Ergodic capacity of coherent optical wireless communications in Gamma-Gamma turbulence

Ergodic capacity is investigated for the optical wireless communications employing coherent detection over the Gamma-Gamma turbulence channels. We consider three different adaptive transmission schemes: (i) variable-power and variable-rate adaptive transmission, (ii) channel inversion with fixed rate, and (iii) truncated channel inversion with fixed rate. Highly accurate series approximations are derived for the ergodic capacity of the coherent optical wireless communications using a series expansion of the modified Bessel function. In addition, we present the asymptotic high signal-to-noise ratio channel capacity expression for the variable-power and variable-rate adaptive transmission scheme. Numerical results confirm high accuracy of our series ergodic capacity expressions in different turbulence regimes.