Mobile Communications Chapter 3 : Media Access

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1 Moivaion Can we apply media access mehods from fixed neworks? Mobile Communicaions Chaper 3 : Media Access Moivaion SDMA, FDMA, TDMA Aloha Reservaion schemes Collision avoidance, MACA Polling CDMA SAMA Comparison Example CSMA/CD Carrier Sense Muliple Access wih Collision Deecion send as soon as he medium is free, lisen ino he medium if a occurs (original mehod in IEEE 802.3) Problems in wireless neworks signal srengh decreases proporional o he square of he disance he sender would apply CS and CD (, bu he s happen a he receiver i migh be he case ha a sender canno hear he, i.e., CD does no work furhermore, CS migh no work if, e.g., a erminal is hidden Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS05 3. Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS Moivaion - hidden and exposed erminals Moivaion - near and far erminals Hidden erminals A sends o B, C canno receive A C wans o send o B, C senses a free medium (CS fails) a B, A canno receive he (CD fails) A is hidden for C Terminals A and B send, C receives signal srengh decreases proporional o he square of he disance he signal of erminal B herefore drowns ou A s signal C canno receive A Exposed erminals B sends o A, C wans o send o anoher erminal (no A or B) C has o wai, CS signals a medium in use bu A is ouside he radio range of C, herefore waiing is no necessary C is exposed o B A B C A B C If C for example was an arbier for sending righs, erminal B would drown ou erminal A already on he physical layer Also severe problem for CDMA-neworks - precise power conrol needed! Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS05 3.4

2 Access mehods SDMA/FDMA/TDMA SDMA (Space Division Muliple Access) segmen space ino secors, use direced anennas cell srucure FDMA (Frequency Division Muliple Access) assign a cerain frequency o a ransmission channel beween a sender and a receiver permanen (e.g., radio broadcas), slow hopping (e.g., GSM), fas hopping (FHSS, Frequency Hopping Spread Specrum) TDMA (Time Division Muliple Access) assign he fixed sending frequency o a ransmission channel beween a sender and a receiver for a cerain amoun of ime FDD/FDMA - general scheme, example GSM (Frequency Division Duplex) 960 MHz MHz 95 MHz f MHz 200 khz The muliplexing schemes presened in chaper 2 are now used o conrol medium access! MHz Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS µs TDD/TDMA - general scheme, example DECT Aloha/sloed aloha Mechanism random, disribued (no cenral arbier), ime-muliplex Sloed Aloha addiionally uses ime-slos, sending mus always sar a slo boundaries Aloha sender A downlink uplink sender B sender C Sloed Aloha sender A sender B sender C Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS05 3.8

3 DAMA - Demand Assigned Muliple Access Channel efficiency only 8% for Aloha, 36% for Sloed Aloha (assuming Poisson disribuion for packe arrival and packe lengh) Reservaion can increase efficiency o 80% a sender reserves a fuure ime-slo sending wihin his reserved ime-slo is possible wihou reservaion also causes higher delays ypical scheme for saellie links Examples for reservaion algorihms: Explici Reservaion according o Robers (Reservaion-ALOHA) Implici Reservaion (PRMA) Reservaion-TDMA Access mehod DAMA: Explici Reservaion Explici Reservaion (Reservaion Aloha): wo modes: ALOHA mode for reservaion: compeiion for small reservaion slos, s possible reserved mode for daa ransmission wihin successful reserved slos (no s possible) i is imporan for all saions o keep he reservaion lis consisen a any poin in ime and, herefore, all saions have o synchronize from ime o ime Aloha reserved Aloha reserved Aloha reserved Aloha Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS Access mehod DAMA: PRMA Implici reservaion (PRMA - Packe Reservaion MA): a cerain number of slos form a frame, frames are repeaed saions compee for empy slos according o he sloed aloha principle once a saion reserves a slo successfully, his slo is auomaically assigned o his saion in all following frames as long as he saion has daa o send compeiion for his slos sars again as soon as he slo was empy in he las frame reservaion ime-slo ACDABA-F frame A C D A B A F ACDABA-F frame 2 A C A B A AC-ABAFframe a 3 A B A F A---BAFD reservaion frame 4 A B A F D aemps ACEEBAFD frame 5 A C E E B A F D Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS05 3. Access mehod DAMA: Reservaion-TDMA Reservaion Time Division Muliple Access N mini-slos every frame consiss of N mini-slos and x daa-slos every saion has is own mini-slo and can reserve up o k daa-slos using his mini-slo (i.e. x = N * k). oher saions can send daa in unused daa-slos according o a round-robin sending scheme (bes-effor raffic) reservaions for daa-slos N * k daa-slos oher saions can use free daa-slos based on a round-robin scheme Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS e.g. N=6, k=2

4 MACA - avoidance MACA examples MACA (Muliple Access wih Collision Avoidance) uses shor signaling packes for avoidance RTS (reques o send): a sender reques he righ o send from a receiver wih a shor RTS packe before i sends a daa packe (clear o send): he receiver grans he righ o send as soon as i is ready o receive Signaling packes conain sender address receiver address packe size Varians of his mehod can be found in IEEE802. as DFWMAC (Disribued Foundaion Wireless MAC) MACA avoids he problem of hidden erminals A and C wan o send o B A sends RTS firs RTS C wais afer receiving from B A B C MACA avoids he problem of exposed erminals B wans o send o A, C o anoher erminal now C does no have o wai for i canno receive from A RTS RTS A B C Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS ACK MACA varian: DFWMAC in IEEE802. idle wai for ACK sender RxBusy ime-ou NAK; RTS packe ready o send; RTS wai for he righ o send ; daa ACK: posiive acknowledgemen NAK: negaive acknowledgemen ime-ou; RTS daa; ACK ime-ou daa; NAK RxBusy: receiver busy receiver idle wai for daa RTS; RxBusy RTS; Polling mechanisms If one erminal can be heard by all ohers, his cenral erminal (a.k.a. base saion) can poll all oher erminals according o a cerain scheme now all schemes known from fixed neworks can be used (ypical mainframe - erminal scenario) Example: Randomly Addressed Polling base saion signals readiness o all mobile erminals erminals ready o send can now ransmi a random number wihou wih he help of CDMA or FDMA (he random number can be seen as dynamic address) he base saion now chooses one address for polling from he lis of all random numbers ( if wo erminals choose he same address) he base saion acknowledges correc packes and coninues polling he nex erminal his cycle sars again afer polling all erminals of he lis Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS05 3.6

5 ISMA (Inhibi Sense Muliple Access) Curren sae of he medium is signaled via a busy one he base saion signals on he downlink (base saion o erminals) if he medium is free or no erminals mus no send if he medium is busy erminals can access he medium as soon as he busy one sops he base saion signals s and successful ransmissions via he busy one and acknowledgemens, respecively (media access is no coordinaed wihin his approach) mechanism used, e.g., for CDPD (USA, inegraed ino AMPS) Access mehod CDMA CDMA (Code Division Muliple Access) all erminals send on he same frequency probably a he same ime and can use he whole bandwidh of he ransmission channel each sender has a unique random number, he sender XORs he signal wih his random number he receiver can une ino his signal if i knows he pseudo random number, uning is done via a correlaion funcion Disadvanages: higher complexiy of a receiver (receiver canno jus lisen ino he medium and sar receiving if here is a signal) all signals should have he same srengh a a receiver Advanages: all erminals can use he same frequency, no planning needed huge code space (e.g ) compared o frequency space inerferences (e.g. whie noise) is no coded forward error correcion and encrypion can be easily inegraed Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS CDMA in heory CDMA on signal level I Sender A sends A d =, key A k = 000 (assign: 0 = -, = +) sending signal A s = A d * A k = (-, +, -, -, +, +) Sender B sends B d = 0, key B k = 00 (assign: 0 = -, = +) sending signal B s = B d * B k = (-, -, +, -, +, -) Boh signals superimpose in space inerference negleced (noise ec.) A s + B s = (-2, 0, 0, -2, +2, 0) Receiver wans o receive signal from sender A apply key A k biwise (inner produc) A e = (-2, 0, 0, -2, +2, 0) A k = = 6 resul greaer han 0, herefore, original bi was receiving B B e = (-2, 0, 0, -2, +2, 0) B k = = -6, i.e. 0 daa A key A key sequence A daa key signal A Real sysems use much longer keys resuling in a larger disance beween single code words in code space. A d A k A s Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS

6 CDMA on signal level II CDMA on signal level III signal A A s daa A 0 A d daa B 0 0 B d key B key sequence B daa key B k A k signal B B s ( ) * A k inegraor oupu comparaor oupu 0 Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS CDMA on signal level IV CDMA on signal level V daa B 0 0 B d wrong key K B k ( ) * B k ( ) * K inegraor oupu inegraor oupu comparaor oupu 0 0 comparaor oupu (0) (0)? Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS

7 SAMA - Spread Aloha Muliple Access Aloha has only a very low efficiency, CDMA needs complex receivers o be able o receive differen senders wih individual codes a he same ime Idea: use spread specrum wih only one single code (chipping sequence) for spreading for all senders accessing according o aloha sender A 0 narrow sender B 0 band send for a shorer period wih higher power spread he signal e.g. using he chipping sequence 00 ( CDMA wihou CD ) Problem: find a chipping sequence wih good characerisics Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS Comparison SDMA/TDMA/FDMA/CDMA Approach SDMA TDMA FDMA CDMA Idea segmen space ino cells/secors Terminals Signal separaion only one erminal can be acive in one cell/one secor cell srucure, direced anennas Advanages very simple, increases capaciy per km² Disadvanages inflexible, anennas ypically fixed segmen sending ime ino disjoin ime-slos, demand driven or fixed paerns all erminals are acive for shor periods of ime on he same frequency synchronizaion in he ime domain esablished, fully digial, flexible guard space needed (mulipah propagaion), synchronizaion difficul segmen he frequency band ino disjoin sub-bands every erminal has is own frequency, uninerruped filering in he frequency domain simple, esablished, robus inflexible, frequencies are a scarce resource spread he specrum using orhogonal codes all erminals can be acive a he same place a he same momen, uninerruped code plus special receivers flexible, less frequency planning needed, sof handover complex receivers, needs more complicaed power conrol for senders Commen only in combinaion sandard in fixed ypically combined wih TDMA, FDMA or neworks, ogeher wih TDMA CDMA useful wih FDMA/SDMA (frequency hopping used in many paerns) and SDMA mobile neworks (frequency reuse) TDMA/FDMA Prof. Dr.-Ing. Jochen Schiller, hp:// MC SS sill faces some problems, higher complexiy, lowered expecaions; will be inegraed wih

Key Issue. 3. Media Access. Hidden and Exposed Terminals. Near and Far Terminals. FDD/FDMA General Scheme, Example GSM. Access Methods SDMA/FDMA/TDMA

Key Issue. 3. Media Access. Hidden and Exposed Terminals. Near and Far Terminals. FDD/FDMA General Scheme, Example GSM. Access Methods SDMA/FDMA/TDMA Key Issue Can we apply media access mehods from fixed neworks? 3. Media Access SDMA, FDMA, TDMA Aloha and Reservaion Schemes Avoidance and Polling MACA, CDMA, SAMA Example CSMA/CD: Carrier Sense Muliple