1、Cellular Wireless NetworksChapter 10Cellular Network OrganizationnUse multiple low-power transmitters(100 W or less)nAreas divided into cellsnEach served by its own antennanServed by base station consisting of transmitter,receiver,and control unitnBand of frequencies allocatednCells set up such that
2、 antennas of all neighbors are equidistant(hexagonal pattern)Frequency ReusenAdjacent cells assigned different frequencies to avoid interference or crosstalknObjective is to reuse frequency in nearby cellsn10 to 50 frequencies assigned to each cellnTransmission power controlled to limit power at tha
3、t frequency escaping to adjacent cellsnThe issue is to determine how many cells must intervene between two cells using the same frequencyApproaches to Cope with Increasing CapacitynAdding new channelsnFrequency borrowing frequencies are taken from adjacent cells by congested cellsnCell splitting cel
4、ls in areas of high usage can be split into smaller cellsnCell sectoring cells are divided into a number of wedge-shaped sectors,each with their own set of channelsnMicrocells antennas move to buildings,hills,and lamp postsCellular System OverviewCellular Systems TermsnBase Station(BS)includes an an
5、tenna,a controller,and a number of receiversnMobile telecommunications switching office(MTSO)connects calls between mobile unitsnTwo types of channels available between mobile unit and BSnControl channels used to exchange information having to do with setting up and maintaining callsnTraffic channel
6、s carry voice or data connection between usersSteps in an MTSO Controlled Call between Mobile UsersnMobile unit initializationnMobile-originated callnPagingnCall acceptednOngoing callnHandoffAdditional Functions in an MTSO Controlled CallnCall blockingnCall terminationnCall dropnCalls to/from fixed
7、and remote mobile subscriberMobile Radio Propagation EffectsnSignal strengthnMust be strong enough between base station and mobile unit to maintain signal quality at the receivernMust not be so strong as to create too much cochannel interference with channels in another cell using the same frequency
8、 bandnFadingnSignal propagation effects may disrupt the signal and cause errorsHandoff Performance MetricsnCell blocking probability probability of a new call being blockednCall dropping probability probability that a call is terminated due to a handoffnCall completion probability probability that a
9、n admitted call is not dropped before it terminatesnProbability of unsuccessful handoff probability that a handoff is executed while the reception conditions are inadequateHandoff Performance MetricsnHandoff blocking probability probability that a handoff cannot be successfully completednHandoff pro
10、bability probability that a handoff occurs before call terminationnRate of handoff number of handoffs per unit timenInterruption duration duration of time during a handoff in which a mobile is not connected to either base stationnHandoff delay distance the mobile moves from the point at which the ha
11、ndoff should occur to the point at which it does occurHandoff Strategies Used to Determine Instant of HandoffnRelative signal strengthnRelative signal strength with thresholdnRelative signal strength with hysteresisnRelative signal strength with hysteresis and thresholdnPrediction techniquesPower Co
12、ntrolnDesign issues making it desirable to include dynamic power control in a cellular systemnReceived power must be sufficiently above the background noise for effective communicationnDesirable to minimize power in the transmitted signal from the mobilenReduce cochannel interference,alleviate healt
13、h concerns,save battery powernIn SS systems using CDMA,its desirable to equalize the received power level from all mobile units at the BSTypes of Power ControlnOpen-loop power controlnDepends solely on mobile unitnNo feedback from BSnNot as accurate as closed-loop,but can react quicker to fluctuatio
14、ns in signal strength nClosed-loop power controlnAdjusts signal strength in reverse channel based on metric of performancenBS makes power adjustment decision and communicates to mobile on control channelTraffic EngineeringnIdeally,available channels would equal number of subscribers active at one ti
15、menIn practice,not feasible to have capacity handle all possible loadnFor N simultaneous user capacity and L subscribersnL N blocking systemBlocking System Performance QuestionsnProbability that call request is blocked?nWhat capacity is needed to achieve a certain upper bound on probability of block
16、ing?nWhat is the average delay?nWhat capacity is needed to achieve a certain average delay?Traffic IntensitynLoad presented to a system:n=mean rate of calls attempted per unit timenh=mean holding time per successful callnA=average number of calls arriving during average holding period,for normalized
17、 hAFactors that Determine the Nature of the Traffic ModelnManner in which blocked calls are handlednLost calls delayed(LCD)blocked calls put in a queue awaiting a free channelnBlocked calls rejected and droppednLost calls cleared(LCC)user waits before another attemptnLost calls held(LCH)user repeate
18、dly attempts callingnNumber of traffic sourcesnWhether number of users is assumed to be finite or infiniteFirst-Generation AnalognAdvanced Mobile Phone Service(AMPS)nIn North America,two 25-MHz bands allocated to AMPSnOne for transmission from base to mobile unitnOne for transmission from mobile uni
19、t to basenEach band split in two to encourage competitionnFrequency reuse exploitedAMPS OperationnSubscriber initiates call by keying in phone number and presses send keynMTSO verifies number and authorizes usernMTSO issues message to users cell phone indicating send and receive traffic channelsnMTS
20、O sends ringing signal to called partynParty answers;MTSO establishes circuit and initiates billing informationnEither party hangs up;MTSO releases circuit,frees channels,completes billingDifferences Between First and Second Generation SystemsnDigital traffic channels first-generation systems are al
21、most purely analog;second-generation systems are digitalnEncryption all second generation systems provide encryption to prevent eavesdroppingnError detection and correction second-generation digital traffic allows for detection and correction,giving clear voice receptionnChannel access second-genera
22、tion systems allow channels to be dynamically shared by a number of usersMobile Wireless TDMA Design ConsiderationsnNumber of logical channels(number of time slots in TDMA frame):8nMaximum cell radius(R):35 kmnFrequency:region around 900 MHznMaximum vehicle speed(Vm):250 km/hrnMaximum coding delay:a
23、pprox.20 msnMaximum delay spread(m):10 snBandwidth:Not to exceed 200 kHz(25 kHz per channel)Steps in Design of TDMA TimeslotGSM Network ArchitectureMobile StationnMobile station communicates across Um interface(air interface)with base station transceiver in same cell as mobile unitnMobile equipment(
24、ME)physical terminal,such as a telephone or PCSnME includes radio transceiver,digital signal processors and subscriber identity module(SIM)nGSM subscriber units are generic until SIM is insertednSIMs roam,not necessarily the subscriber devicesBase Station Subsystem(BSS)nBSS consists of base station
25、controller and one or more base transceiver stations(BTS)nEach BTS defines a single cellnIncludes radio antenna,radio transceiver and a link to a base station controller(BSC)nBSC reserves radio frequencies,manages handoff of mobile unit from one cell to another within BSS,and controls pagingNetwork
26、Subsystem(NS)nNS provides link between cellular network and public switched telecommunications networksnControls handoffs between cells in different BSSsnAuthenticates users and validates accountsnEnables worldwide roaming of mobile usersnCentral element of NS is the mobile switching center(MSC)Mobi
27、le Switching Center(MSC)DatabasesnHome location register(HLR)database stores information about each subscriber that belongs to itnVisitor location register(VLR)database maintains information about subscribers currently physically in the regionnAuthentication center database(AuC)used for authenticati
28、on activities,holds encryption keysnEquipment identity register database(EIR)keeps track of the type of equipment that exists at the mobile stationTDMA Format Time Slot FieldsnTrail bits allow synchronization of transmissions from mobile unitsnEncrypted bits encrypted datanStealing bit-indicates whe
29、ther block contains data or is stolennTraining sequence used to adapt parameters of receiver to the current path propagation characteristicsnStrongest signal selected in case of multipath propagationnGuard bits used to avoid overlapping with other burstsGSM Speech Signal ProcessingGSM Signaling Prot
30、ocol ArchitectureFunctions Provided by ProtocolsnProtocols above the link layer of the GSM signaling protocol architecture provide specific functions:nRadio resource managementnMobility managementnConnection managementnMobile application part(MAP)nBTS managementAdvantages of CDMA CellularnFrequency
31、diversity frequency-dependent transmission impairments have less effect on signalnMultipath resistance chipping codes used for CDMA exhibit low cross correlation and low autocorrelationnPrivacy privacy is inherent since spread spectrum is obtained by use of noise-like signalsnGraceful degradation sy
32、stem only gradually degrades as more users access the systemDrawbacks of CDMA CellularnSelf-jamming arriving transmissions from multiple users not aligned on chip boundaries unless users are perfectly synchronizednNear-far problem signals closer to the receiver are received with less attenuation tha
33、n signals farther awaynSoft handoff requires that the mobile acquires the new cell before it relinquishes the old;this is more complex than hard handoff used in FDMA and TDMA schemesMobile Wireless CDMA Design ConsiderationsnRAKE receiver when multiple versions of a signal arrive more than one chip
34、interval apart,RAKE receiver attempts to recover signals from multiple paths and combine themnThis method achieves better performance than simply recovering dominant signal and treating remaining signals as noisenSoft Handoff mobile station temporarily connected to more than one base station simulta
35、neouslyPrinciple of RAKE ReceiverTypes of Channels Supported by Forward LinknPilot(channel 0)-allows the mobile unit to acquire timing information,provides phase reference and provides means for signal strength comparisonnSynchronization(channel 32)-used by mobile station to obtain identification in
36、formation about cellular systemnPaging(channels 1 to 7)-contain messages for one or more mobile stationsnTraffic(channels 8 to 31 and 33 to 63)the forward channel supports 55 traffic channelsForward Traffic Channel Processing StepsnSpeech is encoded at a rate of 8550 bpsnAdditional bits added for er
37、ror detectionnData transmitted in 2-ms blocks with forward error correction provided by a convolutional encodernData interleaved in blocks to reduce effects of errorsnData bits are scrambled,serving as a privacy maskForward Traffic Channel Processing Steps(cont.)nPower control information inserted i
38、nto traffic channelnDS-SS function spreads the 19.2 kbps to a rate of 1.2288 Mbps using one row of 64 x 64 Walsh matrixnDigital bit stream modulated onto the carrier using QPSK modulation schemeITUs View of Third-Generation CapabilitiesnVoice quality comparable to the public switched telephone netwo
39、rkn144 kbps data rate available to users in high-speed motor vehicles over large areasn384 kbps available to pedestrians standing or moving slowly over small areasnSupport for 2.048 Mbps for office usenSymmetrical/asymmetrical data transmission ratesnSupport for both packet switched and circuit swit
40、ched data servicesITUs View of Third-Generation CapabilitiesnAn adaptive interface to the Internet to reflect efficiently the common asymmetry between inbound and outbound trafficnMore efficient use of the available spectrum in generalnSupport for a wide variety of mobile equipmentnFlexibility to al
41、low the introduction of new services and technologiesAlternative InterfacesCDMA Design ConsiderationsnBandwidth limit channel usage to 5 MHznChip rate depends on desired data rate,need for error control,and bandwidth limitations;3 Mcps or more is reasonablenMultirate advantage is that the system can flexibly support multiple simultaneous applications from a given user and can efficiently use available capacity by only providing the capacity required for each service
