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From: cpshah99 on 16 Jul 2010 06:21 Hi All So far I have worked on time domain equalizers such as LE and DFE. Recently I have started to work on OFDM systems. And I am trying to compare the performance for Proakis channel B which is three tap channel. First system: map the info bits to BPSK, pass the symbols through channel, add noise. The channel output I write it as y=conv(h,x)+noise, where h=[0.407 0.815 0.407] and x= +/- 1. At receiver perform LE using MMSE under the assumption that the channel is knonwn at the receiver. I get the exact BER plot given in Proakis comms edition 4. Second system: map the info bits to BPSK, perform IFFT, add cyclic prefix, pass it through channel and add noise. The channel output y=conv(h,x)+noise, where x=IFFT(X,N), X=+/- 1 and N=2048. At receiver, remove the CP, truncate the signal and performe one tap MMSE equalization i.e. \hat{X}=fft(H,N)/fft(y,N) and then take hard decision on real part. The problem is that the BER of system 2 is way off when compared to the BER of system 1. As a test case, when I removed the noise from system 2, I get 0 BER. Furthermore, when I put h=1 in system 2, I get the exact plot of BPSK on AWGN. Any idea on what I might be doign wrong? Your help is greatly appreciated. Regards Chintan
From: Steve Pope on 16 Jul 2010 08:29 cpshah99 <cpshah99 (a)n_o_s_p_a_m.rediffmail.com> wrote:> >Hi All > >So far I have worked on time domain equalizers such as LE and DFE. Recently >I have started to work on OFDM systems. And I am trying to compare the >performance for Proakis channel B which is three tap channel. > >First system: map the info bits to BPSK, pass the symbols through channel, >add noise. The channel output I write it as y=conv(h,x)+noise, where >h=[0.407 0.815 0.407] and x= +/- 1. At receiver perform LE using MMSE under >the assumption that the channel is knonwn at the receiver. I get the exact >BER plot given in Proakis comms edition 4. > >Second system: map the info bits to BPSK, perform IFFT, add cyclic prefix, >pass it through channel and add noise. The channel output >y=conv(h,x)+noise, where x=IFFT(X,N), X=+/- 1 and N=2048. At receiver, >remove the CP, truncate the signal and performe one tap MMSE equalization >i.e. \hat{X}=fft(H,N)/fft(y,N) and then take hard decision on real part. >The problem is that the BER of system 2 is way off when compared to the BER >of system 1. > >As a test case, when I removed the noise from system 2, I get 0 BER. >Furthermore, when I put h=1 in system 2, I get the exact plot of BPSK on >AWGN. > >Any idea on what I might be doign wrong? Sounds to me that the OFDM system needs coding to work. Its native performance sucks. Add rate 1/2 k=7 Viterbi coding to each, with properly constructed metrics. It should then be comparable. Steve
From: Frank on 16 Jul 2010 08:41 On Jul 16, 12:21 pm, "cpshah99" <cpshah99 (a)n_o_s_p_a_m.rediffmail.com>wrote: > Hi All > > So far I have worked on time domain equalizers such as LE and DFE. Recently > I have started to work on OFDM systems. And I am trying to compare the > performance for Proakis channel B which is three tap channel. > > First system: map the info bits to BPSK, pass the symbols through channel, > add noise. The channel output I write it as y=conv(h,x)+noise, where > h=[0.407 0.815 0.407] and x= +/- 1. At receiver perform LE using MMSE under > the assumption that the channel is knonwn at the receiver. I get the exact > BER plot given in Proakis comms edition 4. > > Second system: map the info bits to BPSK, perform IFFT, add cyclic prefix, > pass it through channel and add noise. The channel output > y=conv(h,x)+noise, where x=IFFT(X,N), X=+/- 1 and N=2048. At receiver, > remove the CP, truncate the signal and performe one tap MMSE equalization > i.e. \hat{X}=fft(H,N)/fft(y,N) and then take hard decision on real part.. > > The problem is that the BER of system 2 is way off when compared to the BER > of system 1. > > As a test case, when I removed the noise from system 2, I get 0 BER. > Furthermore, when I put h=1 in system 2, I get the exact plot of BPSK on > AWGN. > > Any idea on what I might be doign wrong? > > Your help is greatly appreciated. > > Regards > > Chintan You don't say exactly what you mean when you say that your BER measurement is "way off", but one possibility is that you haven't compensated for the rotation on the carriers in the OFDM system. Try choosing a single carrier index and plotting it as a scatter plot (without noise). That will very quickly give you an idea of whether or not that's the root of your problem. Frank
From: Vladimir Vassilevsky on 16 Jul 2010 08:57 cpshah99 wrote: > Hi All > > So far I have worked on time domain equalizers such as LE and DFE. Recently > I have started to work on OFDM systems. And I am trying to compare the > performance for Proakis channel B which is three tap channel. > > First system: map the info bits to BPSK, pass the symbols through channel, > add noise. The channel output I write it as y=conv(h,x)+noise, where > h=[0.407 0.815 0.407] and x= +/- 1. At receiver perform LE using MMSE under > the assumption that the channel is knonwn at the receiver. I get the exact > BER plot given in Proakis comms edition 4. > > Second system: map the info bits to BPSK, perform IFFT, add cyclic prefix, > pass it through channel and add noise. The channel output > y=conv(h,x)+noise, where x=IFFT(X,N), X=+/- 1 and N=2048. At receiver, > remove the CP, truncate the signal and performe one tap MMSE equalization > i.e. \hat{X}=fft(H,N)/fft(y,N) and then take hard decision on real part. > > The problem is that the BER of system 2 is way off when compared to the BER > of system 1. > > As a test case, when I removed the noise from system 2, I get 0 BER. > Furthermore, when I put h=1 in system 2, I get the exact plot of BPSK on > AWGN. > > Any idea on what I might be doign wrong? The channnel is essentially LOWPASS. BPSK has sin(x)/x spectrum. OFDM has flat spectrum. Got the idea? > Your help is greatly appreciated. How much is the great appreciation? > Regards > Chintan STUPIDENT. VLV
From: Laurent Schmalen on 16 Jul 2010 09:22
cpshah99 wrote: > Hi All > > So far I have worked on time domain equalizers such as LE and DFE. Recently > I have started to work on OFDM systems. And I am trying to compare the > performance for Proakis channel B which is three tap channel. > > First system: map the info bits to BPSK, pass the symbols through channel, > add noise. The channel output I write it as y=conv(h,x)+noise, where > h=[0.407 0.815 0.407] and x= +/- 1. At receiver perform LE using MMSE under > the assumption that the channel is knonwn at the receiver. I get the exact > BER plot given in Proakis comms edition 4. > > Second system: map the info bits to BPSK, perform IFFT, add cyclic prefix, > pass it through channel and add noise. The channel output > y=conv(h,x)+noise, where x=IFFT(X,N), X=+/- 1 and N=2048. At receiver, > remove the CP, truncate the signal and performe one tap MMSE equalization > i.e. \hat{X}=fft(H,N)/fft(y,N) and then take hard decision on real part. > shoudln't this be \hat{X}=fft(y,N)/fft(h,N) ? Laurent |