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.

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In this paper, an analog-digital adaptive image-reject (IR) technique for quadrature receivers is presented. In these receivers, I/Q mismatches degrade the image rejection performance. To compensate the mismatches, a complex error signal is generated digitally of which the real and imaginary parts are proportional to the I/Q amplitude and phase mismatches. This error signal is applied to Digitally-Controlled Multiplier-Adder (DCMA) blocks to correct the phase and amplitude mismatches. Simulations show that the imagerejection ratio is improved to more than 57 dB for a low-IF cable modem receiver. This performance is independent of the synchronization and equalization of the receiver.