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