The return signal, written , is an attenuated and time-shifted copy of the original transmitted signal (in reality, Doppler effect can play a role too, but this is not important here.) There is also noise in the incoming signal, both on the imaginary and the real channel.
The received signal, represented , is a diminished and delayed copy of the initial broadcast signal (notably, the Doppler effect can have an impact, although this is not crucial to the explanation). The incoming signal contains noise, both in the imaginary and real components.