Research On Key Technologies Of Multiuser Cooperative Diversity

As the development of wireless communications technology, people's requirement of wireless data services is also growing. The loss of wireless channel is one of the main obstructions for increasing channel capacity and improving service quality. And multi-antenna diversity technology is an effective measure to deal with the channel loss. However, the multi-antenna diversity requires that the communications equipment has multiple antennas, which is difficult to achieve in nodes of many systems because of the limits of size and power. Cooperative diversity technology appeared in this context.Cooperative diversity communication is based on relay channel model, and it is a new spatical diversity technology. It makes use of the broadcast nature of radio waves, each node can receive information which other nodes transmitted and retransmitted it to the destination node after a certain treatment. So different users can share their antennas and form spatial diversity. Cooperative diversity can improve the capacity of wireless communication systems, reduce the outage probability, expand the wireless network coverage and connectivity, and save the energy for data transmission. So it can improve the effectiveness and reliability of wireless communication systems. Cooperative diversity can not only be used in cellular mobile communication network, and it also can be well applied in sensor networks, cognitive radio and other wireless communication systems. And cooperative diversity have also contributed to the practical use of multiple-input multiple-output (MIMO) technology, and thus become an important technology for future research on wireless communications.This article focuses on problems in cooperative diversity system, and three research questions are consideres: when to cooperate, who cooperates with whom and how to cooperate. The opportunity of cooperation, partner selection and the improvement of cooperative strategies have been studied.Firstly, the principle analysis and protocol design of cooperative diversity are introduced. The basic channel model and protocol of cooperative diversity are introduced first and the theoretical cooperative gain is provided. Then decoded-and-forward (DF) and amplify-and-forward (AF) are considered and performances are compared. The flow of cooperative communication is introduced and the particularity of cooperative system is considered. Based on the frame structure in existing communications systems, communications processes and frame structure which can be applied in cooperative diversity systems are designed. The slot and structure of cooperative partner selection, cooperation created are appended in the the new communications processes. And"when to cooperate"is discussed based it.Secondly, the cooperative partner selection methods are proposed for two different scenarios. In high user-density area, combined with"worst-link-first"matching method, the reciprocit and non-reciprocity"channel between users first"matching methods are proposed. With the methods, we do not require solving the equation to computae the possible energy gain for cooperating with other users. We only need to know the channel gain from users to the destination and channel gain between users, we can group the nodes to cooperate. The matching method can reduce the calculation and system overhead with ensuring the energy gain. For high-speed mobile user nodes on the railway or highway, with the analysis of the particularity of high speed nodes, we can divided the nodes into groups based on the users'moving direction and speed, and high-speed mobile user nodes only select partner from the nodes moving by the same direction with themselves. So it can reduce the relative velocity of partners, and which can increase the duration of cooperation, reduce the partner switching frequency, thereby it can reduce system overhead and increase the effectiveness of the cooperation.Thirdly, we study improving the cooperative diversity strategy and analyze the cost of cooperation. As AF and DF both have advantages and disadvantages, so a cooperative method combined DF and AF is proposed. When the channel between the user nods is better and the partner can decode correctly, DF is used, or else AF is used. So this method synthesizes advantages of AF and DF, and it can improve the spectrum efficiency, get better physical layer and MAC layer performance. Then we analyzed the application of adaptive modulation in DF, which can further enhance the spectral efficiency of cooperative diversity system. Finally, a cooperative cost function is designed, which synthesizes energy, time and frenquency, and it can calculate the cooperative gain and cost in starting cooperation. We can determine if cooperation or partner switch is necessary and how much gain we can earn.Fourthly, cooperative diversity in broadband channel is analyzed. First of all, an"equalization and forward"method is proposed for low PAPR single-carrier frequency domain equalization system. It can provide diversity gain which is similar to instantaneous power control in narrowband system. And it also can be combined with DF to further improve system performance. Then the influence to performance of different partners is analyzed, which can support the partner selection. Finally, for orthogonal frequency division multiple access (OFDMA) system, we proposed a modified channel allocated method. By means of setting several thresholds, we can improve the system capacity greatly with less fairness loss. And a tradeoff between fair method and greedy method can be obtained.