Research On Cooperative Transmissions Of Multiple Relays In Cognitive Radio Networks

With the rapid development of communication information technology in recent years, wireless smart devices have attracted a large number of popular, which makes the wireless spectrum become a very valuable technical resource. Cognitive radio (CR) technology can make unlicensed users sense underused spectrum and adjust working parameters dynamically to access the spectrum. As a result, the spectrum efficiency can be improved. However, we notice that in Cognitive Radio Networks (CRNs) the direct transmissions between primary user (PU) transmitter and primary user receiver may be severely damaged due to the unstable environment in wireless communications such as multipath fading and shadowing. In these cases, selecting secondary users (SUs) with good channel quality as relays for cooperative transmissions can improve the system performance effectively in CRNs. Compared with single cooperative relay transmission, multiple cooperative relays transmissions can get larger space diversity gain, which can make cognitive users achieve larger transmission rate. Therefore, relay selection algorithms, especially multiple relay selection algorithms, which are mainly used solving the problem that how to find appropriate relays effectively for the communications between two nodes, are the key technology schemes of cooperative transmissions.In this thesis, we firstly introduce the related research works from domestic and overseas in the field of relay selection algorithms and emphatically elaborate the necessity and innovation of our research in this field. Then we introduce the fundamental concepts and relevant technologies of CRNs technology and cooperative relaying technology. We also give an overview of stopping theory.Next, we begin our research on two major areas:not real-time cooperative transmissions and real-time cooperative transmissions. For the research on not real-time transmissions, this thesis supposes cooperative relays transmit the same packets synchronously. The thesis also considers the factor that the transmissions of SUs should not bring interference to PUs’ communications in CRNs. Based on this limit and the idea of relay ordering, multiple relay selection algorithms with linear complexity and quadratic complexity are proposed. These algorithms can achieve high SNR (Signal-Noise Ratio) gain with a large number of cooperative relays, so as to ensure the integrity of transmitted packets. Extensive simulations are also provided to validate and compare the performance of proposed algorithms. In the study of real-time cooperative transmissions, this thesis constructs a system model and a time slot model based on the background of HD video chats. If the time for selecting cooperative relays could be saved, the transmission rate within a time slot can be improved so that the transmission standard of720P HD video can be satisfied. With this purpose, this thesis proposes a multiple relay selection algorithm based on multiple stopping theory, which makes the primary user can select optimal cooperative relays without observing all relays and comparing them. This thesis also proposes a video pockets allocation algorithm based on0-1Knapsack problem, which makes the selected relays can transmit packets as more as possible. Extensive simulations are also conducted to investigate the performance of proposed algorithms and the transmission rate of video packets.