Optical wireless links with spatial diversity over strong atmospheric turbulence channels
IEEE Transactions on Wireless Communications
BER Performance of Free-Space Optical Transmission with Spatial Diversity
IEEE Transactions on Wireless Communications
Do We Really Need OSTBCs for Free-Space Optical Communication with Direct Detection?
IEEE Transactions on Wireless Communications - Part 2
Optical wireless: the story so far
IEEE Communications Magazine
Part 1: optical communication over the clear turbulent atmospheric channel using diversity
IEEE Journal on Selected Areas in Communications
Optical repetition MIMO transmission with multipulse PPM
IEEE Journal on Selected Areas in Communications
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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 receiver apertures provide an effective solution for fading mitigation. Although MIMO (Multiple-Input Multiple-Output) OW systems have been extensively studied in recent years, most of these works are mainly limited to symbol-by-symbol decoding. Maximum Likelihood Sequence Detection (MLSD) exploits the temporal correlation of turbulence-induced fading and promises further performance gains. In this paper, we investigate MLSD for IM/DD (intensity-modulation/direct-detection) MIMO OW systems over log-normal atmospheric turbulence channels. Even with a low-order modulation scheme such as On-Off keying 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 much smaller than a direct evaluation of the log-likelihood function. The Monte-Carlo simulation results demonstrate that the EM-based algorithm outperforms the symbol-by-symbol decoder and achieves a performance which lies within 0.5 dB of that of the optimal MLSD.