The signal detector only needs to establish which quadrant the signal is in, to recover the value (which is '10', the green circle in the lower right corner). In better signal conditions, the detector can establish the phase and amplitude more precisely, to recover four more bits of data ('1101').
The signal detector only needs to identify the quadrant where the signal is located to retrieve the value (which is '10', the green circle in the lower right corner). In better signal conditions, the detector can also determine the phase and amplitude more precisely, allowing it to recover four additional bits of data ('1101').