wireless mac protocols[无线mac协议](41).ppt

《wireless mac protocols[无线mac协议](41).ppt》由会员分享，可在线阅读，更多相关《wireless mac protocols[无线mac协议](41).ppt（41页珍藏版）》请在得力文库 - 分享文档赚钱的网站上搜索。

1、Polytechnic UniversityM.VeeraraghavanWireless MAC protocolsProf.Malathi VeeraraghavanElec.&Comp.Engg.Dept/CATTPolytechnic University EL604:Wireless&Mobile Networking1Polytechnic UniversityM.VeeraraghavanOutlineNeed for wireless MAC protocolsObtain assignment of resources per call ala circuit switchi

2、ng fixed assignmentObtain assignment of resources per packet ala packet switching CL flavor:random-access CO flavor:demand-assignment2Polytechnic UniversityM.VeeraraghavanNeed for wireless MAC protocolsWireless is naturally a shared mediumPSTNBase stations/cell sitesReversechannelsForward channelsDo

3、wnstreamUpstreamInternetAccess points3Polytechnic UniversityM.VeeraraghavanClassification of wireless MAC protocolsWireless MAC protocolsFixed-assignment schemesRandom-access schemesDemand assignment schemesCircuit-switchedCL packet-switchedCO packet-switched4Polytechnic UniversityM.VeeraraghavanOut

4、lineNeed for wireless MAC protocolsObtain assignment of resources per call ala circuit switching-fixed assignmentFDMA and TDMAObtain assignment of resources per packet ala packet switching CL flavor:random-access CO flavor:demand-assignment5Polytechnic UniversityM.VeeraraghavanFDMA(Frequency Divisio

5、n Multiple Access)Similar to broadcast radio and TV,assign a different carrier frequency per callModulation technique determines the required carrier spacingEach communicating wireless user gets his/her own carrier frequency on which to send dataNeed to set aside some frequencies that are operated i

6、n random-access mode to enable a wireless user to request and receive a carrier for data transmission6Polytechnic UniversityM.VeeraraghavanTDMA(Time Division Multiple Access)Each user transmits data on a time slot on multiple frequenciesA time slot is a channelA user sends data at an accelerated rat

7、e(by using many frequencies)when its time slot beginsData is stored at receiver and played back at original slow rate7Polytechnic UniversityM.VeeraraghavanFrequency vs.timeFrequencyTimeCarrierFDMATimeFrequencyTDMATimeFrequencyHybrid FDMA/TDMABasic principle of communication:Two regions in the time-f

8、requency plane with equal areas can carry the same amount of information8Polytechnic UniversityM.VeeraraghavanCellular System Overview9Polytechnic UniversityM.VeeraraghavanCellular Systems TermsBase Station(BS)includes an antenna,a controller,and a number of receiversMobile telecommunications switch

9、ing office(MTSO)connects calls between mobile unitsTwo types of channels available between mobile unit and BSControl channels used to exchange information having to do with setting up and maintaining callsTraffic channels carry voice or data connection between users10Polytechnic UniversityM.Veerarag

10、havanAMPS cellular systemFDMAAnalog cellular network:Advanced Mobile Phone System(AMPS)Spectrum allocation by FCC:A and B allocations to different providersAB8258451667AB8708901667OriginalReverseForwardAB8258451023667A824849991 1799BAAB8708901023667A869894991 1799BAExtended11Polytechnic UniversityM.

11、VeeraraghavanDuplex techniquesSeparates signals transmitted by base stations from signals transmitted by terminalsFrequency Division Duplex(FDD):use separate sets of frequencies for forward and reverse channels(upstream and downstream)Time Division Duplex(TDD):same frequencies used in the two direct

12、ions,but different time slots12Polytechnic UniversityM.VeeraraghavanQuestionIs AMPS an FDD or TDD scheme?13Polytechnic UniversityM.VeeraraghavanExamplesFDD:Cellular systems:AMPS,NA-TDMA,CDMA,GSMTDDCordless telephone systems:CT2,DECT,PHS14Polytechnic UniversityM.VeeraraghavanCarrier frequencies corre

13、sponding to channel numbers in AMPSEvery mobile phone can be programmed to use channels of corresponding subscriber(A or B)For every phone call,a mobile user uses two channels;one in the forward direction and the second one in the reverse directionHow is a mobile user assigned these channels for the

14、 call?kHzReverse directionkHzForward directionkHz15Polytechnic UniversityM.VeeraraghavanControl channelsA set of channels are set aside as“control channels”channels 313-354(21 channels in each band)forward control channels(FOCC)broadcast channels(one-to-many)reverse control channels(RECC)random acce

15、ss(many-to-one)16Polytechnic UniversityM.VeeraraghavanRECC Access ProtocolA RECC is used by multiple endpoints to request a traffic channelBut before a mobile issues call origination request on an RECC,it needs to make sure that the RECC is idleFirst,endpoints tune into FOCC;find the strongest one a

16、nd then determine whether corresponding RECC is busy or idle17Polytechnic UniversityM.VeeraraghavanBroadcasts on FOCCWhen a mobile handset is switched on,it locks into the strongest FOCCIf in the band of the“other”service provider,then“roaming”signal is displayedIf there are no strong signals,displa

17、y“no service”Basestation broadcasts busy/idle status of each RECC on its corresponding FOCCBasestation sends global action message specifiyingmaximum number of tries if busy(MAXBUSY)maximum number of attempts to seize the RECC(MAXSZTR)18Polytechnic UniversityM.VeeraraghavanRandom access protocol on

18、a RECCbusy/idle=0?NBUSY=NBUSY+1yesnoNBUSY=0Send originateContinueNSZTR=0NBUSY=1Too many failuresAbandonnoyesNBUSY MAXBUSYrandom delay0 200 msMonitor19Polytechnic UniversityM.VeeraraghavanRECC Random access protocol contd.busy/idle=0too soon(before 5.6ms)?ContinueyesNSZTR=NSZTR+1yesNSZTR MAXSZTRrando

19、m delay0 200 msToo many failuresAbandonnonobusy/idle=0before 10.4ms?noApparent success;wait for responseyesMonitor20Polytechnic UniversityM.VeeraraghavanMessages on control channelsSent on FOCC System parameter identifies provider,indicates number of forward and reverse control channelsGlobal action

20、indicates MAXBUSY and MAXSZTRRegistration ident specifies frequency to be used by terminal for sending registration messagesOther messages:Page,Initial voice channel,Release,Confirm registration,etc.21Polytechnic UniversityM.VeeraraghavanMessages on control channelsSent on RECCOriginate(call origina

21、tion)Page responseRegistrationSent on forward voice channelHandoff,Change power level,etc.Sent on reverse voice channelOrder confirmation,etc.22Polytechnic UniversityM.VeeraraghavanMessages exchanged:call originating from mobileMobile Switching Center(MSC)TerminalBase stationscan FOCCs;lock into str

22、ongest one;record system parametersSys.parameter,Global actionRegistration identuser presses send buttonOriginateselect voicechannelInitial voice channeltune to voice channelCONVERSATIONAccess channel info record RECC information23Polytechnic UniversityM.VeeraraghavanMessages exchanged:call terminat

23、ing at a mobileTradeoff between Page and RegistrationIf terminals register,then a Page does not need to be sent to many cellsFor systems with a large number of base stations,volume of page messages can overwhelm the systemPage sent with identification of called mobile on broadcast FOCCLatter respond

24、s with a Page response on RECC24Polytechnic UniversityM.VeeraraghavanDimension of spaceQuestion:Transmission on a wireless link can be heard by all terminals within range of base station What is that range?What controls that range?What parameters determine the number of simultaneous calls within thi

25、s range?25Polytechnic UniversityM.VeeraraghavanPowerRadiated power at a base station is 25W per channel for wide area coverageRadiated power from a terminal is the range of 8dBm(6mW)to 36 dBm(4W)in steps of 4dB(each power level is 2.5 times higher)26Polytechnic UniversityM.VeeraraghavanSpectrum effi

26、ciencyC:Number of conversations per Mhz that would be possible in a single cell with no interference from neighboring cellsN:Channel reuse factor indicates the capacity reduction due to interference from signals transmitted in other cellsSpectrum efficiency Econversations/cell/MhzRequired received s

27、ignal-to-interference ratio is 18dB for high-quality sound reproduction at the receiver27Polytechnic UniversityM.VeeraraghavanReceived signal strengthReceived signal strength depends uponattenuation that increases with distancerandom variations due to environmental fluctuations(shadow or slow fading

28、:obstacles)signal fluctuations due to the motion of a terminal(Rayleigh fading or fast fading)distortions because components of the signal travel along different paths to the receiver28Polytechnic UniversityM.VeeraraghavanHexagonal cell frequency planRDD:Distance between a base station and the neare

29、st base station that uses the same channelsR:Radius of a cellReuse distance=D/RChannel plan:method ofassigning channels to cells to guarantee a minimumreuse distance between cells thatuse the same channel which is the minimum reuse distance for which29Polytechnic UniversityM.VeeraraghavanHexagon pro

30、pertiesRadius of a hexagon,R:radius of the circle that circumscribes it(which is equal to the distance from the center to any vertex;also equal to the length of a side)Distance between adjacent cells is dArea of a hexagon is30Polytechnic UniversityM.VeeraraghavanReuse factorDivide available channels

31、 into N groupsN:reuse factor;select N such that cells assigned the same frequencies will have a D:R ratio greater than(D:R)reqFor hexagons,reuse factor N is given byPractical values of N range from 3 to 21most commonly used:7(D/R=4.6)31Polytechnic UniversityM.VeeraraghavanChannel plan with a reuse f

32、actor N=71246537176232Polytechnic UniversityM.VeeraraghavanService provider AHas a total of 832/2=416 channelsSet aside 21 for controlTherefore 395 traffic channelsPer cell,we can have 56 and 3/7 channels(N=7)Four cells are given 56 channels and three cells are given 57 channelsThe 395 channels fit

33、over 25MhzTherefore,the spectrum efficiency of an AMPS system isconversations/cell/Mhz33Polytechnic UniversityM.VeeraraghavanQuestionSo how many control channels are present in each cell?What is the implication of this question?34Polytechnic UniversityM.VeeraraghavanTypical parametersAverage delay s

34、pread refers to the multipath delay spread(the same signal takes different paths,and there is a difference in delay incurred by the different paths)35Polytechnic UniversityM.VeeraraghavanIncreasing CapacityAdd new channelsDynamic channel allocation frequencies can be taken from adjacent cells by con

35、gested cellsCell splitting cells in areas of high usage can be split into smaller cellsCell sectoring cells are divided into a number of wedge-shaped sectors,each with their own set of channels(typical:3)Microcells antennas move to buildings,hills,and lamp posts36Polytechnic UniversityM.Veeraraghava

36、nRadio channel handoffIntra-cell handoff is when the channel needs to be changed when the user is still within a cellReason:user moves into a sector served by a different directional antennaMore commonly understood meaning for handoff:base-station to base-station(cell site)Neighboring cells use diff

37、erent channels(for interference reasons)As a user moves from cell to cell,channels are changed37Polytechnic UniversityM.VeeraraghavanWhen to handoff?Relative signal strengthhandoff from basestation A to basestation B if signal strength at B exceeds that at ARelative signal strength with thresholdsig

38、nal at A is lower than the thresholdthe signal at B is strongerRelative signal strength with hysteresisif signal at B is stronger by a hystersis amount Hhand back to A only if signal at A is higher by H38Polytechnic UniversityM.VeeraraghavanWhen to handoff?Relative signal strength with hysteresis an

39、d thresholdcurrent signal drops below a thresholdtarget base station is stronger by a hystersis margin HPrediction techniqueshandoff based on the expected future value of the received signal strength39Polytechnic UniversityM.VeeraraghavanAcronymsCT2:Cordless Telephone Second GenerationDECT:Digital E

40、uropean Cordless TelecommunicationsPHS:Personal Handyphone SystemGSM:Groupe Special Mobile(special Mobile group),also Global System for Mobile communicationAMPS:Advanced Mobile Phone SystemTDMA:Time Division Multiple AccessFDMA:Frequency Division Multiple AccessCDMA:Code Division Multiple AccessFDD:Frequency Division DuplexTDD:Time Division Duplex40Polytechnic UniversityM.VeeraraghavanReferenceDavid Goodman,“Wireless Personal Communication Systems,”Prentice Hall,ISBN 0-201-63470-8,1997.41