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 Access wih Deecion Send as soon as he medium is free, lisen ino he medium if a collision occurs: Original mehod of IEEE 802.3 Eherne Problems in wireless neworks: Signal srengh decreases proporionally o he square of he disance The sender would apply CS and CD, bu he collisions happen a he receiver I migh be he case ha a sender canno hear he collision, i.e., CD does no work Furhermore, CS migh no work if, e.g., a erminal is hidden, i can no be seen 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 2 Hidden and Exposed Terminals Near and Far Terminals Hidden erminals (verseckes Endgerä): 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 collision (CD fails) A is hidden for C -> s Terminals A and B send, C receives: Signal srengh decreases proporional o he square of he disance A and B send wih similar signal srengh The signal of erminal B, herefore, drowns ou A s signal a C C canno receive A Exposed erminals (ausgelieferes Endgerä): B sends o A, C wans o send o anoher erminal (D) 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 -> Exended delays 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 3 A B C D If C, for example, was an arbier for sending righs, erminal B would drown ou erminal A already on he physical layer Lack of fairness for any MAC layer scheme Also severe problem for CDMA neworks: Precise power conrol needed! Differen signal srengh monopolize medium! 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 4 Access Mehods SDMA/FDMA/TDMA FDD/FDMA General Scheme, Example GSM 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 The muliplexing schemes presened in module 2 are now used o conrol he access o he medium! Duplex channel: Pre-defined frequencies for up-link and down-link Down-link f 960 MHz 935.2 MHz 95 MHz 890.2 MHz Up-link 24 24 FDD: Frequency Division Duplex GSM: Global Sysem for Mobile Communicaion Duplex connecion 20 MHz 200 khz 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 5 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 6
TDD/TDMA General Scheme, Example DECT Aloha and Sloed Aloha Advanage: Pre-defined frequency no changed, sender becomes simpler, SW-based access Drawback: Fixed symmeric bandwidh assignmens per sender/receiver pair 47 µs 2 3 2 2 3 2 Downlink TDD: Time Division Duplex DECT: Digial Enhanced Cordless Telephone 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 7 Uplink 0 ms Mechanism: Random, disribued (no cenral arbier), ime-muliplex Sloed Aloha addiionally uses ime-slos, sending mus always sar a slo boundaries Aloha: Sender A Sender C Sloed Aloha: Sender A Sender C 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 8 DAMA Demand Assigned Muliple Access Access Mehod DAMA: Explici Reservaion Aloha channel efficiency: Assuming a Poisson disribuion for he packe arrival and he packe lengh: Only 8% for Aloha 36% for Sloed Aloha Reservaion can increase efficiency o 80%: A sender reserves a fuure ime-slo Sending wihin his reserved ime-slo is possible wihou collision Reservaion also causes higher delays Typical scheme for saellie links Explici Reservaion (Reservaion Aloha): Two modes: ALOHA mode for reservaion: compeiion for small reservaion slos, collisions possible Reserved mode for daa ransmission wihin successful reserved slos (no collisions 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 Examples for reservaion algorihms: Explici Reservaion according o Robers (Reservaion ALOHA) Implici Reservaion (PRMA) Reservaion TDMA Aloha reserved (compeiion) Aloha reserved Aloha reserved Aloha Saellie-based reservaion may reserve he nex afer he nex slos! 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 9 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 0 Access Mehod DAMA: PRMA Implici reservaion (PRMA: Packe Reservaion Muliple Access): A cerain number of slos (e.g., 8) 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 Vecor (sen periodically by BS) Reservaion ACDABA-F ACDABA-F AC-ABAF- A---BAFD ACEEBAFD Frame Frame 2 Frame 3 Frame 4 Frame 5 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 2 3 4 5 6 7 8 Time Slo A C D A B A F A C A B A A B A F a A B A F D reservaion aemps A C E E B A F D Access Mehod DAMA: Reservaion TDMA Reservaion Time Division Muliple Access: Every frame consiss of N mini slos and x daa slos (saic reserve/ransmi) 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) or in an Aloha-ype N Mini Slos Reservaions for daa-slos N * k Daa Slos Maximum access ime -> guaraneed bandwidh! Isochronous daa raffic suppor. 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 2 E.g., N=6, k=2 Oher saions can use free daa-slos based on a round-robin scheme 2
MACA Avoidance Soluion o hidden/exposed erminals w/o base saion MACA (Muliple Access wih Avoidance) uses shor signaling packes for collision avoidance: (Reques To Send): A sender reques he righ o send from a receiver wih a shor packe before i sends a daa packe CTS (Clear To Send): The receiver grans he righ o send as soon as i is ready o receive Signaling packes conain: Sender address Receiver address Packe size/duraion of ransmission Pros: W/o base saion, flexible as Aloha, dynamic reservaion Varians of his mehod can be found in IEEE802. as DFWMAC (Disribued Foundaion Wireless MAC) 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 3 MACA Examples MACA avoids he problem of hidden erminals: A and C wan o send o B A sends firs C wais afer receiving CTS from B No addiional erminal moves in! CTS CTS s possible due o muliple, no a problem hough! 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 a CTS ( wrong receiver) CTS Canno receive CTS from A due o disance, C may sen o D 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 4 : Reques o Send CTS: Clear o Send D MACA Varian: DFWMAC in IEEE 802. ACK Idle RxBusy Wai for ACK Sender Time-ou NAK; ACK: Posiive acknowledgemen NAK: Negaive acknowledgemen Packe ready o send; Wai for he righ o send CTS; daa DFWMAC: Disribued Foundaion Wireless MAC 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 5 Time-ou; Daa; ACK Time-ou disurbed daa; NAK RxBusy: Receiver busy C/: Clear/Reques o Send Receiver Idle Wai for daa ; RxBusy ; CTS 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: maser/slave approach) Example: Randomly Addressed Polling: Base saion signals readiness o all mobile erminals Terminals ready o send can now ransmi a random number wihou collision wih he help of CDMA or FDMA (he random number can be seen as dynamic address) The base saion now chooses one address for polling from he lis of all random numbers (collision if wo erminals choose he same address) The base saion acknowledges correc packes and coninues polling he nex erminal This cycle sars again afer polling all erminals of he lis 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 6 ISMA (Inhibi Sense Muliple Access) Access Mehod CDMA Curren sae of he medium is signaled via a busy one : The base saion signals on he downlink (base saion o erminals) if he medium is free or no Terminals mus no send if he medium is busy Terminals can access he medium as soon as he busy one sops The base saion signals collisions and successful ransmissions via he busy one and acknowledgemens, respecively (media access is no coordinaed wihin his approach) Missing ACK idenifies a collision Back-off and reransmission required Mechanism used, e.g., for CDPD (Cellular Digial Packe Daa) in he US, inegraed ino AMPS (Advanced Mobile Phone Sysem) 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 7 CDMA (Code Division Muliple Access): All erminals send on he same frequency, probably a he same ime, and can use he enire bandwidh of he ransmission channel Each sender has a unique random number, he sender XORs he signal wih his random number The receiver can une ino his signal, if i knows he pseudo random number, uning is done via a correlaion funcion Drawbacks: 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., 2 32 ) compared o frequency space Inerference, e.g., whie noise, is no coded Forward error correcion and encrypion can be easily inegraed 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 8 3
CDMA in Theory CDMA on he Signal Level () Sender A wih unique code/key: Sends A d =, key A k = 000 (assign: 0 = -, = +) Sending signal A s = A d * A k = (-, +, -, -, +, +) wih unique code/key: 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 (e.g., noise) in he analog modulaion case = (-2, 0, 0, -2, +2, 0) Receiver wans o receive signal from sender A: Apply key A k bi-wise (inner produc/skalarproduk): A e = (-2, 0, 0, -2, +2, 0) A k = (-2, 0, 0, -2, +2, 0) (0,, 0,0,,) = 2 + 0 + 0 + 2 + 2 + 0 = 6; Resul greaer han 0, herefore, original bi was Receiving B: B e = (-2, 0, 0, -2, +2, 0) B k = -2 + 0 + 0-2 - 2 + 0 = -6, i.e. 0 Code/key = Chipping Sequence Daa A Key A Key Sequence A Daa Key Signal A 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Real sysems use much longer keys resuling in a larger disance beween single code words in code space. IS-95 uses 2 42 - code lengh, chipping sequence,228,800 chips/s. Code repeiion all 4,425 days. A d A k A s 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 9 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 20 CDMA on he Signal Level (2) CDMA on he Signal Level (3) Signal A A s Daa A 0 A d Daa B 0 0 B d Key B Key Sequence B Daa Key 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 B k A k Signal B B s ( ) * A k Inegraor Oupu Comparaor Oupu 0 Daa B are coded and combined wih he resul of Daa A: Reconsrucion of daa sen from A 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 2 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 22 CDMA on he Signal Level (4) CDMA on he Signal Level (5) Daa B 0 0 B d Wrong Key K B k ( ) * K ( ) * B k Inegraor Oupu Inegraor Oupu Comparaor Oupu 0 0 Comparaor Oupu (0) (0)? Reconsrucion of daa sen from B Receiving of a signal wih a wrong key 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 23 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 24 4
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. Base saion/mobile erminal simple Idea: Use spread specrum wih only one single code (chipping sequence) for spreading for all senders accessing according o aloha Sender A 0 0 Narrow-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 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 25 Comparison of SDMA, TDMA, FDMA, CDMA Approach SDMA TDMA FDMA CDMA Idea segmen space ino cells/secors segmen sending ime ino disjoin segmen he frequency band ino spread he specrum using orhogonal codes ime-slos, demand disjoin sub-bands driven or fixed paerns Terminals only one erminal can be acive in one all erminals are acive for shor every erminal has is own frequency, all erminals can be acive a he same place a he cell/one secor periods of ime on uninerruped same momen, he same frequency uninerruped Signal cell srucure, direced synchronizaion in filering in he code plus special separaion anennas he ime domain frequency domain receivers Advanages Disadvanages Commen very simple, increases capaciy per km² inflexible, anennas ypically fixed only in combinaion wih TDMA, FDMA or CDMA useful esablished, fully digial, flexible guard space needed (mulipah propagaion), synchronizaion difficul sandard in fixed neworks, ogeher wih FDMA/SDMA used in many mobile neworks 2005 Burkhard Siller and Jochen Schiller FU Berlin M3 26 simple, esablished, robus inflexible, frequencies are a scarce resource ypically combined wih TDMA (frequency hopping paerns) and SDMA (frequency reuse) flexible, less frequency planning needed, sof handover complex receivers, needs more complicaed power conrol for senders sill faces some problems, higher complexiy, lowered expecaions; will be inegraed wih TDMA/FDMA 5