Multcarrer Modulaton Wha Sook Jeon Moble Computng & Communcatons Lab
Contents Concept of multcarrer modulaton Data transmsson over multple carrers Multcarrer modulaton wth overlappng Chap. subchannels Mtgaton of subcarrer fadng Dscrete mplementaton Orthogonal frequency dvson multpleng Challenges n Multcarrer Systems (Chap. ) Mult-user
Data ransmsson Usng Multple Carrers () he smplest form of multcarrer modulaton dvdes the data stream nto multple substreams to be transmtted over dfferent orthogonal subchannels centered at dfferent frequences Consder a system wth data rate R and bandwdth B If coherence bandwdth (B c ) s assumed to be B c < B, the sgnal eperences frequency-selectve fadng > he effect of ISI cannot be neglgble he wdeband system s broken nto subsystems n parallel Each wth subchannel bandwdth B B / and data rate R R / For suffcently large, B << B c, relatvely flat fadng on each subchannel he symbol tme on each subchannel s much greater than the delay spread of the subchannel > eperences lttle ISI 3
Data ransmsson Usng Multple Carrers () Multcarrer transmtter he th substream s modulated va QAM or PSK wth the subcarrer frequency f he modulated sgnals assocated wth all the subcarrers are summed together s( t) s g( t)cos(f t φ ) where s s the comple symbol, φ s phase offset of the th carrer, and g(t) s rased cosne pulses wth the rolloff factor β. hs system does not change the data rate or sgnal bandwdth relatve to the orgnal system but elmnates ISI for B << B B c B ( β ε ) whereε, s the addtonal bandwdth requred due to tmelmtng of pulse shaper he th subchannel s affected by the flat fadng correspondng to a channel gan H(f )
Multcarrer Modulaton wth Overlappng Subchannels he spectral effcency of multcarrer modulaton by overlappng the subchannel he subcarrer must stll be orthogonal so that they can be separated out by the demodulator n the recever he subcarrer set: Orthonomal bass set: { cos( ( f ) t φ ),,,, } cos( ( f ) t φ)cos( ( f ) t φ) dt Even f the subchannels overlap, the modulated sgnal transmtted n each subchannel can be separated out n the recever he total system bandwdth β ε B for a large Overlappng subcarrers f 5
6 ( ) ( ) ) (.5 ) ( cos cos ) ( cos ) ( cos ) ( cos dt t dt t f dt t dt t f t f δ φ φ φ φ φ φ φ φ ( ) ( ) ( ) ( ) ( ) s s dt t f t f t g s dt t f t g t f t g s s ) ( ) ( cos ) ( )cos ( )cos ( )cos ( ˆ δ φ φ φ φ Aulary Materal
Mtgaton of Subcarrer Fadng () Codng wth nterleavng over tme and frequency Precodng encode data bts nto codeword, nterleave coded bts over both tme and frequency, and then transmt the coded bts over dfferent subcarrers he coded bts wthn a gven codeword eperence ndependent fadng he transmtter has to have knowledge of the subchannel flat fadng gan through channel estmaton If the desred receved sgnal power n the th subchannel s P and the flat fadng gan n the th subchannel s α, the transmtted power n the th subchannel under precodng s P α he receved sgnal power: α P α P qute common on wrelne multcarrer systems lke HDSL wo man problem wth precodng n a wreless settng power neffcent n fadng channel because precodng s bascally channel nverson. he need for accurate estmates at the transmtter (t s very dffcult to obtan) 7
Mtgaton of Subcarrer Fadng () Adaptve Loadng Based on the adaptve modulaton technques It s to vary the data rate and power of each subchannel relatve to ts gan he channel estmaton at transmtter s requred It s commonly used on slowly changng channel lke dgtal subscrber lne, where the channel estmaton at transmtter can be obtaned farly easly. Consder the capacty of multcarrer system wth ndependent subchannel of bandwdth B and gan { α,, } wth total power constrant P α otal capacty: C ma P : P P B log P B 8
Multcarrer Modulaton () Multcarrer modulaton, whch s nvented n the 95s, can be used wdely by the development of the dscrete Fourer transform and nverse DF Multcarrer Modulaton each waveform s orthogonal wth each other for a symbol nterval ( s ) 9
Multcarrer Modulaton () power spectrum of the transmtted sgnal Sgnal spectrum on each subcarrer has snc functon for a rectangular pulse shape Sgnal on each subcarrer can be detected by usng baseband processng (not bandpass flterng) Implementaton va FF/IFF
Dscrete Implementaton of Multcarrer Modulaton () Implementaton [] [] [] [ ], [],, [ ] [3]
Dscrete Implementaton of Multcarrer Modulaton (3) tme cos cos cos cos cos 3 cos cos cos cos cos 3 cos cos cos [ n] [ ] e n cos 9 cos 3 cos 3 -th OFDM symbol -th OFDM symbol
3 Aulary Materal ] [ ] [ n e n { } { } { } { } 8 6 8 6 [3] [] [] [] [3] [3] [] [] [] [] [3] [] [] [] [] [3] [] [] [] [] e e e e e e e e e
Dscrete Implementaton of Multcarrer Modulaton () 8 값이증가하면 D/A 의입력 frequency 가증가한다. 값은 sub-carrer 의개수 (M) 와는무관하며, 단지 > M 이면된다.
5 Dscrete Implementaton of Multcarrer Modulaton (5) -pont DF power of two : FF/IFF (fast processng) ) ( ] [ ] [ n e n ) ( ] [ ] [ n n e n,, ], [,, ], [ M k k n n DF IDF
Dscrete Implementaton of Multcarrer Modulaton (6) Crcular convoluton - h[n] wth duraton M, [n] wth duraton M ( > ma(m,m)) - DF of -pont crcular convoluton of h[n] and [n] s equal to the multplcaton of DF(h[n]) and DF([n]) repetton repetton zero-paddng 6
Dscrete Implementaton of Multcarrer Modulaton (7) Dagonal matr : Y[] depends on only [] 7
Cyclc Pref (CP) () 전송된신호와채널 mpulse response의 convoluton으로주어지는수신신호가전송된신호와채널 mpulse response의 -pont crcular convoluton된신호처럼보이도록해주기위하여 CP를추가한다. CP의길이가 channel mpulse response의길이보다긴경우 y m [ n] h [ n] [ n] η [ n] m m m DF of the receved sgnal Dagonal matr 8
Cyclc Pref (CP) () CP 의길이가 channel mpulse response 의길이보다짧은경우 Y m H Γ m m m H ICI m H ISI m CP 의길이를 channel mpulse response 의길이보다길게잡아주면, receved sgnal 에대해 -pont DF 를하고난후에는 nter-carrer nterference (ICI) 및 nter-symbol nterference (ISI) 가생기지않는다. 9
ransmtter and Recever (OFDM)
Challenges n Multcarrer Systems () Peak-to-Average Power Rato : PAR grows wth the number of subcarrers Average power Mamum power: when all the add coherently he mamum PAR s for subcarrers he observed PAR s typcally less than because full coherent addton of all symbols s hghly mprobable. [ ] ] E[ ] E[ ] E[ E E ma
Challenges n Multcarrer Systems () Frequency Offset Orthogonalty s assured by the subcarrer separaton In practce, the frequency separaton of the subcarrers s mperfect Due to msmatched oscllators, Doppler shfts, or tmng sync error Intercarrer nterference (ICI) he receved samples of the FF wll contan nterference from adacent subcarrers due to the degradaton n the orthogonalty of the subcarrers mng Offset Δf he effect of tmng error s less than that from the frequency offset due to guard tme (cyclc pref), that s, as long as a full -sample OFDM symbol s used at the recever wthout nterference from prevous or subsequent symbols
Multuser OFDM OFDM-DMA: 각각의사용자에게 predetermned tme slot 이할당. tme slot 안에서는모든 sub-carrer 사용. OFDM-FDMA: 각각의사용자에게 predetermned band of sub-carrers 할당. 특정 sub-carrer band 를사용하므로 frequency-selectve fadng 환경 에서사용자간성능차이가많이생길수있다. OFDM Interleaved-FDMA: band of sub-carrers 가아니라서로서로떨어져있는 sub-carrer 들을각각의사용자에게할당. OFDMA (Orthogonal Frequency Dvson Multple Access) OFDM-CDMA 3
OFDM-FDMA (or OFDMA) OFDMA wth Adaptve modulaton Basestaton ransmtter Subcarrer Allocaton and Bt Loadng Algorthm Channel Informaton from K users User data Subcarrer User data... User K data Adaptve Modulator Subcarrer Subcarrer IFF and P/S Add CP and D/A Removal CP and A/D FF and S/P Subcarrer Subcarrer Subcarrer Subcarrer Selecton & Adaptve Demodulator User k data Channel Informaton User k recever from K users
Case Study I IEEE 8.a IEEE 8.a, whch occupes a bandwdth of MHz n the 5GHz band, s based on OFDM 6 subcarrers are generated, although only 8 are actually used for data transmsson he cyclc pref length s / of OFDM symbol tme Possble codng rates are /, /3, 3/ he modulaton types can be used are BPSK, QPSK, 6QAM, 6QAM he subcarrer bandwdth Symbol tme per subcarrer s Mamum data rate s B MHz / 6 3.5 khz / B 5 / us 3 8 6 5 Mbps 6 R Ma 5
Case Study II IEEE 8.6a IEEE 8.6a, whch occupes a bandwdth of MHz, s based on OFDMA subcarrers are generated, although only 768 are actually used for data transmsson he cyclc pref length s /8 of OFDM symbol tme Possble codng rates are /, /3, 3/ he modulaton types can be used are QPSK, 6QAM, 6QAM he subcarrer bandwdth B Symbol tme per subcarrer s Mamum data rate s MHz / 768 3.kHz / B 9 / 8 5. us 3 768 6 3 Mbps 6 5. R Ma 6