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