无线通信原理与应用 第五章 Multiple Access Techniques for Wireless Communications

1、Multiple Access Techniques for Wireless Communications 2022/7/201DuplexingFrequency domain techniques FDDTime domain techniques TDD2022/7/202TransmitterReceiverBPFTransmitterReceiverBPFF1F1Mobile StationBase StationSynchronous SwitchesTDD system2022/7/203Frequency (f)Time (t)Code (c)Space (si)Packet

2、 RadioGoal: multiple use of a shared mediumMultiple AccessMultiple Access TechniquesFDMATDMACDMASDMAPR2022/7/204FDMATDMACDMA2022/7/205Cellular Systems MA Technique AMPS ( Advanced Mobile Phone system ) FDMA / FDD GSM ( Global System for Mobile )TDMA / FDDIS 95 ( U.S Narrowband Spread Spectrum ) CDMA

3、 / FDDWCDMACDMA / FDD,CDMA/TDDTD-SCDMA CDMA / TDDCdma2000CDMA / FDDMultiple ascess technologies used in different wireless systems2022/7/206FDMAEach user is allocated a unique frequency band or channel.These channels are assigned on demand to users who requestservice.The bandwidth of FDMA channels i

4、s 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 spectrum into smaller frequency bandsA channel gets a certain band of the spectrum for the whole time2022/7/207FDM

5、A 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 duplexers. FDMA requires tight RF filtering to minimize adjacent channel interference. Nonlinear Effects in FDMA: In

6、termodulation frequency2022/7/2082022/7/209TDMATDMA systems divide the radio spectrum into time slots and each user is allowed to either transmit or receive in each time slots.Each user occupies a cyclically repeating time slots. Disadvantages:precise synchronization necessary2022/7/2010TDMAUtilized

7、 only in digital systems!Number of time slots (TS) depends on technology2022/7/2011Call 1Call 2Call 3BS AntennaTS1TS2TS3TS4TS5TS62022/7/2012TDMATDMA shares the single carrier frequency with several users, where each user makes use of non-overlapping timeslots.Data Transmission for user of TDMA syste

8、m is discrete bursts The result is low battery consumption, power save. Handoff process is simpler, since it is able to listen for other base stations during idle time slots.Since different slots are used for T and R, duplexers are not required.Equalization is required, since transmission rates are

9、generally very high as compared to FDMA channels.2022/7/20132022/7/2014TDMA&FDMACombination of both methodsA channel gets a certain frequency band for a certain amount of timeExample: GSM 2022/7/2015 If GSM uses a frame structure where each frame consists of eight time slots, and each time slot cont

10、ains 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.Example Solution(a) Th

11、e 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.2022/7/2016Spread Spectrum modulation spreads signal bandwidt

12、h to a bandwidth that is several orders of magnitude wider than the signal bandwidthTwo spectrum spread methodsFrequency Hopping Spread SpectrumDirect Sequence Spread Spectrum SSMA2022/7/2017Spread SpectrumShannons capacity equation2022/7/2018Advantagesfrequency selective fading and interference lim

13、ited to short periodsimple implementationuses only small portion of spectrum at any timeDisadvantagesnot as robust as DSSSDiscrete changes of carrier frequencysequence of frequency changes determined via pseudo random number sequenceFHMA2022/7/2019Each channel has a unique codeAll channels use the s

14、ame spectrum at the same timebandwidth efficientno coordination and synchronization necessaryImplemented using spread spectrum technologyDSSS (CDMA)2022/7/2020Direct Sequence (DS)Modulation(primary modulation)user dataSpreading(secondary modulation)TxBase-bandFrequencyPowerDensityRadioFrequencyPower

15、DensityTIMEdata rate10110100spreading sequence(spreading code)How to spread CDMA2022/7/2021you know the correct spreading sequence (code) ,RadioFrequencyPowerDensityreceived signalspreading sequence(spreading code)you can find the spreading timing which gives the maximum detected power, and Accumula

16、te for one bit durationDemodulated dataBase-bandFrequencygathering energy !101101001011010010110100TIME010010111011010010110100001111111110000000000000000Demodulating2022/7/2022 Unlike TDMA or FDMA, CDMA has a soft capacity limit. Self-jamming is a problem in CDMA system.The near-far problem occurs

17、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 collecting time delayed versions

18、of the required signal .2022/7/2023Received 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 phases01101100001112022/7/2024Packet Radio High spectral efficiency

19、CollisionsService:DATAPacket Radio 2022/7/2025Vulnerable periodNormalized channel trafficNormalized throughputPoisson process2022/7/2026Pure alohaTransmit whenever a message is readyRetransmit when there is a collisionR=0.5 Tmax=0.18392022/7/20272022/7/2028Slotted alohaTime is divided into equal tim

20、e slotsTransmit only at the beginning of a time slotAvoid partial collisionsIncrease delay, and require synchronization2022/7/20292022/7/2030CSMA non-persistent CSMA 1-persistent CSMA p-persistent CSMA CSMA/CD easy in wired LANs: measure signal strengths, compare transmitted, received signalsdifficu

21、lt in wireless LANs: receiver shut off while transmitting CSMA/CA (used in 802.11)2022/7/2031Channel capacity for a radio system is defined as the maximum number of channels or users that can be provided in a fixed frequency band spectrum efficiency of wireless system.For a Cellular Systemm = Bt / (

22、BC * N) Bt = Total allocated spectrum for the systemBC = Channel bandwidthN= Number of cells in frequency reuse patternCapacity of Cellular Systems 2022/7/2032Capacity of Digital Cellular CDMACapacity of FDMA and TDMA system is bandwidth limited.Capacity of CDMA system is interference limited.The li

23、nk performance of CDMA increases as the number of users decreases.2022/7/2033Antenna SectorizationA cell site with 3 antennas,each having a beamwidth of 120 degree.This increase the capacity.Monitoring or Voice activity:Each transmitter is switched off during period of no voice activity. Voice activ

24、ity is denoted by a factor .2022/7/2034W/R processing gainCDMA2022/7/2035Eb/N0 within a sector2022/7/2036If W = 1.25 MHz, R= 9600 bps, and a minimum acceptable Eb/ No is 10 dB, determine the maximum number of users that can be supported in a single cell CDMA system using (a) omni directional base st

25、ation antennas and no voice activity detection (b) 3 sectors at base station and a = 3/8. Assume the system is interference limited. Example therefore N=3*35.7 =107 users/ cell. 2022/7/2037Exercises1. The US Digital Cellular TDMA system uses a 48.6 kbps data rate to support three users per frame. Each user occupies two of the six time slots per frame. What is the raw data rate provided for each user?2.In an unslotted ALOHA system the packet arrival times form a Poisson process having a