 20-30dB

slow fadingslow fading slower changes due to user passing different height slower changes due to user passing different height

buildings, gaps in buildings etc.buildings, gaps in buildings etc. over longer distances than fast fadingover longer distances than fast fading

flat fadingflat fading affects all frequencies in same proportionaffects all frequencies in same proportion

selective fadingselective fading different frequency components affected differentlydifferent frequency components affected differently

Error Compensation Error Compensation MechanismsMechanisms

forward error correctionforward error correction applicable in digital transmission applicationsapplicable in digital transmission applications typically, ratio of total bits to data bits is 2-3typically, ratio of total bits to data bits is 2-3 has a big overheadhas a big overhead

adaptive equalizationadaptive equalization applied to transmissions that carry analog or digital applied to transmissions that carry analog or digital

informationinformation used to combat intersymbol interferenceused to combat intersymbol interference gathering the dispersed symbol energy back together into gathering the dispersed symbol energy back together into

its original time intervalits original time interval techniques include so-called lumped analog circuits and techniques include so-called lumped analog circuits and

sophisticated digital signal processing algorithmssophisticated digital signal processing algorithms

Error Compensation Error Compensation MechanismsMechanisms

diversitydiversity based on fact that individual channels experience based on fact that individual channels experience

independent fading eventsindependent fading events use multiple logical channels between transmitter and use multiple logical channels between transmitter and

receiverreceiver send part of signal over each channelsend part of signal over each channel doesn’t eliminate errorsdoesn’t eliminate errors reduce error ratereduce error rate equalization, forward error correction then cope with equalization, forward error correction then cope with

reduced error ratereduced error rate space diversity involves physical transmission pathsspace diversity involves physical transmission paths more commonly refers to frequency or time diversitymore commonly refers to frequency or time diversity

First Generation AnalogFirst Generation Analog original cellular telephone networksoriginal cellular telephone networks analog traffic channelsanalog traffic channels early 1980s in North Americaearly 1980s in North America Advanced Mobile Phone Service (AMPS) Advanced Mobile Phone Service (AMPS) also common in South America, Australia, also common in South America, Australia,

and Chinaand China replaced by later generation systemsreplaced by later generation systems

AMPS Spectral Allocation In AMPS Spectral Allocation In North AmericaNorth America

two 25-MHz bands are allocated to AMPStwo 25-MHz bands are allocated to AMPS from BS to mobile unit (869–894 MHz)from BS to mobile unit (869–894 MHz) from mobile to base station (824–849 MHz)from mobile to base station (824–849 MHz)

bands is split in two to encourage competitionbands is split in two to encourage competition operator is allocated only 12.5 MHz in each direction operator is allocated only 12.5 MHz in each direction channels spaced 30 kHz apart (416 channels / operator)channels spaced 30 kHz apart (416 channels / operator) control channels are 10 kbps data channels control channels are 10 kbps data channels voice channels carry analog using frequency modulationvoice channels carry analog using frequency modulation control info also sent on voice channels in bursts as datacontrol info also sent on voice channels in bursts as data number of channels inadequate for most major marketsnumber of channels inadequate for most major markets for AMPS, frequency reuse is exploitedfor AMPS, frequency reuse is exploited

OperationOperation AMPS-capable phone has numeric assignment AMPS-capable phone has numeric assignment

module (NAM) in read-only memorymodule (NAM) in read-only memory NAM contains number of phoneNAM contains number of phone serial number of phoneserial number of phone when phone turned on, transmits serial number and when phone turned on, transmits serial number and

phone number to MTSO phone number to MTSO MTSO has database of mobile units reported stolenMTSO has database of mobile units reported stolen MTSO uses phone number for billingMTSO uses phone number for billing if phone is used in remote city, service is still billed to if phone is used in remote city, service is still billed to

user's local service provideruser's local service provider

AMPS Call SequenceAMPS Call Sequence1.1. subscriber initiates call keying in number subscriber initiates call keying in number 2.2. MTSO validates telephone number and checks user MTSO validates telephone number and checks user

authorized to place callauthorized to place call3.3. MTSO issues message to user's phone indicating MTSO issues message to user's phone indicating

traffic channels to usetraffic channels to use4.4. MTSO sends ringing signal to called partyMTSO sends ringing signal to called party5.5. when called party answers, MTSO establishes when called party answers, MTSO establishes

circuit and initiates billing informationcircuit and initiates billing information6.6. when one party hangs up MTSO releases circuit, when one party hangs up MTSO releases circuit,

frees radio channels, and completes billing frees radio channels, and completes billing informationinformation

AMPS Control ChannelsAMPS Control Channels 21 full-duplex 30-kHz control channels21 full-duplex 30-kHz control channels

transmit digital data using FSKtransmit digital data using FSK data transmitted in framesdata transmitted in frames

control information can be transmitted over voice control information can be transmitted over voice channel during conversationchannel during conversation Mmobile unit or the base station inserts burst of data Mmobile unit or the base station inserts burst of data

• turn off voice FM transmission for about 100 msturn off voice FM transmission for about 100 ms• replacing it with an FSK-encoded messagereplacing it with an FSK-encoded message

used to exchange urgent messagesused to exchange urgent messages• change power levelchange power level• handoffhandoff

Second Generation CDMASecond Generation CDMA provide higher quality signals, higher data rates, provide higher quality signals, higher data rates,

support digital services, with overall greater support digital services, with overall greater capacitycapacity

key differences includekey differences include digital traffic channelsdigital traffic channels encryptionencryption error detection and correctionerror detection and correction channel accesschannel access

• time division multiple access (TDMA)time division multiple access (TDMA)• code division multiple access (CDMA)code division multiple access (CDMA)

Code Division Multiple Code Division Multiple Access (CDMA)Access (CDMA)

have a number of 2nd gen systemshave a number of 2nd gen systems for example IS-95 using CDMAfor example IS-95 using CDMA

each cell allocated frequency bandwidtheach cell allocated frequency bandwidth is split in twois split in two

half for reverse, half for forwardhalf for reverse, half for forward uses direct-sequence spread spectrum (DSSS)uses direct-sequence spread spectrum (DSSS)

Code Division Multiple Code Division Multiple Access (CDMA) AdvantagesAccess (CDMA) Advantages

frequency diversityfrequency diversity noise bursts & fading have less effectnoise bursts & fading have less effect

multipath resistancemultipath resistance chipping codes have low cross & auto correlationchipping codes have low cross & auto correlation

privacyprivacy inherent in use of spread-spectruminherent in use of spread-spectrum

graceful degradationgraceful degradation more users means more noisemore users means more noise leads to slow signal degradation until unacceptableleads to slow signal degradation until unacceptable

Code Division Multiple AccessCode Division Multiple Access (CDMA) Disadvantages(CDMA) Disadvantages

self-jammingself-jamming some cross correlation between userssome cross correlation between users

near-far problemnear-far problem signals closer to receiver are received with signals closer to receiver are received with

less attenuation than signals farther awayless attenuation than signals farther away

RAKE ReceiverRAKE Receiver

IS-95IS-95 second generation CDMA schemesecond generation CDMA scheme primarily deployed in North Americaprimarily deployed in North America transmission structures different on transmission structures different on

forward and reverse linksforward and reverse links

IS-95 Channel StructureIS-95 Channel Structure

IS-95 Forward LinkIS-95 Forward Link four types of channelsfour types of channels

Pilot (channel 0)Pilot (channel 0)• allows mobile unit to acquire timing informationallows mobile unit to acquire timing information

Synchronization (channel 32)Synchronization (channel 32)• 1200-bps channel used by mobile station to obtain 1200-bps channel used by mobile station to obtain

identification information about the cellular systemidentification information about the cellular system Paging (channels 1 to 7)Paging (channels 1 to 7)

• Contain messages for one or more mobile stationsContain messages for one or more mobile stations Traffic (channels 8 to 31 and 33 to 63)Traffic (channels 8 to 31 and 33 to 63)

• 55 traffic channels55 traffic channels

all channels use same bandwidthall channels use same bandwidth

Forward Forward Link Link

ProcessingProcessing

Forward Link - ScramblingForward Link - Scrambling after interleaver, data scrambledafter interleaver, data scrambled privacy maskprivacy mask prevent sending of repetitive patternsprevent sending of repetitive patterns

reduces probability of users sending at peak power at reduces probability of users sending at peak power at same timesame time

scrambling done by long code scrambling done by long code pseudorandom number from 42-bit shift registerpseudorandom number from 42-bit shift register initialized with user's electronic serial numberinitialized with user's electronic serial number output at a rate of 1.2288 Mbpsoutput at a rate of 1.2288 Mbps

Forward Link - Power ControlForward Link - Power Control inserts power control info in traffic channelinserts power control info in traffic channel

to control the power output of antennato control the power output of antenna robs traffic channel of bits at rate of 800 bps robs traffic channel of bits at rate of 800 bps

by stealing code bitsby stealing code bits 800-bps channel carries information directing 800-bps channel carries information directing

mobile unit to change output levelmobile unit to change output level power control stream multiplexed to 19.2 kbpspower control stream multiplexed to 19.2 kbps

Forward Link - DSSSForward Link - DSSS spreads 19.2 kbps to 1.2288 Mbpsspreads 19.2 kbps to 1.2288 Mbps using one row of Walsh matrixusing one row of Walsh matrix

assigned to mobile station during call setupassigned to mobile station during call setup if 0 presented to XOR, 64 bits of assigned row sentif 0 presented to XOR, 64 bits of assigned row sent if 1 presented, bitwise XOR of row sentif 1 presented, bitwise XOR of row sent

final bit rate 1.2288 Mbpsfinal bit rate 1.2288 Mbps bit stream modulated onto carrier using QPSKbit stream modulated onto carrier using QPSK

data split into I and Q (in-phase and quadrature) data split into I and Q (in-phase and quadrature) channels channels

data in each channel XORed with unique short codedata in each channel XORed with unique short code

IS-95 Reverse LinkIS-95 Reverse Link up to 94 logical CDMA channelsup to 94 logical CDMA channels

each occupying same 1228-kHz bandwidtheach occupying same 1228-kHz bandwidth supports up to 32 access and 62 traffic channelssupports up to 32 access and 62 traffic channels

traffic channels are mobile uniquetraffic channels are mobile unique each station has unique long code mask based on each station has unique long code mask based on

serial numberserial number• 42-bit number, 242-bit number, 24242 – 1 different masks – 1 different masks• access channel used by mobile to initiate call, respond to access channel used by mobile to initiate call, respond to

paging channel message, and for location updatepaging channel message, and for location update

ReverseReverseLinkLink

Processing Processing

Reverse Link - DSSSReverse Link - DSSS long code unique to mobile XORed with output long code unique to mobile XORed with output

of randomizerof randomizer 1.2288-Mbps final data stream1.2288-Mbps final data stream modulated using orthogonal QPSK modulationmodulated using orthogonal QPSK modulation differs from forward channel in use of delay differs from forward channel in use of delay

element in modulator to produce orthogonalityelement in modulator to produce orthogonality forward channel, spreading codes orthogonalforward channel, spreading codes orthogonal reverse channel orthogonality of spreading codes not reverse channel orthogonality of spreading codes not

guaranteedguaranteed

Third Generation SystemsThird Generation Systems high-speed wireless communications to support high-speed wireless communications to support

multimedia, data, and video in addition to voicemultimedia, data, and video in addition to voice 3G capabilities:3G capabilities:

• voice quality comparable to PSTNvoice quality comparable to PSTN• 144 kbps available to users over large areas144 kbps available to users over large areas• 384 kbps available to pedestrians over small areas384 kbps available to pedestrians over small areas• support for 2.048 Mbps for office usesupport for 2.048 Mbps for office use• symmetrical and asymmetrical data ratessymmetrical and asymmetrical data rates• packet-switched and circuit-switched servicespacket-switched and circuit-switched services• adaptive interface to Internetadaptive interface to Internet• more efficient use of available spectrummore efficient use of available spectrum• support for variety of mobile equipmentsupport for variety of mobile equipment• allow introduction of new services and technologiesallow introduction of new services and technologies

Driving ForcesDriving Forces trend toward universal personal telecommunications trend toward universal personal telecommunications universal communications accessuniversal communications access GSM cellular telephony with subscriber identity module, GSM cellular telephony with subscriber identity module,

is step towards goalsis step towards goals personal communications services (PCSs) and personal personal communications services (PCSs) and personal

communication networks (PCNs) also form objectives for communication networks (PCNs) also form objectives for third-generation wirelessthird-generation wireless

technology is digital using time division multiple access technology is digital using time division multiple access or code-division multiple accessor code-division multiple access

PCS handsets low power, small and lightPCS handsets low power, small and light

IMT-2000 Terrestrial Radio IMT-2000 Terrestrial Radio Alternative InterfacesAlternative Interfaces

CDMA Design Considerations CDMA Design Considerations – Bandwidth and Chip Rate– Bandwidth and Chip Rate

dominant technology for 3G systems is CDMAdominant technology for 3G systems is CDMA 3 CDMA schemes, share some design issues 3 CDMA schemes, share some design issues 

bandwidth (limit channel to 5 MHz)bandwidth (limit channel to 5 MHz) 5 MHz reasonable upper limit on what can be 5 MHz reasonable upper limit on what can be

allocated for 3Gallocated for 3G 5 MHz is enough for data rates of 144 and 384 kHz5 MHz is enough for data rates of 144 and 384 kHz

chip ratechip rate given bandwidth, chip rate depends on desired data given bandwidth, chip rate depends on desired data

rate, need for error control, and bandwidth limitationsrate, need for error control, and bandwidth limitations chip rate of 3 Mbps or more reasonablechip rate of 3 Mbps or more reasonable

CDMA Design Considerations CDMA Design Considerations – Multirate– Multirate

provision of multiple fixed-data-rate channels to userprovision of multiple fixed-data-rate channels to user different data rates provided on different logical channelsdifferent data rates provided on different logical channels logical channel traffic can be switched independently logical channel traffic can be switched independently

through wireless fixed networks to different destinationsthrough wireless fixed networks to different destinations flexibly support multiple simultaneous applications flexibly support multiple simultaneous applications efficiently use available capacity by only providing the efficiently use available capacity by only providing the

capacity required for each servicecapacity required for each service use TDMA within single CDMA channeluse TDMA within single CDMA channel or use multiple CDMA codesor use multiple CDMA codes

CDMA MultirateCDMA MultirateTime and Code MultiplexingTime and Code Multiplexing

SummarySummary principles of wireless cellular networksprinciples of wireless cellular networks operation of wireless cellular networksoperation of wireless cellular networks first-generation analogfirst-generation analog second-generation CDMAsecond-generation CDMA third-generation systemsthird-generation systems