We examine a new problem called the Zone Selection Problem (ZSP) in wireless Orthogonal Frequency-Division Multiple Access (OFDMA)-based systems. We consider the case where the mobile station (MS) informs the base station of its preferences regarding possible working modes supported by the system. In Worldwide Interoperability for Microwave Access (WiMAX) systems, which are used as a practical reference system for constraints and specific modes of operations, each working mode must be assigned at a different time interval called a Zone. Such zone may, in the downlink subframe, define a specific transmission scheme, which can be either single-input single-output or one of the multiple-in multiple-out (MIMO) modes supported by the system. We define ZSP as a problem where the base station assigns MSs to zones in a way that optimizes the system utilization (in terms of resource usage) while trying to satisfy the preferences of the users as much as possible. This makes ZSP a bi-objective optimization problem. We derive assignment costs from physical channel models that integrate specific MIMO schemes and channel conditions. We provide an exact solution to ZSP by using a combinatorial optimization abstraction. We propose a simplified and practical mode ranking approach that enables the MS to define its preferences regarding operation in the optional MIMO modes in the system. We prove that ZSP is NP-hard and describe two heuristic solutions: a greedy approach and a local search approach. Simulation results show the performance of the exact algorithms on a realistic WiMAX system assumptions and present the trade-off between the user preferences and system preferences.
- bi-objective optimization