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

An adaptive subcarrier intensity modulation employing rectangular quadrature amplitude modulation is investigated 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. Highly accurate series approximations for the achievable spectral efficiency, average bit-error rate, and outage probability are derived using a series expansion of the modified Bessel function. In addition, asymptotic bit-error rate and outage probability analyses are also presented. Our asymptotic analysis reveals that the diversity order of the considered system depends only on the smaller channel parameter of the Gamma-Gamma turbulence.