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We investigate the ultimate QAM channel capacity of a laser
diode in a hybrid fiber/coax network which is limited by the
laser-clipping induced nonlinear distortions (NLD''s) and the
relative intensity noise (RIN). Our study includes a spectral
analysis, a complete system simulation, and an experiment. In
the experiment, up to 70 channels of vector-arbitrary-waveform-
synthesizer generated upstream QPSK or 16-QAM signals were used
to modulate an isolated/cooled DFB laser and two unisolated/
uncooled Fabry-Perot lasers, respectively. Our analytical
results show that for an upstream laser diode, over 1000 QPSK
channels or 170 16-QAM channels can be delivered, even in the
presence of a high RIN level of -115 dB/Hz. However, these high
capacities are reduced significantly when we consider the effect
of collision-based medium access control (MAC) protocols. We
found that, in the worst case condition (collisions occur in all
but one channels), the ultimate QPSK channel capacity of an
upstream laser diode is dramatically reduced from over 1000 to
125 for 8 collisions/channel. These results have important
implications to systems transporting frequency-stacked return-
path bands with or without collision-based MAC channels. As
regard to the ultimate capacity of a downstream laser diode with
a RIN level of -135 dB/Hz, we found that as high as 600 and 128
channels of 64-QAM and 256-QAM signals can be transported,
respectively. Through computer simulations, we also propose
and verify the use of a precoding technique to eliminate the
NLD''s in a QAM channel which is transported along with multiple
AM-VSB channels by a laser diode. We show that, for the first
time, the precoding technique can completely remove the
clipping-induced bit-error-rate floor at a cost of a 3 dB
signal-to-noise ratio reduction.
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