Illustration of two forms of PPM. (a) In conventional PPM the time axis is filled with PPM frames, or TF = TI. (b) In BPPM the PPM frame duration TF is decoupled from the interframe interval TI by choosing a very small value for Ts, allowing for blank intervals between frames. (c) The internal structure of each PPM frame consists of M time slots, each with duration equal to the slot time Ts.