The Research On Active Interference Cancellation In Cognitive OFDMA System

The emergence of cognitive radio which brings revolutionary change, is a landmark in the history of development of wireless communication. Non-licensed user opportunistically accesses the spectrum not used by licensed user currently. The OFDM-based spectrum pooling technology can control the spectrum resources agilely through open or close the subcarriers. Combined with cognitive radio technology, the non-licensed user can realize the dynamic spectrum access(DSA) to the licensed spectrum, sharing the spectrum resources with licensed user.Although the cognitive OFDM system can adequately utilize the spectrum not occupied by licensed user, the out of band radiation(OOB) of OFDM should interfere with the vicinal licensed user during its transmission. The thesis mainly focus on the interference introduced by cognitive user to licensed user and the method of how to cancel the interference during the process of the cognitive wireless link transmission which is based on OFDM spectrum polling.Firstly, the mutual interference problem between cognitive user and licensed user in cognitive OFDM wireless link transmission was considered. The basic technology principle of the Non-Continuous OFDM(NC-OFDM), as an OFDM-based spectrum pooling technology was introduced, then combined with cognitive radio, the producing mechanism of out of band interference of cognitive NC-OFDM was derived. After the analysis on "windowing" and "filtering" which are two kinds of general interference suppression technologies, we simulate and compared the interference suppression results of the two technologies in cognitive OFDM system.Next, the research on Active Interference Cancellation(AIC) algorithm in cognitive OFDM system was done for mitigating the interference introduced by cognitive user to licensed user. The AIC models with non-cyclic prefix, adding cyclic prefix or windowing was deduced and the performance was compared through simulation. Instead of placing the Interference Inter-Cancellation Subcarriers(IICS) fixedly by the initial method, the thesis proposed a new method based on dynamic IICS, which choose the best positions according to the OFDM structure dynamically. Simulation results showed that, compared with the initial method, the new method can get better performance with the same parameters. Then the constraint on the energy consumed by IICS was put on the new method, and the simulation results indicate the interference suppression effect was kept close to the best level, when lowering the energy consumed by the HCS and alleviating the fluctuation of PSD.Finally, the AIC algorithm in cognitive single user environment was generalized to cognitive multiuser environment, and the interference suppression in cognitive multiuser OFDMA was considered. All the cognitive users need use AIC to protect the licensed user, the number, the weighting coefficient, especially the position of HCS need to be considered. Placing IICS in the proper position can not only protect the licensed user, but also make the limited subcarriers resource utilized effectively, improving the spectrum efficiency. Compared with not using AIC by cognitive user, the licensed user's BER was lowered much when using AIC.Through the work on interference suppression technology in cognitive OFDMA system, this thesis proposed some physical layer design rules of cognitive radio. The development of cognitive wireless radio should take lots of technologies and realization problems into account, including wideband signal processing techniques, advanced wireless communications methods, artificial intelligence and machine learning techniques, and cognitive radio-aware adaptive wireless networking protocols. There is an increasing interest on this technology among the researchers in both academia and industry and the spectrum policy makers.