ECS455: Chapter 4 Multiple Access

Transcription

1 Spread specrum (SS) ECS455: Chaper 4 Muliple Access Dr.Prapun Suksompong prapun.com/ecs DS/SS Oice Hours: BKD, 6h loor o Sirindhralai building Tuesday 4:20-5:20 Wednesday 4:20-5:20 Friday 9:5-0:5 3 Provide a measure o immuniy o disorion due o mulipah propagaion. In conjuncion wih a RAKE receiver, can provide coheren combining o dieren mulipah componens. Asynchronous muliple-access capabiliy. Wide bandwidh o specrum signals is useul or locaion and iming acquisiion. Applicaions Cordless phones. The basis or boh 2nd and 3rd generaion cellular sysems as well as 2nd generaion wireless LANs (WLAN). [Goldsmih, 2005, Ch 3] Spread specrum (SS) Spread specrum: Deiniion Hisorically specrum was developed or secure communicaion and miliary uses. Diicul o inercep or an unauhorized person. Easily hidden. Can even hide below he noise loor during ransmission For an unauhorized person, i is diicul o even deec heir presence in many cases. Resisan o narrowband jamming and inererence. Spread specrum reers o any sysem ha saisies he ollowing condiions [Lahi, 998, p 406 & Goldsmih, 2005, p. 378]:. The specrum may be viewed as a kind o modulaion scheme in which he modulaed ( specrum) signal bandwidh is much greaer han he message (baseband) signal bandwidh. 2. The specral ing is perormed by a code ha is independen o he message signal. This same code is also used a he receiver o de he received signal in order o recover he message signal (rom he specrum signal). In secure communicaion, his code is known only o he person(s) or whom he message is inended. 2 [Goldsmih, 2005, Ch 3] 4 [R. Pickholz, D. Schilling, L. Milsein, Theory o Spread-Specrum Communicaions - A Tuorial, IEEE Trans. Commun., Vol. 30, pp , May 982.]

2 SS: Processing Gain and BW Sharing 5 Increase he bandwidh o he message signal by a acor N, called he processing gain (or bandwidh ing acor). In pracice, N is on he order o [Goldsmih, 2005, p 379] N = 28 or IS-95 [T&V] Waseul? Bandwidh Sharing: Alhough we use much higher BW or a specrum signal, Muliplexing: we can also muliplex large numbers o such signals over he same band. Muliple Access: many users can share he same specrum bandwidh wihou inerering wih one anoher. Achieved by assigning dieren code o each user. Frequency bands can be reused wihou regard o he separaion disance o he users. [hp:// Two orms o specrum FHSS Example: Blueooh. Frequency Hopping (FH) Hop he modulaed daa signal over a wide BW by changing is carrier requency BW is approximaely equal o NB N is he number o carrier requencies available or hopping B is he bandwidh o he daa signal. The mos celebraed invenion o requency hopping was ha o acress Hedy Lamarr and composer George Anheil in Direc Sequence (DS) The band a 2.4 GHz is divided ino 79 channels. A Blueooh device, hops requency a a rae o 600 hops per second, randomly selecing a channel o MHz o operae. 6 2 [ hps://sies.google.com/sie/nearcommunicaions/adapaive-requency-hopping ]

3 DS/SS Sysem: The Basic ing code/sequence y c m c m de de 5 Compare wih ECS332 s modulaion: carrier modulae demodulae (daa/inormaion signal) (ing sequence) We assume polar signals. In paricular, we assume. 6 DS/SS Sysem (Con ) Observe ha To be able o perorm he deing operaion, he receiver mus know he code sequence c() used a he Tx o he signal synchronize he codes o he received signal and he locally generaed code. The process o deecion (deing) is idenical o he process o specral ing. Recall ha or DSB-SC, we have a similar siuaion in ha he modulaion and demodulaion processes are idenical (excep or he oupu iler). de 8 During he ime ha, he ing code is non-invered in. During he ime ha, he ing code is invered (or negaed) in. (daa/inormaion signal) (ing sequence) - - de

4 9 DS/SS: Specral Spreading Signal c() The ing code c() is design o be pseudorandom Appear o be unpredicable Can be generaed by deerminisic recipe (hence, pseudorandom) This will be sudied in he nex secion. Each recangular pulse in c() is called a chip. The bi rae o c() is hen known as he chip rae. (ing sequence) - T c = duraion o chip Ts Review: Specrum o PAM signal Ts 2 P = [-,-,,-,-,,,-,-,-,,-,-,,-,,,-,-,-,-,,-,-,-,-,-,,-,] - p Ts x s x mn p nt X P mne n n j nt s The bi rae o c() is chosen o be much higher hen he bi rae o m(). In ac, by deiniion, he ing acor T b = duraion o daa bi. Frequency-Domain Analysis T c T b T b - - (daa/inormaion signal) (ing sequence) T c = duraion o chip 22 T c We will simply draw he main lobes.

5 DS/SS: Secure Communicaion SS Modem - - As N increases, he peak o is reduced. Secure communicaion Signal can be deeced only by auhorized person(s) who know he pseudorandom code used a he ransmier. Signal specrum is over a very wide band, he signal s specral level is very small, which makes i easier o hide he signal wihin he noise loor modulae demodulae de DS/SS: Jamming Resisance 24 y c x i c m c i c m i c Jamming Resisance / Narrowband Inererence rejecion The decoder des he signal y() o yield m(). The jamming signal i() is o yield i()c(). Using a, we can recover m() wih only a small racion o he power rom i(). Cauion: Channel noise will no.