Adaptive Symbol-Rate Free-Space-Optical Communications

Free space optical communications through fading atmospheric channels can achieve improved throughput by using adaptive transmission techniques. This paper considers the use of adaptive symbol-rate and transmit-power methods. We use an idealised model of channel coding that assumes zero errors can be achieved above some SNR threshold and adjust the transmission rate and power as the channel varies to operate above the required threshold. Results are presented for two noise models appropriate to non-coherent photodetection. Substantial performance gains are demonstrated compared to non-adaptive transmission. Most of the benefit from adaptive power control can be obtained with modest peak to average power constraints.