Mobile Communications Chapter 2: Wireless Transmission

Transcription

1 This book ocuses on higher layer aspecs o mobile communicaions, he compuer science elemens raher han on he radio and ransmission aspecs, he elecrical engineering par. This chaper inroduces only hose undamenal aspecs o wireless ransmission which are necessary o undersand he problems o higher layers and he complexiy needed o handle ransmission impairmens. Frequencies,.1 Signals,. (read yourselves) Anennas,.3 (read yourselves) Signal propagaion,.4 Muliplexing,.5 Modulaion,.6 Spread specrum,.7 Cellular sysems,.8 Pro. Dr.-Ing. Jochen Schiller, hp:// MC SS0.1 Signal propagaion ranges (Chaper.4) Transmission range communicaion possible low error rae Deecion range deecion o he signal possible no communicaion possible Inererence range signal may no be deeced signal adds o he background noise sender ransmission deecion inererence disance This is an idealisic view! Pro. Dr.-Ing. Jochen Schiller, hp:// MC SS

2 Signal propagaion Propagaion in ree space always like ligh (sraigh line) I such a sraigh line exiss beween sender and receiver i is called Line-o-Sigh Receiving power proporional o 1/d² (d = disance beween sender and receiver) Receiving power addiionally inluenced by ading (requency dependen), he signal is aded due o disances Shadowing or blocking relecion a large obsacles reracion depending on he densiy o a medium These eecs are no only bad, scaering a small obsacles some o hem can be used!! diracion a edges shadowing relecion reracion scaering diracion Pro. Dr.-Ing. Jochen Schiller, hp:// MC SS0.3 Muliplexing (Chaper.5) channels k i k 1 k k 3 k 4 k 5 k 6 Muliplexing in 4 dimensions space (s i ) c ime () c requency () code (c) s 1 s c Goal: muliple use o a shared medium Imporan: guard spaces needed! s 3 Pro. Dr.-Ing. Jochen Schiller, hp:// MC SS

3 Advanced Phase Shi Keying Q BPSK (Binary Phase Shi Keying): bi value 0: sine wave bi value 1: invered sine wave very simple PSK low specral eiciency 10 1 Q 0 I 11 robus, used e.g. in saellie sysems I QPSK (Quadraure Phase Shi Keying): bis coded as one symbol symbol deermines shi o sine wave needs less bandwidh compared o BPSK more complex A Pro. Dr.-Ing. Jochen Schiller, hp:// MC SS0.5 Quadraure Ampliude Modulaion Quadraure Ampliude Modulaion (QAM): combines ampliude and phase modulaion i is possible o code n bis using one symbol n discree levels, n= idenical o QPSK bi error rae increases wih n, bu less errors compared o comparable PSK schemes Q Example: 16-QAM (4 bis = 1 symbol) 0011 φ a 0000 I Symbols 0011 and 0001 have he same phase φ, bu dieren ampliude a and 1000 have dieren phase, bu same ampliude Pro. Dr.-Ing. Jochen Schiller, hp:// MC SS

4 Eecs o spreading and inererence dp/d dp/d i) dp/d ii) sender dp/d user signal broadband inererence narrowband inererence dp/d iii) iv) v) receiver Pro. Dr.-Ing. Jochen Schiller, hp:// MC SS0.7 Spreading and requency selecive ading channel qualiy 1 narrow band signal 3 4 guard space 5 6 requency narrowband channels channel qualiy 1 spread specrum channels spread specrum requency Pro. Dr.-Ing. Jochen Schiller, hp:// MC SS

5 DSSS (Direc Sequence Spread Specrum) I XOR o he signal wih pseudo-random number (chipping sequence) many chips per bi (e.g., 18) resul in higher bandwidh o he signal Advanages reduces requency selecive ading in cellular neworks base saions can use he same requency range several base saions can deec and recover he signal so handover Disadvanages precise power conrol necessary b 0 1 c b : bi period c : chip period user daa XOR chipping sequence = resuling signal Pro. Dr.-Ing. Jochen Schiller, hp:// MC SS0.9 FHSS (Frequency Hopping Spread Specrum) I Discree changes o carrier requency sequence o requency changes deermined via pseudo random number sequence Two versions Fas Hopping: several requencies per user bi Slow Hopping: several user bis per requency Advanages requency selecive ading and inererence limied o shor period simple implemenaion uses only small porion o specrum a any ime Disadvanages no as robus as DSSS simpler o deec Pro. Dr.-Ing. Jochen Schiller, hp:// MC SS

6 End Pro. Dr.-Ing. Jochen Schiller, hp:// MC SS