An improved AGC subsystem for preferred use in quadrature amplitude modulation receivers offers improved automatic gain control of received signals in fading channels by generating an error signal only from constellation signal points having large signal-to-noise ratios for increase the signal-to-noise ratio of the gain control error signal for lowering dispersion of symbol signals in the constellation signal space for improving data detection of symbol signals with the improved performance with as much as one dB in the signal-to-noise ratio.