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. In the architecture we consider, the central base station can broadcast to the nodes (downlink), and the nodes can transmit simultaneously to the base station (uplink). The base station employs an imaging receiver, in which signals arriving from different directions are detected by different pixels, mitigating ambient light noise and interference between simultaneous uplink transmissions from different nodes (provided that the transmissions are imaged onto disjoint sets of pixels). We describe a low-complexity asynchronous reception scheme 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. During each frame interval, the imaging receiver samples a linear combination of two adjacent bits, which is a form of intersymbol interference. Our heterochronous detection algorithm uses maximum-likelihood sequence detection (MLSD), which can be implemented using the Viterbi algorithm. We assume that the receiver has prior knowledge of the bit rate of the reception, but not of the starting time offset. The receiver makes an initial estimate of the time offset, then considers several quantized values of the time offset centered at this initial estimate. The algorithm performs MLSD on several trellises corresponding to the different time offset values, thereby jointly estimating the received data and the time offset. In addition, the receiver needs to estimate pixel gain coefficients for MRC; these are estimated by incorporated per-survivor processing in the MLSD algorithm.