Resource Allocation For Relay Based Orthogonal Frequency Division Multiplexing Networks

The next-generation of wireless communication networks will bring a real sense of freedom tocommunicate, and change the way we live completely, thus, the performance of the system hadbeen put forward higher requirements. Orthogonal frequency division multiplexing (OFDM) is amature technique to provide higher spectrum utilization while cooperative relaying can provide anefficient way to overcome the disadvantages brought by wireless fading channel. The combine ofthe advantages of both technologies has become an effective solution to the technical problems ofwireless communication networks. This thesis focuses on the research of the resource allocation formulti-user relay enhanced OFDM systems. The emphasis is laid on the resource allocation of thesingle-cell single-relay, the single-cell multi-relay and the multi-cell multi-relay OFDM systems.Different resource allocation algorithms are proposed for the different scenarios. These algorithmsare based on the core objective to maximize system capacity and taking the complexity and fairnessof different users into account as well.For the single relay scenario, a dual-hop OFDM relaying system employing decode-and-forward(DF) relaying protocol is considered and a low-complexity subcarrier and power allocation (PA)scheme with four steps is proposed. First, the dual-hop subcarriers are matched according to theiractual strength, then we allocate the total transmit power between source and relay with respect totheir average channel attenuation of first and second hop. Afterwards, we reformulate the originalmax-min problem into a standard maximization problem and give the optimal PA at relay for agiven source PA to maximize the instantaneous rate. Finally, the sum transmit power of each givensubcarrier pair is reallocated to further enhance the system capacity. This thesis also investigatessubcarrier allocation problem of relay aided OFDM systems with PF constraint and proposes anefficient PF scheduling priority by using the user’s channel condition of past scheduling time andcurrent scheduling time more comprehensively and uniformly which can both maximize the systemtransmission rate and enhance the system fairness no less than traditional ones. The simulationresults show the validity.This thesis applies the random geometry theory to DF relay-enhanced single-cell OFDM systemand derives a new system capacity model finally. There are some assumptions that multiple usersdistribute uniformly in a ring region around Base Station (BS) and frequency is reused existingamong users in access zone. By using the upper bound of interfere integral value to alternate the actual interfere value when considering the co-channel interference, the closed expression of systemcapacity is obtained of the system capacity model under the power constraints of BS and relaynodes. Simulation results show the validity of the system capacity model.To maximize the transmit rate of system, a joint interference coordination, subcarrier and powerallocation algorithm is proposed. To reduce the complexity, this semi-distributed algorithm dividesthe primal optimization into three sub-optimization problems, which transforms the mixed binarynonlinear programming problem (BNLP) into standard convex optimization problems. The firstlayer optimization problem is used to get the optimal subcarrier distribution index. The second is tosolve the problem that how to allocate power optimally in a certain subcarrier distribution order.Based on the concept of equivalent channel gain (ECG) we transform the max-min function intostandard closed expression. Subsequently, with the aid of dual decomposition, water-filling theoremand iterative power allocation algorithm the optimal solution of the original problem can be gotwith acceptable complexity. The third sub-problem considers dynamic co-channel interferencecaused by adjacent cells and redistributes resources to achieve the goal of maximizing systemthroughput.Finally, a low-complexity algorithm taking into account of interference coordination, subcarrierand power allocation is proposed for multi-cell OFDM-relay networks with frequency planning. Wedivide each cell into three sectors and allow different subcarrier set can be used by the users of onesector, such a method can help to increase the distance among users that using the same subcarriersof adjacent cells, which can reduce the impact of co-channel interference to a certain extent.Therefore, the original problem can be decoupled into three independent sub-problems by means offrequency dividing and adaptive power allocation at base station nodes which can reduce thecomputing complexity greatly. In the process of resource allocation for single sector, therelationship of transmission power between base station and relay node is used to transform themax-min problem into standard closed expression. With the help of dual decomposition approach,water-filling theorem and iterative power allocation algorithm, the suboptimal solution of the primalproblem can be achieved finally. Simulation results confirm the effectiveness of our proposedalgorithm.