Comparison analysis of adaptive free-space optical transmissions over turbulence channels

Short range free-space optical (FSO) communication is vulnerable to atmospheric turbulence (AT). In urban environment, weak turbulence is popularly existed in light windy clear weather condition during both daytime and night, which deteriorates the performance of such an FSO communication system. However, impacts of the weak AT on the performance of adaptive FSO transmissions are rarely reported. In this study, both exact closed-form and asymptotic expressions of the ergodic capacity for three transmission schemes are derived. From the asymptotic analysis, it is revealed that the ergodic capacity linearly increases with signal-to-noise ratio (SNR) in dB, while it is affected by the AT in the way of an additive degradation term which is, however, independent of the SNR.