Wireless Networkig ad ommuicatios Group Wireless Networkig ad ommuicatios Group What is Multiple Access? ode Divisio Multiple Access for Wireless ommuicatios Prof. effre G. Adrews Wireless Networkig ad ommuicatios Group (WNG) Electrical ad omputer Egieerig Dept. Uiversit of Texas at Austi Multiple users wat to commuicate i a commo geographic area ellular Example: Ma people wat to talk o their cell phoes. Each phoe must commuicate with a ase statio. Imagie if ol oe perso could talk o their cell phoe at a time! Prolem: How should we share our resources so that as ma users as possile ca commuicate simultaeousl? WNG Wireless Networkig ad ommuicatios Group Freq. Divisio Multiple Access (FDMA) AMPS (aalog), the First Geeratio (G) used 0 KHz for each user. Pros Ver Simple to desig Narrowad (o ISI) Schroizatio is eas No iterferece amog users i a cell os Narrowad iterferece Static spectrum allocatio Freq. reuse is a prolem High Q aalog filters or large guard ad required Wireless Networkig ad ommuicatios Group Time Divisio Multiple Access (TDMA) a also partitio time: users take turs usig the chael IS-5 (G) used same 0 KHz chaels, ut with three users sharig them ( slots) GSM has 8 slots/70 KHz Pros Better suited for digital Ofte gets higher capacit ( times higher here) Relaxes eed for high Q filters os Strict schroizatio ad guard time eeded Still susceptile to jammig, other-cell iterferece Ofte requires equalizer
Wireless Networkig ad ommuicatios Group Alterative to FDMA ad TDMA? What if we could allow users to share time ad frequec? Elimiates eed for tight schroizatio amog ma differet users Elimiates eed for expesive aalog filters Ma have favorale impact o capacit (?) But: How do we separate the users? Wo t the iterfere with each other? Wireless Networkig ad ommuicatios Group ode Divisio Multiple Access (DMA) k (t): its for user k k : spreadig code : spreadig factor s k (t): trasmitted sigal for user k (t): received sigal for all users K ( t) hk ( t) sk ( t) ( t) k h k (t): chael impulse respose for user k (t): oise 5 WNG Spreadig odes Wireless Networkig ad ommuicatios Group Wireless Networkig ad ommuicatios Group How Walsh odes Work The spreadig code k must e uique for each user. Ideall, the are orthogoal to oe aother, i.e. < i, k > 0, uless i k < i, k >, if i k Example: Walsh odes For a spreadig factor, there are Walsh codes I geeral there are alwas Walsh codes, as log as,, 8, 6,, 6, 8,... -Ar Walsh odes Assume a oiseless chael with uit gai Walsh codes allow four users to e trasmitted at the same time ad frequec usig four times the umer of its, sg sg 0 0 0 sg( )
9 Wireless Networkig ad ommuicatios Group A umerical example ( ) ( ) ( ) ( ) sg sg, sg sg sg sg, sg sg Decoded Bits : : Received sigal Spreadig odes :, - Bits : Wireless Networkig ad ommuicatios Group Properties of Walsh odes There are some issues with Walsh odes Schroizatio of all users is required I a multipath chael, delaed copies ma e received which are ot orthogoal a loger! Ol codes exist with a adwidth expasio of, so as far as capacit, we are right ack where we started with TDMA ad FDMA! Advatages relative to TDMA ad FDMA No guard ads or guard times are tpicall required No equalizer is tpicall required, whe a RAKE receiver is used Wireless Networkig ad ommuicatios Group The IS-95 DMA (G) Forward Lik This trasmitter structure is used i the ase statio Sprit PS, Verizo, ad worldwide Total spreadig gai 8 WNG Wireless Networkig ad ommuicatios Group The IS-95 Reverse Lik The reverse lik is quite differet Istead of Walsh odes, psuedoradom oise (PN) codes PN codes are determiistic Beroulli sequeces of {-,} While ot orthogoal, the have low cross-correlatio, e.g. < i, k >, uless i k < i, k >, if i k These codes have good properties eve whe ot schroized Ver strog error correctig codes make up the differece
Wireless Networkig ad ommuicatios Group The Near-Far Prolem Users ma e received with ver differet powers: Users ear the ase statio are received with high power Users far from the ase statio are received with low power For a path loss expoet of ad a cell size of km, example: P 000 60,000 5dB! P 50 Near users will completel swamp far awa users Solutio: Power otrol DMA Issues Wireless Networkig ad ommuicatios Group So far, DMA looks like a step ackwards: Tight schroizatio is required to use orthogoal codes, which the reak i a multipath chael awa Quasi-orthogoal codes cause self-iterferece, which domiates the performace i most DMA sstems Near-far prolem is a serious hidrace, requirig fast ad accurate power cotrol (that uses up its we could otherwise sed iformatio with) Ad for all this, the required adwidth is ow times larger tha it was efore, so there does t appear to e a capacit gai How did Qualcomm covice people to use this stuff? WNG Wireless Networkig ad ommuicatios Group Iterferece Averagig It turs out there are serious advatages to DMA i a multicell sstem Ulike FDMA ad TDMA, DMA does ot rel o orthogoal frequec ad time slots that are compromised eighorig cells DMA sstems ca reuse frequecies ever cell! (FDMA ad TDMA usuall eed reuse factors of - 7) Voice Activit Wireless Networkig ad ommuicatios Group I TDMA ad FDMA sstems: If a user does t have athig to sed, the time/frequec slot allocated to them is wasted It is tpicall ver difficult to damicall allocate time ad frequec slots I DMA sstems: If a user does t have athig to sed, it causes less iterferece to other users of the sstem Tpicall, each user eeds to trasmit less tha half the time Sice iterferece-limited, this doules the capacit apacit icreased -7 fold
Wireless Networkig ad ommuicatios Group Sectorized Ateas ells ca use directioal ateas to sectorize the cell At right, 0 degree ateas create -sector cells ver commo For DMA, this reduces the iterferece a factor of three apacit is icreased a factor of three! FDMA/TDMA also use sectored ateas, ut just to decrease reuse distace Wireless Networkig ad ommuicatios Group apacit ompariso omparig the capacit of TDMA/FDMA/DMA is ver cotroversial I 99, a famous (otorious?) Qualcomm paper claimed that due to voice activit, frequec reuse, ad sectorizatio, DMA icreased capacit : Factor of 8 relative to AMPS Factor of 6 relative to US TDMA (ad similar for GSM) This tured out to e optimistic, aout / of this gai actuall happeed (depeds o who ou ask) Still, twice as ma users is othig to sear at! All G sstems use DMA for multiple access 7 WNG Wireless Networkig ad ommuicatios Group The Future of DMA DMA has overcome most cicism to domiate the worldwide wireless voice market What aout data services? Schedulig vs. Iteferece Averagig DMA appears to e a uderdog for G, ut still ma wi Ogoig research o DMA Icrease capacit joit decodig (multiuser detectio & iterferece cacellatio) Applig DMA to other applicatios: optical DMA, ad hoc etworks, dese wireless LANs MultiDMA : multiple atea DMA, multicarrier DMA, multicode DMA Further Readig Wireless Networkig ad ommuicatios Group. Prof. Adrews s DMA wepage : http://www.ece.utexas.edu/~jadrews/cdma.html. A tutorial: R. Koho, R. Meida, ad L. Milstei, Spread spectrum access methods for wireless commuicatios, IEEE omm. Magazie, a. 995.. The Qualcomm capacit paper K.S. Gilhouse et al, O the capacit of a cellular DMA sstem,'' IEEE Tras. o Vehicular Tech., Ma 99.. The defiitive text (theoretical) A. Viteri, DMA: Priciples of Spread Spectrum ommuicatio, Addiso Wesle, 995.