## Abstract

In this paper, a closed-form suboptimal maximum-likelihood sequence detection (MLSD) metric and its low-complexity version are proposed for free space optical communications systems operating over lognormal fading and high signal-to-noise channel. This algorithm provides a simplification to the algorithm reported by Riediger *et al.* In comparison, firstly, the parameters of this algorithm are independent of the scintillation index’s variation, and secondly this algorithm just contains conventional computations; it saves computational time significantly. Bit error rate performance results confirm that the proposed algorithm performs comparably well as optimal MLSD. Moreover, the low-complexity version of this algorithm consumes much less time than the previously suboptimal MLSD metric presented by Riediger *et al.*

© 2012 Optical Society of America

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