Effect of imperfect slot synchronization on direct-detection optical synchronous CDMA communication systems with PPM signaling

Effect of imperfect slot synchronization between the transmitter and the receiver on optical synchronous code-division multiple-access using pulse position modulation as data modulation (PPM/CDMA) is investigated. Optical orthogonal codes (OOC's) are employed as signature sequences, and parallel optic-fiber delay line encoders and correlators are adopted in transmitters and receivers, respectively. We derive the upper bound on the bit error probability of PPM/CDMA on condition that the receiver slot timing shifts from the transmitter timing clock. We show the bit error probability performance for some values of parameters. It is found that as the number of slots per frame increases, the timing offset should be restricted to be smaller. Further, the bit error probability performance under photocounts per second constraint is also found to be more improved than under the fixed photocounts per symbol as the number of slots in one frame increases in the small range of the timing offset.