Research On Interference Suppression Algorithm For Downlink MIMO Cognitive Radio System

As a non-renewable resource, the scarcity of radio spectrum becomes moreand more significant due to the rapid development of wireless communicationapplications. According to FCC’s survey, It suggested that the traditional fixedspectrum allocation method occupied a large amount of existing frequencyspectrum, at the same time, its spectral efficiency is at a very lower level.Therefore, in recent years the theory of cognitive radio has been proposedfrequently and received extensive attention. As an new effective technique to solvethe radio spectrum scarcity problem, cognitive radio technology reuse thespectrum resource by allowing the secondary users to access the licensed sp ectrumwith a lower priority in which they wouldn’t interfere with the primary users’communication, and it has become a trend of wireless communication technologydevelopment.In order to further improve the system capacity and take full advantage ofspace-frequency domain resources, it is proposed to combine the multiple-inputmultiple-output technology with cognitive radio system. With multi-antennasequipped at the base station and cognitive users, the system earns increasingdegree of spatial freedom and more multiplexing gain, so as to achieve a moredesirable performance without any increasing in spectrum bandwidth andtransmitting power. However, transmission with multi-antennas can causeincreasing of the co-channel interference, thus the interference suppression methodhas become a key technology in the application of multi-antennas. Conventionalinterference suppression algorithm in multi-user MIMO system has theshortcoming of computing expensive, strict requirements on the number ofantennas and it doesn’t distinguish the users’ priority level. But in cognitiveMIMO system, it is more essential to guarantee the primary user’s quality ofcommunication than suppress interference between secondary users and thesecondary link’s transmission should be transparent to the primary link. Theoptimization problem become a coupling one which worthy of further study.This paper base on the scenario in the downlink of multi-user cognitiveMIMO system, aimed at studying on the interference suppression algorithmbetween primary user and secondary users, then achieving the spectrum sharingusing cognitive radio technology. This thesis firstly focus on several conventionalinterference suppression method in the multi-antenna system, then proposed acognitive MIMO beamforming design based on generalized Rayleigh quotient, signal-to-leakage-and-noise ratio criterion and matrix dimensionality reductiontheory. Theoretical analysis and simulation demonstrate that the proposed designachieve interference avoidance at the primary user as well as improve theperformance of secondary link obviously without constraint on the number ofantennas which is required in traditional method. On the other hand, an improvedprecoding scheme has been presented based on simultaneous diagonalization andSVD decomposition with multiple data streams for per cognitive user. It can beproved that our proposed precoding achieves considerable gains in BERperformance and the multi-streams’ channel gain are now less unbalanced.Simulation results also suggest that our proposed scheme can achieve a desirableperformance in the downlink of cognitive MIMO system.