On the Structure of the Multiuser QoS Region

The quality-of-service (QoS) feasible region (the set of jointly feasible QoS) of a multiuser system is limited by mutual interference. In this paper, we characterize the region under the assumption that the interference coupling between the users can be described by a parameter-dependent matrix, where the parameter stands for some receive (or transmit) strategy. This model holds for a large class of practically relevant system designs (e.g., beam- forming, CDMA). In general, interference coupling is strongly influenced by the receive strategy. Thus, classical results based on the assumption of fixed irreducible coupling matrices cannot be transferred directly. New techniques are required to describe systems, where users are partially decoupled because of the impact of the receive strategy. We develop an analytical framework, which considers the close interaction between receive strategy and power allocation, but without the restricting assumption of irreducibility. We characterize the structure of the boundary, and explain the system behavior in the high signal-to-noise ratio (SNR) regime. We provide conditions for existence and uniqueness of an optimal power allocation, given an optimal choice of receive strategy. We also discuss the duality between interference mitigation at the receiver and at the transmitter. Finally, we propose a globally convergent algorithm, which computes the min-max optimal power allocation under the assumption of block-irreducibility (independent irreducible subsystems).