Research On Power Allocation Algorithms In Relay OFDM Systems

With the development of the society, wireless communication technology has made great progress with the needs of people. However, the existing technologies of telecommunications is now unable to meet the requirements for higher communication quality and it’s imminent to construct the next generation wireless networks. As is known to all, long distance and obstacles cause a bad influence on the communication quality, so more and more attention has been paid on the wireless relay technology and a lot of researches have been made as it can weaken the influence. The OFDM technology has also been used in various communication environments because of its high spectrum efficiency. By combining both relay and OFDM technologies, we can make full use of their advantages. As a result, the communication quality can be improved when the channel state is not ideal, the capacity of the whole networks can be improved, and people’s requirement for high speed data transmission can be satisfied.The resource allocation schemes for wireless relay OFDM systems are studied in this thesis. Schemes are proposed with various emphasis, such as modified relay, power allocation and subcarrier pairing. The main content are as follows:(1) Resource allocation schemes in multi- relay multi-user OFDM system are studied. There are one source node, several relay nodes and several user nodes in this system. Subcarrier pairing between the source node and the user node is used. In order to avoid the interference between different subcarrier pairs, different relays should use different subcarrier pairs and one subcarrier pair can only be allocated to one relay and one user. A resource allocation scheme with power allocation, subcarrier pairing, relay selection and user selection is studied to maximize the capacity of the system. All the relay nodes in the system are worked under decode-and-forward mode. Unlike the traditional mode, an improved relay mode is used which allows the source node to retransmit the same signal to the user node in the second phase, thus improving the system performance. The detailed steps of the algorithm are designed and simulated. The influence of power, number of subcarriers and number of relay on system performance is analyzed.(2) A joint optimization scheme for power allocation and subcarrier pairing under high SNR in two-way multi-relay OFDM system is proposed. Unlike those schemes in which relays use subcarriers separately, we allow all the relays to forward signal on each subcarrier pair for providing more diversity gain. With the constraint of total system power, the proposed scheme firstly allocates all the relay power with Cauchy inequality when the total relay power is fixed, then allocates source power and the total relay power with dichotomy when the subcarrier is fixed and lastly allocates the subcarrier pair power with convex programming. There are none optimal power allocation schemes in two-way multi-relay systems yet because of the high computation complexity. Simulation results show that the proposed scheme not only outperforms the relay selection scheme and the subcarrier-pairing scheme, but also has a very low computation complexity.(3) A resource allocation scheme based on energy-efficiency in single-relay multi-user OFDM system is proposed. Under the constraints of maximum total power and minimum transmission rate, we formulate a joint optimization scheme for subcarrier pairing and power allocation to maximize the system energy efficiency. As the optimization problem based on energy efficiency is too hard to compute, the Dinkelbach’s method is used to depress the complexity and to transform the problem to a linear programming problem, then we use the dual decomposition approach solve the problem and use the Hungarian algorithm and the sub-gradient algorithm to calculate the optimal solution. The proposed scheme uses the improved DF relay, which means the source can disuse the relay node when the channel state is very bad, and retransmit the same signal to the user node in the second phase again. Simulation results show that, under the constraint of minimum system transmission rates, compared with the spectral-efficient algorithm, the proposed scheme can achieve higher energy efficiency with low computation complexity.