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.
Yes, I think this is a critical step. small letters are time domain and
capital letters are frequency domain.
x(n) -----------> X(k)
x(n-n’) ---------> e^(j2pi*n’*k/N) * X(k)
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
almost 2pin’. Therefore for a QPSK, when you map symbols to bits,
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.
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
effect you in DQPSK case. Please look at this link for relevant papers
Also for frequency offset, what really matters is not the frequency
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
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
is more sensitive to these errors than QPSK.
Please correct me if I am wrong somewhere. I am sharing what I have
learnt about these things …