Abstract
A major performance-limiting factor in terrestrial optical wireless (OW) systems
is turbulence-induced fading. Exploiting the additional degrees of freedom in the
spatial dimension, multiple laser transmitters combined with multiple receive apertures
provide an effective solution for fading mitigation. Although multiple-input
multiple-output (MIMO) OW systems have been extensively studied in recent years, most of
these works are mainly limited to symbol-by-symbol decoding. MLSD exploits the temporal
correlation of turbulence-induced fading and promises further performance gains. In this
paper, we investigate MLSD for intensity-modulation/direct-detection MIMO OW systems
over log-normal atmospheric turbulence channels. Even with a low-order modulation scheme
such as OOK, which is typically used in OW systems, the complexity of MLSD might be
prohibitive. We therefore present an iterative sequence detector based on the
expectation–maximization (EM) algorithm. The complexity of the proposed algorithm is
considerably less than a direct evaluation of the log-likelihood function, and it is
independent of the channel's fading statistics. The Monte Carlo simulation results
demonstrate that the EM-based algorithm outperforms the symbol-by-symbol decoder and
achieves a performance, which lies within 0.3 dB of that of the optimal MLSD.
© 2010 IEEE
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