
3.3.6 Phase Estimation
Since the linewidth of semiconductor distributed feedback (DFB) lasers used as
the transmitter and LO typically ranges from 100 kHz to 10 MHz, the optical
carrier phase
n
(t) varies much more slowly than the phase modulation, whose
symbol rate is 10 Gsymbol/s in the experiments described in Section 3.4. There-
fore, by averaging the carrier phase over many symbol intervals, it is possible to
obtain an accurate phase estimate. In the following, assuming the M-ary PSK
modulation, we explain the phase-estimation procedure, but such procedure can
easily be extended to quadrature amplitude modulation (QAM).
The phase of the complex ...