Outage Capacity With Non-Uniform Signaling For Free-Space Optical Channels

The outage capacity of slow-fading free-space optical (FSO) channels is derived with pulse amplitude modulation (PAM) signaling. Both uniform and non-uniform discrete input distributions are considered. For practical implementation, a nonuniform distribution with quantized-level probabilities is proposed. The combined effects of atmospheric turbulence, pointing errors and transmitter beamwidth are considered in the analysis. An upper and achievable lower bounds on the outage capacity are presented. For a given outage probability, a significant increase in the transmitted data rate, 5–10 times, is noticed when the non-uniform signaling scheme is employed over the widely used uniform 2-PAM signaling.