Suppression of adjacent-channel and cochannel FM interference via extended Kalman filtering

The problem of demodulating an FM signal in the presence of an adjacent-channel or cochannel interferer is addressed. The pertinent measurement equation and a first-order state-space model of the underlying message processes are incorporated in an optimal estimation framework. Due to the nonlinearity of the measurement equation the extended Kalman filter is used. The resulting system consists of two interconnected second-order phase-locked loops (PLLs), each of which tracks one of the two FM signals. The system is contrasted with the previous state of the art in the area, which combined maximum a posteriori estimation with adaptive filtering and also yielded a cross-coupled PLL structure. The significant differences in the new system are variable loop gains, a different amplitude estimator structure, and an interconnection between the internal states of the PLLs. Computer simulation results are presented comparing the relative effectiveness of the Kalman filter and the maximum a posteriori estimator