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From: burky on 12 Aug 2007 07:04
Hi everbody,

I am doing my M.SC thesis on the topic "ICI cancellation in OFDM". I
have very limited time, if you can help me I will be grateful to
you....

I can implement the OFDM structure in AWGN channel, it works very
well..

But when i try to implement the system in a "rayleighchan", I obtain
very bad BER results.

--------------------------


N=256 ; % carriers
GI=N/4; % Guard ınterval=Cyclic prefix
DataBit = randint(N,1,2); % obtain 0 and 1's
DataMod = qammod(DataBit,2); % obtain modulated symbols (BPSK)

Ifft=Ifft (DataMod, N); %Take ifft of the symbols
x_t = [Ifft(N- GI + 1 : N);Ifft];

%Here x_t enters the channel

%CHANNEL

pathgains=[1,0.63,0.1,0.01];
gain= 10*log10(pathgains); % convert to dB

pathdelays=[0,0.2,0.4,0.6]*1E-6;
samplingtime=0.2E-6;
fdoppler=50 ; % 50 Hz
chan=rayleighchan(samplingtime,fdoppler,pathdelays,gain);

chan.ResetBeforeFiltering=0;

y_t=filter(chan,x_t);
......
.....

------------------------------------

I have some questions:

1) Which parameters determine the sampling time of x_t and sampling
time of the channel parameters? (It is a basic question sorry but I am
confused...)

(I think sampling time=0.2E-6 and x_t's sampling rate are not
correlated in my code, may be it is the problem)

2) I do not determine any parameters like ; carrier frequency =2
MHz , OFDM symbol duration = 200us etc...
May I determine them, then how i can I put them in the code...(Again
it is a basic question sorry but I am confused...)

2)If i use only "chan=rayleighchan(samplingtime, fdoppler)" then
there is no problem, BER results are good...So am I choosing wrong
pathdelays and samplingtime or am I doing wrong channel
estimation&equalization?

3) How can I get channel impulse response? (Because chan.PathGains
gives the path gains , not channel impulse response complex values)

4)I am taking chan.ResetBeforeFiltering=0 but not sure about it, am I
doing this wrong?


Burkay Etiler

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