Research On Diversity Technique In Cooperative Communications Systems

Space diversity technique has attracted wide attentions since it can improve the transmission speed as well as the good fading resistance, In early research, the space diversity gain needs to be installed multiple antennas in a transmitter and a receiver, namely through multiple input multiple output system. In actual transceivers, however, especially the small hand-held mobile terminal, often due to the limitations of volume, power, and electromagnetic interference and other factors, it is often difficult to install multiple antennas.Cooperative communication system without multiple antenna installed at the terminal for space diversity gain, originated in the relay communication, it’s through multi-user sharing antennas and other network resources to form a virtual antenna array, and cooperation has been generated by distributed processing to achieve space diversity gain. Since the technology was proposed, it has attracted the attention of scholars around the world. A variety of different cooperative transport protocols and mechanisms appeared, and the key technology of the traditional end-to-end communication system was gradually evolving into the cooperative communication system. At present, the cooperative communication technology has become a hot research field of wireless communications. This paper studies the diversity reception technology of cooperative communication systems with spread and non-spread environment, and the innovative achievements are as follows:1. The author in this paper investigates the space-time 2D-RAKE receiver into the asynchronous cooperative communication systems with two relays under Rician multipath fading channels with large delay, and the author further proposes interference cancellation to improve the performance of the space-time 2D-RAKE. First, initial detection result of the transmitted information is obtained by conventional space-time 2D-RAKE, and then the multipath signals are reconstituted and eliminated to obtain the line of sight component in the received signals. Finally, the LOS components from different relays are combined by MRC criterion in space diversity combining in order to obtain spatial diversity gain. Further, iterative detection with multiple detection stages is employed in the paper to improve the performance. The algorithm can effectively eliminate inter-symbol interference in the received signal, and it’s suitable for asynchronous cooperative communications system. Simulation results show that the proposed algorithm can eliminates ISI of the received signal, and reduces the error rate "platform" effect of original system, so it significantly improve the performance of traditional space-time RAKE receiver, and makes space-time 2D-RAKE receiver to be applicable technologyin in cooperative communication system.2. ISI is eliminated by the interference cancellation technique, but multipath component which can provided multipath diversity gain also was eliminated at the same time. In order to solve this problem, the author propose full diversity space-time 2D-RAKE receiving technology to acquire the diversity gain of multipath components, it makes further improvement on the detection performance. First, initial detection result of the transmitted information is obtained by conventional space-time 2D-RAKE, and then the multipath signals that pass through all the paths of the channel are reconstructed and canceled all the other multipath signals component from the received signal. Parallel interference cancellation technology eliminates multipath signals which through others channels, leaving only the signal component by the multipath branch of the received signals, and then take the same process to obtain each path multipath component. Finally, we carry out the space combination to each multipath component and then implement multipath combination to the results of the space combining, after the hard decisions on the final output, we can get the detecting result of the new algorithm. New algorithm from space diversity combining to multipath diversity combining by loop iteration manner, in other words, it’s interference cancellation-diversity combining iterative structure. The proposed algorithm obtains not only the space diversity gain but also multipath diversity gain. so it’s a full diversity receiver. In addition, the proposed algorithm can be completed in the receiver, while the transmitter and all the working principle of relay node structure does not need to do any change, so the algorithm has a good system compatibility, good prospect of popularization and application.3. Space-time 2D-RAKE only can be applied in spread spectrum system, so the application is restricted. For non-spreading cooperation communication systems, space-time 2D-RAKE technology can not be applied directly, aiming at this problem, the fifth chapter studies diversity reception technique based on single carrier frequency domain equalization technology in cooperative communications systems. From the perspective of interference offset:STBC-SC-FDE is applied in order to get initial solution of the original signals, using the initial solution combined with CIR reconstruct path of multipath signals and offset them from the received signals, only leaves the LOS. Then, LOS components are combined by MRC, after that hard decision is carried out to recover the original signals. The method eliminates the interference of multipath fading and gets cooperative diversity gain of relays. From the multipath and relay diversity combining perspective:With similar to full diversity of space-time 2D-RAKE, by decomposing the received signals, obtainment of the components of each multipath signal. Then according to the order of the space diversity combining and multipath diversity combining in criterion of MRC, the method gets two dimensional diversity gain, transmission performance of the system can be further improved. The bit error rate of the initial detection effect is obvious for the algorithm, finally the author also puts forward an improved full diversity consolidation structure:First the interference cancellation algorithm is carried out to obtain a better initial test results, and then multipath components are reconstructed by the initial solution of interference cancellation algorithm and counteracted from the received signals, and two dimensional consolidation are carried out in the final. The simulation results show that the proposed new algorithm can eliminate the ISI of received signal, the new algorithm significantly reduces the BER "platform" of traditional systems, and the new one uses linear computational complexity, so the cost of the algorithm is acceptable.