Abstract
The availability of Channel State Information (CSI) and the
effects of channel memory on the capacities and the achievable rates of free-space
optical communication channels are investigated. For memoryless channels,
the capacities and achievable rates are computed and compared for both uniform
and “positive” Gaussian inputs subject to different assumptions
on the CSI availability. For the strong turbulence regime, it is shown that
the knowledge of CSI both at the transmitter and the receiver increases the
achievable rates for low-to-moderate Signal-to-Noise Ratios (SNRs) in comparison
to the cases for which the CSI is known only at the receiver. For the weak
turbulence regime however, the availability of CSI at both ends of the link
does not provide any improvement over a system with CSI known at the receiver
alone, and we find that a simple channel inversion technique suffices. In
addition, for low SNRs, Pulse Amplitude Modulation (PAM) with $M \geq 4$ levels outperforms Gaussian-distributed inputs
regardless of the knowledge of CSI at the transmitter. For high SNRs, a Gaussian
distribution gives superior results, implying the need for new, more efficient
positive signal constellations. For channels with memory and without knowledge
of CSI, a change in the channel quasifrequency has negligible effects on the
capacity for any turbulence regime.
© 2008 IEEE
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