A digital decision-based adaptive image-reject technique for QAM receivers

In this paper, a fully digital, decision-based, adaptive image-reject (IR) technique for QAM receivers is presented. In these receivers, I/Q mismatches degrade the image rejection performance. To compensate for the mismatches, two simple error signals, which are proportional to the I/Q amplitude and phase mismatches, are generated digitally. These error signals are applied to digital multiplier-adder blocks in order to correct the mismatches. Simulations show that the image-rejection ratio (IRR) is improved to 57dB for a low-IF cable modem receiver. This performance is achieved in a time period less than 1,000 symbol periods.