Rearches On Efficient Cooperative Relay Technologies For Wireless Networks

In wireless communication system, channel fading can decrease the reliability of communications. Cooperative diversity, in which users share their antennas to obtain space diversity, becomes a new valuable diversity technique. To date, cooperative diversity has been widely used in wireless sensor networks, wireless Ad-hoc networks, wireless Mesh networks and wireless celllar networks. Obviously, cooperative communication will be the key technology for convergence of various heterogeneous networks.On the other hand, as wireless technologies continue to expand, more and more spectrum resources will be needed. Cognitive radio (CR) is an exciting emerging technology that has the potential of dealing with the stringent requirement and scarcity of the radio spectrum. Although cooperative diversity can increase the ability of combating channel fading and the cognitive radio can enhance the untization of licensed spectrum in the wireless communication systems, it is not enough to solve above two problems. The cooperative relay and cognitive radio techniques based convergence mechanism which is a potential solution to increase the performance of combating the detrimental effects of fading channels, channel capacity and frequency ustilization for wireless system has attracted more attentions in recent researches.This dissertation considers the special issues of cooperative relay communication and investigates optimized design of cooperative diversity. Furthermore, how to cooperative diversity works in the cognitive radio (CR) networks is investigated.First, the cooperative and power allocation strategies based on energy-constrained licensed wireless networks, in which the frequency bands are exclusively allocated to the users, is provided in Chaper 3. For energy-constrained wireless networks, such as wireless sensor networks (WSN), MIMO (Multi-input-multi-output) cooperative energy efficient protocols to prolong the network lifetime are investigated. Then, a new partner choice algorithm based on cooperative MIMO transmission is proposed. Compared with existing work, cooperative nodes choice in every transmission is not based on minimizing energy consumption but balancing residual energy among participant nodes including source node and relays for one special application so as to maximize network lifetime.Second, cooperative diversity which works in the underlay sharing-spectrum environment, in which peak transmit-power of CR system must be below the interference temperature constraint of primary user, is investigated in the Chaper 4,5 and 6 of this thesis. The main content includes the following aspects:1) For energy-constrained CR networks, such as wireless sensor networks (WSNs), the user cooperative strategy in the spectrum sharing environment is investigated in Chaper 4. Optimal power allocation strategy in order to minimize the energy consumption of CR system and meanwhile guarantee the quality of service (QoS) of the primary system is proposed. Simulation results showed that the proposed method obtains higher energy efficiency in CR system and better performance of primary link than those of the direct transmission.2) For multi-relays spectrum sharing cooperative CR system, Chaper 5 proposes a distributed transmit power allocation algorithm based on Amplify and Forward (AF) cooperative model. By this algorithm, the relays adjust their transmit-power based on broadcast information from destination without central node to control. The simulations validate the efficiency of proposed algorithm by comparing it with the direct transmission and the scheme where equal weights are assigned to the relays.3) The performance of capacity, outage probability and bit errors rate (BER) has recently been investigated only in licensed cooperative wireless networks. The outage performance of wireless cognitive radio relay networks based on decoding and forward (DF) mode in a spectrum sharing environment is investigated in Chaper 6. In particular, the relation between the outage performance of the secondary relay link and the interference inflicted on the primary user is quantified. Numerical results are provided to validate this anlysis.From the dissertation, it can be found that, in cooperative communications system, selecting suitable cooperative schemes and power allocation algorithms for special transmission strategies can achieve cooperative diversity, which can significantly improve the performance and reduce the energy cost of systems.