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.htmlAlso 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