Research Of Spectrum Sensing In Cognitive Radio System

With the rapid development of wireless communication technologies and the proliferation of wireless users, more and more spectrum resources are needed to support the numerous merging wireless services.Thus, the wireless spectrum resource appears rarity. However, many studies have shown that the wireless communication spectrum resource is scarce in one aspect and in the other aspect, it is wasted very much. The reason is the existing static spectrum allocation policies and the dynamic spectrum utilization system is not matched very well. In order to solve the problem of low spectrum utilization, Dr.Mitola proposed the idea of cognitive radio which can dynamically access the spectrum, once there are free spectrum resources, the cognitive user can dynamically use them, and so it can extend the bound of spectrum usage efficiently. The cognitive users must accurately detect the existence of licensed user signals in time to avoid harmful interference. So the spectrum sensing technology is an important prerequisite in cognitive radio networks. The spectrum detection in the cognitive radio system is presented as the main content in this paper. Several algorithms of spectrum detection are researched in the paper,such as energy detection, cooperation spectrum detection. cooperative spectrum detection with double threshold, a large number of MATLAB simulations are performed for each algorithm. In the penultimate chapter, briefly descriptions of the spectrum detection schemes in IEEE 802.22 draft are given. The main content and innovation can be summarized as following:1. Detailed analysis of the energy detection principle is carried out. Then we derive the false alarm probability and detection probability formula in energy detection, followed with the MATLAB simulation of energy detection performance under different signal to noise ratio or different testing time. In this section, we also have studied the energy detection algorithm over fading channels and derived the average detection probability over Nakagami,Rice and Rayleigh fading channels. Finally we analyze the influence of noise uncertainty in energy detection and give the computer simulation of performance.2. Collaborative spectrum sensing is proposed and studied in this paper as a means to improve the traditional energy detection algorithm. First of all, we introduce some basic terms of collaborative spectrum sensing:concept, categories, data fusion rules and so on, then we take the "OR" rule as an example, carry out MATLAB simulation. Our analysis and simulation results suggest that collaboration may improve sensing performance significantly compared with single-node energy detection. But further simulations show when a cognitive user involved in collaborative is in low SNR environment or a malicious user join the collaboration detection, detection performance decrease. We need weight the dates instead of using them directly.3. We propose a cooperative spectrum sensing scheme with double threshold in order to improve sensing performance. Principle of the algorithm and detailed theoretical analysis are also given in this paper.Simulation results prove that the spectrum sensing performance in Rayleigh channels is improved under the proposed scheme and also prove that the proposed scheme can significantly reduce the adverse effects of noise uncertainty.4. The IEEE 802.22 system topology, service coverage, service capacity are briefly introduced, following that the paper lists some spectrum detection algorithms proposed in IEEE 802.22 draft. A spectrum sensing algorithm for wireless microphone signals using FM modulation is described. Finally, we conducted computer simulations of the wireless microphone signals detection with spectrum sensing algorithm described above and DVB-T signals detection with collaborative spectrum sensing algorithm. Simulation results reveal the spectrum sensing algorithms for TV and wireless microphone signals which achieve the sensing requirement specified by FCC.