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?