Research About Spectrum Sensing Algrithm In Cognitive Radio

With the rapid development of wireless communication services and technology,the number of wireless terminal equipment is also increasing,while the spectrum resources are limited.On one hand,the rapid growth of wireless communication traffic is hard to be satisfied by the traditional spectrum allocation;on the other hand,the usage rate of the spectrum allocated is seriously low,and most of spectrum allocated are often in the idle state.The “ Cognitive Radio” technology was proposed at the historic moment,which not only alleviated the problem of spectrum shortage effectively,but also improve the utilization of the spectrum significantly.In the cognitive radio technology,the cognitive users are allowed to use the authorized spectrum which is assigned to the primary user in a dynamic way,after they find the spectrum holes by sensing wireless communication environment.And all of that should be established on the basis that the movement of cognitive users must not disturb the primary user to use their bands.One of the key technologies of congnitve radio technology is spectrum sensing.The results of the spectrum sensing by a single cognitive user is uncertain,and it could affect the performance of system.Multi-user cooperative sensing can solve these problems,and they can improve the detection performance of the system effectively.But in reality,objective factors can not be ignored such as the effect of fading and the building block.Meanwhile,the security problems is increasingly serious.In this thesis,we mainly study how to overcome the impact of the wireless environmental in cooperative spectrum sensing,and how to overcome the malicious attacks,and two effective schemes were proposed.Firstly,this thesis introduces the cognitive radio technology,and focuses on the spectrum sensing technology.Then it introduces several classical methods of sensing,sensing model collaboration and fusion algorithm In this thesis,we'll also introduce potential safety problem in the presence of the security spectrum sensing,classify malicious attack and analyze existing defense system.Secondly,due to fading effect,building block and other objective factors in a complex wireless communication environment,we proposed a cooperative spectrum sensing algorithm based on the credibility of the dynamic threshold.The algorithm follows three steps: the firststep is in accordance with the detected signal to noise ratio of authorized users filter out reliable user;the second step is dynamically adjust the detection threshold based on signal to noise ratio;a third step is detect the correct degree of confidence weighted fusion according to the user's participation as well as the ability to.This algorithm could reduce system overhead compared to traditional confidence-weighted algorithm.The simulation results show that in the case of fading in the environment,the algorithm can obtain a high detection probability.In the meanwhile,the fusion phase is based on the cognitive ability of users,so it obtains a better performance to malicious attacks.Finally,to defend the SSDF attack,a modified reputation value updated mechanism for wighted sequential probability ratio test is proposed.The algorithm computes the local reputation function by using recent sensing information of cognitive user's.In order to eliminate the influence of the transmission efficiency on primary user brought by malicious users,its credibility value and the stability of the performance during the sensing process also need to be considered.The simulation results show that the new algorithm is better than the typical cooperative spectrum sensing algorithms in malicious environment.