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We suggest an iterative power and greedy subcarrier allocation algorithm to improve rate-sum capacity in an uplink OFDMA system. For the downlink, it has been accepted as an optimal solution that each subcarrier is allocated to the user with the best channel condition and power is allocated by water-filling over subcarriers. However, it is not true in an uplink OFDMA system which has distributive power constraints. We formulate the optimization problem having constraints power and subcarriers in uplink, then draw two necessary conditions for optimality. Using the conditions, we propose a greedy subcarrier allocation algorithm based a marginal rate junction and iterative power allocation algorithm based on water-filling. Simulation results show that the enhanced system capacity is achieved by our proposed scheme.