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1、Multiple Access Techniques for Wireless Communications2024/7/291DuplexingFrequency domain techniques FDDTime domain techniques TDD2024/7/292TransmitterReceiverBPFTransmitterReceiverBPFF1F1Mobile StationBase StationSynchronous SwitchesTDD system2024/7/293Frequency (f)Time (t)Code (c)Space (si)Packet
2、RadioGoal: multiple use of a shared mediumMultiple AccessMultipleMultiple Access TechniquesAccess TechniquesFDMAFDMATDMATDMACDMACDMASDMASDMAPRPR2024/7/294FDMATDMACDMA2024/7/295Cellular Systems MA Technique AMPS ( Advanced Mobile Phone system ) FDMA / FDDFDMA / FDD GSM ( Global System for Mobile )TDM
3、A / FDDTDMA / FDDIS 95 ( U.S Narrowband Spread Spectrum ) CDMA / FDDCDMA / FDDWCDMACDMA / CDMA / FDD,CDMA/TDDFDD,CDMA/TDDTD-SCDMA CDMA / TDDCDMA / TDDCdma2000CDMA / FDDCDMA / FDDMultiple ascess technologies used in different wireless systems2024/7/296FDMAEach user is allocated a unique frequency ban
4、d or channel.These channels are assigned on demand to users who requestservice.The bandwidth of FDMA channels is narrow (30 KHz) since it supports only one call/ carrier.ISI is low since the symbol time is large compared to average delay spread No equalization is required.Separation of the whole spe
5、ctrum into smaller frequency bandsA channel gets a certain band of the spectrum for the whole time2024/7/297FDMA Since FDMA is a continuous transmission scheme, fewer bits are needed for overhead purposes (such as synchronization and framing bits) as compared to TDMA. The FDMA mobile unit uses duple
6、xers. FDMA requires tight RF filtering to minimize adjacent channel interference. Nonlinear Effects in FDMA: Intermodulation frequency2024/7/2982024/7/299TDMATDMA systems divide the radio spectrum into time slots and each user is allowed to either transmit or receive in each time slots.Each user occ
7、upies a cyclically repeating time slots. Disadvantages:precise synchronization necessary2024/7/2910TDMAUtilized only in digital systems!Number of time slots (TS) depends on technology2024/7/2911Call1Call2Call3BSAntennaTS1TS2TS3TS4TS5TS62024/7/2912TDMAl lTDMA shares the single carrier frequency with
8、TDMA shares the single carrier frequency with several users, where each user makes use of non-several users, where each user makes use of non-overlapping timeslots.overlapping timeslots.l lData Transmission for user of TDMA system is Data Transmission for user of TDMA system is discrete burstsdiscre
9、te bursts The result is low battery consumption, power The result is low battery consumption, power save.save. Handoff process is simpler, since it is able to Handoff process is simpler, since it is able to listen for other base stations during idle time listen for other base stations during idle ti
10、me slots.slots.l lSince different slots are used for T and R, duplexers Since different slots are used for T and R, duplexers are not required.are not required.l lEqualization is required, since transmission rates Equalization is required, since transmission rates are generally very high as compared
11、 to FDMA are generally very high as compared to FDMA channels.channels.2024/7/29132024/7/2914TDMA&FDMACombination of both methodsA channel gets a certain frequency band for a certain amount of timeExample: GSM 2024/7/2915 If GSM uses a frame structure where each frame consists of eight time slots, a
12、nd each time slot contains 156.25 bits, and data is transmitted at 270.833 kbps in the channel, find (a) the time duration of a bit, (b) the time duration of a slot, (c) the time duration of a frame, and (d) how long must a user occupying a single time slot wait between two successive transmissions.
13、Example Solution(a) The time duration of a bit,(b) The time duration of a slot, Tslot = 156.25 x Tb = 0.577 ms.(c) The time duration of a frame, Tf= 8 x Tslot= 4.615 ms.(d) A user has to wait 4.615 ms, the arrival time of a new frame, for its next transmission.2024/7/2916Spread Spectrum modulation s
14、preads signal bandwidth to a bandwidth that is several orders of magnitude wider than the signal bandwidthTwo spectrum spread methodsFrequency Hopping Spread SpectrumDirect Sequence Spread Spectrum SSMA2024/7/2917Spread SpectrumShannons capacity equation2024/7/2918Advantagesqfrequency selective fadi
15、ng and interference limited to short periodqsimple implementationquses only small portion of spectrum at any timeDisadvantagesqnot as robust as DSSSDiscrete changes of carrier frequencyqsequence of frequency changes determined via pseudo random number sequenceFHMA2024/7/2919Each channel has a unique
16、 codeAll channels use the same spectrum at the same timebandwidth efficientno coordination and synchronization necessaryImplemented using spread spectrum technologyDSSS (CDMA)2024/7/2920Direct Sequence (DS)Modulation(primary modulation)user dataSpreading(secondary modulation)TxBase-bandFrequencyPowe
17、rDensityRadioFrequencyPowerDensityTIMEdata rate10110100spreading sequence(spreading code)How to spread CDMA2024/7/2921you know the correct spreading sequence (code) ,RadioFrequencyPowerDensityreceived signalspreading sequence(spreading code)you can find the spreading timing which gives the maximum d
18、etected power, and Accumulate for one bit durationDemodulated dataBase-bandFrequencygathering energy !101101001011010010110100TIME010010111011010010110100001111111110000000000000000Demodulating2024/7/2922 Unlike TDMA or FDMA, CDMA has a soft capacity limit. Self-jamming is a problem in CDMA system.T
19、he near-far problem occurs at a CDMA receiver if an undesired user has a high detected power as compared to the desired user.CDMA power controlSince CDMA uses co-channel cells, it can use macroscopic spatial diversity to provide soft handoff.A RAKE receiver can be used to improve reception by collec
20、ting time delayed versions of the required signal .2024/7/2923Received multipath signalReceived signal consists of a sum of delayed (and weighted) replicas of transmitted signal.:Signal replicas: same signal at different delays, with different amplitudes and phases01101100001112024/7/2924Packet Radi
21、oHighspectralefficiencyCollisionsService:DATAPacket Radio2024/7/2925Vulnerable periodNormalized channel trafficNormalized throughputPoisson process2024/7/2926Pure aloharTransmit whenever a message is readyrRetransmit when there is a collisionR=0.5 Tmax=0.18392024/7/29272024/7/2928Slotted alohamTime
22、is divided into equal time slotsmTransmit only at the beginning of a time slotmAvoid partial collisionsmIncrease delay, and require synchronization2024/7/29292024/7/2930CSMA non-persistent CSMA 1-persistent CSMA p-persistent CSMA CSMA/CD measy in wired LANs: measure signal strengths, compare transmi
23、tted, received signalsmdifficult in wireless LANs: receiver shut off while transmitting CSMA/CA (used in 802.11)2024/7/2931l lChannel capacity for a radio system is defined as Channel capacity for a radio system is defined as the maximum number of channels or users that can the maximum number of cha
24、nnels or users that can be provided in a fixedbe provided in a fixed frequency band frequency band spectrum spectrum efficiency of wireless system.efficiency of wireless system.For a Cellular SystemFor a Cellular Systeml lm = Bm = Bt t / (B / (BC C * N) * N) l lB Bt t = Total allocated spectrum for
25、the system = Total allocated spectrum for the systeml lB BC C = Channel bandwidth = Channel bandwidthl lN= Number of cells in frequency reuse patternN= Number of cells in frequency reuse patternCapacity of Cellular Systems2024/7/2932Capacity of Digital Cellular CDMACapacity of FDMA and TDMA system i
26、s bandwidth limited.Capacity of CDMA system is interference limited.The link performance of CDMA increases as the number of users decreases.2024/7/2933Antenna SectorizationA cell site with 3 antennas,each having a beamwidth of 120 degree.This increase the capacity.Monitoring or Voice activity:Monito
27、ring or Voice activity:Each transmitter is switched off during Each transmitter is switched off during period of no voice activity. Voice activity is period of no voice activity. Voice activity is denoted by a factor .denoted by a factor .2024/7/2934W/R processing gainCDMA2024/7/2935Eb/N0 within a s
28、ector2024/7/2936l lIf W = 1.25 MHz, R= 9600 bps, and a minimum If W = 1.25 MHz, R= 9600 bps, and a minimum acceptable acceptable E Eb b/ N/ No o is 10 dB, determine the maximum is 10 dB, determine the maximum number of users that can be supported in a single number of users that can be supported in
29、a single cell CDMA system using cell CDMA system using (a) omni directional base station antennas and no (a) omni directional base station antennas and no voice activity detection voice activity detection (b) 3 sectors at base station and a = 3/8. Assume (b) 3 sectors at base station and a = 3/8. As
30、sume the system is interference limited. the system is interference limited. Example thereforeN=3*35.7=107users/cell.2024/7/2937Exercises1.TheUSDigitalCellularTDMAsystemusesa48.6kbpsdataratetosupportthreeusersperframe.Eachuseroccupiestwoofthesixtimeslotsperframe.Whatistherawdatarateprovidedforeachus
31、er?2.InanunslottedALOHAsystemthepacketarrivaltimesformaPoissonprocesshavingarateof103packets/sec.Ifthebitrateis10Mbpsandthereare1000bits/packet,find(a)thenormalizedthroughputofthesystem,and(b)thenumberofbitsperpacketthatwillmaximizethethroughput.(c)repeat(b)foraslottedALOHAsystem. 2024/7/29383 In an
32、 omnidirectional (single-cell, single-sector) CDMA cellularsystem,Eb/N0=20dBisrequiredforeachuser. If100users,eachwith a baseband data rate of 13 kbps, are to be accommodated,(a)determinetheminimumchannelbitrateofthespreadspectrumchipsequence.Ignorevoiceactivityconsiderations.(b)repeattheproblemforthecasewherevoiceactivityisconsideredandisequalto40%.(c)repeat the problem for the case of a tri-sectored CDMA system.Includetheeffectsofvoiceactivity,whereitisassumedthateachuserisactive40%ofthetime.2024/7/29392024/7/2940
