网络控制外文文献翻译(编辑修改稿)

网络控制外文文献翻译(编辑修改稿)内容摘要：

trum in US is not utilized [1]. In order to increase the efficiency of spectrum utilization, cognitive radio technology was recently proposed [2], [3]. A requirement of cognitive radios is that their transmission should not cause harmful interference to primary users. Namely, the secondary users can use the licensed spectrum as long as the primary user is absent. However, when the primary user es back into operation, the secondary users should vacate the spectrum instantly to avoid interference with the primary user. Accordingly, spectrum sensing is a crucial phase in building a cognitive radio system. One of the great challenges of implementing spectrum sensing is the hidden terminal problem which caused by the fading of the channels and the shadowing effects. In order to deal with the hidden terminal problem, cooperative spectrum sensing has 黄石理工学院 毕业设计（论文）外文文献翻译 been studied to improve the spectrum sensing performance [4], [5]. In[6], due to control channel for each cognitive radio to report its sensing result is usually bandwidth limited, a censoring method which has two thresholds is given to decrease the average number of sensing bits to the mon receiver. By censoring the collected local observations, only the secondary users with enough information will send their local decisions to the mon receiver. In this paper, we present a new double threshold cooperative spectrum sensing method with reputation. In our system, every cognitive user will firstly obtain an observation independently and only the users with reliable information send their local decisions to the mon receiver based on double thresholds. If no user is reliable, only the cognitive user with the highest reputation is selected to sense the spectrum. Simulation results show that the spectrum sensing performance under AWGN channels is improved and the munication traffic is also reduced as opposed to the conventional method. The rest of the paper is organized as follows. In section Ⅱ , system model is briefly introduced. Sensing performance is analyzed in Section Ⅲ . In Section Ⅳ , we present the simulation results of our cooperative spectrum sensing method. Finally, we draw our conclusions in Section Ⅴ . II. SYSTEM MODEL In cognitive radio systems, spectrum sensing is a critical element as it should be firstly performed before allowing secondary users to access a vacant licensed channel. Cooperative spectrum sensing has been widely used to detect the primary user with a high agility and accuracy. The essence of spectrum sensing is a binary hypothesistesting problem: 0H :primary user is absent。 1H :primary user is present. For implementation simplicity, we restrict ourselves to energy detection in the spectrum sensing. The local spectrum sensing is to decide between the following two hypotheses: 黄石理工学院 毕业设计（论文）外文文献翻译 01( ) , ,() ( ) ( ) ( ) ,n t Hxt h t s t n t H   (1) Where )(tx is the signal received by secondary user, )(ts is primary user’s transmitted signal, )(tn is AWGN, and )(th is the temporary amplitude gain of the channel. According to energy detection theory [7], we have the following distribution: 122022 ),(,~ HX HXmm  (2) Where  is the energy value collected by secondary user, is instantaneous SNR and follows exponentially distribution with the mean value _ , m is the time bandwidth product of the energy detector, 22m represents a central chisquare distribution with 2m degrees of freedom and. )(22 m represents a noncentral chisquare distribution with m2 degrees of freedom and a noncentrality parameter 2 . In conventional energy detection method, the local decision is made by paring the observation with a prefixed threshold as (a). When the collected energy  exceeds the threshold  , decision 0H will be made. Otherwise decision 1H will be made. In contrast, the system model which has two thresholds of our interest is shown (b). Where ― Decision 0H ‖ and ―Decision 1H ‖ represent the absence and the presence of licensed user, respectively.“ No decision‖ means that the observation is not reliable enough and the i th cognitive user will send nothing to the mon receiver. But when all the secondary users don’t。