Capacity of adaptive free-space optical channel using bi-directional links

ABSTRACT Free space optical (FSO) communication experiences severe fading due to optical scintillation in long-range links. Adaptive schemes based on channel variation can utilize the channel capacity efficiently. In this paper, we propose a truncated variable rate adaptive scheme for FSO channel. We determined the improvement in channel capacity for perfect channel side information (CSI) at the transmitter and receiver using M-ary pulse position modulation (M-PAM) format. We also determined the improvement in capacity for an erroneous feedback channel using one link of a correlated bi-directional FSO link as a feedback link to the transmitter. Both synthetic and experimental channel samples are used to determine the gain. For perfect CSI we have 1-2 dB improvement in channel capacity compared to non-adaptive schemes using 2-PAM and 4-PAM modulation. For an erroneous feedback channel the improvement depends on the channel correlation coefficient. With medium to high correlation it gives good capacity improvement.