Multiplexing. Structure of the Lecture. Channels. Frequency Multiplexing

Transcription

1 Srucure o he Lecure Muliplexing Chaper Technical Basics: Layer Mehods or Medium Access: Layer Channels in a requency band Saic medium access mehods (Muliplexing) Flexible medium access mehods (Muliple Access) Chaper 3 Wireless Neworks: Blueooh, WLAN, WirelessMAN, WirelessWAN Mobile Neworks: GSM, GPRS, UMTS Saellies and Broadcas Neworks Chaper 4 Mobiliy on he nework layer: Mobile IP, Rouing, Ad-Hoc Neworks Mobiliy on he ranspor layer: reliable ransmission, low conrol, QoS Mobiliy suppor on he applicaion layer Page Goal: muliple use o a shared medium Wireless communicaion sysems usually are broadcas neworks Wih muliplexing schemes, we do a coordinaion o several ransmission aemps by assigning a cerain channel exclusively o one sender Muliplexing needs he creaion o channels in one o our dimensions (or wih a combinaion): ime () requency () code (c) and maybe: space (s i ) Channels k i s k c k k 3 k 4 k 5 k 6 Muliplexing in space: creae cells s 3 c c s Page Channels Muliplexing For a wireless communicaion sysem, we ge assigned a cerain requency band We need o subdivide our requency band somehow in areas (channels) which can be used or dieren ransmissions simulaneously. To avoid inererence beween he channels, guard spaces are needed. Division Muliple Access (FDMA)... Time Division Muliple Access (TDMA) Code Division Muliple Access (CDMA) Code c k k k 3 k 4 k 5 k 6 Principle: Separae he whole requency band ino sub-bands One sub-band corresponds o one channel, assigned o one communicaion pary Guard spaces beween he sub-bands are necessary o avoid inererence o neighbored channels Time Each ransmission is assigned a cerain sub-band o he whole requency band Each ransmission is assigned he whole requency band or cerain ime slos Time Time All ransmissions ake place all he ime on he whole requency band, bu using a code Page 3 Advanage: Simpliciy Easy o implemen (Radio, TV principle) No coordinaion necessary Disadvanage: Inlexibiliy Wase o bandwidh i a pary is no sending coninuously Bursy raic canno be served Page 4

2 Division Duplex (FDD) FDD/FDMA in GSM FDD is a sandard (since early sysems o mobile communicaion) o use requency muliplexing or a duplex communicaion Principle: For a duplex communicaion, wo sub-bands are used, one or sending, he oher or receiving (uplink resp. downlink) On echnical reasons o avoid inererence beween he sen and he received in he anennas: a guard space beween he uplink- and he downlink requencies is needed (e.g. 45MHz in GSM) wo separae anennas have o be used, or a duplexer is needed which consiss o wo band ilers which suppress he unwished s Downlink Uplink 960 MHz 935. MHz 95 MHz 890. MHz MHz 00 khz D d A d B d C d... D u A u B u C u... X MHZ X+45 MHz Page 5 Page 6 Time Muliplexing TDMA Guard Times Principle: A ransmission is assigned he whole requency band exclusively or he duraion o a cerain duraion (ime slo) Advanages: No guard spaces beween requency bands hus, compared wih FDMA, more eicien usage o bandwidh Low power consumpion (deacivaion o sending/receiving device or k k k 3 k 4 k 5 k 6 unused ime slos) c Disadvanage: Precise synchronizaion beween all communicaion parners Guard imes beween he ime slos Timing advance needed For avoiding overlapping o he s o dieren senders, a precise synchronizaion is necessary Problem: Assume a cenral base saion o which all mobile saions communicae and which sends ou synchronizaion s. A mobile saion near he base saion can sar sending earlier as saions ar away, because i receives he synchronizaion earlier (speed o propagaion) Example: Disance mobile saion base saion: 35 km Duraion o synchronizaion : m base saion mobile saion: = 7μs m / s round-rip-ime: 34μs Soluion: inser a guard ime beween ime slos o compensae he dierence in he round-rip-imes Slo i Slo i+ guard ime Page 7 Page 8

3 TDMA - Guard Times and Timing Advance Time Division Duplex (TDD) Bu: decreasing eiciency, because wihin guard imes no ransmission is possible Example: slo duraion in GSM: 577μs guard ime o 34μs causes a loss o 40% o ransmission capaciy! Principle: Only a single carrier requency Separaion o downlink and uplink in erms o ime Flexible: i you need more capaciy in he downlink han in he uplink, jus spend larger ime slos in he downlink par and give he uplink par smaller ime slos Soluion: Timing Advance: he base saion measures he round-rip-ime o a mobile saion and insrucs saions ar away o sar sending earlier Reducion o he guard ime o 30μs Downlink - i ms i ms Uplink - i ms D d A d B d C d D u A u B u C u variable sender/receiver separaion ime Page 9 Page 0 TDD/TDMA - Example DECT Combining Time and Muliplexing Combinaion o FDMA and TDMA A channel ges cerain ime slos on a cerain requency sub-band Example: GSM 47 µs 3 3 Downlink Uplink Advanages: Proecion agains apping Proecion agains requency selecive inererences High daa raes, adapaion o he raic amoun is possible c k k k 3 k 4 k 5 k 6 Disadvanages: Precise coordinaion is necessary Page Page

4 FDD vs. TDD Code Muliplex Flexibiliy Complexiy Eiciency and Throughpu FDD (-) wo carriers per connecion, separaion beween hem (-) eicien symmery only possible wih wo equal bandwidhs o he carriers (-) asymmeric connecions need dieren bandwidhs (-) duplexer needed (anenna) (-) FDD wases requencies (+) easy o implemen (+) high specral eiciency wih QAM modulaion (+) wo carriers enable a high hroughpu in boh direcions TDD (+) one carrier per connecion (+) eicien symmery by using equal uplink/downlink division (+) eicien asymmeric connecions wih variable borderlines beween uplink and downlink (+) no duplexer needed (+) TDD only needs small requency bands (-) synchronizaion is needed (+) high specral eiciency wih QAM modulaion (+) high capaciy For an eicien usage o he bandwidh, he whole requency band is used by all ransmissions in parallel The ransmissions are separaed by codes The code realizes a spreading over he whole bandwidh. When choosing codes, i mus be guaraneed ha hey are dieren enough o separae he ransmissions (orhogonal codes) Advanages: Eicien bandwidh usage No coordinaion needed beween he saions i appropriae codes are chosen Good proecion agains apping (miliary) Redundancies proec agains inererences Disadvanages: Synchronizaion beween all saions needed More complex regeneraion Lower user daa raes by coding he daa Page 3 Page 4 Code Division Muliplex (CDM) CDMA - Compuaion All saions use he same requency and uilize he whole bandwidh o he ransmission channel simulaneously The is combined wih a pseudo random number (XOR); each sender needs a unique number The receiver is able o reproduce he i he knows he sender s random number and perorms a correlaion uncion Bi rae r i << Bi rae r c (e.g. i used in opical sysems: r i /r c 5000) Sender A Sends A d =, Code A k = 000 (se: 0 = +, = -) Sen A s = A d * A k = (-, +, -, -, +, +) Sender B Sends B d = 0, Code B k = 00 (se: 0 = +, = -) Sen B s = B d * B k = (-, -, +, -, +, -) Signal Bi rae r i Signal n Bi rae r i X Code A Bi rae r c X Code B + Mixing o he daa sreams on one carrier requency X Code A Bi rae rc X Code B Boh s inerere in he air: A s = (-, 0, 0, -, +, 0) Remark: he sequences o he orm (-, +, -, -, +, +) are called chip sequences Receiver wans o receive ransmission o A: perorm code A k biwise (inner produc) (-, 0, 0, -, +, 0) A k = = -6 Resul is smaller han 0, sen bi mus have been a Analogously or B (-, 0, 0, -, +, 0) B k = = 6, hus a 0 Page 5 Page 6

5 CDMA on Signal Level CDMA - on Signal Level daa A 0 A d A A s code A chip sequence A A k daa B 0 0 B d daa code code B A A s chip sequence B daa code B k B B s Real sysems use much longer chip sequences resuling in a larger disance beween single code words in code space A s Page 7 Page 8 CDMA - on Signal Level CDMA - on Signal Level daa A 0 A d daa B 0 0 B d A s A s A k B k (A s ) * A k (A s ) * B k inegraor oupu comparaor oupu 0 inegraor oupu comparaor oupu 0 0 Page 9 Page 0

6 CDMA - on Signal Level CDMA - Example A s wrong code K (A s ) * K inegraor oupu comparaor oupu (0) (0)? Page Principle: Each saion is assigned an 8 bi chip sequence *) For ranserring a 0 i sends is chip sequence, or ranserring a i sends is complemen Bipolar noaion wih 0 as + and as - All chip sequences are pairwise orhogonal, i.e. i S and T are orhogonal chip sequences (S T), hen holds: m S T SiTi = 0, S T = 0, S S = m I wo or more saions are sending a he same ime, heir s are added linear Chip sequences o our saions: A: ( ) B: ( ) C: ( ) D: ( ) i= Example ransmissions: (in each case exacly one bi is ranserred) C E = ( ) -- B + C E = ( ) A + B E 3 = ( ) 0 - A + B + C E 4 = ( ) A + B + C + D E 5 = ( ) 0 A + B + C + D E 6 = ( ) *) simpliied example, normally a leas 0 bis E i = ranserred chip sequence or case i Page CDMA - Example On receiver side: For ilering ou he bi sream o a cerain saion, he receiver mus know he chip sequence o his saion For exracing he bis o he saion wih chip sequence C rom he received sequence E, i compues E C For example: E = A + B + C E C = ( A + B + C) C = A C + B C + C C = = For he six example ransmissions one receives: Resul o orhogonal codes E C = = -8 < 0 saion C has ransmied a E C = = -8 < 0 saion C has ransmied a E 3 C = = 0 saion C has ransmied nohing E 4 C = = -8 < 0 saion C has ransmied a E 5 C = = -8 < 0 saion C has ransmied a E 6 C = = 8 > 0 saion C has ransmied a 0 CDMA and Robus Transmission Oher advanage o CDMA: CDMA can also deal wih one large problem o radio ransmission: requency dependen ading can wipe ou narrowband s or duraion o he inererence Use o a bandwidh which is much larger han he one o he modulaed Spread o he over he complee bandwidh using a pseudo random sequence power inererence spread power deecion a receiver spread inererence Tha means: spread a narrowband ino a broadband using a special code o proec agains narrowband inererence Page 3 Page 4

7 Spreading and Robus Transmission Eecs o Spreading and Inererence channel qualiy narrow band 3 4 guard space 5 6 requency Narrowband channels: Signal qualiy depends on carrier requency Unair sysem: depending on he assigned channels, users ge dieren ransmission capaciy power power Original o a sender: Usage o a narrow requency band Spreading o he o a broad requency band; a he same ime, he power is disribued channel qualiy spread specrum requency Spread specrum channels: Narrowband inererence aecs all channels he same way Uniorm channel qualiy or all ransmissions, hus air reamen o all concurren ransmissions Page 5 power During ransmission, narrowband and broadband inererence occur User Broadband inererence Narrowband inererence Page 6 Eecs o Spreading and Inererence Spread Specrum Technology power power The receiver reconsrucs he disurbed ; as a resul he narrowband inererence is spread Using a band pass iler, hose pars o he can be removed which are ouside he bandwidh o he original Advanages: Several s can be ranserred wihou coordinaion wihin he same bandwidh a he same ime Small suscepibiliy or eecs o mulipah ransmission: due o he high bandwidh in any case only a small par o he requency specrum is aeced, so ha he ypical power decrease is smaller han wih narrowband sysems Small inluence o environmenal disurbances Exisence o ransmissions (and as a resul heir decoding) is diicul o deec (o special relevance or miliary sysems) Procedures: Direc Sequence Spread Specrum (DSSS, also: DS-CDMA) Hopping Spread Specrum (FHSS) Page 7 Page 8

8 Direc Sequence bi code word chip Original Code sequence + Spread - Division o he ino redundan inormaion unis (chips), he ransmier sends several (a leas 0) bis or one bi o inormaion Boh, sender and receiver mus know he chip sequence (code) Spreading is a resul o he higher bandwidh he chip sequence causes For oher users, he ransmission appears o be background noise Re-esablishmen o he original, possibly disurbed is possible due o he redundancy Power Power Page 9 Direc Sequence Principle: Doubled modulaion: a. Modulaion o he daa o a spread wide-band b. Modulaion o his o he carrier requency The receiver processes he invered procedure wih idenical chip sequences Inegraion over he bi period user daa X chipping sequence spread specrum modulaor ransmier radio carrier ransmi received radio carrier demodulaor receiver lowpass ilered chipping sequence X correlaor producs decision inegraor sampled sums daa Page 30 Hopping Hopping Time Original Hopping Power Power Principle: Vary he carrier requency in discree levels: Level sequences are deermined by pseudo random sequence Receiver mus use idenical sequence Two caegories regarding he number o ranserred bis per level: Max. one bi: Fas Hopping Several bis: Slow Hopping Carrier requency is changed in cerain ime inervals in accordance o a code sequence (synchronous change o he requency by sender and receivers) hops o he in ixed imes o approx ms Collisions are possible, i wo or more senders use by coincidence he same requency. Thereore suiable codes mus be used. Inererence are limied o shor periods, simple implemenaion No as robus as DSSS, easier o ap Page 3 user daa modulaor ransmier narrowband modulaor requency synhesizer hopping sequence spread received demodulaor requency synhesizer hopping sequence narrowband demodulaor receiver daa Page 3

9 Hopping Two varians: Fas change (as hopping): several requencies per bi Slow change (slow hopping): several bis per requency Daa Orhogonal Division Muliplexing (OFDM) Oher kind o muliplexing is OFDM: insead o modulaing a broadband on a single carrier (causing he use o a channel wih bandwidh b), subdivide b in a number o subbands on which you are ransmiing several s simulaneously. Tha means: When provided a channel o bandwidh b, subdivide i ino n subchannels Send n parallel ransmissions on he subchannels You could use simple requency division muliplexing (FDM) or deining subchannels Bu: use small variaion: Subchannels are orhogonal o each oher, i.e. maximum power o one subchannel is on same requency as minimum o he wo neighbor subchannels: n = 5 b Page 33 Page 34 OFDM and FDM OFDM has a higher specral eiciency han subdividing a requency band wih FDM: -F OFDM F b = F b = 3 F/ n = n = -F FDM F b = F b = F Orhogonal Division Muliplexing (OFDM) Advanages o OFDM: Robus agains narrow-band co-channel inererence Robus agains inersymbol inererence and ading caused by mulipah propagaion High specral eiciency Eicien implemenaion using as Fourier ransorm (FFT) Low sensiiviy o ime synchronizaion errors Can easily adap o severe channel condiions as e.g. done wih ADSL (Discree Mulione modulaion, DMT) -F F -F b = 4 F/3 n = 3 -F F -F b = (n+)/n F he more subchannels, he lower he bandwidh F b = F F Page 35 Enhancemen: Orhogonal Division Muliple Access (OFDMA): muliple access is achieved by assigning dieren OFDM sub-channels o dieren users Also possible: combine OFDM wih CDMA OFDM-CDMA, also known as Muli-Carrier Code Division Muliple Access (MC-CDMA) Page 36

10 Space Division Muliple Access (SDMA) Cell Planning Space Muliplexing means: base saions serve a cerain space (cell) only, dieren base saions have a disance large enough o supply dieren regions Mobile saions communicae only wih he base saion in range Advanages o a cell srucure: The same carrier requencies can be used in dieren cells or dieren users, i.e. higher capaciy, higher number o users possible Less ransmission power because smaller range has o be served More robus agains break-down Propagaion o s is (relaively) easy o handle Problems: Fixed nework needed or connecing base saions Handover (changing rom one cell o anoher) necessary Inererence wih neighbored cells a cell borders avoid using he same requencies in neighbored cells! Cell sizes rom 300 m in ciies o 35 km on he counry side or GSM even less or higher requencies, e.g m or Wireless LAN reuse only wih a cerain disance beween he base saions Sandard model considers hexagonal cells, k3 here using 7 carrier requencies k5 k Maximize disance beween wo cells using k4 k6 k k4 he same carrier requency oherwise, k3 k7 inererence beween wo cells renders he sysem unusable! k An area in which all requencies are used, is called cluser Saic requency assignmen: Cerain carrier requencies are assigned o a cerain cell Problem: dieren raic load in dieren cells Dynamic requency assignmen: Base saion chooses requencies depending on he requencies already used in neighbor cells More capaciy in cells wih more raic Assignmen can also be based on inererence measuremens k5 k Page 37 Page 38 Cell Planning and Clusers Cell Planning D 3 R How o plan clusers? Try o maximize disance D beween wo cells using he same carrier requency Guideline: D = R* 3k cell cluser cell cluser or k cells per cluser and cells o radius R 3 h 3 h 3 h h g g h 3 h 3 g g g g 3 g g 3 g 3 3 cell cluser wih 3 secor anennas And: depending on he expeced load, maybe cells have o have dieren size! requency planning is a complex process Page 39 Page 40

11 Furher Complexiy wihin Cells: Power Conrol Saions A and B boh send o he base saion The srengh decreases proporionally wih he square o he disance Thereore, he rom saion B drowns ou A s The base saion is no able o receive A s s A B Precise power conrol needed! The base saion has o do power measuremens and o adjus he sending power o he conneced saions Cell Breahing Also, speciic problems arise depending on he medium access echnology, here e.g. CDMA sysems: Cell size depends on curren load Addiional raic appears as noise o oher users all oher users have o increase heir ransmi power o overone he new noise (done by power conrol) I he noise level is oo high, disan users come up o maximum sending power and drop ou o cells Varying cell size complicaes he cell planning process Page 4 Page 4 Comparison o SDMA/TDMA/FDMA/CDMA Channel Access 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 inlexible, anennas ypically ixed only in combinaion wih TDMA, FDMA or CDMA useul segmen sending ime ino disjoin ime-slos, demand driven or ixed paerns all erminals are acive or shor periods o ime on he same requency synchronizaion in he ime domain esablished, ully digial, lexible guard space needed (mulipah propagaion), synchronizaion diicul sandard in ixed neworks, ogeher wih FDMA/SDMA used in many mobile neworks segmen he requency band ino disjoin sub-bands every erminal has is own requency, uninerruped ilering in he requency domain simple, esablished, robus inlexible, requencies are a scarce resource ypically combined wih TDMA (requency hopping paerns) and SDMA (requency reuse) all erminals can be acive a he same place a he same momen, uninerruped code plus special receivers lexible, less requency planning needed, so handover complex receivers, needs more complicaed power conrol or senders sill aces some problems, higher complexiy, lowered expecaions; will be inegraed wih TDMA/FDMA Page 43 FDMA and simple TDMA are in general o inlexible or daa communicaion CDMA is very complex DSSS and FHSS do no access conrol, bu increase he robusness o a ransmission Maybe we could use prooed mechanisms rom daa communicaion in ixed neworks insead o or in combinaion wih saic mechanisms? Le s ry or Eherne: CSMA/CD Simply send i he medium is ree, recognize i a collision occurs Problem in wireless neworks The srengh decreases a leas quadraic wih he disance CS/CD are used by he sender, bu collisions occur a he receiver Possibly, he sender can recognize his collision, i.e. CD ails Furhermore, CS can ail i a erminal is o ar away (Hidden Saion) Page 44

12 Hidden Saion and Exposed Saion Hidden Saion A sends o B, C canno receive A C wans o send o B, C senses a ree medium (CS ails) Collision a B, A canno receive he collision (CD ails) A is hidden or C Exposed Saion B sends o A, C wans o send o D C has o wai CS s ha he medium is in use A is ouside he radio range o C waiing is no necessary! C is exposed o B A B A B C D Bu: is i possible o design a CDMA/CD varian or wireless neworks? C Page 45 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 Collision Sender A Sender B Sender C Sloed Aloha Sender A Sender B Sender C Collision Page 46 DAMA - Demand Assigned Muliple Access Channel eiciency only 8% or Aloha, 36% or Sloed Aloha (assuming Poisson disribuion or packe arrival and packe lengh, see lecure Modelling and Evaluaion o Communicaion Sysems) Reservaion can increase eiciency o 80% A sender reserves a uure ime slo Sending wihin his reserved ime slo is possible wihou collision Reservaion also causes higher delays Typical scheme or saellie links Examples or reservaion algorihms: Explici Reservaion according o Robers (Reservaion-ALOHA) Implici Reservaion (PRMA) Reservaion-TDMA DAMA: Explici Reservaion Explici Reservaion (Reservaion Aloha): Two modes: ALOHA mode or reservaion: compeiion or small reservaion slos, collisions possible Reserved mode or daa ransmission wihin successul reserved slos (no collisions possible) I is imporan or all saions o keep he reservaion lis consisen a any poin in ime and, hereore, all saions have o synchronize rom ime o ime Collision Aloha Reserved Aloha Reserved Aloha Reserved Aloha Aloha Aloha Aloha Aloha Page 47 Page 48

13 DAMA: PRMA DAMA: Reservaion-TDMA Implici reservaion (PRMA - Packe Reservaion MA): A cerain number o slos orm a rame, rames are repeaed Saions compee or empy slos according o he sloed aloha principle Once a saion reserves a slo successully, his slo is auomaically assigned o his saion in all ollowing rames as long as he saion has daa o send Compeiion or his slos sars again as soon as he slo was empy in he las rame Reservaion Time Division Muliple Access Every rame consiss o 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 roundrobin sending scheme (bes-eor raic) Reservaion ACDABA-F AC-ABA-- A---BAF- A---BAFD Time slo: rame rame rame 3 rame 4 rame A C D A B A F A C A B A A B A F A B A F D A C E E B A F D collision a reservaion aemps N mini-slos reservaions or daa-slos N * k daa-slos e.g. N=6, k= oher saions can use ree daa-slos based on a round-robin scheme Page 49 Page 50 MACA Collision Avoidance MACA Collision Avoidance MACA (Muliple Access wih Collision Avoidance) uses shor ing packes or collision avoidance RTS (reques o send): a sender reques he righ o send rom a receiver wih a shor RTS packe beore 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 B asks C wheher C will be able o receive a ransmission using a RTS Saion A receives he RTS and knows how long medium will be blocked I C agrees o receive, i sends ou a CTS D receives CTS and is blocked or he included busy ime RTS indicaes busy medium A B C D RTS Daa CTS CTS indicaes busy medium Varians o his mehod can be ound in IEEE80. as DFWMAC (Disribued Foundaion Wireless MAC) Also called CSMA/CA: Carrier Sense Muliple Access wih Collision Avoidance Page 5 Page 5

14 MACA Examples MACA avoids he problem o hidden saions A and C wan o send o B A sends RTS irs C wais aer receiving CTS rom B RTS CTS CTS A B C MACA avoids he problem o exposed erminals B wans o send o A, C o some oher erminal C does no wai unnecessarily because i canno receive he CTS rom A RTS CTS RTS A B C Polling Mechanisms I 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 rom ixed neworks can be used (ypical mainrame - erminal scenario) Example: Randomly Addressed Polling Base saion s readiness o all mobile erminals Terminals ready o send can now ransmi a random number wihou collision wih he help o CDMA or FDMA (he random number can be seen as dynamic address) The base saion now chooses one address or polling rom he lis o all random numbers (collision i wo erminals choose he same address) The base saion acknowledges correc packes and coninues polling he nex erminal This cycle sars again aer polling all erminals o he lis Page 53 Page 54 ISMA (Idle Sense Muliple Access) Curren sae o he medium is ed via a busy one The base saion s on he downlink (base saion o erminals) i he medium is ree or no Terminals mus no send i he medium is busy Terminals can access he medium as soon as he busy one sops The base saion s collisions and successul ransmissions via he busy one and acknowledgemens, respecively (media access is no coordinaed wihin his approach) Page 55 Oher Mechanisms wih Busy Tone Collision avoidance by Ou-o-Band Signaling Use an addiional channel or ing inormaion Busy Tone Muliple Access (BTMA) Each saion hearing an ongoing ransmission on he daa channel sends a busy one on he addiional conrol channel All erminals in he range o hops o a sending erminal will wai No hidden saions, bu many exposed saions Receiver iniiaed Busy Tone Muliple Access (RI-BTMA) Only he receiver sends busy one Nearly no exposed saions, bu he busy one only can be sen when he receiver has decoded he ransmission reques Wireless Collision Deec (WCD) Proocol Combinaion o BTMA and RI-BTMA: wo ypes o busy ones Firs like BTMA: erminals send a busy one collision deec Aer recognizing a ransmission reques, he receiver sends a eedback-one, he oher erminals sop he collision deec busy one Page 56

15 Resource Aucion Muliple Access (RAMA) A deerminisic algorihm is used or random access: All saions have a unique ID (e.g. MAC address) For access o he medium, he ime axis is subdivided ino aucion phases and ransmission phases In an aucion phase, all saion simulaneously ransmi heir ID bi by bi Aer each bi, he base saion echos wha i has receviced a i i were sen only s a 0 i i were sen only 0s a i i were sen boh, 0s and s Only hose saions receiving heir own bi value coninue A saion receiving is own value ill he end o he aucion phase, has access o he ransmission phase Because o inererence o he bis o all saions, a all he ime overrules a 0 very unair algorihm! Cerain saions can sarve because o ohers! SAMA - Spread Aloha Muliple Access Aloha has only a very low eiciency, CDMA needs complex receivers o be able o receive dieren senders wih individual codes a he same ime Idea: use spread specrum wih only one single code (chipping sequence) or spreading or all senders accessing according o aloha Collision Sender A 0 Sender B 0 narrow band send or a shorer period wih higher power Spread he, e.g. using he chipping sequence 00 ( CDMA wihou CD ) Problem: ind a chipping sequence wih good characerisics! Page 57 Page 58 Conclusion Layer Common modulaion echniques GMSK, QPSK, QAM or all wireless neworks Signal propagaion and robusness depend on he requency band Layer Saic access mehods TDMA/FDMA/CDMA: suiable or voice ransmission because ixed capaciies can be assigned Dynamic access mehods or daa communicaion exis in several varians: wih reservaion, using special ing packes, polling, busy ones on addiional channels, Addiionally: CDMA echniques (DSSS, FHSS) can increase he robusness o a ransmission his can be combined wih dynamic access mehods! Page 59