Gain control circuitry for QAM demodulator

In a 2.sup.2n+3 -QAM (guadrature amplitude modulation) demodulator including first and second gain-controlled amplifiers for amplifying first and second, mutually orthogonal baseband components of the QAM signal, respectively, first and second (A/D) analog-to-digital converters for converting output signals from the first and second gain-controlled amplifiers to corresponding digital signals, a first threshold decision circuit for comparing the digital signal from the first A/D converter with decision thresholds to produce first-channel data bits and a first-channel error bit, and a second threshold decision circuit for comparing the digital signal for the second A/D converter with decision thresholds to produce second-channel data bits and a second-channel error bit, a gain control circuit comprising:

• first decision means for defining along a first axis of the signal constellation of the QAM signal two outermost regions A.sub.1, an innermost region B.sub.1, and two intermediate regions C.sub.1 respectively lying between the outermost regions A.sub.1 and the innermost region B.sub.1 and defining along said first axis of said constellation two outer regions E.sub.2 and an inner region D.sub.2 therebetween, and determining the location of the signal point of said first baseband component with respect to said regions A.sub.1, B.sub.1, C.sub.1, D.sub.2 and E.sub.2 in dependence on said first-channel data bits and said first-channel error bit to derive a first logical output signal (ERP, DRP) from the determination made with respect to the regions A.sub.1, B.sub.1 and C.sub.1, and a second logical output signal (XDRP) from the determination made with respect to the regions D.sub.2 and E.sub.2 ;
• second decision means for defining along a second orthogonal axis of the signal constellation two outermost regions A.sub.2, an innermost region B.sub.2 and two intermediate regions C.sub.2 respectively lying between the outermost regions A.sub.2 and the innermost region B.sub.2 and defining along said second axis of said constellation two outer regions E.sub.1 and an inner region D.sub.2 therebetween, and determining the location of the signal point of said second baseband component with respect to said regions A.sub.2, B.sub.2, C.sub.2, D.sub.1 and E.sub.1 in dependence on said second-channel data bits and said second-channel error bit to derive a third logical output signal (ERQ, DRQ) from the determination made with respect to the regions A.sub.2, B.sub.2 and C.sub.2, and a fourth logical output signal (XDRQ) from the determination made with respect to the regions D.sub.2 and E.sub.1 ;
• and
• control means for controlling said first gain-controlled amplifier in accordance with said first and fourth logical output signals and controlling said second gain-controlled amplifier in accordance with said third and second logical output signals.

In a 2.sup.2n+3 -QAM (quadrature amplitude modulation) demodulator including first and second gain-controlled amplifier for amplifying in-phase and quadrature-phase baseband components of the QAM signal, respectively, first and second (A/D) analog-to-digital converters for converting output signals from the first and second gain-controlled amplifiers to corresponding digital signals;

• a first threshold decision circuit for comparing the digital signal from the first A/D converter with decision thresholds to produce first-channel data bits and a first-channel error bit;
• and a second threshold decision circuit for comparing the digital signal from the second A/D converter with decision thresholds to produce second-channel data bits and a second-channel error bit, a method for controlling said first and second gain controlled amplifiers, the method comprising the steps of:
• a) defining along a first axis of the signal constellation of the QAM signal two outermost regions A.sub.1, an innermost region B.sub.1, and two intermediate regions C.sub.1 respectively lying between the outermost regions A.sub.1 and the innermost region B.sub.1 and defining along said first axis of said constellation two outer regions E.sub.2 and an inner region D.sub.2 therebetween;
• b) defining along a second, orthogonal axis of the signal constellation two outermost regions A.sub.2, an innermost region B.sub.2, and two intermediate regions C.sub.2 respectively lying between the outermost regions A.sub.2 and the innermost region B.sub.2 and defining along said second axis of said constellation two outer regions E.sub.1 and an inner region D.sub.1 therebetween;
• c) determining the location of the signal point of said in-phase baseband component with respect to said regions A.sub.1, B.sub.1, C.sub.1, D.sub.2 and E.sub.2 in dependence on said first-channel data bits and said first-channel error bit to make a first decision with respect to the regions A.sub.1, B.sub.1 and C.sub.1, and a second decision with respect to the regions D.sub.2 and E.sub.2 ;
• d) determining the location of the signal point of said quadraturephase baseband component with respect to said regions A.sub.2, B.sub.2, C.sub.2, D.sub.1 and E.sub.1 in dependence on said second-channel data bits and said second-channel error bit to make a first decision with respect to the regions A.sub.2, B.sub.2 and C.sub.2, and a second decision with respect to the regions D.sub.1 and E.sub.1 ;
• and
• e) controlling said first gain-controlled amplifier in accordance with said first decision of the step (c) and the second decision of the step (d) and controlling said second gain-controlled amplifier in accordance with said first decision of the step (d) and the second decision of the step (c).