Weighted sum-rate maximization in multiuser-OFDM systems under differentiated quality-of-service constraints

We consider the maximization of the weighted sum-rate on a multiuser-OFDM downlink with adaptive modulation and power under a total transmit power constraint and a user-wise target BER. We allow each subcarrier to be shared by more than one user. We show that this optimization problem can be decomposed into two subproblems: a subcarrier assignment and a power allocation. We prove that the optimal subcarrier assignment is exclusive, that is each subcarrier is allocated to only one user. The optimal power allocation corresponds to a multilevel water-filling. When the achievable rate region is convex, the optimality of the exclusive subcarrier assignment for arbitrary weights means that the OFDMA is the optimal sharing scheme for various performance criteria.