Research On Resource Allocation In OFDMA Based Wireless Broadband Communication Systems

With the fast growing demand for high data rate communication, the spectrum resource in wireless communication systems becomes more and more scarce. This stimulates the development of new technology to achieve efficient utilization of radio resources. OFDMA is capable of enhancing the spectrum efficiency of the system. Furthermore, it can combat the inter-symbol interference causing by the multi-path effect. Therefore, OFMDA has become one of the key technologies in broadband wireless systems. This thesis focuses on the problem of resource allocation in the OFDMA systems. By utilizing the tools of convex optimization and intelligent computing, the resource allocation algorithms in three kinds of networks including the cellular network, the relay cellular network and the cognitive network based on OFDMA are proposed. The main work and contributions of this thesis are as follows:1) The problem of Rate Adaptive (RA) in multiuser OFDMA system is studied. A distributed resource allocation algorithm based on dual decomposition is proposed. The complicated problem is decoupled into several subproblems with regard to the users that can be solved parallelly. Each user can obtain its own subcarriers and power allocation scheme by solving the subproblem according to its local channel state information and the corresponding parameters provided by the base station. The proposed algorithm has a low computational complexity, and is able to reduce the calculation payload of the base station. Simulation results show that the proposed algorithm can converge quickly to an approximated optimal solution in a small number of iterations.2) The problem of Margin Adaptive (MA) in multiuser OFDMA system is studied. An ant colony algorithm based adaptive resource allocation is proposed. The problem is modelled as finding a minimum cost path in a graph. The ants'solution construction is guided by pheromone trail and heuristic information. Due to the unique heuristic searching mechanism of the ant colony algorithm, the proposed algorithm is guaranteed to converge quickly to an approximately optimal solution. Simulation results show that the proposed algorithm only loses 1 dB in comparison with the optimal solution, and gains 1-2dB in comparison with the fixed allocation scheme.3) Resource allocation in multihop relay OFDMA systems is studied. The problem is formulated as a sum rate maximization problem subject to each access node's transmission power and each user's minimum rate constraints. A dual decomposition based resource allocation algorithm is proposed. The problem is decomposed into several subproblems with regard to the subcarriers. By solving the subproblems, the optimal relay selection scheme, power allocation and subcarrier assignment is obtained. Simulation results show that the proposed algorithm can efficiently improve the capacity of the system, and guarantee each user's QoS requirement at the same time.4) Resource allocation in Decode and Forward (DF) relay OFDMA systems is studied. A proportional fairness based resource allocation algorithm is proposed. The proposed algorithm performs a good tradeoff between system capacity and fairness. The system capacity is further improved by subcarrier matching. According to the proportional fairness criterion, the problem is formulated as a sum logarithmic rate maximization problem subject to total transmission power. In order to reduce the computational complexity, the original optimization problem is decomposed into two steps. In the first step, the subcarrier assignment and relay selection scheme are determined based on equal power allocation. In the second step, the power allocation is achieved by waterfilling theorem. Simulation results show that the proposed algorithm can efficiently improve the capacity of the system, and guarantee the fairness among users simultaneously.5) Resource allocation in cognitive OFDMA system is studied. In practical systems, it may not be possible for the cognitive transmitter to know the perfect channel state information value of the primary user in the authorized network. So the resource allocation with imperfect channel state information is studied in this thesis. The problem is formulated as a weighted sum rate maximization problem subjected to each primary user's outage possibility constrains. A dual decomposition based resource allocation algorithm is proposed to obtain the optimal power and subcarrier allocation scheme. The influence of imperfect channel state information to the system is analyzed through simulation, and the efficiency of the proposed algorithm is also proved.