Performance of Non-Adaptive and Adaptive Subcarrier Intensity Modulations in Gamma-Gamma Turbulence

We investigate a variable-rate, constant-power adaptive subcarrier intensity modulation employing M-ary phase shift keying and rectangular quadrature amplitude modulation for optical wireless communication over the Gamma-Gamma turbulence channels. The adaptive schemes offer efficient utilization of optical wireless communication channel capacity 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 series solutions are presented for the achievable spectral efficiency, average bit-error rate, and outage probability using a series expansion approach of the modified Bessel function. In addition, asymptotic bit-error rate and outage probability analyses are presented. Our asymptotic bit-error rate analysis shows that the diversity order of both non-adaptive and adaptive systems depends only on the smaller channel parameter of the Gamma-Gamma turbulence. Numerical results demonstrate high accuracy of our series solutions with finite number of terms and improved spectral efficiency achieved by the adaptive systems without increasing the transmitter power or sacrificing bit-error rate requirements.