A technique for word timing recovery in a direct detection optical pulse position modulation (PPM) communication system is described. It tracks on back-to-back pulse pairs in the received random PPM data sequences with the use of a phase locked loop. The experimental system consisted of an AlGaAs laser diode transmitter ( lambda =833 nm) and a silicon avalanche photodiode photodetector, and its used Q=4 PPM signaling at a source data rate of 25 Mb/s. The mathematical model developed to characterize 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/sup -6/ at less than 60 average detected photons per information bit.>

Slot timing recovery in a direct-detection optical PPM communication system can be achieved by processing the photodetector output waveform with a nonlinear device whose output forms the input to a phase-locked loop. The choice of a simple transition detector as the nonlinearity is shown to give satisfactory synchronization performance. The RMS phase error of the recovered slot clock and the effect of slot timing jitter on the bit error probability were directly measured. The experimental system consisted of an AlGaAs laser diode ( lambda =834 nm) and a silicon avalanche photodiode photodetector. The system used Q=4 PPM signaling and operated at a source data rate of 25 Mb/s. The mathematical model developed to compute the RMS phase error of the recovered clock is shown to be in good agreement with results of actual measurements of phase errors. The use of the recovered slot clock in the receiver resulted in no significant degradation in receiver sensitivity compared to a system with perfect slot timing. The system achieved a bit error probability of 10/sup -6/ at a received optical signal energy of 55 detected photons per information bit.>

The coding gain of a constraint-length-three, rate one-half convolutional code over a long clear-air atmospheric direct-detection optical communication channel using binary pulse-position modulation signalling is directly measured as a function of interleaving delay for both hard- and soft-decision Viterbi decoding. Maximum coding gains theoretically possible for this code with perfect interleaving and physically unrealizable perfect-measurement decoding were about 7 dB under conditions of weak clear-air turbulence, and 11 dB at moderate turbulence levels. The time scale of the fading (memory) of the channel was directly measured to be tens to hundreds of milliseconds, depending on turbulence levels. Interleaving delays of 5 ms between transmission of the first and second channel bits output by the encoder yield coding gains within 1.5 dB of theoretical limits with soft-decision Viterbi decoding. Coding gains of 4-5 dB were observed with only 100 mu s of interleaving delay. Soft-decision Viterbi decoding always yielded 1-2 dB more coding gain than hard-decision Viterbi decoding.>

A 25 Mbit/s direct-detection optical communication system that used Q=4 PPM signaling was constructed and its performance measured under laboratory conditions. The system used a single mode AlGaAs laser diode ( lambda =834 nm) and low-noise silicon avalanche photodiode (APD). A procedure is given to numerically compute system performance which uses the nearly exact Webb's approximation of the true Conradi distribution for the APD output that does not require excessive amounts of computer time (a few CPU minutes on VAX 8600 per system operating point). Comparison revealed that modeling the APD output as a Gaussian process under conditions of negligible background radiation and low (less than 10/sup -12/A) APD bulk leakage currents leads to substantial underestimates of optimal APD gain and overestimates of system bit error probability. Examples are given which illustrate the breakdown of the Gaussian approximation in assessing system performance. The measured performance of the system was found to be in excellent agreement with the performance predicted by the nearly exact computational procedure. This system achieved a bit error probability of 10/sup -6/ at a received signal energy corresponding to an average of 60 absorbed photons/bit and optimal APD gain of 700.>

A remarkably simple stochastic equation of evolution for approximately minimum mean square error estimates of the intensity of a Gaussian optical field is obtained, and its performance experimentally demonstrated. The estimator is based on observations of individual photoelectron occurrence times.