SEP of dual-ring star-QAM over FSO channels with atmospheric turbulence

Accurate and analytically tractable symbol-error probability (SEP) expressions have been derived for a free-space optics (FSO) communication system that employs M-ary dualring star-quadrature amplitude modulation (QAM) and operates over atmospheric turbulent channels. The turbulence-induced fluctuations of the optical signal intensity are modelled, for weak and strong turbulence conditions, through log-normal and gamma-gamma probability density functions (pdfs), respectively. Resultant end expressions are in the form of summation of single integrals which can be easily computed through numerical integration methods. The accuracy of the theoretical framework is validated through extensive Monte Carlo simulations. It was found that although star-QAM may be used for FSO links extending several kms under weak turbulence, its error performance under strong turbulence is unsatisfactory beyond 1 km.