无线通信基础双语教学ppt课件.ppt

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1、6 6、Channel assignment strategiesChannel assignment strategies4 4、Call blocking and delay at the cell-siteCall blocking and delay at the cell-site3 3、CochannelCochannel and adjacent channel interference and adjacent channel interference5 5、Other mechanisms for capacity increaseOther mechanisms for c

2、apacity increase1 1、Frequency reuse and mobility managementFrequency reuse and mobility management2 2、Cell cluster conceptCell cluster conceptl Fixed Channel Assignment(FCA)l Dynamic Channel Assignment(DCA)In FCA,each cell is allocated a predetermined set of voice channels.Any call attempt within th

3、e cell can only be served by the unused channels in the particular cellA borrowing option may be considered,if all of a cells own channels are already occupied and the neighboring cell has spare channels.In DCA,voice channels are not allocated to different cells on a permanent basis.Each time a call

4、 request is made,the serving base station requests a channel from the MSC.Multiple access is a signal transmission situation in which two or more users wish to simultane-ously communicate using the same propagation channel.Shared MultipleAccess ChannelNode 4Node 3Node 2Node 1Node NEach node is attac

5、hed to a transmitter/receiver which communicates via a channel shared by other nodesTransmission from any node is received by other nodesShared MultipleAccess ChannelNode 4Node 3Node 2Node 1Node N5.2 5.2 Random AccessRandom Access5.1 5.1 Multiple access in a radio cellMultiple access in a radio cell

6、5.35.3 Conflict-free multiple access technologies Conflict-free multiple access technologies 5.45.4 Spectral efficiency Spectral efficiencyl Orthogonality between two signalsl Conflict-Free Mulitiple Accessl Random Access and VariantsThe transmission in the downlink can be heard by each and every us

7、er in the cell,this mode of transmission is referred to as broadcastingThe transmission in the uplink to the base station is many-to-one,and is referred as multiple accssTransmission in the uplink have the following attributes:a.Multiple mobile users want to access the common resource(base station)r

8、andomlyb.If the transmissions from two or more users arrive at the base station receiver at the same time,there will be destructive interference,unless the multiple arriving signals are mutually orthogonalOrthogonality between two signals xi(t)and xj(t)means that their inner product over the signali

9、ng interval 0 T vanishes.That isThe key element in multiple access is to make the transmitted signals from the different users orthogo-nal to each otherlMultiple access protocolsSolving multiple access issuesDifferent types:Channel Partitioning(FDMA TDMA CDMA)Taking Turns(Taking Ring)Random Access(A

10、loha CSMA)Orthogonality can be mechanized usinga.Space division multiple access(SDMA)b.Frequency division multiple access（FDMA）c.Time division multiple access（TDMA）d.Code division multiple access（CDMA）In non-conflict-free multiple access,transmissions by the different users are either uncoordinated

11、or are only partially coordinatedUncoordinated-random accessIn random access,a user contends for usage of the same resource,independent of any other users.For this reason,random access is also referred to as contention accessl Alohal CSMAl In certain situations,it may be advantageous to employ a non

12、-conflict-free multiple access scheme.l In conflict-free multiple access system,random access is often used to gain the initial access to the systemsRandom access was used by a research group from the University of Hawaii in the late 1960s and early 1970s,which was called AlohalPure AlohalSlotted Al

13、ohaThe time interval required to transmit one packet is called a slot1.Transmit whenever you have data to send2.Listen to the broadcastBecause broadcast is fed back,the sending host can always find out if its packet was destroyed just by listening to the downward broadcast one round-trip time after

14、sending the packet3.If the packet was destroyed,wait a random amount of time and send it againThe waiting time must be random to prevent the same packets from colliding over and over again2Node 2 Packet32Time1Node 1 PacketRetransmissionRetransmissionCollision mechanism in ALOHAWaiting a random time3

15、Node 3 PacketThe maximum interval over which two packets can overlap and destroy each other is called the vulnerable periodCollision1.If a host has a packet to transmit,it waits until the beginning of the next slot before sending2.Listen to the broadcast and check if the packet was destroyed3.If the

16、re was a collision,wait a random number of slots and try to send again12&32TimeCollisionRetransmissionRetransmission3SlotNode 1 PacketNodes 2&3 PacketsCollision mechanism in slotted ALOHALet S be the throughput,defined as the successfully transmitted traffic load,and G be the total offered channel t

17、raffic load.The probability of successful transmission is defined as:Psuccess=Pno other packet transmission occurs within a vulnerable periodAssuming that traffic generated for transmission obeys a Possion distribution,thenP no other packet transmission occurs=e GWhere is the vulnerable period.Thus,

18、we have:andCombining the two equations,we have the through-put equation of the Aloha systems given by:The maximum value of S occurs when the slope of the throughput curve is 0,and we getSlotted AlohaAloha0.3680.184GSThe delay experienced by a packet in the system is measured from the instant of the

19、packets arrival until the instant the sender receives confirmation.Thus,the packet delay is a function of the number of the transmissions,the retransmission delay,and the time required for the sender to receive confirmation of successful transmission.LetlR be the number of slots to receive an acknow

20、ledgmentlDret be the mean retransmission delaylE be the mean number of transmissions until successHence,the average packet delay is given bywherelMax throughput achievable by slotted ALOHA is 0.368.lCSMA gives improved throughput compared to Aloha protocols.lListens to the channel before transmittin

21、g a packet(avoid avoidable collisions).123TimeCollision4Node 4 senseDelayNode 5 senseDelayNode 1 PacketNode 2 PacketNode 3 PacketNode 5 PacketNode 4 Packet1-Persistent CSMAl Sense the channel.If busy,keep listening to the channel and transmit immediately when the channel becomes idle.If idle,transmi

22、t a packet immediately.l If collision occurs,Wait a random amount of time and start over again.Non-Persistent CSMAl Sense the channel.If busy,wait a random amount of time and sense the channel againIf idle,transmit a packet immediatelyl If collision occurswait a random amount of time and start all o

23、ver againP-Persistent CSMAlStep1:Sense the channelIf channel is idle,transmit a packet with probability pif a packet was transmitted,go to step 2if a packet was not transmitted,wait one slot and go to step 1If channel is busy,wait one slot and go to step 1.lStep2:Detect collisionsIf a collision occu

24、rs,wait a random amount of time and go to step10 1 2 3 4 5 6 7 8 9G1.00.90.80.70.60.50.40.30.20.1 0SAlohaSlotted Aloha1-persistent CSMA0.5-persistent CSMA0.1-persistent CSMA0.01-persistent CSMANonpersistent CSMACSMA/CD(Collision detection)lSense the channelIf idle,transmit immediatelyIf busy,wait un

25、til the channel becomes idlelCollision detectionAbort a transmission immediately if a collision is detectedTry again later after waiting a random amount of timeABT0A begins transmissionABB begins transmissionTimeT0+-ABB detects collisionT0+ABA detects collision just before end of transmissionT0+2 -C

26、SMA/CD(Collision detection)Problem in WLANlDifficult in Collision DetectionlHidden terminal problemlExposed terminal problemCSMA/CA(Collision Avoidance)lAdd acknowledgementsReceiver acknowledges receipt of dataAvoids hidden terminal problemlAvoid collisions explicitly via channel reservationSender s

27、ends“request-to-send”(RTS)messagesReceiver sends“clear-to-send”(CTS)messagesSender sends data packet using reservationExplicitly indicates length of data so others know how long to back offlUsed in 802.11 wireless LAN networksCSMA/CA(Collision Avoidance)RTSCTSDataACKNode ANode BPropagation delayCSMA

28、/CA(Collision Avoidance)DIFSNext FrameCTSRTSOtherSourceDestinationDIFS SIFSContention windowDefer accessBackoff after deferSIFSDataSIFSACKTime802.11e802.11elService Differentiation lAdmission Control and Bandwidth ReservationlLink Adaptation 802.11e802.11e2 2、TDMATDMA 1 1、FDMAFDMA3 3、CDMACDMA In FDM

29、A,the total bandwidth is divided into non-overlapping frequency subbands,each user is allocated a unique frequency subband for the duration of the connection,whether the connection is in an active or idle state.This type of multiple access support is narrowband,and is not suitable for multimedia com

30、munications with various transmission rates.AMPSIn a TDMA system,the channel time is partitioned into frames,which is further partitioned into time slots.Users have to transmit in their assigned slots from frame to frame.The slot assignment can be fixed or dynamic.lSynchronous TDMA(STDMA)lAsynchrono

31、us TDMA(ATDMA)If the assigned slot is fixed from frame to frame for the duration of the connection,the users have to synchronize to their respective assigned slots.This mode of TDMA is referred to as STDMA.Transmission slots are dynamically assigned from frame to frame is referred to as ATDMASTDMADe

32、pending on the manner in which frequency is allocated,STDMA can be:lWideband TDMAlNarrowband TDMASTDMAWideband TDMANarrowband TDMASTDMAlIn STDMA,the slots is long enough to transmit or receive one information unit.lTo provide the required separation between the transmissions in the uplink and downli

33、nk channels,TDMA can use TDD or FDD.STDMAGSMATDMAATDMA is implemented using a reservation access mechanism.An ATDMA frame contains two segments:lA leading segmentlA trailing segmentATDMAThe leading segment contains mini request slots for active users to submit requests for slot allocation.The traili

34、ng segment contains information slots for packet transmission.ATDMASuperframe#m-1Superframe#mSuperframe#m+1Beacon#mFrom PNCCAPMCTA1GTS1GTSn-1 GTS n.CTAP19.2ks/s1.2288McpsDownLinkUpLinklUniversal frequency reuse significantly reduces the complexity of frequency planning in Cellular systemlHigh transm

35、ission accuracy use Rake receivers to mitigate the fading dispersivelSoft handoff this will improve handoff performancelSoft capacity the maximum number of users that can be supported in each cell limited by MAIlFlexibility a high resource utilization via statistical multiplexingA voice process is i

36、n a talk state only 40%of the timelRequires stringent power control near-far effectlWith a large processing gain this limited the maximum transmission rate in each code channellOperate at a high chip rate and require accurate PN synchronization the complexity of the transmitter and receiver(Rake)is higher than that of TDMA and FDMA systems5.2 5.2 Random AccessRandom Access5.1 5.1 Multiple access in a radio cellMultiple access in a radio cell5.35.3 Conflict-free multiple access technologies Conflict-free multiple access technologies l P 6-2 a.b.