Research On Time-domain Windowing In OFDM-based Cognitive Radio Systems

With the rapid development of the wireless communication, more spectrum resources are needed and the current bandwidth seems more and more scarce. Cognitive radio(CR) provides an opportunists way of sharing and utilizing the spectrum which can effectively solve this problem. Non-contiguous OFDM (NC-OFDM) is a candidate data transmission method of CR. It is an improved system based on the traditional OFDM. In NC-OFDM system, the rental users transmit their data through the non-contiguous unused subcarriers by closing the subcarriers which are occupied by the licensed users.In NC-OFDM system, the signal transmitted on each subcarrier can be considered as a non-return-to-zero rectangular signal. It can produce serious sidelobe power leakage in frequency domain which will interfere the adjacent subcarriers. Besides, this violates the premise of the existence of the CR system that the rental users'communication can't produce any harmful interference to the licensed users. Moreover, the licensed users need not to do any modifications on technical or equipment to support this function at all. Therefore, not only the performance requirement for the system itself, but also the corresponding interference power of the rental users to the licensed users should be considered in NC-OFDM system.In this paper, it aimed at the interference power from the rental users to the licensed users. The paper is including the following contents. Firstly, it gives a brief introduction to the emergence background and research meaning of CR, then the studies status of the sidelobe suppressions in NC-OFDM system at home and abroad. Secondly, it mainly summarizes the CR system based on OFDM. And the OFDM system principle diagram is given. The feasibility and technical challenges is analyzed when the OFDM system is applied to the CR. The simulation analysis of using the time-domain windowing to reduce the out-of-band power leakage of traditional OFDM is given in the third part. This thesis presents to apply the windows with smaller first sidelobe, such as BTRC window,SOCW window,Franks window and Double-jump window instead of the RC window in the window's choosing. The fourth part of this thesis focuses on the sidelobe suppression results of the rental users in NC-OFDM system by applying these five windows. Moreover, the detailed simulation and analysis of the results are given in this part. In the end, it also summarizes the work of this full thesis and points out the future research directions.