Impact of Timing Offset on Optical Spatial Pulse Position Modulation

This work investigates the impact of timing offset on the performance of an optical spatial modulation (OSM) technique termed spatial pulse position modulation (SPPM). SPPM combines optical spatial shift keying (OSSK) with pulse position modulation (PPM), exploiting the spatial domain to improve spectral the efficiency of PPM. In most works related to OSM, a perfect synchronization between the transmit and receive units is assumed. However, timing offset can result from clock jitter and variation in propagation delay of the transmit paths. Timing offset introduces intersymbol interference which can degrade system performance, hence the need to investigate its effect. Using the union bound technique, we derive the analytical upper bound on the symbol error rate (SER) of an SPPM-based optical wireless communication system that is impaired by timing offset. The analysis is validated via closely matching simulation results. Results show that error performance degrades as timing offset increases. For a small range of timing offset values, OSSK shows a better tolerance compared to SPPM.