Abstract-We propose three encoding strategies for a two user cooperative Orthogonal Frequency Division Multiple Access (OFDMA) system, based on block Markov superposition encoding (BMSE). We obtain the expressions for the resulting achievable rate regions for all three encoding strategies. We show that, by allowing for re-partitioning and re-encoding of the cooperative messages across subchannels, it is possible to better exploit the diversity created by OFDMA, and higher rates can be achieved. We demonstrate potential rate gains attained by cooperative OFDMA, through simulations.

For a two user cooperative orthogonal frequency division multiple access (OFDMA) system with full channel state information (CSI), we obtain the optimal power allocation (PA) policies which maximize the rate region achievable by a channel adaptive implementation of inter-subchannel block Markov superposition encoding (BMSE), used in conjunction with backwards decoding. We provide the optimality conditions that need to be satisfied by the powers associated with the users' codewords and derive the closed form expressions for the optimal powers. We propose two algorithms that can be used to optimize the powers to achieve any desired rate pair on the rate region boundary: a projected subgradient algorithm, and an iterative waterfilling-like algorithm based on Karush-Kuhn-Tucker (KKT) conditions for optimality, which operates one user at a time and converges much faster. We observe that, utilization of power control to take advantage of the diversity offered by the cooperative OFDMA system, not only leads to a remarkable improvement in achievable rates, but also may help determine how the subchannels have to be instantaneously allocated to various tasks in cooperation.

Abstract-We propose two encoding strategies for a two user cooperative Orthogonal Frequency Division Multiple Access (OFDMA) system, based on block Markov superposition encoding. We obtain the expressions for the resulting achievable rate regions for both encoding strategies, and evaluate them through simulations. We show that, by allowing for re-partitioning and re-encoding of the cooperative messages across subchannels, it is possible to better exploit the diversity created by OFDMA, and higher rates can be achieved.

Abstract-For a cooperative orthogonal frequency division multiple access (OFDMA) system with two transmitters (TXs), and full channel state information (CSI), we obtain the optimal power allocation (PA) policies which maximize the rate region achievable by a recently introduced version of block Markov superposition encoding (BMSE): inter-subchannel cooperative encoding (ISCE) [1]. We provide the optimality conditions that need to be satisfied by the powers associated with the transmitted codewords. We propose two algorithms that yield the optimal power distribution: a subgradient algorithm which achieves an arbitrary rate point on the achievable rate region boundary, and an iterative waterfillinglike algorithm which maximizes the sum rate, and converges much faster. We observe that, utilization of power control to take advantage of the diversity offered by the cooperative OFDMA system, not only leads to a remarkable improvement in achievable rates, but also may help determine how the subchannels have to be instantaneously allocated to various tasks in cooperation. I. INTRODUCTIONThe ability of OFDMA to cope with both intersymbol and interuser interference, combined with its low complexity of implementation have made it a popular choice for the next generation wireless networks. As a result, the problem of resource allocation in OFDMA systems was studied extensively in the literature. One example is [2], where it was proved that in an OFDMA uplink system, allocating subcarriers to the users with the maximum marginal rate is a necessary condition for maximizing the system throughput. A similar problem was solved in [3] using KKT conditions, by optimizing a utility function which was assumed to be a function of the rates. In [4], a low-complexity algorithm for subcarrier, power, and rate allocation for OFDMA was proposed, to maximize the sum rate under individual rate constraints to guarantee fairness. These works, as well as many others on OFDMA, naturally assume orthogonal multiple access, thereby choosing to avoid interference. However, like all orthogonal transmission techniques, OFDMA incurs some rate penalty, caused by the orthogonalization. Moreover, in wireless channels, "interference" is in fact free side information, and gives rise to the concept of user cooperation, if taken advantage of by the system design. Hence, in this paper, we focus on resource allocation for a cooperative OFDMA model, which allows subchannels to be shared by pairs of transmitters, thereby treating interference as information, and allowing mutual cooperation among the transmitters over each subchannel.Besides its natural use in the uplink within each cell, the cooperative OFDMA system analyzed in this paper is especially useful in multicell cooperation scenarios, some examples of which are illustrated in Figure 1:

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