Wireless Cooperative Transmission And The Application In Cognitive Radio Networks

Cognitive radio is a promising way to improve the spectral efficiency. The cooperative transmission can improve the system capacity. Cognitive and cooperation are two key techniques for the future wireless communication. In cognitive radio networks, cooperate transmission can make full use of space diversity, and improve the system performance.In this dissertation, we mainly engage in the key techniques for the cooperation transmission in cognitive radio systems. The main contributions of the dissertation are as follows:1. Under the circumstance of one source, one relay, and one destination, an incremental relaying scheme based on adaptive relaying protocol is presented. Only the signal noise ratio (SNR) at the destination below a threshold, can the relay take part in the cooperative transmission. Moreover, the relay deploys decode and forward (DF) protocol, if the relay can decode the message from the source node correctly. Otherwise, the amplified and forward (AF) protocol is deployed at the relay. The destination decodes the message with the maximal ratio combining (MRC) scheme. Simulation results show the adaptive relaying protocol is better than the existing protocol.2. Under the circumstance of one source, one relay, and one destination, an adaptive modulation incremental relaying protocol is presented. As the SNR at the destination is below a threshold, the source and the relay will select the appropriate modulation mode according to the SNR at the relay and destination, respectively. Simulation results show the proposed protocol outperforms the existing adaptive modulation protocol.3. Focusing on the problem that co-channel interferences affect the relay selection in the wireless networks, a selection cooperation protocol is firstly proposed. In the protocol, MRC, selection combining (SC) and hybrid MRC and SC schemes are deployed at the destination. The exact outage probability of selection cooperation was derived in Rayleigh fading channels. Moreover, the bit error rate of selection cooperation was derived. Then we propose an opportunistic relaying which selects the relay with maximum instantaneous end-to-end signal to noise-interference ratio (SINR) forwarding the source message. Moreover, as the direct transmission is taken into account, MRC and SC schemes are deployed at the destination to combine the direct transmission and relay transmission, which can make full use of space diversity and decrease the system outage probability. Compared with the existing relay selection protocols, the proposed protocols do not need the information exchange between any two relays, and the data transmission is finished by competition. So the proposed protocols have the low system cost and good practicability.4. Under the constraint of spectrum-sharing, we propose a threshold based incremental relaying protocol to minimize the secondary user outage probability in cognitive radio systems, where the interferences caused by the selected relay and the secondary user transmitter are below than an optimal interference threshold. When the proposed protocol is applied in the cognitive radio systems, the outage probability of secondary user is derived in the Rayleigh fading channels. Simulation results show that compared with the direct the transmission, the protocol can decrease the secondary user outage probability.5. It is often the case that the source node transmits the message to the destination by several hops in wireless networks, where all nodes are powered by battery with limited energy supply. Cooperative transmission can save the transmission power significantly by making full use of the space diversity, and therefore it is crucial to design the energy efficient cooperative routing and power allocation algorithm in the wireless networks. However, the centralized cooperative routing and power allocation algorithms require the global location knowledge at each node, which is hard to realize in large sensor networks. Hence, in this paper, based on the optimal power allocation in cooperative transmission, two distributed algorithms, Distributed separate route and power allocation algorithm (D-SRPA) and Distributed joint route and power allocation algorithm (D-JRPA), are proposed to minimize the total transmission power. Results show that the proposed distributed algorithms can get the similar performance as the corresponding centralized algorithms. Furthermore, compared with D-JRPA, D-SRPA can save the total transmission power more efficiently.6. Based on the optimal power allocation in cooperative transmission, a modified cumulative increment algorithm (MCIA) was proposed. The algorithm permits multiple nodes simultaneously transmitting the message to other nodes in the networks, and can reduce the total transmission power for broadcast. Moreover, considering that the centralized MCIA is hard to realize in large wireless networks, the distributed MCIA was presented. The distributed MCIA requires only 1-hop neighborhood information, and can adaptively determine the number of nodes for transmission. Results show that in terms of total transmission power, MCIA can get a better performance than the existing broadcast algorithms. Furthermore, the performance of distributed MCIA is close to that of centralized MCIA.