Pulse-position modulation

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. 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, which tend to have little or no multipath interference.