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In this paper, algorithms on distributed resource (spectrum and power) sharing for relay stations are investigated for downlink transmissions in an OFDMA-based relay-aided cell. Both system capacity and user fairness are considered. By grouping the relay stations into coalitions according to the set of users they are relaying, the optimal resource allocation can be solved by considering resource allocation within and among the coalitions. The algorithm for intra-coalition resource allocation is proposed by utilizing the key observation: for each data symbol transmitted from the base station to a user (in a subcarrier), only one among all the available relay stations is required to relay the symbol. The inter-coalition resource allocation is modeled by both a non-cooperative and a cooperative game, where the cooperative game is solved by a nonsymmetric Nash bargaining solution. Simulation results show that the non-cooperative algorithm outperforms random allocation by approximately 50% in system capacity with 3 relay stations in each coalition. The cooperative algorithm has approximately 5% loss in system capacity comparing with the non-cooperative algorithm, but achieves a significant gain in terms of fairness performance.