Question

Why ppm is not generally used in digital communication system?

Answer 1

Pulse-position modulation (PPM) is a form of signal modulation in which M message bits are encoded by transmitting a single pulse in one of {\displaystyle 2^{M}} possible required time shifts.[1][2]This is repeated every T seconds, such that the transmitted bit rate is {\displaystyle M/T} bits per second. It is primarily useful for optical communications systems, where tends to be little or no multipath interference