Research On Key Technologies Of Cognitive Radio System

Spectrum bands are allocated in a fixed manner for the traditional wireless communications, and the available bands are becoming rare. However, the recent survey shows a low spectral efficiency in licensed band. The spectral holes in licensed band can be sensed and used to transmit information by employing cognitive radio (CR) technology. As a result, CR becomes a key to deal with this dilemma. This thesis mainly focuses on the spectrum sensing and spectrum sharing technologies in CR. CR users recognize their radio environments and detect primary user by spectrum sensing technology which is a precondition for deployment of CR technology. And the CR users realize coexistence with primary users by spectrum sharing technology which is the ultimate objective of CR. Our major works include the following.Firstly, we model the spectrum sensing and simulate the receiver operating characteristics (ROC) of both single user and cooperative spectrum sensing. In order to evaluate the performance of spectrum sensing and spectrum sharing, two important metrids are introduced. They are average detection time (ADT) and idle channel utilization ratio (ICUR). These two metrics are correlated and both depend on ROC and the time schedule of cognitive system, so joint optimization is needed. We focus on how to minimize ADT by adjusting spectrum sensing time duration while a certain fixed proportion of system time is allocated to spectrum sensing. Another more realistic research is to maximize the ICUR of spectrum sharing while the ADT is constrained. We also design a more practical way to adjust the sensing cycle and sensing time to achieve optimization. The simulation results show that the ICUR can be raised by 10 percents by using our proposed scheme while maintaining the agility of cognitive system.Secondly, although cooperative sensing has solved the problem of "hiding terminal" and deep fading channel, how to exchange the sensing information among multiple CR users is still difficult in practical deployment. Hard combination schemes underperform soft combination ones in sensing accuracy, yet with much less sensing information needed to be transmitted. A novel distributed variable quantization scheme is proposed to find a trade-off between the sensing performance and the transmission of sensing information. The essence of this proposed scheme is that CR users with more confidence send more precise sensing information back to fusion center while less confident ones send less information even nothing, and the specific amount of information is set by the CR users themselves to maintain downlink overhead at a low level. In detailed, the centralized calculation of sensing log-likelihood ratio equation at the fusion center is simplified and split into several distributed calculations which can be achieved at local CR users. Then the CR users quantize the calculation results into bits with different length based on their confidence, at the same time, the system overall overhead is controlled by the quantization factor broadcasted by the fusion center. In order to further minimize the overhead, the circular differential quantization method is devised. The simulation results show that our proposed scheme has similar sensing performance as optimal soft combination scheme, but the average overhead only costs one to two bits.Thirdly, the reliable cooperative sensing must have stable channel to exchange the sensing information with only a few bits. However, CR system itself has no licensed band. If it directly occupies the primary band, the CR and the primary users may strongly interfere each other. It seems that the primary systems whose band CR may access are largely broadband non-spectrum-spread ones. Therefore, we assume to introduce spectrum spread technology into CR system, which spread the low rate overhead to broader bandwidth with lower power spectral density. Additionally, spatial isolation is also applied to realize spectrum sharing between primary users and CR system. Our idea is validated through two practical examples. The first one is the coexistence scenario between Wireless Regional Area Network (WRAN) and DTV, and the second one is the one between a normal CR system and LTE. The WRAN is at the coverage edge of DTV, and the CR system is also at the cell edge of LTE. The primary transmission and the spread CR overhead can well coexist with each other under acceptable low interference.Finally, the multi-antenna technology is introduced to enhance the spectrum sharing and spectrum sensing. We find a way to maximize the downlink throughput by selecting a better MIMO mode based on the channel condition between CR transmitter and primary users, while the interference to primary users is constrained. The correlation among the received signals at multiple antennas is utilized to employ spectrum sensing. The proposed spectrum sensing scheme calculate the singular value decomposition (SVD) of auto-correlation matrix of parallel received signal flows. This method concentrates the signal power on one singular value, yet the noise power is averagely distributed in all singular values. Therefore, its sensing accuracy outperforms energy sensing with multi-antenna.