1 Introduction

When two or more signals use the same frequency band, they create cochannel interference. Such interference arises from in airborne reception where multiple ground transmitters are visible at the receiver, and in frequency reuse in cellular systems, where it is one of the major factors limiting system capacity [1].

The complex baseband representation of a sampled cochannel FM signal is

$$r[n] = A[n] e^{j\theta[n]} + B[n]e^{j\phi[n]}.$$ (1)

When no confusion can result, the subscript $n$ will be dropped. Initially, assume that there is no noise and that $A$ and $B$ are known positive quantities and vary slowly compared to $\theta$ and $\phi$. These assumptions will be dropped later. Given $A$, $B$ and $r$, accurate estimates of $\theta$ and $\phi$ are desired.