无线网络复习wireless comm final review

1、1Wireless Communications Cellular SystemWireless Signal Processing & Networks Lab (WSPN)BUPTXing Zhang 2A Cellular NetworkPublic SwitchedTelephoneNetwork(PSTN)MobileTelephoneSwitchingCenter(MTSC)Base Transceiver Station (BTS) Mobile UserCell 1Cell 2Cordless connectionWired connectionHLRVLRHLR = Home

2、 Location RegisterVLR = Visitor Location Register 3Principle of cellular networksUsing small cells to cover the service area;Reusing frequency among cellsSeparated by large enough distancesImproving the system capacity with limited radio spectrum4Frequency reuseAdjacent cells assigned different freq

3、uencies to avoid interference or crosstalkReuse frequency in nearby cellsMinimum distance between co-channel cells?Number of frequencies assigned to each cell?Reuse factor?56Frequency reusingLayoutEquilateral triangleSquareHexagonleast cells needed given an coverage areaClusterA group of cells using

4、 the complete frequencyCluster size = NFrequency reuse factor = 1/NCapacity of an areaC=M*k*NM: clusters in the areaK: allocated channels for each cellN: cells in a cluster719-cell reuse example (N=19)Figure 3.2 Method of locating co-channel cells in a cellular system. In this example, N = 19 (i.e.,

5、 I = 3, j = 2). (Adapted from Oet83 IEEE.)N=i2+ij+j28Example If a total of 33MHz bandwidth is allocated to a particular FDD cellular telephone system which uses two 25KHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system

6、 uses (1) four-cell reuse(2) seven-cell reuse(3) 12-cell reuse.(4) If 1MHz of the allocated spectrum is dedicated to control channels, determine an equitable distribution of control channels and voice channels in each cell for each of the three systems.9Sol:Total channels=33MHz/(2*25KHz)=660For 4,7,

7、12-cell reuse, we have 660/4=165 chs/cell660/7=94 chs/cell660/12=55 chs/cellIf 1MHz of the spectrum is dedicated to control channels, we have 1MHz/50KHz=20 control channels.For 4,7,12-cell reuse, we have640/4=160 chs/cell for traffic channels and 20/4=5 chs/cell for control channels;640/7=91 chs/cel

8、l for traffic channels and at least 20/7=2 chs/cell for control channels;640/12=53 chs/cell for traffic channels and at least 20/12=1 ch/cell for control channels.10Ways to increase the capacityAdd new channelsFrequency borrowing from adjacent cellsCell splitting-cells in areas of high usage can be

9、split into smaller cellsCell sectoring-cell is divided into a number of wedge-shaped sectors, each with a subset channels of the cells channelsMicrocells-cell radius of 0.1-1km11Channel assignment strategiesGoalIncreasing capacityMinimizing interferenceFixed assignmentPredetermined voice channels fo

10、r each cell Borrowing neighbors unused channelDynamic assignmentChannels pool managed by MSCAllocating a channel to a request call by MSCConsidering the follows to allocate a channelLikelihood of future blocking within the cellFrequency of use of the candidate channelReuse distance of the channelInc

11、reasing channel utilizationDecreasing blocking probabilityIncreasing complexity of MSC 12Wireless radio channelLarge-scale propagation effectsPath lossDissipation of the radiated poweri.e. effects of the propagation channelReceived power variation scale: 100-1000mShadowingCaused by obstaclesAbsorpti

12、on, reflection, scattering, diffractionReceived power variation scale: 10-100mSmall-scale propagation effectsMulti-pathReceived power variation scale: half-several wavelength13Wireless radio channel14Path loss PL15Free-space path lossdPtPr16Line-of-Sight EquationsOptical line of sightEffective, or r

13、adio, line of sightd = distance between antenna and horizon (km)h = antenna height (m)K = adjustment factor to account for refraction, rule of thumb K = 4/317Line-of-Sight EquationsMaximum distance between two antennas for LOS propagation:h1 = height of antenna oneh2 = height of antenna two18Two ray

14、 modelIf we consider the transmitted signal to be narrowband, we have For large d, GlGr , R-1 19Emperical modelsDifficult to model the complex propagation environment by ray- tracing technique.Curve fitting by emperical measurements of different environment, such asLarge urban macrocellUrban microce

15、llModelsOkumuraHataCOST23120Co-channel interferenceCo-channel reuse ratioSignal Co-channel interference ratio (S/I)Propagation in a mobile radio channelAn approximated estimation of S/IWhere i0 is the number of adjacent co-channel cells, n is the path loss exponent.21Co-channel cells for 7-cell reus

16、e22Smaller N is greater capacity23Example.For the U.S. AMPS system, which uses FM and 30KHz channels, subjective tests indicate that sufficient voice quality is provided when S/I is greater than or equal to 18dB, what should the cluster size be in order to meet this requirements? (Assume the path lo

17、ss exponent n=4)24Sol:Using the formula of S/I, we have 25Calculation of co-channel interferenceExact cell geometry layoutConsidering the “worst” userSimulation analysisActual cell coverage is amorphous due to radio propagation environment.Possible different cell sizes due to different propagation e

18、nvironment.26Handoff (handover) strategies GoalAs infrequently as possibleUser imperceptibleUsually prioritize over call requestChoose proper thresholdToo highBurden the MSCToo lowNo enough time to complete a handoff27Handover28Some points of handover (handoff)FadingUnnecessary handoffVehicle speed

19、estimationHelpful to handoff decisionSignal average durationMSC decided by measured RSSIRSSI: radio signal strength indicationBase station monitors the signal strength of all the calls.Locator receiver monitors the neiboring users RSSI.Mobile assisted handoff (MAHO)MS measures the received power fro

20、m surrounding BSs.Report these measurements to the serving BS.Much faster handoff rateIntersystem handoffMS moves to a different MSC controlled cellular system.29Power controlWhy is it desirable to include dynamic power control in a cellular system?Received power must be sufficiently above the backg

21、round noise for effective communication.Desirable to minimize power in the transmitted signal from the mobileCo-channel interferenceBattery powerHealth concerns30System capacityTraffic intensity (Erlang)One Erlang one call occupied a channel one hourExampleA call is 3 minutes long=3/60=0.05 ErlangUs

22、er traffic intensityAu=H : the call requested rate (calls per unit time)H : average duration of a call31Grade of serviceAssume that one system has C channelsSystem capacity ( C Erlang )If offered traffic ACSomeone shouldnt obtain access due to congestion (we call it block someone)GOSExample:Given C

23、channels with 2% blocking Given C channels with 10% blockingBlocking more modating more users (worse service grade)Blocking less modating less users (better service grade)32Trunking and grade of serviceTrunking modates a large number of users in a limited radio spectrum;Exploits the statistical beha

24、vior of usersNumber of telephone circuits for hundreds of telephonesGrade of service (GOS)A measure of the ability of a user to access a trunked system during the busiest time;Probability of a user obtaining channel access given a specific number of channels.Wireless designers jobEstimate the maximu

25、m required channels to meet a given GOS33Trunked system 1Blocked calls cleared (Erlang B system)User access a channel immediately if it is availableUser is blocked and free to try later if channel is not available34Erlang B Trunking GOS35Erlang B36Example 1How many users can be supported for 1% bloc

26、king probability for a blocked calls cleared system with 20 channels? Assume that each user generates a call every 10 minutes and holds 2 minutes.37Example 3.5An urban area has a population of 2,000,000 residents. Three trunked mobile networks (A,B,C) provide cellular service in this area. System A:

27、 394 cells, 19 channels/cellSystem B: 98 cells, 57 channels/cellSystem C: 49 cells, 100 channels/cellEach user: 2 calls/hour, 3 mins/callGOS: 2% blockingQuestion: percentage market penetration of each cellular provider?38Trunked system 2Blocked calls delayed (Erlang C system)A queue to hold calls wh

28、ich are blocked;Delay the call until a channel is available;GOSProbability of a call is blocked after a specific length of time in the queue39Erlang C40Example 3.7A hexagonal cell within a four-cell system has a radius of 1.387km. A total of 60 channels are used within the entire system. If the load

29、 per user is 0.029Erlangs, and =1 call/hour, compute the following for an Erlang C system that has a 5% probability of a delayed call:How many users per square kilometer will this system support?What is the probability that a delayed call will have to wait for more than 10s?What is the probability t

30、hat a call will be delayed for more than 10s?41Trunking efficiencyGOS=0.01, Erlang B 10 channels 4.46 Erlangs5 channels x 2 1.36 x 2=2.72 ErlangsMore trunked channels supports more traffic at a given GOS!42Improving coverage and capacityCell splittingSectoring43Cells are split to add channels with n

31、o new spectrum usage 44Cell Splitting increases capacity45Sectoring improves S/I46Sectoring improves S/I47IEEE 802.11 standardsStandardDateScope802.111997Medium access control(MAC) :One common MAC for WLAN applicationsPhysical layer: Infrared at 1and 2 MbpsPhysical layer: 2.4GHz FHSS at 1 and 2 Mbps

32、Physical layer: 2.4GHz DSSS at 1 and 2Mbps802.11a1999Physical layer: 5-GHz OFDM at rates from 6 to 54Mbps802.11b1999Physical layer: 2.4GHz DSSS ar 5.5 and 11Mbps802.11c2003Bridge operation at 802.11 MAC layer802.11d2001Physical layer: Extend operation of 802.11 WLANs to new regulatory domains (countries)802.11eOngoingMAC: Enhance to improve quality of service and enhance security mechanisms802.