无线通信系统.pptx

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1、2023/3/171 Cellular Networks第1页/共46页2023/3/172Cell shapeCircle:Equilateral triangle:Square:Hexagon:第2页/共46页2023/3/173Cellular coverage representation第3页/共46页2023/3/174lMacrocell large,covering a wide are range of several hundred kilometres(km)to ten km mostly deployed in rural and sparsely populated

2、 areaslMicrocell medium cell,coverage are a smaller than in macro cells range of several hundred metres to a couple of metres deployed mostly in crowded areas,stadiums,shopping mallslPicocell small,covering a very small area range of several tens of metres low power antennas,indoorThe size of a cell

3、第4页/共46页2023/3/175第5页/共46页2023/3/176Base station transmitterslIn hexagonal cells,base stations transmitters are either:centre-excited,base station is at the centre of the cell edge-excited,base station at 3 of the 6 cell verticescentre-excitededge-excited第6页/共46页2023/3/177 Cellular coverage represen

4、tation C canbeusedasameasureofcapacity 第7页/共46页2023/3/178A mobile communication system uses a frequency reuse factor of 1/7 and 416 channels available.If 21 channels are allocated as control channels,compute its system capacity.Assume a channel supports 8 usersChannels available for allocation=416-2

5、1=395Channel Number of a cell=395/7=57Number of simultaneous users per cell=8 x 57=456Number of simultaneous users in system=7 x 456=3192Example 1第8页/共46页2023/3/179If a total of 33 MHz of bandwidth is allocated to a particular FDD cellular telephone system which uses two 25 kHz simplex channels to p

6、rovide full duplex voice and control channels,compute the number of channels available per cell if a system uses four-cell reuse.If 1 MHz of the allocated spectrum is dedicated to control channels,determine an equitable distribution of control channels and traffic channels in each cell for each of t

7、he system.Example 2第9页/共46页2023/3/1710Method of locating co-channel cells第10页/共46页2023/3/1711Channel Assignment StrategiessThree assignment approacheslFixed and static(most common)lDynamicl HybridsFixed channel assignmentlall channels in a cell could be in use all the timenew calls are then blocked(

8、no channels left)may be solved by borrowing spare channels from nearby cells第11页/共46页2023/3/1712Dynamic channel assignmentMSC allocates frequencies when a call is madeProvides high channel utilisationTo do this it needs real-time information onchannel occupancy traffic distribution and radio signal

9、strength indication(RSSI)high computational load and increased storage第12页/共46页2023/3/1713Handover(Handoff)Provides continuity of communication across cellsDifficultydropping a call before reconnecting is unacceptabledifferent cells use different frequenciesmobile phone users usually move from place

10、 to place and very quickly too第13页/共46页2023/3/1714Handover ProcessParties in communication share the same channelsReceived signal weakens as mobile moves out of cellCell site at some point requests handover to cell with stronger signal strengthMSC switches call to new cell after allocating channels.

11、第14页/共46页2023/3/1715Handover Process第15页/共46页2023/3/1716Handover ProcessHandover must not be too frequent or system is kept busy servicing handover requestshandover threshold is set slightly strongerMinimum usable signal level is normally set to be between-90 dBm and-100 dBm第16页/共46页2023/3/1717Choos

12、ing Handover MarginsHandover margin =P r handover -Pr minimum usable If is too large unnecessary handover will occur,burdening the MSCIf is too small,there maybe insufficient time to complete the handover before a call is lost due to weak signalsTherefore is chosen carefully第17页/共46页2023/3/1718Setti

13、ng Handover Thresholds第18页/共46页2023/3/1719Handoff in different system1G signalstrengthmeasurementsaremadebythebasestationsandsupervisedbytheMSC GSM MAHO handoffdecisionsaremobileassisted CDMA pilotchannel,softhandoff第19页/共46页2023/3/1720Interference and System Capacity第20页/共46页2023/3/1721SixEffective

14、InterferingCellsofCell1 第21页/共46页2023/3/1722Co-channelreuseratioQ 第22页/共46页2023/3/1723Co-channel reuse ratio vs.frequency reuse pattern 第23页/共46页2023/3/17247-cell reuse pattern第24页/共46页2023/3/172512-cellreusepattern 第25页/共46页2023/3/172619-cellreusepattern 第26页/共46页2023/3/1727Co-channelreuseratiovs.f

15、requencyreusepattern 第27页/共46页2023/3/1728Propagation model第28页/共46页2023/3/1729Co-channel interference ratio 第29页/共46页2023/3/1730Example 3第30页/共46页2023/3/1731Moreexactgeometrylayout 第31页/共46页2023/3/1732Closely approximately as 第32页/共46页2023/3/1733Trunking and Grade of Service(GOS)第33页/共46页2023/3/1734

16、Grade of Service(GOS)第34页/共46页2023/3/1735About Erlang第35页/共46页2023/3/1736Erlang B formulawhereC isthenumberoftrunkedchannelsofferedbyatrunkedradiosystemandA isthetotalofferedtraffic.第36页/共46页2023/3/1737Trunked mobile network provide cellular service in this area.System has 394 cells with 19 channels

17、 each.Find the number of users that can be supported at 2%blocking if each user averages two calls per hour at an average call duration of three minutes.Compute the total number of subscribers that can be supported by this system.Probability of blocking=2%=0.02Number of channels per cell used in the

18、 system,C=19Traffic intensity per user,Au=H=2 x(3/60)=0.1 Erlangsfrom the Erlang B chart,the totalcarried traffic,A,is obtained as 12 Erlangs.Therefore,the number of users that can be supported per cell isU=AIAu=12/0.1=120Since there are 394 cells,the total number of subscribers that can be supporte

19、d by System A is equal to 120 x 394=47280Example 4第37页/共46页2023/3/1738sOften large cells need to be split into smaller ones because the population of users in the big cell has increased beyond what it can supportlCell splitting increases system capacitylIs used in high density subscriber areas lResu

20、lts to increased costs(eg.new base stations)ImprovingCapacityinCellularSystem第38页/共46页2023/3/1739第39页/共46页2023/3/1740第40页/共46页2023/3/1741Cell Splitting 第41页/共46页2023/3/1742Sectoring 第42页/共46页2023/3/1743A cellular system:an average call lasts two minutes,and the probability ofblockingistobenomorethan

21、1%.Everysubscribermakesonecallperhour,onaverage.Ifthereareatotalof395trafficchannelsforaseven-cellreusesystem,therewillbeabout57trafficchannels per cell.Assume that blocked calls are cleared so the blocking isdescribedbytheErlangBdistribution.FromtheErlangBdistribution,itcanbefoundthattheunsectoreds

22、ystemmayhandle44.2Erlangsor1326callsperhour.Ifemploying120sectoring,thereareonly19channelsperantennasector(57/3antennas).Forthesameprobabilityofblockingandaveragecalllength,itcanbefoundfromtheErlangBdistributionthateachsectorcanhandle11.2Erlangsor336callsperhour.Sinceeachcellconsistsofthreesectors,t

23、hisprovidesacellcapacityof3x336=1008callsperhour.Thus,sectoringdecreasesthetrunkingefficiencywhileimprovingtheS/I foreachuserinthesystem.Example 5第43页/共46页2023/3/17441.Wirelesshasrapidlyemergedinthecommunicationfield.Pleaseuselibraryorinternettofindtheanswersofthefollowingquestions.(a)Describeatleas

24、tonecellularsystemthatbeingusedtoday,thecarrierfrequency,thedatarate,theRFbandwidthormore.(b)ExplainwhatisWLAN?(IEEE802.11,wi-fi,etc.)2.If20MHzoftotalspectrumisallocatedforaduplexwirelesscellularsystemandeachsimplexchannelhas25kHzRFbandwidth,find:(a)thenumberofduplexchannels,(b)thetotalnumberofchann

25、elspercellsite,ifN=4cellreuseisused.HOMEWORK第44页/共46页2023/3/17453.AcellularserviceproviderdecidestouseadigitalTDMAschemewhichcantolerateasignal-to-interferenceratioof15dBintheworstcase.FindtheoptimalvalueofNforomnidirectionalantennas(Assumeapathlossexponentofn=4.)4.ForaN=7systemwithaPrBlocking=1%andaveragecalllengthoftwominutes,findthetrafficcapacitylossduetotrunkingfor57channelswhengoingfromomnidirectionalantennasto60sectoredantennas.(Assumethatblockedcallsareclearedandtheaverageperusercallrateis=1perhour.)第45页/共46页2023/3/1746感谢您的观看！第46页/共46页