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1、Cellular Wireless NetworksChapter 10Cellular Network Organizationn Use multiple low-power transmitters(100 W or less)n Areas divided into cellsn Each served by its own antennan Served by base station consisting of transmitter,receiver,and control unitn Band of frequencies allocatedn Cells set up suc
2、h that antennas of all neighbors are equidistant(hexagonal pattern)Frequency Reusen Adjacent cells assigned different frequencies to avoid interference or crosstalkn Objective is to reuse frequency in nearby cellsn 10 to 50 frequencies assigned to each celln Transmission power controlled to limit po
3、wer at that frequency escaping to adjacent cellsn The issue is to determine how many cells must intervene between two cells using the same frequencyApproaches to Cope with Increasing Capacityn Adding new channelsn Frequency borrowing frequencies are taken from adjacent cells by congested cellsn Cell
4、 splitting cells in areas of high usage can be split into smaller cellsn Cell sectoring cells are divided into a number of wedge-shaped sectors,each with their own set of channelsn Microcells antennas move to buildings,hills,and lamp postsCellular Systems Termsn Base Station(BS)includes an antenna,a
5、 controller,and a number of receiversn Mobile telecommunications switching office(MTSO)connects calls between mobile unitsn Two types of channels available between mobile unit and BSn Control channels used to exchange information having to do with setting up and maintaining callsn Traffic channels c
6、arry voice or data connection between usersSteps in an MTSO Controlled Call between Mobile Usersn Mobile unit initializationn Mobile-originated calln Pagingn Call acceptedn Ongoing calln HandoffAdditional Functions in an MTSO Controlled Calln Call blockingn Call terminationn Call dropn Calls to/from
7、 fixed and remote mobile subscriberHandoff Performance Metricsn Cell blocking probability probability of a new call being blockedn Call dropping probability probability that a call is terminated due to a handoffn Call completion probability probability that an admitted call is not dropped before it
8、terminatesn Probability of unsuccessful handoff probability that a handoff is executed while the reception conditions are inadequateHandoff Performance Metricsn Handoff blocking probability probability that a handoff cannot be successfully completedn Handoff probability probability that a handoff oc
9、curs before call terminationn Rate of handoff number of handoffs per unit timen Interruption duration duration of time during a handoff in which a mobile is not connected to either base stationn Handoff delay distance the mobile moves from the point at which the handoff should occur to the point at
10、which it does occurPower Controln Design issues making it desirable to include dynamic power control in a cellular systemn Received power must be sufficiently above the background noise for effective communicationn Desirable to minimize power in the transmitted signal from the mobilen Reduce cochann
11、el interference,alleviate health concerns,save battery powern In SS systems using CDMA,its desirable to equalize the received power level from all mobile units at the BSTraffic Engineeringn Ideally,available channels would equal number of subscribers active at one timen In practice,not feasible to h
12、ave capacity handle all possible loadn For N simultaneous user capacity and L subscribersn L N blocking systemBlocking System Performance Questionsn Probability that call request is blocked?n What capacity is needed to achieve a certain upper bound on probability of blocking?n What is the average de
13、lay?n What capacity is needed to achieve a certain average delay?Factors that Determine the Nature of the Traffic Modeln Manner in which blocked calls are handledn Lost calls delayed(LCD)blocked calls put in a queue awaiting a free channeln Blocked calls rejected and droppedn Lost calls cleared(LCC)
14、user waits before another attemptn Lost calls held(LCH)user repeatedly attempts callingn Number of traffic sourcesn Whether number of users is assumed to be finite or infiniteFirst-Generation Analogn Advanced Mobile Phone Service(AMPS)n In North America,two 25-MHz bands allocated to AMPSn One for tr
15、ansmission from base to mobile unitn One for transmission from mobile unit to basen Each band split in two to encourage competitionn Frequency reuse exploitedAMPS Operationn Subscriber initiates call by keying in phone number and presses send keyn MTSO verifies number and authorizes usern MTSO issue
16、s message to users cell phone indicating send and receive traffic channelsn MTSO sends ringing signal to called partyn Party answers;MTSO establishes circuit and initiates billing informationn Either party hangs up;MTSO releases circuit,frees channels,completes billingDifferences Between First and S
17、econd Generation Systemsn Digital traffic channels first-generation systems are almost purely analog;second-generation systems are digitaln Encryption all second generation systems provide encryption to prevent eavesdroppingn Error detection and correction second-generation digital traffic allows fo
18、r detection and correction,giving clear voice receptionn Channel access second-generation systems allow channels to be dynamically shared by a number of usersMobile Wireless TDMA Design Considerationsn Number of logical channels(number of time slots in TDMA frame):8n Maximum cell radius(R):35 kmn Fr
19、equency:region around 900 MHzn Maximum vehicle speed(Vm):250 km/hrn Maximum coding delay:approx.20 msn Maximum delay spread(m):10 sn Bandwidth:Not to exceed 200 kHz(25 kHz per channel)GSM Network ArchitectureMobile Stationn Mobile station communicates across Um interface(air interface)with base stat
20、ion transceiver in same cell as mobile unitn Mobile equipment(ME)physical terminal,such as a telephone or PCSn ME includes radio transceiver,digital signal processors and subscriber identity module(SIM)n GSM subscriber units are generic until SIM is insertedn SIMs roam,not necessarily the subscriber
21、 devicesBase Station Subsystem(BSS)n BSS consists of base station controller and one or more base transceiver stations(BTS)n Each BTS defines a single celln Includes radio antenna,radio transceiver and a link to a base station controller(BSC)n BSC reserves radio frequencies,manages handoff of mobile
22、 unit from one cell to another within BSS,and controls pagingMobile Switching Center(MSC)Databasesn Home location register(HLR)database stores information about each subscriber that belongs to itn Visitor location register(VLR)database maintains information about subscribers currently physically in
23、the regionn Authentication center database(AuC)used for authentication activities,holds encryption keysn Equipment identity register database(EIR)keeps track of the type of equipment that exists at the mobile stationTDMA Format Time Slot Fieldsn Trail bits allow synchronization of transmissions from
24、 mobile unitsn Encrypted bits encrypted datan Stealing bit-indicates whether block contains data or is stolenn Training sequence used to adapt parameters of receiver to the current path propagation characteristicsn Strongest signal selected in case of multipath propagationn Guard bits used to avoid
25、overlapping with other burstsGSM Signaling Protocol ArchitectureFunctions Provided by Protocolsn Protocols above the link layer of the GSM signaling protocol architecture provide specific functions:n Radio resource managementn Mobility managementn Connection managementn Mobile application part(MAP)n
26、 BTS managementAdvantages of CDMA Cellularn Frequency diversity frequency-dependent transmission impairments have less effect on signaln Multipath resistance chipping codes used for CDMA exhibit low cross correlation and low autocorrelationn Privacy privacy is inherent since spread spectrum is obtai
27、ned by use of noise-like signalsn Graceful degradation system only gradually degrades as more users access the systemDrawbacks of CDMA Cellularn Self-jamming arriving transmissions from multiple users not aligned on chip boundaries unless users are perfectly synchronizedn Near-far problem signals cl
28、oser to the receiver are received with less attenuation than signals farther awayn Soft 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 Considerationsn RAKE receiver
29、when multiple versions of a signal arrive more than one chip interval apart,RAKE receiver attempts to recover signals from multiple paths and combine themn This method achieves better performance than simply recovering dominant signal and treating remaining signals as noisen Soft Handoff mobile stat
30、ion temporarily connected to more than one base station simultaneouslyPrinciple of RAKE ReceiverTypes of Channels Supported by Forward Linkn Pilot(channel 0)-allows the mobile unit to acquire timing information,provides phase reference and provides means for signal strength comparisonn Synchronizati
31、on(channel 32)-used by mobile station to obtain identification information about cellular systemn Paging(channels 1 to 7)-contain messages for one or more mobile stationsn Traffic(channels 8 to 31 and 33 to 63)the forward channel supports 55 traffic channelsForward Traffic Channel Processing Stepsn
32、Speech is encoded at a rate of 8550 bpsn Additional bits added for error detectionn Data transmitted in 2-ms blocks with forward error correction provided by a convolutional encodern Data interleaved in blocks to reduce effects of errorsn Data bits are scrambled,serving as a privacy maskForward Traf
33、fic Channel Processing Steps(cont.)n Power control information inserted into traffic channeln DS-SS function spreads the 19.2 kbps to a rate of 1.2288 Mbps using one row of 64 x 64 Walsh matrixn Digital bit stream modulated onto the carrier using QPSK modulation schemeITUs View of Third-Generation C
34、apabilitiesn V oice quality comparable to the public switched telephone networkn 144 kbps data rate available to users in high-speed motor vehicles over large areasn 384 kbps available to pedestrians standing or moving slowly over small areasn Support for 2.048 Mbps for office usen Symmetrical/asymm
35、etrical data transmission ratesn Support for both packet switched and circuit switched data servicesITUs View of Third-Generation Capabilitiesn An adaptive interface to the Internet to reflect efficiently the common asymmetry between inbound and outbound trafficn More efficient use of the available
36、spectrum in generaln Support for a wide variety of mobile equipmentn Flexibility to allow the introduction of new services and technologiesAlternative InterfacesCDMA Design Considerationsn Bandwidth limit channel usage to 5 MHzn Chip rate depends on desired data rate,need for error control,and bandwidth limitations;3 Mcps or more is reasonablen Multirate 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