Timing recovery in free space direct detection optical communication systems with PPM signaling

A practical timing recovery subsystem for a direct detection optical communication system that uses Q=4 pulse position modulation (PPM) signaling at 25 Mb/s is described. The system consists of an AlGaAs laser diode transmitter and an avalanche photodiode (APD) photodetector. The PPM slot clock is recovered with a transition detector followed by a phase lock loop (PLL). The PPM word clock is recovered by tracking pairs of back-to-back PPM pulses in the received random PPM data sequences with another PLL. The system is capable of acquiring and maintaining slot and word synchronization at greater than 20 average detected photons per information bit (40 photons/PPM pulse)