Multiuser OFDMA optimization: Algorithms and duality gap analysis

In this paper we address the problem of power allocation in OFDM multi-user systems, if each subcarrier has to be allocated exclusively to a single user. Such transmit strategies are of particular interest for practical systems because users are orthogonalized and thus signal processing is extremely simple. Due to the combinatorial structure the considered problems are non-convex. We propose algorithms based on the Lagrangian dual avoiding a combinatorial optimization. The resulting power allocations are surprisingly close to the information-theoretic optimum. Moreover, the performance loss can be upper bounded by an expression based on the the Lagrangian duality gap, which yields a valuable statement about any arbitrary resource allocation fulfilling the constraints.