WORD TIMING RECOVERY IN DIRECT DETECTION OPTICAL PPM COMMUNICATION-SYSTEMS WITH AVALANCHE PHOTODIODES USING A PHASE LOCK LOOP

A techique for word timing recovery in a direct detection optical PPM communication system is described which tracks on back-to-back pulse pairs in the received random PPM data sequences with the use of a phase lock loop. The experimental system consisted of an AIGaAs laser diode transmitter (X = 833 nm), a silicon avalanche photodiode photodetector, and used Q = 4 PPM signaling at a source data rate of 25 Mbits/s. The mathematical model developed to characterize the system performance is shown to be in good agreement with the experimental measurements. Use of this recovered PPM word clock along with a slot clock recovery system described previously caused no measurable penalty in receiver sensitivity when compared to a receiver which used common transmitter/receiver clocks. The completely self-synchronized receiver was capable of acquiring and maintaining both slot and word synchronizations for input optical signal levels as low as 20 average detected photons per information bit. The receiver achieved a bit error probability of 10-6 at less than 60 average detected photons per information bit. © 1990 IEEE