Research On Resource Allocation Algorithms In OFDM Relay Systems

Relay technique has been accepted as one of the most effective solutions to increase capacity and enhance coverage. Orthogonal frequency division multiplexing (OFDM), has emerged as one of the prime modem schemes for the future broadband wireless systems. By dividing the transmission channel into multiple orthogonal sub-channels, the inter-symbol interference (ISI) induced by multi-path fading is overcome. Considering the advantages of both the relay and OFDM techniques, the integration of relay and OFDM into the future wireless systems has received much attention.This thesis mainly researches the resource allocation algorithm based on wireless-relaying OFDM system. The power allocation algorithm for non regenerative OFDM relay links is presented. Three different utility functions for resource allocation are studied according to the hot topic of energy-efficiency communication:First, an improved Decode-and-Forward (DF) two-hop multi-relay assisted OFDM cooperative system is considered. We propose a resource allocation algorithm and the goal is to minimize the sum power of the system by jointly optimizing subcarrier pairing, subcarrier pair-relay assignment and power allocation under total transmission rate constraint. To reduce the complexity, the mixed integer programming problem is transformed into standard convex optimization problem, and solved in the dual domain.Then the optimizationis transformed into three sub-optimization problems.The first sub-optimization is used to get optimal power allocation for given subcarrier pairing and relay assignment. The second is to get optimal relay selection for given subcarrier pairing. The third is to get optimal subcarrier pairing by using Hungarian method.Finally.The simulation results vertify the theoretical analysisand illustrate the power-reduction benefit.Then, the energy-efficiency problem in a single-relay multiple-access system is investigated. The problem is to maximize the energy-efficiency measured using the "throughput per Joule" metric subject to the total transmit power. It is then transformed into an equivalent convex problem using parametric programming. By observing the special structure of a three-dimensional assignment problem derived from the original problem, a low-complexity algorithm based on continuity relaxation and dual minization is proposed. And the fairness is also considered.Finally, a multi-relay and multi-user assisted OFDM cooperative network is considered. The average "Bits-per-Joule" is defined as the metric, a low-complexity subcarrier-power allocation and relay selection scheme for maximizing system energy efficiency is proposed. The numerical result demonstrates that the proposed scheme can achieve similar performance in much lower computational complexity as compared with the exhaustive search scheme.