QPSK constellation from baseband signal
The following depicts a QPSK Carrier Recovery loop that works on the baseband I and Q samples following proper decisions from a Timing Recovery Loop. For more information on this implementation see 

High modulation index PSK - carrier recovery



And for more information on a Timing Recovery implementation that can precede this see: 

Symbol timing synchronization using a high sampling rate

(Copying the links here since the question asked would not lead you easily to the prior titles)