Research For Link Performance Of Cooperative Wireless Relay Sysytem With Network Coding

The next-generation broadband cellular mobile communication system (IMT-Advanced) introduced the wireless relay technology to expand coverage. And the cooperative relay technology is also applied to provide spatial diversity gain, thereby enhancing spectral efficiency and data transfer rate. In order to solve the problem that the additional need for radio resource in the relay transmission cannot guarantee high spectrum efficiency, the network coding technique has been proposed in order to further improve the wireless relay transmission performance of IMT-Advanced system. Network coding technology breaks the traditional way of information processing. With network coding technology, the relay nodes conduct the corresponding signal processing and then forward the regenerated signals, instead of forwarding the information received. The application of network coding in the relay network increase the throughput of the wireless network, reduce times of the data packets are forwarded, and improve the spectral efficiency of the system.In the wireless environment, due to the fading characteristics of the radio channel, the transmission errors will inevitably exist. Therefore, in the actual fading channel, there is a big gap between the capacity of the relay system using network coding and the theoretical capacity sector of multicast system, and the situation that the system capacity is degraded using network coding may occur. In order to ensure that the wireless network coding scheme has better performance gain and is able to be applied in actual radio relay system, the enhancement techniques of the physical layer with the network coding, which suitable for different application scenarios, are studied in this paper.The researches in this paper focus on the following areas: Firstly, the network coding schemes in two-way relay channel and multiple-access relay channel are full studied. Several commonly used network coding schemes based on AF (Amplify-and-forward) or DF (Decode-and-forward) are introduced in this paper. The outage probabilities and bit error rates of various schemes are provided to assess the strengths, weaknesses and the application scope of different schemes.Secondly, in the two-way relay channel, in order to improve the accuracy of network coding mapping, a Sum-Difference network coding mapping scheme based on the dual-antenna relay is proposed in this paper. Both theoretical derivation and simulation results validate that the proposed scheme effectively improve the accuracy of the network coding mapping in relay node. In addition, a simplified algorithm is provided to reduce the computational complexity of the proposed scheme when quadrature amplitude modulation (QAM) is applied. The performance evaluation are given to illustrate and justify the computational complexity of mapping scheme is reduced, while maintaining the performance gain.Finally, in the multiple-access relay channel, if the decision of the source signals at the relay node is incorrect, and the incorrect signal is applied in reconstructing the network coded signal, the error diffusion will occur and result in the sharp decline of the performance at the destination node. Therefore, a mutual information based soft forwarding network coding scheme is proposed in this paper. In the proposed scheme, the relay node determines the reliability of the signal decision by calculating the mutual information in the process of signal decision. The relay node forwards not only the network coded signal, but also the mutual information of the signal. Simulation results are given to justify the proposed scheme performs better on decreasing BER in a variety of channels, comparing to existing schemes including the AF-based, DF-based, EF (Estimate-and-forward)-based network coding scheme.