Investigation Of User Pairing Algorithms And Cooperative Diversity Performance In Wireless Communications Systems

Diversity is an effective method to mitigate fading induced by multipath propagation in wireless networks. By transmitting redundant signals over essentially independent channel realizations in conjunction with suitable receiver combining, diversity can be achieved to improve the transport performance of the the time variant channel. Cooperative diversity, taking another terminal or a relay, allows a collection of radio terminals that relay signals for each other to emulate an antenna array and exploit spatial diversity to improve communications reliability in wireless fading channels. Cooperative diversity has been recognized as a low-cost and efficient technique to combat fading. Additionally, the newly emerging cooperative technology network coding has found its applications in wireless communications due to the broadcast nature of the radio propagation environment.This dissertation focuses on the research on the theoretical performance and key techniques of cooperative communications, including multi-user cooperative pairing algorithms, the theoretical performance of heterogeneous cooperation schemes, cooperative network coding schemes, etc. According to this, the dissertation is composed of three parts.The first part of this dissertation is to study the multi-user cooperative pairing algorithms aiming at the user pairing algorithm analysis and designing in virtual multiple input multiple output (MIMO) systems. Determinant pairing algorithm (DP) has been the classical method for uplink Virtual MIMO systems. However, its complexity is too high for any practical implementations when the number of users is large. To solve this problem, three simplified methods are proposed in this dissertation which are semi orthogonal pairing (SP), correlation angle-based pairing (CAP), and Frobenius norm-based pairing (FNP), respectively. Both analysis and simulation results show that the correlation angle-based pairing algorithm performs close to the optimum with proper threshold and Frobenius norm-based paring algorithm achieves a fairly good performance with little channel state information. Accordingly, the proposed correlation angle-based pairing algorithm and Frobenius norm-based paring algorithm are more practical and effective for the virtual MIMO transmission.The second part is to research and evaluate the performance of the heterogeneous relaying scheme utilized to complete the hierarchical convergence of multiple radio access networks (RANs), and show key factors impacting on the performance as well. Based on an outage probability analytical model, the diversity gains related to the multiple access schemes and power constraints are analyzed theoretically and simulation results are presented as well. Our analytical and simulation results show that the outage probability mainly depends on the number of cooperative relay nodes, multiple access scheme, normalized power, and radio channel characteristics. Meanwhile, the power constraint for relay nodes does not have much impact on the outage performance especially when the broadcast/multicast radio channel condition is good enough.The third part of this dissertation is to research cooperative wireless network coding. Network coding has recently been applied to wireless networks for its ability to improve network throughput. Early research works in wireless network coding mainly focused on the information theory analysis on bit level exclusive or in Galois field which has been proved well suited to wire networks where the channel is almost invariable. Nevertheless, traditional network coding does not work well in the scenario of wireless networks due to the time variable property of the wireless channel. In order to make network coding work well in the scenario of wireless networks, the physical layer network coding and the analogue network coding as well as the complex field network coding has been proposed in succession. In this paper, analogue network coding in cooperative relay networks is focused on and an optimized transmission scheme utilizing retransmission is presented. By user data retransmission and optimized power allocation, the system capacity can be improved theoretically. And by comparing with traditional cooperative network coding in a simple two-user scenario, the proposed scheme shows better system performance.