Research On Key Technologies Of Multiuser Cooperative Transmission In Woreless Networks

The future of mobile communication networks need to support high-speed multimedia services of different QoS guarantee. The ever-growing demand for data applications over wireless domain has prompted a need to develop technologies that utilize the available resource more efficiently. However, the mobile environment has its own characteristics, such as the change of topology is caused by the mobility of the node, the resource and the ability of the node is limited, the bandwidth of the mobile networks is limited and the connection of the networks is unstable, which cause the increasing acute contradiction between high-speed transmission and coverage. Recently, cooperative relaying emerged as a viable option for future wireless networks. Cooperative relaying has been shown to decrease the link outage probability (or alternatively, increase the link stability) significantly. In the past few years, cooperative communication technology try to improve the channel situation, promote the practical application of MIMO technology and solve the contradiction between high-speed transmission and coverage, which caused widespread concern from academe and enterprises, has been recognized as a candidacy of the future mobile communication system. In the past, the communication system only focus on point to point link, but we now should re-survey the communication system from the perspective of networks. In this cooperative system, every node can use the inherent broadcasting characteristic of wireless communication system to make mutual cooperation. In the past few years, cooperative communiacation thechnology try to improve the channel situation and to solve the contradiction between high-speed transmission and override, which caused a lot focus from acdame and enterprises. this has been accepted as the candidacy of the future mobile communication system. in the past,the communication system only focus on point to point link.but now,we should survey the communication system from the aspect of network.in this cooperative system,every node can use the broadcasting characteristic of wireless commnication system to make mutual cooperation. This thesis studies on physical layer key technology of multi-user cooperative transmission in wireless network. My research focuses on existing cooperative diversity protocols in correlated fading channel, such as cooperative scheme, performance, space-time cooperative process and etc. We also studied self-adaptive select method which can be used in multi-relay (also applies to single-relay) cooperative communications, provides a better way forward adaptive selection scheme and adaptive power distribution strategy for various relay node. The final part, we investigate analyzes the efficiency, reliability and robustness of cooperative communication in a wireless cellular downlink environment by employing a new scheme based on combining a Raptor code with distributed space-time block code (DSTBC), that when channel state information (CS1) is not available at the transmitter. The main contributions of thesis are as follows:1). Differed from existing literatures on cooperative diversity that the fading channels are idealistically assumed to be Rayleigh fading channel, the performance of existing cooperative diversity protocols for virtual MIMO system in Geometry-Based Stochastic Channel Models is analyzed, which has taken into account the realistic propagation environments in the presence of spatial fading correlation. Cooperative diversity is effectively used to combat the correlated fading. The two source nodes (SNs) and two destination nodes (DNs) form a 2×2 virtual MIMO cooperative system. The time division multiple access scheme is proposed with half duplex mode. Cooperative scheme is proposed, because cooperative diversity can combat the fading correlation by spatial decorrelation which is produced by independent multipaths and scattering paths between source nodes (SNs) and destination nodes (DNs). Cooperative relaying is shown to increase the link stability (or, alternatively, decrease the link outage probability) significantly. Therefore, the bit error rate (BER) of cooperation at transmitter is presented in the interference relay channel by applying various cooperative schemes. Based on theoretical analyses and simulation results, the cooperative diversity yields significant performance gain in correlated channel, i.e., reducing the spatial correlation of each path and enhancing the overall performance of virtual MIMO system by high quality of cooperative interuser channel.2). On the basis of the second paragraph, taking into account the realistic propagation environments in the presence of spatial fading correlation, the performance of cooperative protocols using distributed space-time block code (DSTBC) in the two-ring scattering model was analyzed. Considering the cooperative scheme characteristic, a new measurement, total time slices, was found to evaluate the performance. Therefore, the number of total time slices at transmitting exactly and the average bit error rate (BER) of cooperation are presented by applying various cooperative schemes in the correlated channel. The time division multiple access scheme is proposed with half duplex mode. Based on theoretical analyses and simulation results, the cooperative diversity yields significant performance gain in correlated channel, i.e., reducing the spatial correlation of each path and enhancing the overall performance of virtual MIMO system by high quality of cooperative interuser channel.3). We study self-adaptive select method which can be used in multi-relay (also applies to single-relay) cooperative communications, the method is used to support the cooperative wireless communication networks. For different users, it can choose the different number of relay and cooperative mode respectively. According to the current channel information state, taking into account the reliability and effectiveness of communication, each relay node choose the best cooperative mode, allocate the power between various source nodes and relay nodes, and determine whether the relay node can join relay process to achieve the maximum channel capacity and provide users with the reliability to meet the conditions under the efficient communication. The approach of evaluation standards is channel capacity for multi-relay system to maximize), through exhaustive method to choose a different cooperative mode for different relay node. It is found that the method provides a better way about forward adaptive selection and adaptive power distribution strategy for various relay node for multi-relay cooperative communications system, and put forward the judge standard about whether the relay node can work or not. This can make multi-relay system more intelligent, and further enhance its transmission performance and promote the practical application. This study is not only used to support the cooperative wireless communications network uplink transmission, but also used to downlink transmission. Cooperative communication technology has been recognized as a candidacy of the future mobile communication system, therefore, cooperative wireless communications related technologies will be developed and improved unceasingly,. There will have a very great promotion and application prospects.4). Differed from literatures on cooperative relay system adopting a conventional fixed-rate code (traditional code), the rateless code, i.e., Raptor code, to cooperative wireless cellular system is studied. In particular, this paper analyzes the efficiency, reliability and robustness of cooperative communication in a wireless cellular downlink environment by employing a new scheme based on combining a Raptor code with distributed space-time block code (DSTBC), when channel state information (CSI) is not available at the transmitter. Considering the flexible code rate characteristic, a new measurement (total time slices) to evaluate the performance is found. Therefore, the number of total time slices at transmitting exactly and the average bit error rate (BER) of cooperation are presented by applying various encoding schemes. The time division multiple access scheme is proposed with half duplex mode. Based on theoretical analyses and simulation results, we show that without channel state information at the transmitter, the cooperative wireless cellular system using Raptor code and DSTBC yields significant performance gain, i.e., reducing the total transmission time and energy consumption, even in a low signal-to-noise ratio (SNR) environment.