In small satellite communication technique we can use optical spectrum (optical (laser) communication system). The common and simplest modulation techniques used are On and Off keying (OOK), Q-ary PPM (Quaternary Pulse Position modulation) and Frequency Shift Keying (FSK).
Could someone explain why the PSK - modulations (BPSK, QPSK) could be not used in the optic communication in satellite? Why is any modulation based on phase information difficult to implement in an optical link?
to demodulate phase information you have to phase lock to underlying carrier. It is fairly expensive to phase lock an optical carrier. Can use differential phase measurements... still difficult to align phase of optical carrier. can be done.... I worked on a few start-up designs that used fiber. Most of the start-ups failed.
thank you for your reply. Could you explain what you mean under "phase lock to underlying carrier"?
Not my area of expertise, but I'll comment anyway. Optical frequencies are many orders of magnitude higher than radio frequencies. That means that a very very tiny time delay can change the phase of the modulated signal, and the resulting phase change is essentially random because even a slight time delay will shift the phase by many many cycles.
The three common modulation techniques you mentioned do not care about phase and are insensitive to very small delays.
Remember that "phase" is measured in metres (or millimetres, or microns) as it is 'fractions of a wavelength'.
So shifting to optical frequencies, where the wavelength is sub-micron is a real precision engineering and stability challenge to maintain that accurate 'zero phase' position at the receiver (from which to measure received phase).
It's not usually worth it without sneaky techniques.
It's not that it's impossible, rather it is just that you need either sneaky techniques or careful selection of the method.
I don't think the laser terminal is going to be some low cost thing..
A quick google for "Homodyne BPSK modulation" suggests that homodyne is the best method from the options. Also Binary psk can accept a lot of phase noise with the 0/180 deg shift keying.