PPM versus PCM for optical local-area networks

Pulse-position modulation (PPM) is presented within the context of a local-area network (LAN) application. A simplified analysis is used to calculate the advantage of converting from pulse-code modulation (PCM) to PPM signalling in a LAN, assuming in each case the same fibre network and the same optical transmitting device. Although PPM exploits its wide bandwidth to give greater sensitivity in noise, the true system advantage is obtainable only by using a large peak-to-mean-power ratio in the transmitter. It is shown that this performance is possible for both a laser diode and for an edge-emitting LED. An experimental system is reported, operating at a data rate of 4 Mbit/s, and good correspondence with theory is obtained. Design procedures are given for broad classes of situation.<>