Mobile and Ubiquitous Compu3ng. Mul3plexing for wireless. George Roussos.

Transcription

1 Mobile and Ubiquious Compu3ng Mul3plexing for wireless George Roussos

2 Overview Sharing he wireless (mul3plexing) in space by frequency in 3me by code PuEng i all ogeher: cellular mobile nes

3 Mul3plexing Goal: mul3ple use of a shared medium Take urns (requires organiza3on) Mul3plexing using signal parameers/ characeris3cs space (s i ), 3me (), frequency (f) code (c) Imporan: guard spaces needed!

4 Mul3plexing channels k i in space Each channel spa3ally separaed by ohers Only one channel a each loca3on Guard space is he disance beween he regions where communica3on occurs s 1 k 1 c k 2 k 3 k 4 k 5 k 6 f c s 3 s 2 c f f

5 Time mul3plex A channel ges he whole specrum for a cerain amoun of 3me Advanages: only one carrier in he k 1 medium a any 3me c hroughpu high even for many users Disadvanages: precise synchroniza3on necessary k 2 k 3 k 4 k 5 k 6 f

6 Frequency mul3plex Separa3on of he whole specrum ino smaller frequency bands A channel ges a cerain band of he specrum for he whole 3me Advanages: no dynamic coordina3on necessary works also for analog signals c k 1 k 2 k 3 k 4 k 5 k 6 f Disadvanages: wase of bandwidh if he raffic is disribued unevenly inflexible guard spaces

7 Time and frequency mul3plex Combina3on of boh mehods A channel ges a cerain frequency band for a cerain amoun of 3me Example: GSM Advanages: beuer proec3on agains apping proec3on agains frequency selec3ve inerference higher daa raes compared o code mul3plex bu: precise coordina3on required c k 1 k 2 k 3 k 4 k 5 k 6 f

8 Example: Cellular Mobile Implemens space division mul3plex: base sa3on covers a cerain ransmission area (cell) Mobile sa3ons communicae only via he base sa3on Cell sizes range from less han 100m in ci3es o as much as 35kn in rural areas for GSM The same frequency f can be reused in differen cells as long as hey are far away from each oher

9 Separa3ng cells: Frequency planning 3 cell cluser f 7 f 5 f 4 f 6 f 5 f 4 f 7 h h 2 1 h 3 h 1 h 2 h 3 g 1 g 2 g 3 g 1 g 2 g 3 g 1 g 2 g 3 3 cell cluser wih 3 secor anennas f 6 f 5 7 cell cluser

10 Example: GSM frequency planning Frequency reuse only wih a cerain disance beween he base sa3ons Sandard model using 7 frequencies: f 5 f 4 f 6 f 5 f 4 f 7 MS: Mobile Sa3on BSC: Base sa3on conroller MSC: Mobile swiching cener PSTN: Public swiched elephone nework

11 GSM FDMA 25 Mhz for BSC and 25 Mhz for MS khz guard band Each frequency carrier is 200 khz wide Toal carriers 25 Mhz / 200 khz = 125

12 Source: GSM TDMA

13 Dynamic GSM frequency planning Fixed frequency assignmen: cerain frequencies are assigned o a cerain cell problem: differen raffic load in differen cells Dynamic frequency assignmen: base sa3on chooses frequencies depending on he frequencies already used in neighbor cells more capaciy in cells wih more raffic assignmen can also be based on inerference measuremens

14 Advanages Advanages of cell srucures: higher capaciy, higher number of users less ransmission power needed more robus, decenralized base sa3on deals wih inerference, ransmission area ec. locally Problems: fixed nework needed for he base sa3ons handover (changing from one cell o anoher) necessary inerference wih oher cells

15 Code mul3plexing Each channel has a unique code All channels use he same specrum a he same 3me Advanages: bandwidh efficien no coordina3on and synchroniza3on necessary good proec3on agains inerference and apping Disadvanages: lower user daa raes more complex signal regenera3on Implemened using spread specrum echnology k 1 k 2 k 3 k 4 k 5 k 6 c f

16 TDD/TDMA - general scheme 417 µs downlink uplink

17 Mechanism random, disribued (no cenral arbier), 3me- mul3plex Aloha/sloUed aloha SloUed Aloha addi3onally uses 3me- slos, sending collision mus always sar a slo boundaries Aloha sender A sender B sender C collision SloUed Aloha sender A sender B sender C

18 Demand Assigned Mul3ple Access Channel efficiency only 18% for Aloha, 36% for SloUed Aloha (assuming Poisson disribu3on for packe arrival and packe lengh) Reserva3on can increase efficiency o 80% a sender reserves a fuure 3me- slo sending wihin his reserved 3me- slo is possible wihou collision reserva3on also causes higher delays ypical scheme for saellie links Examples for reserva3on algorihms: Explici Reserva/on Implici Reserva/on (PRMA) Reserva/on- TDMA

19 Reserva3on- TDMA Reserva3on Time Division Mul3ple Access every frame consiss of N mini- slos and x daa- slos every sa3on has is own mini- slo and can reserve up o k daa- slos using his mini- slo (i.e. x = N * k). oher sa3ons can send daa in unused daa- slos according o a round- robin sending scheme (bes- effor raffic) N mini-slos N * k daa-slos e.g. N=6, k=2 reservaions for daa-slos oher saions can use free daa-slos based on a round-robin scheme