Review Wireless Communications

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1 EEC173B/ECS152C, Winer 2006 Review of las week s maerial Wireless Channel Access Challenges: Hidden/Exposed Terminals Access Mehods: SDMA, FDMA, TDMA, CDMA Random Access: Aloha, CSMA/CD, Reservaion Chuah, Winer 06 Review Wireless Communicaions Wha is muliplexing? Which 4 dimensions can we use for muliplexing? Wha is frequency reuse? Wha is he relaionship beween cluser size, N, frequency reuse disance, D, and cell radius, R? Can N be any number? Wha equaion mus N saisfy? How do you calculae co channel inerference? Wha is handoff? 2 Page 1

2 Page 2 3 Basic hings o remember abou signals for HW#1 Le ransmied signal x, received signal y, channel funcion, h d h x y h x y 0 τ τ τ A convoluion is an inegral ha expresses he amoun of overlap of one funcion h as i is shifed over anoher funcion x As far as his class is concerned, when we discuss mulipah fading, h is usually represened as impulse rain. K i k h 1 θ δ 4 Special Propery of Dela Funcion and 0 oherwise, 1.2 1when 1.2 and 0 oherwise 0, 1when excep when 0 b T b T δ δ α α δ b T A A h + e.g., for HW#1: ] [ for HW#1 e.g., b b b b T Ap Ap T A A p T p T p p p x x + + δ δ δ δ θ θ δ

3 EEC173B/ECS152C, Winer 2006 Review of las week s maerial Wireless Channel Access Challenges: Hidden/Exposed Terminals Access Mehods: SDMA, FDMA, TDMA, CDMA Random Access: Aloha, CSMA/CD, Reservaion * Acknowledgmen: Seleced slides from Prof. Schiller Chuah, Winer 06 Medium Access Sublayer Nework Daa Link Physical Medium Access Sublayer Medium Access Conrol MAC Proocols disribued algorihm ha arbirae access o a common shared channel among a populaion of users, i.e., deermine when a node can ransmi 6 Page 3

4 Remarks on MAC Sublayer MAC is no imporan on poin o poin links MAC is only used in broadcas or shared channel neworks Only one can send successfully a a ime Two or more simulaneous ransmissions > inerference How o share a broadcas channel? Communicaion abou sharing mus use he channel iself! Examples: Packe swiched Radio Nework Aloha Eherne IEEE CSMA/CD Token Ring IEEE 802.5, FDDI Token Passing Cellular, Saellie, Wireless LAN MACAW 7 Moivaion Can we apply media access mehods from fixed neworks? Example CSMA/CD Carrier Sense Muliple Access wih Collision Deecion Send as soon as he medium is free, lisen ino he medium if a collision occurs original mehod in IEEE Page 4

5 Problems in wireless neworks Signal srengh decreases proporional 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 9 Hidden Terminals Hidden erminals A sends o B, C canno receive A C wans o send o B, C senses a free medium CS fails Collision a B, A canno receive he collision CD fails A is hidden for C A B C 10 Page 5

6 Exposed Terminals 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, waiing is no necessary! C is exposed o B A B C D 11 Near and Far Terminals Terminals A and B send, C receives Signal srengh decreases proporional o he square of he disance The signal of erminal B herefore drowns ou A s signal C canno receive A 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! 12 Page 6

7 Taxonomy of MAC Proocols 13 Conrolled Access mehods 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 before are now used o conrol medium access! 14 Page 7

8 FDD/FDMA e.g., GSM 960 MHz f MHz 915 MHz MHz 200 khz MHz 1 15 TDD/TDMA, e.g., DECT 417 µs downlink uplink 16 Page 8

9 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 The receiver can une ino his signal if i knows he pseudo random number, uning is done via a correlaion funcion 17 CDMA Con d 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 18 Page 9

10 CDMA: An Example Sender A Sends A d 1, key A k assign: 0 1, 1 +1 Sending signal A s A d * A k 1, +1, 1, 1, +1, +1 Sender B Sends B d 0, key B k assign: 0 1, 1 +1 Sending signal B s B d * B k 1, 1, +1, 1, +1, 1 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 Resul greaer han 0, herefore, original bi was 1 Receiving B B e 2, 0, 0, 2, +2, 0 B k , i.e Anoher Example: CDMA on signal level 1 daa A key A key sequence A daa key A d A k signal A A s Real sysems use much longer keys resuling in a larger disance beween single code words in code space. For his example, 0 is high, 1 is low. 20 Page 10

11 CDMA on signal level 2 signal A A s daa B B d key B key sequence B daa key B k signal B B s A s + B s 21 CDMA on signal level 3 daa A A d A s + B s A k A s + B s * A k inegraor oupu comparaor oupu Page 11

12 CDMA on signal level 4 daa B B d A s + B s B k A s + B s * B k inegraor oupu comparaor oupu CDMA on signal level 5 A s + B s wrong key K A s + B s * K inegraor oupu comparaor oupu 0 0? 24 Page 12

13 Implicaions for High Speed Wireless Daa Conrolled muliple access performs well wih coninuous sream raffic bu inefficien for bursy raffic Complexiy: frequency division < ime division < code division Muliple daa raes Muliple frequency bands Muliple ime slos Muliple codes 25 Comparison SDMA/TDMA/FDMA/CDMA Approach SDMA TDMA FDMA CDMA Idea segmen space ino spread he specrum cells/secors using orhogonal codes Terminals Signal separaion Advanages Disadvanages Commen only one erminal can be acive in one cell/one secor cell srucure, direced anennas very simple, increases capaciy per km² inflexible, anennas ypically fixed only in combinaion wih TDMA, FDMA or CDMA useful 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 sandard in fixed neworks, ogeher wih FDMA/SDMA used in many mobile neworks 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 ypically combined wih TDMA frequency hopping paerns and SDMA frequency reuse 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 sill faces some problems, higher complexiy, lowered expecaions; will be inegraed wih TDMA/FDMA 26 Page 13

14 Taxonomy of MAC Proocols 27 Random Access Techniques Design Goals Fully decenralized Fairness among users High efficiency Low delay Faul olerance Techniques Aloha Carrier Sense Techniques Reservaion Proocols 28 Page 14

15 Pure Aloha Originally developed for packe radio communicaions a he campuses of U. of Hawaii in 1970 beween a cenral compuer & various daa erminals All nodes ransmi on one frequency, f o Cenral node relays packes on he oher frequency, f 1 Cenral Node f o f 1 node 1 node 2 node 3 node n 29 Pure Aloha Algorihm 1. Daa is packeized and users ransmi whenever hey have somehing o send Random, disribued no cenral arbier, ime muliplex If more han one node ransmis a he same ime > Collision! If here is a collision, boh nodes need o reransmi Cenral Node X 30 Page 15

16 Pure Aloha Algorihm con d 2. Lisen o he broadcas Assume he receiver rebroadcass he received signal, so he sender can find ou if is packe was desroyed jus by lisening o downward broadcas one round rip ime afer sending i 3. If packe was desroyed, wai a random amoun of ime, and send i again preven he same packe from colliding over and over again 31 Conenion Period in Pure Aloha Jus send: no waiing for beginning of slo A collision occurs for any parial overlap of packes If firs bi of a new packe overlaps wih las bi of a packe almos finished, boh packes are desroyed. : one packe ransmission ime L/R Vulnerable period: 2 32 Page 16

17 Sloed Aloha How o reduce vulnerabiliy/conenion period? Time is divided ino equal size slos Avoids parial overlap of packes Nodes mus always sar ransmi a he beginning of a slo Packes mus be ransmied wihin a slo If collision, reransmi laer Success S, Collision C, Empy E slos 33 Conenion Period in Sloed Aloha : one packe ransmission ime L/R slo duraion Packe could arrive in he middle of a slo, bu can send unil he beginning of a slo Poenial collision wih packes sen wihin he same slo Vulnerable period: 34 Page 17

18 Pure Aloha vs. Sloed Aloha Pure Aloha collision sender A sender B sender C Sloed Aloha collision sender A sender B sender C 35 Performance of Aloha Channel efficiency only 18% for Aloha, 36% for Sloed Aloha assuming Poisson disribuion for packe arrival and packe lengh S Throughpu per Packe Time Pure Aloha Sloed Aloha G Aemps per Packe TIme Commens Inefficien for heavily loaded sysem Capure effec improves efficiency Combining SS wih ALOHA reduces collisions 36 Page 18

19 Demand Assigned Muliple Access DAMA Use a common reservaion channel o assign bandwidh on demand A sender reserves a fuure ime slo > Sending wihin his reserved ime slo is possible wihou collision Typical scheme for saellie links Reservaion can increase efficiency o 80% Very efficien if overhead raffic is a small percenage of he message raffic 37 DAMA 2 Issues Reservaion also causes higher delays Reservaion channel requires exra bandwidh Example reservaion algorihms: DAMA w/ Explici Reservaion: Reservaion ALOHA DAMA w/ Implici Reservaion: PRMA Packe Reservaion MA Reservaion TDMA Time Division MA 38 Page 19

20 DAMA w/ Explici Reservaion 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 collision Aloha reserved Aloha reserved Aloha reserved Aloha 39 DAMA w/ Implici Reservaion PRMA 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 40 Page 20

21 Access mehod DAMA: PRMA reservaion ACDABA F ACDABA F AC ABAF A BAFD ACEEBAFD frame 1 frame 2 frame 3 frame 4 frame ime slo A C D A B A F A C A B A A B A F collision a A B A F D reservaion aemps A C E E B A F D 41 DAMA Reservaion TDMA Time Division Muliple Access TDMA 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 beseffor raffic N mini-slos N * k daa-slos e.g. N6, k2 reservaions for daa-slos oher saions can use free daa-slos based on a round-robin scheme 42 Page 21

22 Carrier Sense Techniques Channel is sensed before ransmission o deermine if i is occupied More efficien han ALOHA Fewer reransmissions Carrier sensing is ofen combined wih collision deecion in wired neworks e.g., Eherne NOT POSSIBLE in a radio environmen Collision avoidance is used in curren wireless LANs. WLAN, Specral Eiquee 43 MACA Collision Avoidance MACA Muliple Access wih Collision Avoidance uses shor signaling packes for collision 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 CTS 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 IEEE as DFWMAC Disribued Foundaion Wireless MAC 44 Page 22

23 MACA examples MACA avoids he problem of hidden erminals A and C wan o send o B A sends RTS firs C wais afer receiving CTS from B 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 CTS from A RTS CTS A B C RTS CTS CTS RTS A B C 45 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 46 Page 23

24 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 47 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 collisions 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 48 Page 24

25 Example Wireless Packe Daa Services ARDIS Advanced Radio Daa Inegraed Sysem Join venure of Moorola/IBM Sloed CSMA RAM Mobile Daa Mobiex Join venure of Ericsson/BellSouh Provides communicaion link beween base and mobile Typical applicaions: hos access and dispach Sloed CSMA CDPD DSMA/CD Digial Sense Muliple Access Collisions deeced a receiver and ransmied back WaveLAN CSMA/CA 49 Implicaions for High Speed Wireless Daa Reransmissions are power and specrally inefficien. ALOHA canno saisfy high speed daa hroughpu requiremens. Reservaion proocols are also ineffecive for shor messaging. Delay consrains impose hroughpu limiaions. 50 Page 25

Mobile Communications Chapter 3 : Media Access

Mobile Communications Chapter 3 : Media Access 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