Inter-Symbol Guard Time for Synchronizing Optical PPM

An inter-symbol guard time has been proposed as a means of synchronizing the symbol and slot clocks of an optical pulse-position modulation (PPM) receiver with the symbol and slot periods of an incoming optical PPM signal. (Such synchronization is necessary for correct identification of received symbols.) The proposal is applicable to the low-flux case in which the receiver photodetector operates in a photon-counting mode and the count can include contributions from incidental light sources and dark current. The use of the inter-symbol guard time would be an alternative to a prior synchronization method based on the periodic transmission of a fixed pilot symbol.