16-03-2012, 01:48 PM
Adaptive Modulation and Coding for Free-Space Optical Channels
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Abstract—
Adaptive modulation and coding can
provide robust and spectrally efficient transmission
over terrestrial free-space optical channels. Three
adaptive modulation schemes are considered in this
paper: (i) variable-rate variable-power adaptation,
(ii) channel inversion, and (iii) truncated channel inversion
schemes. It is shown that a simple channel inversion
scheme performs comparable to a variablerate
variable-power adaptation scheme in the weak
turbulence regime but faces significant performance
degradation in the strong turbulence regime. We further
study adaptive coding based on large-girth
quasi-cyclic low-density parity-check- (LDPC-) coded
modulation. It is shown by simulation that deep fades
of the order of 30 dB and above in the regime of
strong turbulence can be tolerated with the proposed
scheme.
INTRODUCTION
Free-space optics (FSO) communication is a technology
that can address any connectivity needs in
future optical networks, be they in the core, edge, or
access [1,2]. In metropolitan area networks (MANs),
FSO can be used to extend the existing MAN rings; in
enterprise, FSO can be used to enable local-areanetwork-
(LAN-) to-LAN connectivity and intercampus
connectivity; and FSO is an excellent candidate
for last-mile connectivity [2].
DESCRIPTION OF THE PROPOSED FSO COMMUNICATION
SYSTEM
The adaptive FSO communication system, shown in
Fig. 1, consists of a transmitter, a propagation path
through the atmosphere, and a receiver. The optical
transmitter includes a semiconductor laser of high
launch power, an adaptive modulation and coding
block, and a power control block. To reduce the system
cost, direct modulation of a laser diode is used.
ADAPTIVE MODULATION
There are many parameters that can be varied at
the transmitter side relative to the FSO channel intensity
gain, including data rate, power, coding rate,
and combinations of different adaptation parameters.
The transmitter power adaptation, similar to wireless
communications, can be used to compensate for the
signal-to-noise ratio (SNR) variation due to atmospheric
turbulence, with the aim to maintain a desired
bit error rate (BER). The power adaptation
therefore inverts the FSO channel scintillation so that
the FSO channel behaves similarly to an AWGN channel
to the receiver.
V. CONCLUSION
We have described three adaptive modulation and
adaptive coding schemes with RF feedback, which
provide robust and spectrally efficient transmission
over free-space optical channels, namely, (i) the
variable-rate variable-power adaptation scheme, (ii)
the channel inversion with a fixed rate scheme, and
(iii) the truncated channel inversion scheme. It was
demonstrated that the simple channel inversion
scheme is sufficient in the weak turbulence regime.