电子科技大学：《数字通信基础 Digital Communications》课程教学资源（课件讲稿，图片版）Chapter 06 Digital transmission through band-limited AWGN channels 6.3 OFDM

Contents 5 Hrs ISI and zero-ISI condition (Refp380-381,p393-394) Design of BL signals for zero-ISI (Refp396-399) ■OFDM UESTC-Digital transmission thro BLAWGN channels Prof李晓峰 2

6.3.1 Basics If the channel is not flat,an alternate approach to avoid ISI is to subdivide the channel into a number of narrow sub-channels,which are nearly flat. And the data is transmitted in a multicarrier or FDM manner. x(r)K sub-channels (1)=Cx (1)+n (t) Bit stream Bit stream C=C S2P P2S C UESTC-Digital transmission thro BLAWGN channels Prof李晓峰 3

6.3.1 Basics Let Rs be the symbol rate and Ts the interval.Let K be the number of sub- channels,T=KTs the symbol interval of the parallel system. Suppose that the channel time-dispersion is Tc,which satisfies Te KTs Thus ISI has very limited effect on the multicarrier system x(r)K sub-channels (1)=Cx (1)+n (t) Bit stream Bit stream S2P P2S Narrow pulse h.(t) Channel resp wide pulse (T>>t) T=KT T T=KT UESTC-Digital transmission thro BLAWGN channels Prof李晓峰 4

6.3.1 Basics Let be the carrier freq of the kth sub-channel and A/the space of adjacent subcarriers,where=0,1,...K-1.By selecting symbol interval,T'=1/Af, the subcarriers are orthogonal regardless of the initial phases,that is,. ∫sin(2mf1+4)sin(2πf1+)dh=0. where i≠j,0≤i,j<K. In this case,we refer the comm. system to as orthogonal freq-division multiplexing (OFDM)system.OFDM has highest spectral efficiency due to smallest△f UESTC-Digital transmission thro BLAWGN channels Prof李晓峰 5

6.3.2 An OFDM system with FFT algorithms 1.Parallel-bank structure The modulation and demodulation in an OFDM system can be implemented by use of a parallel bank of filters. ak X:xk(t) X +ng P channel Dem Det c(t) n(t) On each sub-channel,QAM is adopted in general. Mapping:a=(S1,52),denoted as a complex X=S+jsx2 QAM: xx()=Sk1cos2πf1-Sk2sin2πft=Re(Xe2r) Overall signal:))-Re UESTC-Digital transmission thro BLAWGN channels Prof李晓峰 6

6.3.2 An OFDM system with FFT algorithms 2.Calculated by IFFT It can be easily The modulation and demodul calculated with X s() K- by use of a parallel bank of fil IFFT IFFT,the fast Inv- DFT algorithm. x (1 P channe Dem Det c(t) n(t) On each sub-channel,QAM is adopted in general. Mapping:=(S1,52),/deroted as a complex=+js2 QAM: xx()=Sk1cos2πflsk2sn2πft=Re(Xe2fr) Overall signal::st0=∑0飞cx,easr UESTC-Digital transmission thro BLAWGN channels Prof李晓峰 7

6.3.2 An OFDM system with FFT algorithms 3.Cycle prefix When transmit,there is ISI,through relatively small. mhmum UESTC-Digital transmission thro BLAWGN channels Prof李晓峰 8

6.3.2 An OFDM system with FFT algorithms 3.Cycle prefix When transmit,there is ISI,through relatively small. T wMM A simple way to completely avoid ISI is to insert a time guard of m points, then a slot is N+m points,of which N are for effective data. On reception,we discard the guard before demodulation. UESTC-Digital transmission thro BLAWGN channels Prof李晓峰 9

6.3.2 An OFDM system with FFT algorithms 3.Cycle prefix When transmit,there is ISI,through relatively small. T。 A simple way to completely avoid ISI is to insert a time guard of m points, then a slot is N+m points,of which N are for effective data. On reception,we discard the guard before demodulation. More analysis finds that empty guard will cause other problem.A good way is to append a so-called cycle-prefix(cp)as time guard. UESTC-Digital transmission thro BLAWGN channels Prof李晓峰 10

6.3.3 OFDM systems 3.The block diagram of a OFDM system Serial-to- Multicarrier Add cyclic Input prefix and D/A Output to parallet modulator data inverse DFT) parallel-to- converter buffer transmitter serial convert Remove Parallel- Multicarrier cyclic Output to- Detector demodulator prefix and A/D bits serial (DFT) serial-to- converter converter parallel convert Fig 6.50 on p471 Note FFT (or DFT)serves as the demodulator. UESTC-Digital transmission thro BLAWGN channels Prof李晓峰 11