Research On Beamforming Technology In MIMO Cognitive Radio Networks

Throughout the tendency of rapid development of wireless mobile communication technology, the scarcity of radio spectrum resources has become an outstanding problem. In order to alleviate the spectrum resource scarcity problem and improve bandwidth efficiency, Dr. J.Mitola proposed the concept of cognitive radio( CR) in 1999. Multiple Input Multiple Output(MIMO) techniques use multiple antennas at both the transmitter and receiver to increase the capacity of system. The dissertation of this thesis aims to link the MIMO techniques into CR system. Cognitive beamforming techniques are studied to suppress the interference between secondary users and primary users and improve the spectrum efficiency in cognitive system.In this thesis, we study the downlink beamforming technology and joint beamforming technology in multi-user cognitive MIMO system by using the convex optimization theory. In this study, the uncertainty of the channel state information is introduced into the process of the robust beamforming algorithm design. Firstly, the downlink beamforming technology has been studied in the cognitive MIMO system. Due to the limitation of the transmit power of antennas at the cognitive base station, we design a robust downlink beamforming algorithm in CR-MIMO system to minimize the transmit power of the cognitive base station. Simulation results show that the new algorithm can calculate the optimal beamforming vectors under the imperfect channel state information and then achieve the transmit power allocation at cognitive base station. Secondly, we give a robust iterative algorithm based on up-down link duality on the basis of the up-down link duality theory of multi-antenna system. We focuse on joint beamforming technology in cognitive MIMO system and more comprehensive inter-system interference is considered in this study, such as the interference from the primary base station to secondary user. According to the up-down link duality theory of multi-antenna system, the up-down link duality problem of the original cognitive beamforming problem is derived. Simulation results show that the algorithm can control the interference between the primary system and cognitive system effectively under the imperfect channel state information, and then optimize the transmit power allocation at secondary base station. Finally, we propose a robust iterative algorithm based on variable separation on the basis of the variable separation theory of the convex optimization theory. In multi-user cognitive MIMO system, we present the iterative problem of the original problem by using the fixed variable method while the interference and signal to interference plus noise power ratio are limited. Simulation shows that the proposed algorithm has high speed of convergence, adapts to the change of the channel state information and has certain robustness.