
and the SNR penalty is less than 1 dB at BER = 10
6
for k = 5, 10, and 25. However,
the phase-noise increase gradually neutralizes the advantage of phase estimation
compared to differential demodulation. This is because the phase difference between
the first and last samples of (2k þ1) samples becomes significant. For example, in
the case of
p
= 4.3 10
2
, the error floor appears even when k = 5, and the merit
of phase estimation is small. Also note that independently of
p
,theBERperfor-
mance when k = 0 is identical to that of differential detection. These results indicate
that if the laser linewidth is as low as 150 kHz, the acceptable performa