Multiple-Symbol Detection for Shot-Noise Limited Free-Space Optical Communications

Free-space optics (FSO) have received increased attention recently for last-mile, line-of-sight wireless links. In this research, we investigate noncoherent detection, i.e. detection assuming the absence of channel state information at the receiver, of on-off keying (OOK) in an FSO system. To partially recover the performance loss associated with symbol-by-symbol noncoherent detection, we consider the application of multiple-symbol detection (MSD), in which block-wise decisions are made using an observation window of many bit intervals. Significantly, we develop a fast decoding algorithm for optimal MSD, which avoids the complexity of an exhaustive search over all candidate sequences. The conclusion is reached that the proposed MSD technique provides an attractive low complexity mechanism for performing optimal noncoherent detection of OOK in an FSO system.