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Cooperative transmissions have been shown to be able to greatly improve system performance by exploring the broadcasting nature of wireless channels and cooperation among users. We focus, in this paper, on leveraging cooperation for resource allocation among users such that the network performance can be improved. Two important questions are answered-who should help whom among the distributively located users, and how many resources the users should use for cooperation to improve the performance. To answer these questions, a power-optimization, subcarrier-allocation, and relay-selection problem is formulated over a multiuser orthogonal frequency-division multiplexing (OFDM) network, which is applicable to systems such as wireless local area networks (WLANs). In the multiuser OFDM network, cooperation among different users is conducted by assigning the subcarriers of the helping users to relay a certain part of the helped users' data, while maintaining the desired rates of both helping users and helped users by means of power control and rate adaption. This way, the bandwidth efficiency of the multiuser OFDM system with cooperation is the same as that of the noncooperative OFDM system. The formulated optimization problem is an assignment problem for subcarrier usage and corresponding bit loading as well as power control. We provide an approximate closed-form solution for a two-user two-subcarrier case. Then, a suboptimal heuristic algorithm for a multiple-user multiple-subcarrier case is proposed and implemented in the base station to solve the formulated NP-hard problem. From the simulation results, the proposed scheme achieves up to 50% overall power saving for the two-user system and 19%-54% overall power saving for the multiuser case with random locations, compared with the current multiuser OFDM system without cooperative diversity. The proposed scheme is also compared to a much more complicated orthogonal frequency-division multiple access (OFDMA) system.