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 return signal, signified , is a reduced and time-delayed version of the original transmitted wave (it's worth noting that the Doppler effect might also be relevant, but it's not essential to this context). There is also interference present in the incoming signal, both on the imaginary and real axes.