Multipath Propagation. Outline. What is OFDM? (OFDM) for Broadband Communications and. Orthogonal Frequency Division Multiplexing

Transcription

1 Orthogonal Dvson Multplexng (OFDM) for Broadband Communcatons and Dgtal Audo Broadcastng (DAB) Klaus Wtrsal VL: Dgtale Audotechnk, 21. März, 2002 What s OFDM? Modulaton technque Requres channel codng Solves multpath problems Transmtter: Info Source Source codng codng / nterleavng OFDM modulaton I/Q I/Q-mod., RF upconverter Recever: Decodng / denterleavng Radochannel Info Snk Source decodng OFDM demodulaton Downconverter, I/Q-demod. I/Q RF Outlne Introducton What s OFDM? Multpath fadng rado-channel Prncple of OFDM OFDM Implementaton and System Model Advantages and Dsadvantages OFDM n Practce Multpath Propagaton Reflectons from walls, etc. Tme dspersve channel Impulse response: p (τ) (PDP) τ [ns] Problem wth hgh rate data transmsson: nter-symbol-nterference Multpath Rado

2 Inter-Symbol-Interference Transmtted sgnal: Receved Sgnals: Lne-of-sght: Reflected: The symbols add up on the channel Dstorton! Delays Multpath Rado Concept of parallel transmsson (1) mpulse response Tme 1 (seral) 2 s s are transmtted at dfferent frequences (sub-carrers) 8 s In practce: s (sub-carrers) OFDM Technology Outlne Introducton What s OFDM? Multpath fadng rado-channel Prncple of OFDM OFDM Implementaton and System Model Advantages and Dsadvantages OFDM n Practce The -Selectve Rado 20 Power response [db] Interference of reflected (and LOS) rado waves -dependent fadng Multpath Rado

3 Concept of parallel transmsson (2) mpulse response Tme transfer functon 1 (seral) Sgnal s broadband 2 s 8 s s are narrowband OFDM Technology Generatng the OFDM sgnal (1) Symbol (QPSK) of sub-carrer at tme k Other symbol-alphabets can be used as well (BPSK, m-qam) Baseband sgnal s generated by DSP [ j 2 f ( t ) ] s BB k, k ( t ) = w ( t kt ) x, exp π kt Wndow functon Sub-carrer x,k Im Re Outlne Introducton What s OFDM? Multpath fadng rado-channel Prncple of OFDM OFDM Implementaton and System Model Advantages and Dsadvantages OFDM n Practce Spectrum of the modulated data symbols Rectangular Wndow of duraton T 0 Has a snc-spectrum wth zeros at 1/ T 0 Magntude T 0 Other carrers are put n these zeros sub-carrers are orthogonal N sub-carrers: N 1 s BB, k ( t ) = w ( t kt ) x k e = 0 2 π ( kt ) resembles IDFT!,, j f t

4 Generatng the OFDM sgnal (2) x 0,k seral-toparallel x 1,k IDFT x n (IFFT) s 0,k s 1,k parallelto-seral s n x N,k s N,k N data symbols: (n frequencydoman) x,k Im Base-band sgnal (tme-doman) Re Guard nterval (2) - Cyclc extenson Idea of Guard Interval (GI) Inserton of guard nterval (cyclc prefx): 1 OFDM symbol FFT-part mpulse response (shorter than GI): Cyclc convoluton of transmtted sgnal wth channel mpulse response multplcaton n frequency-doman Introducton OFDM Symbol Confguraton (1) Transmtter pulse prototype w(t) T T wn T guard T FFT Prefx effectve TX-tme Postfx k T mpulse response tme τ max τ excess delay tme Recever flter (mplemented by FFT) TFFT tme System Proposal tme t

5 OFDM System Model Multplcaton of data symbols wth (complex-valued) channel transfer-functon: x-n/2,k h-n/2,k n-n/2,k y-n/2,k xn/2-1,k hn/2-1,k nn/2-1,k yn/2-1,k y = x h + n Introducton Desgn of an OFDM System mpulse response Guard nterval length x(4 10) FFT symbol length Data rate; modulaton order Nr. of carrers Parameters are needed Other constrants: Nr. of carrers should match FFT sze and data packet length consderng codng and modulaton schemes Introducton OFDM Block Dagram Transmtter codng / nterleavng Symbol mappng (modulaton) OFDM modulaton (IFFT) I/Q Guard I/Q nterval 0110 Recever RF modulaton; TX antenna; Multpath channel; RX antenna; RF demodulaton Decodng / denterleavng symbol demappng (detecton) OFDM demod. (FFT) Guard nterval removal I/Q I/Q est. Tme sync. OFDM Technology Outlne Introducton What s OFDM? Multpath fadng rado-channel Prncple of OFDM OFDM Implementaton and System Model Advantages and Dsadvantages OFDM n Practce

6 Advantages of OFDM Solves the multpath-propagaton problem Smple equalzaton at recever Computatonally effcent For broadband systems more effcent than SC Supports several multple access schemes TDMA, FDMA, MC-CDMA, etc. Supports varous modulaton schemes Adaptablty to SNR of sub-carrers s possble Elegant framework for MIMO-systems All nterference among symbols s removed Correlaton-based -sync. Correlaton of duplcated parts of OFDM sgnal e.g.: Cyclc prefx (Guard nterval - GI): s : Guard nterval (M samples) FFT-part (L samples) P M 1 * opt = r r = 0 + L... conj. (M tmes) conj.... Σ Peak at optmum poston Phase frequency-offset Receved sgnal wth f-offset: r = s exp( j 2 π δ f / N Constant phase offset between samples spaced by L ) Introducton Problems of OFDM (Research Topcs) Synchronzaton ssues: Tme synchronzaton Fnd start of symbols synchr. Fnd sub-carrer postons Non-constant power envelope Lnear amplfers needed tme doman sgnal (baseband) magnary real sample nr. ampltude estmaton: To retreve data s tme-varant frequency δf frequency offset OFDM Technology Outlne Introducton What s OFDM? Multpath fadng rado-channel Prncple of OFDM OFDM Implementaton and System Model Advantages and Dsadvantages OFDM n Practce

7 OFDM systems (1) Wreless LANs OFDM systems (2) - Broadcastng Used n HIPERLAN/2, IEEE a/g, MMAC 5 GHz carrer (2.4 GHz: g) 64 pont FFT; 48 data sub-carrers per symbol bandwdth ~ 16.5 MHz Transmsson rates from 6 up to 54 Mbt/s Varous codng schemes (rates) and modulaton schemes (symbol alphabets are used) accordng to channel condton Trade-off: data rate range Dgtal audo and vdeo broadcastng (DAB, DVB) DAB: 2.4 Mbt/s; 1.5 MHz BW; >1500 f- spacng of 1 khz, D-QPSK DVB: 34 Mbt/s; 7 MHz BW; up to 8000 sub-carrers Sngle frequency networks: Multple transmtters use same carrer OFDM can resolve for long multpath delays DAB Status (1) DAB-Status (2) Germany 70% of populaton About 150 statons Regular servces snce sprng 1999 Austra 19% of populaton 3 tx n Venna 2 tx n Tyrol (Inntal)

8 Dscrete Mult Tone (DMT) - ADSL Multcarrer-modulaton s used Called DMT n stead of OFDM No complex sgnal: no frequency-synchronzaton Adaptve bt-loadng s used Optmum number of bts used on sub-carrers s (almost) tme-nvarant Summary Essental Ingredents IFFT & FFT For effcent mplementaton Guard nterval nserton Obtanng smple equalzaton Removng all IS- & IC-nterferences Error correcton codng To restore bts that are lost on weak sub-carrers