The Deep Space Optical Communication (Dsoc) between earth and the psyche spacecraft uses large-M PPM for this reason! This mission is currently ongoing.
They send optical pulses in one of up to 128 possible time slots, thereby carrying 7 bits each. And each optical pulse on earth may only be received by 5-10 photons.
They send optical pulses in one of up to 128 possible time slots, thereby carrying 7 bits each. And each optical pulse on earth may only be received by 5-10 photons.