Joint uplink and downlink resource optimization for OFDMA systems: When base station serves as a relay

In this paper, we are concerned with intra-cell communications in OFDMA networks for which the base station serves as a decode-and-forward relay to assist for information flows within the cell. Unlike the existing resource allocation problems which separate the uplink and the downlink designs, the metrics considered in this paper, i.e., the information flow rate between each pair of intra-cell users, couples the uplink and the downlink rates and calls for joint optimization between the uplink and the downlink resources. Two problems for jointly designing subcarrier/user assignment and power allocation for OFDMA uplink and downlink are considered: weighted sum information flow rate maximization problem and weighted sum power minimization problem. Using Lagrange dual decomposition method, we derive the asymptotically optimal solutions for the proposed problems. The derived solutions allow flexible and efficient subcarrier allocation and user assignment. In particular, different from the subcarrier assignment and pairing (SAP) techniques in the literature, the proposed schemes do not restrict the uplink subcarriers to be paired with the downlink subcarriers, thus they are applicable to the case when uplink and downlink have different number of subcarriers. Furthermore, the proposed schemes decouple the uplink and downlink designs, thus have much lower computational complexity as compared to the SAP techniques. The simulation results have validated that the proposed algorithms outperform the existing schemes.