On 4/4/06, Robert McGwier [email protected] wrote:

AHA! Now we are getting some place. This last paragraph was very

revealing. Time sync is not a problem because of the guard interval BUT

varying depth in to the guard interval will IMMEDIATELY translate into

phase shifts on each and every bin that will vary with frequency and

depth into the guard interval. I am sorry I have not had time to spend

on the code but I have a lot going at the moment. There is simply no

such thing as a free lunch anywhere anytime, not even with OFDM.

Bob

Yes, I think this is a critical step. small letters are time domain and

capital letters are frequency domain.

x(n) -----------> X(k)

FFT

x(n-n’) ---------> e^(j*2*pi*n’*k/N) * X(k)

FFT

for one OFDM symbol, 0 <= k <= N-1

Therefore as you can see the phase shift will increase with k. For k=0,

there will be no phase shift and for k=N-1, there will be a phase shift

of

almost 2*pi*n’. Therefore for a QPSK, when you map symbols to bits,

there

will be more errors for higher subcarrier (index). You have to start

sampling exactly after the cyclic prefix ends. In general if your timing

error is towards cyclic prefix, you effectively cyclic shift your data.

In

case your shift is away from the end of the cyclic prefix, you take some

samples from next symbol and introduce ISI. You need to see how this

will

effect you in DQPSK case. Please look at this link for relevant papers

http://www.cds.caltech.edu/~yasi/publications.html

Also for frequency offset, what really matters is not the frequency

offset

in Hz, but the relative frequency offset delta_f/F_s, where F_s is the

subcarrier spacing. Please look at

*BER sensitivity of OFDM systems to carrier frequency offset andWiener*

phase

noise

Pollet, T.; Van Bladel, M.; Moeneclaey, M.

Communications, IEEE Transactions on

Volume 43, Issue 234, Feb/Mar/Apr 1995 Page(s):191 - 193

Digital Object Identifier 10.1109/26.380034

In general if your relative frequency offset is .01 or less, I think you

will not be affected much. But again I am not too sure for DQPSK. Again

QAM

is more sensitive to these errors than QPSK.

Please correct me if I am wrong somewhere. I am sharing what I have

recently

learnt about these things …

Regards

Prateek Dayal

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