Abstract
Cooperative communications have emerged as a popular technique to combat fading and to exploit spatial diversity gains inherent in wireless multiuser systems. When combined with orthogonal frequency division multiplexing (OFDM), additional degrees of freedom in frequency can be further exploited to enhance system performance. In cooperative OFDM systems, efficient resource allocation among users and subcarriers is necessary to fully utilize the available degrees of freedom in both frequency and space. However, the dimensions of the problem often pose great challenges in determining the optimal design and, thus, efficient and innovative approaches must be proposed to address these issues. Moreover, to perform these resource allocation policies, accurate channel state information must be obtained at the terminals. Without accurate channel estimates, the system performance will be dominated by channel estimation errors, causing potential diversity gains to diminish. The main contribution of this thesis is to propose (1) an innovative approach to determine efficient subcarrier allocation, power allocation, and partner selection policies for cooperative OFDM systems and (2) a feedback-aided pilot placement scheme to help enhance channel estimation performance. Specifically, in the first part of the thesis, subcarrier allocation, power allocation, and partner selection algorithms are examined for cooperative multicarrier systems. In general, subcarrier allocation and partner selection problems are known to be intractable for systems with large number of users and subcarriers. In this thesis, a suboptimal but mathematically tractable approach is proposed to address these issues. This is done by considering the power allocation over a relaxed problem formulation, where each subcarrier is allowed to retransmit or relay for all other subcarriers in the cooperative phase over virtual orthogonal channels. Based on the results of the relaxed power allocation problem, an efficient subcarrier allocation algorithm for the original problem can then be derived by allowing subcarriers to forward the data that is allocated the most power. The results of the relaxed problem also yield a necessary and sufficient condition on when cooperation is helpful among two users, and the partner selection scheme is then devised based on the criterion given in the condition. In the second part of the thesis, the use of channel feedback to better determine pilot placement in cooperative OFDM systems is examined. With channel feedback from previous symbol periods, pilot locations can be determined to reduce channel estimation errors and to make available favorable subcarriers for data transmission. Based on the feedback gain, which is defined as the ratio between the minimum effective SNR achieved with the optimal feedback-aided pilot placement scheme and that achieved with the equal-spaced pilot placement scheme, we provide an analytic lower bound on the optimal feedback gain and show its rate of increase with respect to the number of pilot subcarriers. Due to the complexity of finding the optimal placement, a suboptimal but efficient algorithm, called the iterative pilot relocation (IPR) scheme, is proposed. The analysis and the proposed IPR scheme are also extended to systems with subcarrier pairing at the relay and also to systems with multiple relays. The efficacy of the proposed resource allocation and pilot placement schemes are demonstrated through numerical simulations.