Study Of Secondary User Cooperation Based Spectrum Sensing Method

Presently, the demand for wireless spectrum resource is growing with the rapidly developing wireless service, and the increasing demand makes wireless resources more and more scarce. However, only authorized users (primary users) are allowed to use the licensed band with the current fixed spectrum allocation policy, which makes a large number of licensed spectrum resources sometimes are idle. That is to say, the fixed spectrum allocation policy results in inefficient usage of spectrum and can not well satisfy the growing demand for spectrum resources.Cognitive radio is one of effective ways to improve the efficiency of spectrum usage, while spectrum sensing is the key basis for realizing cognitive radio. The objective of spectrum sensing is to acquire the information of spectrum hole in spatial domain and temporal domain quickly and reliably, so as to support the follow-up functions of cognitive radios, such as spectrum management, spectrum sharing and spectrum mobility. Up to now, although some researchers have carried out a lot of research and made a mighty advance in spectrum sensing, there still exist some challenging issues.In this dissertation, secondary user cooperation based spectrum sensing is investigated, mainly including sensing task allocation, sensing information fusion, and jointly spatial-temporal cooperative spectrum sensing. The some theoretical models and relative algorithms are proposed, and extensive simulations are also presented to demonstrate the validity of the proposed methods.The main contents and contributions of this dissertation are listed as follow.1) Under the constraints of limited hardware capabilities of secondary users, how to rapidly sense multi-channel spectrum is studied. Based on information theory, the relationship between sensing accuracy and the system entropy is analyzed. Then a least entropy based strategy for cooperative spectrum sensing is proposed. The main idea behind the proposed strategy is based on the least entropy criteria to allocate secondary users to sense different channel according to the feedback of sensing results, which can adaptively adjust sensing interval of each channel to maximumly reduce the uncertainty of the estimation of channel state, and improve the accuracy of the channel estimation.2) Taking the impact of shadow fading and hidden node problem into consideration, how to increase the reliability of spectrum sensing for single-channel is studied. In the dissertation, the cooperative sensing with error information is modeled as hidden Markov model. Based on hidden Markov model theory, an online algorithm for sensing information fusion is proposed. The basic idea of the proposed algorithm is to learn the model parameters through the observation sequence, then collect sensing information of secondary users to estimate the channel state, which can improve the reliability of single-channel spectrum sensing. Meanwhile, the selection strategy of secondary user and a selection method are also given. Theoretical analysis and simulations demonstrate the efficiency of proposed algorithm. It is shown that the proposed approach outperforms the existing methods for cooperative spectrum sensing.3) Considering the character of spectrum hole with spatial-temporal distribution, how to jointly sense the spatial-temporal spectrum is studied. The joint sensing problem is modeled as parameter-state combined estimation, and a particle filtering based solution strategy is proposed. The proposed method can sense the spectrum in spatial domain and temporal domain simultaneously, which means to online estimate the location of primary user in spatial domain and the channel state in temporal domain at the same time. In addition, a secondary user selection method is presented, which combines the estimated spatial and temporal information to select secondary users for cooperative sensing, according to the distance between primary user and secondary users as well as the spatial distribution symmetry of secondary users. Proper secondary user selection can obviously improve the performance of spectrum sensing in temporal and spatial domain.