Free-space heterochronous imaging reception of multiple optical signals

We consider free-space optical communication between a distributed collection of nodes (e.g., a distributed network of sensor nodes) and a central base station with an imaging receiver. This paper studies both synchronous and asynchronous reception of the optical signals from the nodes at the imaging receiver. Synchronous reception is done using a symbol-by-symbol maximal-ratio combining technique. We describe a low-complexity asynchronous reception scheme for the uplink that allows the nodes to transmit at a bit rate slightly lower than the frame rate. Since the two rates are nominally different, the scheme is said to be heterochronous. Our heterochronous detection algorithm uses joint maximum-likelihood sequence detection of multiple trellises, which can be implemented using the Viterbi algorithm, as well as the per-survivor processing technique. We develop an approximate upper bound for the average bit-error probability and compare it to Monte Carlo simulation results.