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OFDM demodulation



Here's my question:

In doing the OFDM (like DVB-T) demodulation, why do people just talk
about
carrier frequency sync and carrier phase noise but not carrier phase
itself?
In OFDM you have to synchronise accurately in frequency, because if you
don't then you won't be able to demodulate successfully. Each subcarrier
is a sinc (sin x/x) function, as can be seen on here:

If you have a frequency offset then you won't be sampling the sinc
curves at the top, and the further away in frequency you sample the
greater the interference from other subcarriers (inter-carrier
interference (ICI)).

For example, take frequency A in the figure. At that frequency the red
curve, which is the curve for that subcarrier is maximum and the
responses from all the other subcarriers are zero. This is the ideal
situation where there's no inter-carrier interference. If there's an
offset then you don't sample the curve at the top and responses from
subcarriers B, C, D and E all add up and interfere, which lowers the
signal to noise ratio.

Also, if you have a frequency offset then the received constellation
points rotate in time, i.e. if you transmitted QPSK symbols all with a
zero phase angle then the received point would rotate with time, and the
rotation speed is proportional to the frequency offset.

Does that mean OFDM is not coherently demodulated?
OFDM can use either coherent or differential modulation. 16-QAM has to
be phase synchronised, whereas the DAB system uses differential QPSK.


Thank you for your help!