Automatic gain control using the extreme points of a constellation

Claim 1

In a modem for receiving a QAM encoded data signal which corresponds to a received vector, and having a decoder adapted to generate an ideal vector corresponding to said received vector, said ideal vector defining a point of a signal constellation, an error corrector comprising: means for obtaining the radial component in the direction of said ideal vector of an error vector, said error vector being a difference vector between said received and ideal vectors; means for obtaining an instantaneous gain error signal determined by said radial component; a secondary detector for generating an adjustment signal when said ideal vector corresponds to an extreme point of said signal constellation; adjustment means responsive to said adjustment signal for modifying said instantaneous gain error signal to generate a final gain error signal; and means for modifying said received vector in accordance with said final gain error signal and wherein said instantaneous gain error signal is given by gain error signal g=E x P x -E y P y where E x and E y are the real and imaginary components of the error vector; P x =cos s; P y =sin s; s being an instantaneous phase angle of the ideal vector.