Research On Beamforming Algorithms For Cognitive Two-way Relay System

Nowadays, wireless communication services develop rapidly, the use of traditional radio spectrum resources has led to the shortage and waste of spectrum resources, which has seriously restricted the development of wireless services. Cognitive radio is an effective method to solve the spectrum crisis, which has become the focus of the study. Beamforming technology, combined with the application of relay network, can increase the distance of communications, overcome fading and reduce interference, ensure the reliability and security of communication. This Thesis aims to study the beam forming technology in the cognitive two-way relay system.Firstly, the robust relay beamforming in TDD cognitive two-way relay network is studied in this thesis. Considering the imperfect channel state information(CSI) between relays and primary receiver and power constraints of each relay, we propose a robust cognitive two-way relay beamforming algorithm to minimize the interference to primary users while satisfying the SINR thresholds of second users, the total transmit power constraint in the relay side and the transmit power of each relay. Simulation shows that the new algorithm can effectively ensure the system operate normally with a negligible performance loss. Secondly, the beamforming in cognitive two-way relay with hybrid relay strategy is studied. Due to the problem that the relay amplify the noise in the cognitive two-way relay with AF relay strategy, a beamforming algorithm of cognitive two-way relay with hybrid relay strategy is proposed to minimize the interference to primary users while satisfying the SINR thresholds of second users and the total transmit power constraint in the relay side. Simulation shows that the new algorithm can effectively reduce the interference to primary users, and improve system performance. At last, a beamforming algorithm based on multiple relay selection for cognitive two-way relay system is studied. Considering that using all of the relays in the cognitive two-way relay system at the same time to assist the transmission may lead to high bit error rate and distortion, which reduces the performance of the system. Therefore, we propose a cognitive two-way relay beamforming algorithm based on multi-relay selection to maxmum the sum-tate of cognitive users while satisfying the interference constraint to primary users and the total transmit power constraint. Simulation results show that the new algorithm improves the maximum the sum-rate of the cognitive users and improves the performance of the system.