DVB-T Synchonization


Hi all,

I have a doubt concerning the timing synchronization (in acquisition mode)
in an OFDM system. Normally, this synchronization is divided in 2 stages:
coarse and fine. From the literature that I have seen, the coarse timing
estimation is performed before the FFT is applied in the receiver, whereas
the fine synchronization is applied at the end of the synchronizer (after
the FFT).


If I got it rigth, the FFT window in the receiver should start between the
instants -Tg+tau and 0, where 0 indicates the start of the FFT window in
the transmitter and:

Tg: duration of the GI
tau: maximum propagation delay of the channel

since his way, both Inter Symbol and Inter Carrier interference are
avoided, and only a phase offset remains.

OFDM symbol
Lg FFT at transmitter
/----/------------------/
------------------------
| GI | Data |
------------------------
/-------------------/
Estimated FFT at receiver

My question is: which is the acceptable range for the coarse timing
estimator?

If the above reasoning is correct, we would get an error (ICI and ISI)
everytime the coarse estimation is outside the range [-Tg+tau, 0] since
this estimation is the one that determines the FFT window in the receiver
(because the fine estimation has not been done yet). Is this right?


Should I consider that that I get an error if the estimation is outside
this region? I ask this because all the papers I have found just measure
the quality by looking at the ABSOLUTE error between the actual start of
the FFT window and the estimated one, no matter if this error happens in
one direction or the other.


Any help would be very welcome,
Thanks,

Javi



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