Research On The Key Technology For Wireless Cooperative And Cognitive Radio Networks

Cooperative communication is an emerging and effective technology in the IMT-Advanced system to combat signal fading caused by multiple radio propagation, improve channel capacity, enhance the transmission reliability, and enlarge the coverage. The basic idea behind cooperative communication rests on the observation that, due to the broadcasting nature of wireless channels, the antennas of randomly distributed users can be shared to form a virtual antenna array, by which the limitation of installing multiple antennas on small mobile terminals can be overcome and the spatial diversity can be obtained; On the other hand, with the development of wireless broadband and intelligent businesses, the contradiction of the rapid growth of wireless spectrum resource demand and the relative scarcity of spectrum resource supply is increasing. However, a recent survey of spectrum utilization made by the Federal Communications Commision (FCC) has shown that a significant amount of licensed spectrum bands are used only sporadically and furthermore, the usage varies significantly in spatial and temporal dimensions, which is mainly caused by fixed spectrum allocation. Cognitive radio (CR), which could break the traditional rigid spectrum management, is taken as a promising technology to realize Dynamic Spectrum Access (DSA), and alleviate the shortage of spectrum resource. Recently, the combination of cooperative communication and CR has been attracting significant research interests, which can effectively improve the transmission performance and spectrum utilization.In this dissertation, we have investigated multiple relay node selection and resource allocation in wireless cooperative systems, the TCP performance optimization as well as the multimedia transmission performance optimization in wireless cooperative and cognitive networks. Correspondingly, we have proposed several innovative schemes and further verificated the performance through theoretical analysis and simulation results. The main contributions of this dissertation are listed as follows:1. A joint opportunistic multiple relay nodes selection and optimal power allocation scheme in decode-and-forward (DF) cooperative communication networks is proposed.Firstly, we analyzed that single relay node selection is unable to obtain "emergence" gain. Besides, if all relay nodes are involved in the cooperation, the implementation complexity will be increased and the synchronization is difficult to be realized. In addition, in the wireless sensor network and the mobile Ad Hoc network, the relay nodes are battery-powerd with limited energy, and the network lifetime is closely related to nodes’residual energy.Secondly, in order to extend the network lifetime, we proposed an opportunistic multiple relay nodes selection based on instantaneous channel state information (ICSI) and residual energy information, which could avoid excessive use of relay nodes with better channel conditions, and balance the resource utilization of each node.Thirdly, by exploting the Lagrangian dual decomposition and subgradient projection method, we proposed an optimal algorithm to allocate tranmit power for the source node and cooperative relay nodes to maximize the total channel capacity.Finally, simulation results show that the proposed mechanism can significantly extend the average network lifetime with low complexity, and the balance between channel capacity and network lifetime can be achieved by adjusting the weight coefficient.2. The TCP end-to-end throughput optimization scheme in the underlay cooperative and cognitive networks is proposed.Firstly, we analyzed the necessity of optimizing TCP performance in underlay cooperative and cognitive radio networks, and discussed the effects of spectrum’s dynamic characteristics, PHY-Layer modulation and coding mechanism, MAC-Layer automatic repeat-request (ARQ) mechanism on TCP end-to-end throughput.Secondly, we considered the quality of service (QoS) demand of primary user (PU). Secondary user (SU) could access the PU’s licensed spectrum band under the condition that the primary link is provided with a minimum-rate for a certain percentage of time.Thirdly, we formulate the cooperative and cognitive network as a restless bandit system. Due to the priority-indexable property, the optimal policy can be obtained through the linear programming relaxation and primal-dual heuristic.Finally, simulation results show that the TCP end-to-end throughput can be improved dramatically.3. Jointly condering the channel sensing and access strategy, the multimedia transmission performacen optimization scheme in cooperative and cognitive networks is proposed using cross-layer design approach.Firstly, from the perspective of user’s experience, we analyzed the necessity of optimizing multimedia performance in cooperative and cognitive networks, and discussed the effects of channel sensing and access stragegy on multimedia transmission performance.Secondly, with the objective of minimizing the end-to-end video distortion, the application-layer parameter, i.e., intra-refreshing rate, is optimized to adapt the current network status.Thirdly, since the spectrum occupancy states cannot be observed directly, we formulte the process of channel sensing and access as a partially observable Markov process (POMDP). The optimal policy can be obtained through dynamic programming and the value function.Finally, simulation results show that the proposed scheme can reduce the end-to-to video distortion significantly.