Hello Garav
You have raised a very good question about LO power selection in DP Qpsk syste.As far as i know the LO is used to provide gain and used for phase matching .Also in optisysyem 13 the DSP is used for the carrier phade estimation and frequency phase estimation for more enhanced results snd estimation.Hoever the LO power depends upon RIN of oscillator and thermal noise inversely.So by fixing the receiver power you can calculate the LO power.total power at receiver from signal +LO is taken as the power from LO.so it is very important to select the optimum Lo power