multiple access techniques in wireless network

Multiple Access Techniques for Wireless CommunicationFDMA TDMASDMAPDMAA Presentation by Schffner Harald

Introductionmany users at same timeshare a finite amount of radio spectrumhigh performanceduplexing generally requiredfrequency domaintime domain

Frequency division duplexing (FDD)two bands of frequencies for every userforward bandreverse bandduplexer neededfrequency seperation between forward band and reverse band is constantfrequency seperationreverse channelforward channelf

Time division duplexing (TDD)uses time for forward and reverse linkmultiple users share a single radio channelforward time slotreverse time slotno duplexer is requiredtime seperationtforward channelreverse channel

Multiple Access TechniquesFrequency division multiple access (FDMA)Time division multiple access (TDMA)Code division multiple access (CDMA)Space division multiple access (SDMA)grouped as:narrowband systemswideband systems

Narrowband systemslarge number of narrowband channelsusually FDDNarrowband FDMANarrowband TDMAFDMA/FDDFDMA/TDDTDMA/FDDTDMA/TDD

Logical separation FDMA/FDDftuser 1user nforward channelreverse channelforward channelreverse channel...

Logical separation FDMA/TDDftuser 1user nforward channelreverse channelforward channelreverse channel...

Logical separation TDMA/FDDftuser 1user nforwardchannelreversechannelforward channelreversechannel...

Logical separation TDMA/TDDftuser 1user nforwardchannelreversechannelforward channelreversechannel...

Wideband systemslarge number of transmitters on one channelTDMA techniquesCDMA techniquesFDD or TDD multiplexing techniquesTDMA/FDDTDMA/TDDCDMA/FDDCDMA/TDD

Logical separation CDMA/FDDcodefuser 1user nforward channelreverse channelforward channelreverse channel...

Logical separation CDMA/TDDcodetuser 1user nforward channelreverse channelforward channelreverse channel...

Multiple Access Techniques in use Multiple Access TechniqueAdvanced Mobile Phone System (AMPS) FDMA/FDDGlobal System for Mobile (GSM) TDMA/FDDUS Digital Cellular (USDC) TDMA/FDDDigital European Cordless Telephone (DECT) FDMA/TDDUS Narrowband Spread Spectrum (IS-95) CDMA/FDDCellular System

Frequency division multiple access FDMAone phone circuit per channelidle time causes wasting of resourcessimultaneously and continuously transmittingusually implemented in narrowband systems for example: in AMPS is a FDMA bandwidth of 30 kHz implemented

FDMA compared to TDMAfewer bits for synchronization fewer bits for framinghigher cell site system costshigher costs for duplexer used in base station and subscriber unitsFDMA requires RF filtering to minimize adjacent channel interference

Nonlinear Effects in FDMAmany channels - same antennafor maximum power efficiency operate near saturationnear saturation power amplifiers are nonlinearnonlinearities causes signal spreading intermodulation frequencies

Nonlinear Effects in FDMAIM are undesired harmonicsinterference with other channels in the FDMA systemdecreases user C/I - decreases performanceinterference outside the mobile radio band: adjacent-channel interferenceRF filters needed - higher costs

Number of channels in a FDMA systemN number of channels Bt total spectrum allocationBguard guard bandBc channel bandwidthN=Bt - BguardBc

Example: Advanced Mobile Phone SystemAMPSFDMA/FDDanalog cellular system12.5 MHz per simplex band - BtBguard = 10 kHz ; Bc = 30 kHzN=12.5E6 - 2*(10E3)30E3= 416 channels

Time Division Multiple Accesstime slotsone user per slotbuffer and burst methodnoncontinuous transmissiondigital datadigital modulation

Repeating Frame StructureSlot 1Slot 2Slot 3 Slot NPreamble Information Message Trail BitsOne TDMA FrameTrail Bits Sync. Bits Information Data Guard BitsThe frame is cyclically repeated over time.

Features of TDMAa single carrier frequency for several userstransmission in burstslow battery consumptionhandoff process much simplerFDD : switch instead of duplexervery high transmission ratehigh synchronization overheadguard slots necessary

Number of channels in a TDMA systemN number of channelsm number of TDMA users per radio channelBtot total spectrum allocationBguard Guard BandBc channel bandwidth N=m*(Btot - 2*Bguard)Bc

Example: Global System for Mobile (GSM)TDMA/FDDforward link at Btot = 25 MHz radio channels of Bc = 200 kHzif m = 8 speech channels supported, andif no guard band is assumed :

N=8*25E6200E3= 1000 simultaneous users

Efficiency of TDMApercentage of transmitted data that contain informationframe efficiency fusually end user efficiency < f ,because of source and channel codingHow get f ?

Repeating Frame StructureSlot 1Slot 2Slot 3 Slot NPreamble Information Message Trail BitsOne TDMA FrameTrail Bits Sync. Bits Information Data Guard BitsThe frame is cyclically repeated over time.

Efficiency of TDMAbOH number of overhead bitsNr number of reference bursts per framebr reference bits per reference burstNt number of traffic bursts per framebp overhead bits per preamble in each slotbg equivalent bits in each guard time intervallbOH = Nr*br + Nt*bp + Nt*bg + Nr*bg

Efficiency of TDMAbT total number of bits per frameTf frame durationR channel bit ratebT = Tf * R

Efficiency of TDMAf frame efficiencybOH number of overhead bits per framebT total number of bits per framef = (1-bOH/bT)*100%

Space Division Multiple AccessControls radiated energy for each user in spaceusing spot beam antennasbase station tracks user when movingcover areas with same frequency: TDMA or CDMA systemscover areas with same frequency:FDMA systems

Space Division Multiple Accessprimitive applications are Sectorized antennas in future adaptive antennas simultaneously steer energy in the direction of many users at once

Reverse link problemsgeneral problemdifferent propagation path from user to basedynamic control of transmitting power from each user to the base station required limits by battery consumption of subscriber unitspossible solution is a filter for each user

Solution by SDMA systemsadaptive antennas promise to mitigate reverse link problemslimiting case of infinitesimal beamwidthlimiting case of infinitely fast track abilitythereby unique channel that is free from interferenceall user communicate at same time using the same channel

Disadvantage of SDMAperfect adaptive antenna system: infinitely large antenna neededcompromise needed

SDMA and PDMA in satellites INTELSAT IVASDMA dual-beam receive antennasimultaneously access from two different regions of the earth

SDMA and PDMA in satellitesCOMSTAR 1PDMAseparate antennassimultaneously access from same region

SDMA and PDMA in satellitesINTELSAT VPDMA and SDMAtwo hemispheric coverages by SDMAtwo smaller beam zones by PDMAorthogonal polarization

Capacity of Cellular Systemschannel capacity: maximum number of users in a fixed frequency bandradio capacity : value for spectrum efficiency reverse channel interferenceforward channel interferenceHow determine the radio capacity?

Co-Channel Reuse Ratio QQ co-channel reuse ratioD distance between two co-channel cellsR cell radiusQ=D/R

Forward channel interferencecluster size of 4D0 distance serving station to userDK distance co-channel base station to user

Carrier-to-interference ratio C/IM closest co-channels cells cause first order interferenceC =ID0-n0Mk=1DK-nkn0 path loss exponent in the desired cell nk path loss exponent to the interfering base station

Carrier-to-interference ratio C/IAssumption:just the 6 closest stations interfereall these stations have the same distance Dall have similar path loss exponents to n0

C I=D0-n6*D-n

Worst Case Performancemaximum interference at D0 = R(C/I)min for acceptable signal qualityfollowing equation must hold:1/6 * (R/D) (C/I)min=>-n

Co-Channel reuse ratio QD distance of the 6 closest interfering base stationsR cell radius(C/I)min minimum carrier-to-interference ration path loss exponentQ = D/R = (6*(C/I)min)1/n

Radio Capacity mBt total allocated spectrum for the systemBc channel bandwidthN number of cells in a complete frequency reuse clusterm =BtBc * Nradio channels/cell

Radio Capacity mN is related to the co-channel factor Q by:Q = (3*N)1/2m=BtBc * (Q/3)=BtBc *6CI3n/2(*()min)2/n

Radio Capacity m for n = 4m number of radio channels per cell(C/I)min lower in digital systems compared to analog systemslower (C/I)min imply more capacityexact values in real world conditions measured

m =BtBc *2/3 * (C/I)min

Compare different Systemseach digital wireless standard has different (C/I)minto compare them an equivalent (C/I) neededkeep total spectrum allocation Bt and number of rario channels per cell m constant to get (C/I)eq :

Compare different SystemsBc bandwidth of a particular system(C/I)min tolerable value for the same systemBc channel bandwidth for a different system(C/I)eq minimum C/I value for the different systemCI=CIBcBc()()min)eq*(

C/I in digital cellular systemsRb channel bit rateEb energy per bitRc rate of the channel codeEc energy per code symbolC Eb*Rb Ec*RcI I I==

C/I in digital cellular systemscombine last two equations:(C/I) (Ec*Rc)/I Bc(C/I)eq (Ec*Rc)/I Bc==()The sign marks compared system parameters

C/I in digital cellular systemsRelationship between Rc and Bc is always linear (Rc/Rc = Bc/Bc )assume that level I is the same for two different systems ( I = I ) :Ec BcEc Bc= ()

Compare C/I between FDMA and TDMAAssume that multichannel FDMA system occupies same spectrum as a TDMA system FDMA : C = Eb * Rb ; I = I0 * BcTDMA : C = Eb * Rb ; I = I0 * BcEb Energy per bitI0 interference power per HertzRb channel bit rateBc channel bandwidth

ExampleA FDMA system has 3 channels , each with a bandwidth of 10kHz and a transmission rate of 10 kbps.A TDMA system has 3 time slots, a channel bandwidth of 30kHz and a transmission rate of 30 kbps.Whats the received carrier-to-interference ratio for a user ?

ExampleIn TDMA system C/I be measured in 333.3 ms per second - one time slotC = Eb*Rb = 1/3*(Eb*10E4 bits) = 3*Rb*Eb=3*CI = I0*Bc = I0*30kHz = 3*IIn this example FDMA and TDMA have the same radio capacity (C/I leads to m)

ExamplePeak power of TDMA is 10logk higher then in FDMA ( k time slots)in practice TDMA have a 3-6 times better capacity

Capacity of SDMA systemsone beam each userbase station tracks each user as it movesadaptive antennas most powerful formbeam pattern G() has maximum gain in the direction of desired userbeam is formed by N-element adaptive array antenna

Capacity of SDMA systemsG() steered in the horizontal -plane through 360G() has no variation in the elevation plane to account which are near to and far from the base stationfollowing picture shows a 60 degree beamwidth with a 6 dB sideslope level

Capacity of SDMA systems

Capacity of SDMA systemsreverse link received signal power, from desired mobiles, is Pr;0 interfering users i = 1,,k-1 have received power Pr;Iaverage total interference power I seen by a single desired user:

Capacity of SDMAi direction of the i-th user in the horizontal planeE expectation operatorI = E { G(i) Pr;I}K-1i=1

Capacity of SDMA systemsin case of perfect power control (received power from each user is the same) :Pr;I = PcAverage interference power seen by user 0:I = Pc E { G(i) }K-1i=1

Capacity of SDMA systemsusers independently and identically distributed throughout the cell: I = Pc *(k -1) * 1/DD directivity of the antenna - given by max(G()) D typ. 3dB 10dB

Capacity of SDMA systemsAverage bit error rate Pb for user 0:Pb = Q ( )3 D NK-1D directivity of the antennaQ(x) standard Q-functionN spreading factorK number of users in a cell

Capacity of SDMA systems