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From: Chintan on 16 Jul 2010 13:00
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(y,N)/fft(h,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 doing wrong?

Your help is greatly appreciated.

Regards

Chintan
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