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 BL AWGN channels Prof李晓峰 2
6.2 Design of BL signals for zero-ISI Most channels are practically BAND-LIMITED.Examples such as, Subscribe-line in a standard telephone system is limited to 4kHz. Uplink channel in GSM is for data from mobile user to the base and is a bandpass channel limited to 200kHz. The freq response of channel,C() 1.For BB,limited to some w,and C()=0,f>W 2.For PB,limited to some w about its central freq f,and C(f)=0,f-f≤W12 UESTC-Digital transmission thro BLAWGN channels Prof李晓峰 4
6.2 Design of BL signals for zero-ISI Think about the channel with BW of WHz. Case 1:W<Rs/2 三AU-z Flat?Impossible! -R -R/2 W Case 2:W=Rs/2 十2U-回 Flat?If and only if H(t)is rect. -R, -R/2 R -W UESTC-Digital transmission thro BLAWGN channels Prof李晓峰 7
6.2 Design of BL signals for zero-ISI Think about the channel with BW of WHz. Case 1:W<Rs/2 三U-) Flat?Impossible! -R -R/2 W Case 2:W=Rs/2 一 Flat?If and only if H(t)is rect. -R, -R/2 R.V2 R -W Case 3:W>Rs/2 Flat?Many chances, once H(t)is complement about -R-m-R/2 公 R Rg/2. UESTC-Digital transmission thro BLAWGN channels Prof李晓峰 8
6.2 Design of BL signals for zero-ISI Think about the channel with BW of WHz. Case 1:W<Rs/2 十芝U-) Conclusion for BL transmission: A digital communication system for zero-ISI must Flat?Impossible! meet Rs<2W C: Nyquist Rate:The maximum symbol rate for zero- ISI transmission over a WHz-channel is 2W baud. Flat?If and only if The highest rate can only be reached when the H(t)is rect. overall system response h(t)is a LPF of WHz. Case 3:W>Rs/2 Flat?Many chances, once H(t)is complement about -R.-F-R/2 R Rg/2. UESTC-Digital transmission thro BLAWGN channels Prof李晓峰 9