Research On Network Coding In Cooperative Communication Systems

In wireless communication networks, MIMO, with the usage of multipath propagation, is an effective technology which can reduce the negative effect of wireless fading channels and improve system spectrum resource utilization and increase the channel capacity. However, the performance of MIMO system is limited by the correlations between the antennas. Furthermore in practice, MIMO is difficult to be implemented due to the restriction on user terminals of size, mass and power. Under this background, cooperative diversity is proposed, where the user terminals can share the antennas with each other to form cooperative MIMO. As a result, the single-antenna terminals can obtain MIMO performance. However, with the growing communication demands, the multiple-antenna terminal is an inevitable trend in the development of wireless communications. On the other hand, network coding is another hot topic in wireless communication field. Network coding, which breaks the store and forward working mode of the traditional routers, can significantly enhance the network throughput due to expanding relay nodes’ features. As a result, network coding can approach the capacity upper bound defined by max-flow min-cut theorem. We can expect that the performance of communication networks can be further improved when cooperative diversity and network coding are combined.Under this background, the dissertation makes deep studies on network coding in two-way relay channels, analog network coding, physical-layer network coding and its combining with space-time coding in multi-user cooperative communications.First, the basic principles of cooperative diversity and network coding are introduced, which are the basis of the subsequent research. In MIMO system model, combining criteria of receiving diversity and channel capacity of MIMO systems are analyzed. As well, some typical cooperative diversity methods are introduced, and the achievable rate regions and system outage probability are analyzed in a two-user cooperative model. In two-way relay channels, the two-way relay communication methods and the demodulation and mapping scheme with physical-layer network coding are elaborated.Then, the intensive research on network coding schemes in two-way relay communications is done and pseudo physical-layer network coding scheme is proposed. The signal processing procedure is analyzed. Similar to physical-layer network coding, the relay node in PPNC scheme just need one demodulation procedure and one channel decoding procedure, but without the synchronization problem. The PPNC scheme can be seen as the physical-layer implementation of network coding, and it is also a three-time slot scheme. The computational complexity, bit error rate and achievable rate are analyzed, compared with other network coding schemes. Also the application scenarios of PPNC scheme are discussed.To solve the problem of transmission delay in a multi-user cooperative system, analog network coding is introduced to a multi-user cooperative system, and a cooperative analog network coding scheme is proposed. The cooperation procedure of this scheme is analyzed and the channel capacity of the scheme is derived when the relay node works in amplify-and-forward mode. Compared with traditional cooperation, the influence of the nodes’ antenna number on channel capacity is analyzed and the BER performance of cooperative analog network coding is studied under different simulation scenarios.After that, the problem about how to apply physical-layer network coding into multi-user cooperative systems is deep studied. Employing polarization diversity, cooperative quadrature physical-layer network coding scheme is proposed and the cooperation procedure is analyzed. This proposed scheme is a cooperative method based on decode-and-forward and physical-layer network coding. Compared with some other cooperation schemes, the BER and system throughput performances are analyzed under different transmission conditions. Also the effect of different decoding methods on the performance of this proposed scheme is studied. The simulation results reveal that the proposed scheme can effectively improve the system performances.Finally, to further improve the performance of multi-user cooperative systems, the application of PNC in a multi-user cooperative system with multiple-antenna terminals is studied and the cooperative quadrature physical-layer network coding jointed with space-time coding scheme is proposed. The effect of the nodes’ antenna number on BER and system throughput performances is analyzed. Also it is compared with traditional decode-and-forward scheme and cooperative network coding scheme. The simulation results reveal that the proposed scheme can effectively improve the system performances, and its performance is the best.