Capacity analysis of free-space optical communication channels with multiple receiver apertures

A laser beam experiences random wavefront distortion, beam wander, and beam broadening along its path through the turbulent atmosphere. These atmospheric effects cause excessive average power loss and random power fluctuation, called fading, at the receiver aperture. Using multiple receiver apertures is an effective approach to mitigate channel fading by exploring spatial diversity. This paper studies the free-space optical communication systems with single transmitter aperture and multiple receiver apertures from an information-theoretical point of view. The system is modeled as a discrete-time binary-input slowly fading Poisson channel. The maximum channel capacity and the corresponding optimal binary distribution are derived for signal dominated binary-input Poisson channels. An information-theoretical justification of using equiprobable binary distribution for the widely used OOK modulated optical system is provided. It is proved that fading always decreases average channel capacity if channel is signal dominated. However, fading-free capacity can be approached by using large number of receiver apertures. For systems employing low dark current photodetectors, multiple receiver apertures can both increase channel capacity and decrease outage probability. However, if dark current level is high, more receiver apertures may result in lower channel capacity. The connections between single and multiple receiver aperture channels are studied.