Pulse Position Modulation(PPM) is one of several pulse modulation methods. The classic PPM modulation has the clock signal as reference; there is also the Differential Pulse Position Modulation, which works without a reference signal.
In conventional pulse position modulation, the pulses have a constant amplitude and also a constant pulse width; the position of the pulses is modulated in relation to the clock signal. The modulated pulses constantly change their position in relation to the beginning of the clock signal. The distance of the pulse edge to the clock signal determines whether it is a digital "0" or "1". Small distances correspond to "0", large ones to a "1".
It is different with differential pulse position modulation, which works without a reference signal.