Demodulation of cochannel QAM signals

The authors describe a technique for demodulating a quadrature amplitude modulated (QAM) signal in the presence of a second cochannel QAM interferer and intersymbol interference. The method consists of a two-step procedure whereby tentative estimates of both the desired and interfering signals are made and then converted into final estimates by detecting and correcting errors. The tentative decisions are formed using a structure consisting of a primary demodulator followed by a secondary demodulator incorporating an adaptive interference canceller. Errors in the tentative decisions are detected using an adaptive channel estimator and then corrected using a maximum-likelihood sequence estimator. Signature curves quantifying the symbol error rate of the tentative demodulator as a function of signal-to-noise ratio and signal-to-interference ratio are presented. A simulation of the error detection/correction method is also presented. Simulations indicate that the method will yield substantial improvement in performance relative to conventional demodulation