Research On Interference Suppression In Spectrum Sharing Cognitive Radio Systems

Cognitive radio technology can evidently improve spectrum efficiency from the point of view of dynamic spectrum access through reusing the underutilized spectrum by sensing spectrum holes, and thus effectively solve the problem of the scarcity of spectrum resources. Therefore cognitive radio technology has been extensively concerned and studied as the future trend of communication technologies. In the meantime, as the cognitive users sharing the same spectrum with the primary users in cognitive radio systems, interference will be involved with no doubt, which challenges the cognitive radio technology in resolving the related interference problems.Based on the current international research works, this dissertation investigates some key theories and technologies for the related interference problems of spectrum sharing cognitive radio systems in depth, and presents some new contributions, specifically in the following research topics:transmission strategies with interference coordination in MIMO CR systems, interference modeling for CR systems, and spectrum sharing MAC protocol design.On the research of transmission technologies with interference coordination in spectrum sharing MIMO cognitive radio networks, an optimal transmit strategy of cognitive users is determined to maximize the cognitive user rate based on both a PU average interference constraint and a PU outage probability constraint to protect PUs under the imperfect channel state information. The proposed non-convex problem is approximated as a mix binary integer programming problem and solved by branch and bound algorithm with low complexity. A key result theoretically proved is that the rank of the optimal transmit covariance matrix is one, and thus CR beam forming is the optimal strategy under PU outage constraints. In order to lower the complexity, a heuristic algorithm is presented to provide a suboptimal solution to the proposed problem efficiently.On the research of interference modeling and analysis for CR networks, a practical interference model is presented under consideration of network deployment and wireless environment conditions such as spatial density of cognitive users, PU protection region, path loss, shadowing, and fading, etc. The statistical distribution for aggregate interference of a CR network is analyzed based on a power control scheme with assurance of the minimum QoS for cognitive users. Furthermore, considering MAC protocol, the interference model and statistical distribution are studied based on the random access protocol of CSMA/CA scheme in a WLAN system. The SIR outage probability and capacity outage probability of PUs are studied based on the interference model.On the research of MAC protocol for spectrum sharing with interference control, a two-dimensional protocol based on OFDMA and CSMA/CA is proposed. Under backoff scheme and hand shaking, multiple users can contend and access different channels with fairness, and transmit without interference and collisions. The system throught of proposed protocol is analyzed from the sub-channel point of view and simulation results are presented to show that the performance and MAC efficiency are evidently increased with decrased collision probability under heavy load network conditions.