Performance of MIMO adaptive subcarrier QAM intensity modulation in Gamma-Gamma turbulence

We investigate a constant-power, variable-rate multiple-input multiple-output adaptive subcarrier intensity modulation employing rectangular quadrature amplitude modulation for optical wireless communication over the Gamma-Gamma turbulence channels. The adaptive scheme is implemented by adapting the modulation order according to the received signal-to-noise ratio and a pre-defined target bit-error rate requirement. Novel closed-form expressions for the achievable spectral efficiency, average bit-error rate, and outage probability are presented in terms of a fast convergent infinite series. In addition, asymptotic bit-error rate and outage probability analyses are presented. Our asymptotic analysis reveals that the diversity order of the considered multiple-input multiple-output system depends on the number of transmit and receive apertures, and on the smaller channel parameter of the Gamma-Gamma turbulence.