Development of Pulse Position Modulation/Optical CDMA (PPM/O-CDMA) for Gb/s Fiber Optic Networking

Abstract

Pulse position modulation (PPM) in lasercom systems is known to provide potential advantages over other modulation schemes. In PPM, a periodic time frame is established and data is transmitted by placing a pulse in any one of several subintervals (or "slots") within each frame. In PPM/O-CDMA all users use the same frame structure and each transmits its unique address code in place of the PPM pulse. The advantage of PPM as a pulsed signal format is that 1) a single pulse can transmit multiple bits during each frame; 2) decoding (determining which subinterval contains the pulse) is by comparison rather than threshold tests (as in on-off-keying); 3) each user transmits in only a small fraction of the frame, hence the multi-access interference (MAI) of any user statistically spreads over the entire frame time, reducing the chance of overlap with any other user; and 4) under an average power constraint, increasing frame time increases the peak pulse power (i.e., PPM trades average power for peak power)