ELEG 5693 Wireless Communications Ch. 7 Multicarrier Modulation

Transcription

1 Dpartmt of Elctrical Egirig Uivrsit of Arasas ELEG 5693 Wirlss Commuicatios C. 7 Multicarrir Modulatio Dr. Jigia Wu wu@uar.du

2 OUTLIE Itroductio OFDM Callgs i Multicarrir Sstms

3 ITRODUCTIO 3 Multicarrir Modulatio MCM) Acivs broadbad commuicatio b modulatig a larg umbr of arrow-bad data strams ovr closl spacd sub-carrirs. Divid a broadbad cal ito ma arrow-bad sub-cals i t frquc domai Eac sub-cal is calld a sub-carrir Divid t trasmittd data stram ito ma sub-strams, ad trasmit o sub-stram i o sub-cal T data is trasmittd ovr ma sub-carrir multicarrir modulatio

4 ITRODUCTIO W MCM? Multipat Multipat Multipat Multipat Allviat t impairmts causd b itr-smbol itrfrc ISI) Sigl-carrir broadbad commuicatio suffrs srious ISI s s s 3 s s 4 s 5 s 6 s7 s 8 s s3 s4 s5 s6 s7 will itrfr wit ad ISI s s s 3 I MCM, ac subcarrir dlivrs a arrow-bad sigal arrow-bad logr smbol priod Lss ISI s 8 s 9 s 9 s s ISI ca b compltl rmovd b usig guard itrval btw adact smbols s 7 s 7 Tim

5 ITRODUCTIO 5 W MCM Cot d) Origial sigl-carrir sstm prics frquc slctiv fadig Data rat >> corc badwidt Smbol priod << rms dla sprad frquc slctiv fadig MCM: o ac subcarrir: data rat dcrass, t smbol priod icrass data rat << corc badwidt Smbol priod >> rms dla sprad frquc flat fadig o qualizatio is cssar!

6 ITRODUCTIO 6 MCM tciqus Ortogoal frquc divisio multiplig OFDM) 4G cll po sstms Most wirlss LAs IEEE 8.) IEEE8.5 UWB Digital audio ad vido broadcastig T most popular MCM tciqu Discrt multito DSL Vctor codig

7 OUTLIE 7 Itroductio OFDM Callgs i Multicarrir Sstms

8 OFDM 8 Ortogoal Frquc Divisio Multiplig OFDM) Ortogoal: t subcarrirs ar mutuall ortogoal tr is o itrcarrir itrfrc ICI) amog t subcarrirs T most popular MCM tciqu Ca b implmtd wit fast Fourir trasform FFT) ad ivrs fast Fourir trasform IFFT)

9 OFDM: DFT AD IDFT Rviw: DFT ad IDFT discrt Fourir trasform DFT) Ivrs discrt Fourir trasform IDFT) Liar covolutio Circular covolutio 9 / i i X / i i i X modulo

10 OFDM: DFT AD IDFT Liar covolutio v.s. circular covolutio Liar covolutio,,, 3 3,,, 3,,, 3,,, 3,,, circular covolutio,,, 3, 3,,,, 3,,,, 3 3,,,

11 OFDM: DFT AD IDFT DFT ad Circular Covolutio Circular covolutio i t tim domai multiplicatio i t frquc domai DFT X i H i Circular covolutio is prfrrd i discrt tim implmtatio Howvr, practical sstm as t ffct of liar covolutio.

12 OFDM: CYCLIC PREFIX Discrt-tim modl of a sstm wit frquc slctiv fadig ) z Liar covolutio Ca w covrt liar covolutio to circular covolutio?,,, 3,,, 3 3,,,, 3,,, 3,, 3,,,, 3,,,, 3, 3,,,, 3,, 3,,,, 3,, Liar covolutio btw,,, 3 ad,, 3,,,, 3,,, 3 3,,, Circular covolutio btw,,, 3 ad,,, 3,, 3,,,, 3 circular prfi

13 OFDM: CYCLIC PREFIX 3 Cclic prfi Circular covolutio btw lgt: ) ad ca b acivd if. Add circular prfi to, gt ~ ~. Prform liar covolutio btw ad,,, ~,,,,,, ~ cclic prfi data I a practical sstm, w ca. Add circular prfi to t data. Trasmit t cclic-prfid data 3. T cal fuctios li a liar covolutio 4. At t rcivr, rmov cclic prfi, w gt a circular covolutio.

14 OFDM: TRASMITTER 4 OFDM Trasmittr R Modulatd data: X X, X,, X Aftr IFFT: i X i i/,,, Add cclic prfi: ~,,,,,,

15 OFDM: RECEIVER 5 ~ z z z Rcivr Rcivd data sampl: Aftr FFT: i Z i X i H i Y,, i t data smbol trasmittd o t i-t sub-carrir Hi: t cal cofficit of t i-t sub-carrir T i-t sub-carrir is quivaltl modld as a flat fadig cal witout ISI Tr is o itrfrc amog sub-carrirs. Xi:

16 OFDM: MATRIX REPRESETATIO Matri rprstatio of DFT Dfi: DFT matri 6 ) ) ) ) ) Q i i Y DFT DFT i matri format ) ) ) ) ) Y Y Y Q Y

17 OFDM: MATRIX REPRESETATIO Matri rprstatio of IDFT Dfi: IDFT matri 7 H ) ) ) ) ) Q i i i Y IDFT IDFT i matri format ) ) ) ) ) Y Y Y Y Q H

18 OFDM: MATRIX REPRESETATIO Matri rprstatio of OFDM Trasmittr: Data frquc domai): IDFT: Cclic prfi: Cal tim domai): 8 z z z z X X Q ~ ~ z z z H H is a cclic matri Eac row is a cclic sift of t prvious row

19 OFDM: MATRIX REPRESETATIO 9 Matri rprstatio of OFDM Cot d) Rcivr Tim domai: H z Frquc domai: appl DFT at t rcivd sigal Propositio Y Q QH Qz QHQ H X Z Λ X Z T frquc domai cal matri, Λ QHQ matri, wit t i-t diagoal lmt big Proof: H i i/ H, is a diagoal

20 OFDM: MATRIX REPRESETATIO Matri rprstatio of OFDM Trasmittr: Data frquc domai): IDFT: Q X Cclic prfi: Rcivr: Aftr rmoval of cclic prfi: H z DFT: Y ΛX Z Iformatio o t i-t sub-carrir: Y i H i X i Z i X ~ X IDFT ~ Cclic Prfi Cal Rmov Prfi DFT Y

21 OFDM: MATRIX REPRESETATIO Eampl Cosidr a simpl two-tap discrt-tim cal wit impuls rspos H z).9z fid t matri rprstatio i tim ad frquc domai of a OFDM sstm wit = 8.

22 MCM: OFDM OFDM: frquc domai Ortogoal i frquc domai o itr-carrir itrfrc ICI) i static cal. s s s s3 T 3 T T T Frquc /36

23 OUTLIE 3 Itroductio OFDM Callgs i Multicarrir Sstms

24 CHALLEGES: PAPR 4 Pa-to-avrag powr ratio PAPR) ma t t) PAPR E t) A low PAPR t tim domai amplitud dos ot cag dramaticall w ca us mor fficit o-liar powr amplifir A ig PAPR tr is larg variatio i t tim domai amplitud a lot of iformatio is carrid i t amplitud w av to us liar powr amplifir, wic usuall as low fficic

25 CHALLEGES: PAPR 5 Eampl Wat is t PAPR of a si wav?

26 CHALLEGE: PAPR 6 PAPR i OFDM T tim domai sampls ar obtaid from IDFT of modulatd smbols i/ X i i Basd o ctral limit torm, w is larg, t follows zroma Gaussia distributio T tortical valu of t PAPR is ifiit w?) I practic, t PAPR icrass approimatl liarl wit Larg larg PAPR Larg small ovrad du to cclic prfi Tr ar ma was to rduc PAPR i practical sstm Clippig t pa sigal Spcial codig tciqus

27 CHALLEGE: FREQUECY AD TIMIG OFFSET 7 Frquc ad timig offst cos f c t) Frquc offst: Matcd Filtr Du to t iaccurac of t carrir frquc oscillator, tr migt b a small rror i f c E.g. if t oscillator is accurat to part pr millio, t t frquc offst for a 5GHz sstm is f f c 6 T frquc offst will dgrad t ortogoalit amog t subcarrirs.

28 8 CHALLEGE: FREQUECY AD TIMIG OFFSET Frquc ad timig offst cos f c t) Timig offst Matcd Filtr Durig t samplig of t tim domai sigal, tr migt b a small offst durig t samplig If t tim offst is larg, it will caus bot ISI ad ICI If t tim offst is small, it will caus ICI Grall spaig, t ICI causd b t timig offst is smallr ta tat causd b frquc offst.

29 CASE STUDY: IEEE 8.a 9 IEEE 8.a: Wirlss LA stadard Carrir frquc: 5 GHz Total BW: 3 MHz Cal BW: MHz umbr of subcarrirs: = 64 Data: 48; zros: ; pilot: 4 Cclic prfi: 6 Subcarrir spacig: MHz/64 = 3.5 Hz Sampl priod: Ts = / MHz = 5 s Maimum dla sprad tat ca b adld b t sstm: OFDM smbol tim: Cod: covolutioal cod wit rats /, /3 or 3/4 Modulatio: BPSK, QPSK, 6QAM, 64QAM Miimum data rat: Maimum data rat:

30 CASE STUDY: IEEE 8.a 3 Eampl Fid t data rat of a 8.a sstm wit 6QAM ad /3 covolutioal codig.