An approach to multipath correction in FM transmission is proposed. It is based on the factorization of the channel transfer function in terms of the roots of the underlying polynomial. A time-recursive algorithm for the identification of the roots of the all-zero channel transfer function at the receiver is presented. It allows an efficient adaptive design of special equalizer structures with low computational burden compared to existing solutions. The multipath correction method was tested in connection with a baseband FM receiver and a modulating signal consisting of a bandlimited stereo-multiplex signal produced by combining sinusoids with random phases distributed uniformly in the interval 0-2 pi . The identification of the channel roots was accomplished by means of a stochastic gradient algorithm with constant step size alpha =0.0005. The trajectories of the identified roots in the z-plane and the corresponding modulus of the equalizer output signal are illustrated for three different channel configurations.>

Multipath propagation is a severe problem in conventional FM broadcasting since this phenomenon causes nonlinear distortions after demodulation. Several proposals for the correction of multipath corrupted FM signals were given in some recent papers, but up to now there exist no satisfactory solutions with sufficiently low hardware complexity for commercial application. The main idea of the present paper is to take advantage of some a priori knowledge of the multipath transfer function with the goal to develop a nonrecursive equalizer structure with minimum hardware expense. For adaptive adjustment of the equalizer, we use a recently introduced constant modulus algorithm (CMA) in a modified form. Instead of direct coefficient adjustment, we regard the parameters of the transmission channel as unknowns which are updated adaptively. The equalizer coefficients are uniquely defined by the channel parameters and can be determined in one step. The advantage of this method is a large improvement of the convergence behavior in comparison to existing solutions. The methods presented here are tested by means of an FM hardware system including FM transmitter, multipath simulation, FM demodulator, and an FIR multipath equalizer, which allows real-time investigations of the equalizer performance under well-defined multipath conditions.