辅导案例-ELEC6021
Resear h Methods, ELEC6021 (EZ619) S Chen What's New in Communi ations Imagine a few s enarios: { In holiday, use your fan y mobile phone to take pi ture and send it to a friend { In airport waiting for boarding, swit h on your laptop and go to your favourite web side { Or play game via Internet with your mobile phone These are not Resear h, they are not even Development, they are Developed Do you know these words: CDMA, multi arrier, OFDM, spa e-time pro essing, MIMO, iterative or turbo oding, intelligent network, smart antenna There are plenty of opportunities for R & D In this introdu tory ourse, we only go through some A B C 1 Resear h Methods, ELEC6021 (EZ619) S Chen Wireless and Mobile Networks Current/future: 2G GSM, 3G UMTS (universal mobile tele ommuni ation system), and MBS (mobile broadband system) being developed for 4G mobile system G S M / G P R S G S M G S M / H S C S D G S M / E D G E MBS HIPERLAN UMTS B−ISDNISDNFixed Movable mobile Solow Fast mobile mobility User data rate Service (bps) 9 . 6 k 6 4 k 1 4 4 k 3 8 4 k 2 M 2 0 M 1 5 5 M Some improved 2G, HSCSD: high-speed ir uit swit hed data, GPRS: general pa ket radio servi e, EDGE: enhan ed data rates for GSM evolution. Also, HIPERLAN: high performan e radio lo al area network 2 Resear h Methods, ELEC6021 (EZ619) S Chen For Those Keen to Read Any good text books on digital ommuni ations, e.g. I.A. Glover and P.M. Grant, Digital Communi ations. 2nd edition, Pearson, 2004. J.D. Gibson, Prin iples of Digital and Analog Communi ations. 2nd edition, M Millan, 1993 More advan ed level, e.g. L. Hanzo, W. Webb and T. Keller, Single- and Multi-Carrier Quadrature Amplitude Modulation. Wiley, 2000. L. Hanzo, M. Munster, B.J. Choi and T. Keller, OFDM and MC-CDMA. Wiley, 2003 A. Paulraj, R. Nabar and D. Gore, Introdu tion to Spa e-Time Wireless Communi ations. Cambridge University Press, 2003 3 Resear h Methods, ELEC6021 (EZ619) S Chen General Comments on Communi ations Aim of tele ommuni ations: to ommuni ate information between geographi ally separated lo ations via a ommuni ations hannel of adequate quality (at ertain rate reliably) channel input output The transmission will be based on digital data, whi h is obtained from (generally) analogue quantities by 1. sampling (Nyquist: sampling with at least twi e the maximum frequen y), and 2. quantisation (introdu tion of quantisation noise through rounding o) Transmitting at ertain rate requires ertain spe tral bandwidth Here hannel means whole system, whi h has ertain apa ity, the maximum rate that an be used to transmit information through the system reliably 4 Resear h Methods, ELEC6021 (EZ619) S Chen General Transmission S heme A digital transmission s heme generally involves: input source encoding channel encoding lation modu output channel lation channel decodingdecoding source demodu Input/output are onsidered digital (analogue sampled/quantised) CODEC, MODEM, hannel (transmission medium) Your 2G mobile phone, for example, ontains a pair of transmitter and re eiver (trans eiver), onsisting of a CODEC and MODEM 5 Resear h Methods, ELEC6021 (EZ619) S Chen Sour e En oding / De oding A digital sour e is hara terised by: { sour e alphabet; symbol rate; symbol probabilities; and probabilisti interdependen e of symbols Sour e oding is about how to ode symbols (samples) into bits; sour e de oding is about how to ode bits ba k into symbols (samples) Ideally, sour e en oding should remove any redundan y from the signal to be transmitted, and the sour e de oder has to restore the original sour e signal distortionless (lossless oding) In pra ti e, a small (potentially imper eptible) error may be allowed in the oding/en oding (lossy oding) e.g. spee h oding an be lossy, but medi al diagnosis images annot tolerate lossy 6 Resear h Methods, ELEC6021 (EZ619) S Chen Channel En oding / De oding The hannel exhibits impairments: { hannel distortion (e.g. multipath and fading, insuÆ ient bandwidth), and { noise (e.g. thermal noise in re eiver amplier ir uit) input output channel Depending on the severity of impairment, transmission errors may o ur Channel oding adds redundan y to the transmitted signal to allow error dete tion and/or orre tion on the re eiving side 7 Resear h Methods, ELEC6021 (EZ619) S Chen Channel Chara teristi s Channel may introdu e amplitude and phase distortion { Bandwidth B, signal-to-noise ratio S N , and { maximum rate for possible error-free transmission ( hannel apa ity C) C = B log 2 1 + S N [bits/se ℄ ChannelTx Mod RxDem analogue channel digital channel Channel hara teristi s depend on design of transmission system: { transmission power, bandwidth, speed (data rate), reliability (error rate) 8 Resear h Methods, ELEC6021 (EZ619) S Chen Modulation / Demodulation Modulation requires: { en oding of a bit stream into a symbol stream; { ltering (pulse shaping) to limit the bandwidth; { modulation with a arrier frequen y Demodulation requires: { arrier re overy: orre t arrier phase have to be found; { syn hronisation (timing re overy): orre t sampling instan es have to be found; { equalisation: to ope with hannel distortions 9 Resear h Methods, ELEC6021 (EZ619) S Chen Human Spee h glottal sound vocal tract transmission lip radiation i n t e n s i t y frequency vowel F F F F 4 321 Spee h is either voi ed or unvoi ed, whi h results in quasi-periodi or noise-like signals, respe tively; both types have a degree of redundan y { Voi ed spee h: vo al folds vibrate at a fundamental frequen y (100 . . . 150 Hz for male, 200 . . . 300 Hz for female) { Arti ulation: spe tral shaping through the spe i resonan es of the vo al tra t (peaks: formant frequen ies, whi h determine the vowel /a/,/i/,et .) 10 Resear h Methods, ELEC6021 (EZ619) S Chen Sour e Coding for Spee h Methods of sour e oding for spee h: { Waveform oding tries to adapt the quantiser hara teristi