Bounds and algorithm for direction finding of phase modulated signals

In many cases where direction finding is of interest, the signals impinging on an antenna array are known to be phase modulated, hence to have a constant modulus (CM). This is a strong property, by itself already sufficient for source separation, and can be used to construct improved direction finding algorithms. We first present the relevant Cramer-Rao bounds for arbitrary array configurations, and arbitrary number of signals and then specialize to uniform linear arrays with a single source. We then propose a simple suboptimal direction estimation algorithm, in which the signals are separated using the CM property only. Compared to the ESPRIT algorithm and to the Cramer-Rao bound for arbitrary signals, the algorithm shows good results.