Performance Analysis of FSO Communications With Diversity Methods: Add More Relays or More Apertures?

This paper targets the performance analysis of multiple-input-multiple-output (MIMO) point-to-point single-aperture relay-assisted and MIMO relay-assisted free-space optical (FSO) communications. The objective of the paper is to compare the different FSO diversity methods via an outage probability analysis over gamma-gamma turbulence channels in the case of independent fading among the different apertures of the communicating nodes. Through a well-tailored calculation method, we approximate the gamma-gamma distribution and the gamma-gamma sum-distribution by appropriate gamma distributions. This method resulted in simple closed-form outage probability expressions that are accurate for average-to-large values of the signal-to-noise ratio (SNR) and in asymptotic expressions that are useful for evaluating the diversity gains and coding gains. In particular, we derived the diversity gains of MIMO repetition coding (RC), MIMO transmit laser selection (TLS), MIMO all-active relaying (AR) and MIMO selective-relaying (SR). We prove that MIMO-RC and MIMO-TLS, on one hand, and MIMO-AR and MIMO-SR, on the second hand, achieve the same diversity gain and we evaluate the coding gain of TLS with respect to RC and of SR with respect to AR. By comparing the MIMO and cooperative methods with point receivers, we reach the central result that it is always better, from a diversity gain point of view, to add more apertures to the source and destination rather than adding more relays in their vicinity despite the fact that the fading variance along FSO links decreases with the distance. For receivers that implement aperture averaging, we derive conditions under which MIMO techniques are unconditionally better than cooperative techniques with any network topology. [ABSTRACT FROM PUBLISHER]