Reduced-Complexity Multiple-Symbol Detection for Free-Space Optical Communications

In this paper, we investigate noncoherent detection, i.e. detection assuming the absence of channel state information at the receiver, of on-off keying in a free-space optical 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 N bit intervals. Specifically, we develop a fast search algorithm for optimal MSD and propose a reduced-complexity decision metric suitable for suboptimal MSD; performance results confirm that the optimal and suboptimal MSD metrics perform equally well. Significantly, the complexity of our MSD receiver, on a per bit-decision basis, is effectively independent of the length of the observation window. Furthermore, bit-error-rate results clearly indicate that the performance of the MSD receiver approaches the coherent detection lower bound with increasing N.