Signal space partitioning versus simultaneous water filling for mutually interfering systems

We consider a communication system with multiple independent user-base pairings in a white Gaussian noise environment, and for which a simultaneous water filling condition is satisfied by users at their respective bases. We focus on the weak mutual interference case for which the simultaneous water filling solution is unique and corresponds to complete user overlap in signal space. We show that when users at other bases are treated as Gaussian noise, simple separation of users in signal space usually offers better performance than simultaneous water filling, and present a distributed algorithm which separates users in signal space.