| Recently, Cognitive Radio (CR) has been suggested as a new technology for increasing spectrum utilization. This approach calls for allowing cognitive users (CUs) to operate within licensed bands that are not heavily occupied by licensed users (LUs). Multi-carrier modulation technology which is highly flexible, adaptable and efficient is considered to be the best candidate for PHY in CR system. For example, orthogonal frequency division multiplexing (OFDM) and filtered multi-tone modulation (FMT) based on filter banks are the two cases of multi-carrier modulation technology. Generally, cognitive users and licensed users operate in two adjacent frequency bands respectively, so there is adjacent channel interference between them. The achievement of sharing spectrum resources should be conducted on the premise of kindly co-existence between cognitive users and licensed users, that is to say, it is required that the interference power caused by cognitive users to licensed users be limited to be tolerable. Consequently, the cognitive users can only operate within licensed band given they do not interfere the licensed users. In order to making full use of spectrum, it is required that the cognitive system itself have a strong ability of anti-interference. Therefore, the suppression of interference becomes a key issue that must be taken into account in the CR system design.In this dissertation, the suppression of interference in CR system based on OFDM and FMT modulation technology are investigated. The cancellation of interference caused by transmitting antenna to spectrum sensing antenna in CR system is studied and verified in FPGA additionally. The dissertation consists of six parts.In the first part, a general developing background and principles of CR is introduced and some key technologies in CR are mentioned.In the second part, the basic principle of multi-carrier communication system is introduced, and after that the elementary theory of OFDM and FMT is represented. Then, an analysis of the differences and relationship between OFDM and FMT is conducted. Finally, the CR system framework based on the two types of modulation and how they work are studied respectively, and meanwhile the problems exist in OFDM and FMT system is pointed out.In the third part, the suppression of out-of-band radiation in OFDM system is investigated. Considering that the proposed Cancellation Carriers (CCs) algorithm which attempts to suppress the sidelobe of OFDM spectrum has high complexity and cost when implemented in hardware, an improved CCs algorithm is proposed in this part. The evaluation about the effect of sidelobe suppression and the complexity of hardware implement between these two methods is conducted.In the forth part, the cancellation of inter carrier interference (ICI) in Non-overlap FMT and Half-overlap FMT system is investigated. A new improved Half-overlap FMT system based on cascaded integrator-comb (CIC) filter is proposed. The ICI model in FMT multi-carrier system based on CIC filter banks (CICFB-FMT) is derived, and then a comparison of the anti-ICI ability among CICFB-FMT, OFDM and Half-overlap FMT systems is performed on the condition that there is frequency offset in system.In the fifth part, the suppression of interference caused by transmitting antenna to receiving antenna which is used to perform spectrum sensing in CR system is studied. A method for canceling interference based on adaptive filter is proposed and verified in FPGA by means of System Generator.In the sixth part, a summary of the dissertation including the deficiencies in the work and the emphasis of further study is made. |
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