We consider an amplify-and-forward (AF) cooperative diversity network consisting of one source, multiple relays, one destination, and multiple interferers. It is assumed that only partial channel information of the interferers is known at the system. For this scenario, we derive an optimum/suboptimum detection rule. Furthermore, we propose an optimum relay power allocation scheme to maximize the instantaneous signal-to-interference-plus-noise-ratio (SINR). An exact and closed-form instantaneous BER expression is also presented. Numerical results demonstrate that the proposed scheme considerably outperforms the existing schemes.