【正文】 reputation Ⅰ . INTRODUCTION Due to the increasingly development of wireless applications, more and more spectrum resources are needed to support numerous emerging wireless service. However, recent measurements by Federal Communication Commission (FCC) have shown that 70% of the allocated spectrum 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 黃石理工學(xué)院 畢業(yè)設(shè)計（論文）外文文獻(xiàn)翻譯 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。當(dāng)認(rèn)知用戶數(shù)量很大的時候，控制信道的帶寬將不夠用。仿真參數(shù)設(shè)置如表 1 所示。 3 頻譜感知性能分析 網(wǎng)絡(luò)開銷 在 1bit 量化條件下， avgK 代表歸一化平均感知位數(shù) ， KT 和 KNT? 分別代表 K 個已向認(rèn)知無線電網(wǎng)絡(luò)基站發(fā)送數(shù)據(jù)和 NK 個未向認(rèn)知無線電網(wǎng)絡(luò)基站發(fā)送報告。 在能量檢測算法本地判決中，每個認(rèn)知用戶把接收到的能 量 ? 跟 預(yù)設(shè)的門限 ?進(jìn)行比較，如圖 2（ a） 所示。該算法中每個認(rèn)知用戶基于雙檢測門限獨立進(jìn)行頻譜感知，但只有部分可靠的認(rèn)知用戶通過控制信道向認(rèn)知無線網(wǎng)絡(luò)基站發(fā)射感知報告。通常它又可以分為能量檢測法、匹配濾波器法和循環(huán)平穩(wěn)特征法 [4]。首先每個認(rèn)知用戶基于雙檢測門限獨立進(jìn)行頻譜感知，但只有部分可靠的認(rèn)知用戶通過控制信道向認(rèn)知無線網(wǎng)絡(luò)基站發(fā)送本地感知結(jié)果。據(jù) FCC 的調(diào)查統(tǒng)計， 70%的已分配頻譜資源沒有得到有效利用 ]1[ 。協(xié)同頻譜感知算法性能較好，但是當(dāng)認(rèn)知用戶數(shù)量很大的時候，控制信道的帶寬將不夠用。 圖 1. 認(rèn)知無線電網(wǎng)絡(luò)示意圖 頻譜感知的實質(zhì)是一個二元假設(shè)問題，即 01( ) , ,() ( ) ( ) ( ) ,n t Hxt h t s t n t H?? ? ??? （ 1） 其中 x(t)代表認(rèn)知用戶接收到的信號， s(t)表示授權(quán)用戶的發(fā)送信號， h(t)代表授權(quán)用戶與認(rèn)知用戶之間信道的衰落因子。 圖 2.（ a）一般能量檢測算法本地判決示意圖 （ b）雙門限能量檢測算法本地判決示意圖 信用度獲取方法采取文獻(xiàn) [9]的方法： 在最開始階段，認(rèn)知無線電網(wǎng)絡(luò)基站把每個認(rèn)知用戶數(shù)目的可信度設(shè)為 0，當(dāng)某認(rèn)知用戶本地判決結(jié)果與認(rèn)知無線電網(wǎng)絡(luò)基站的最終判決結(jié)果一致時，該認(rèn)知用戶可信度加 1，否則減 1。假設(shè) 0? ， 1? 分別代表在授權(quán)用戶在工作和授權(quán)用戶未工作情況下沒有認(rèn)知用戶發(fā)送感知報告，即當(dāng) K=0 時，則： NNFFHKP 01200 ))()((}|0{ ??????? ??? (12) NNGGHKP 11211 ))()((}|0{ ??????? ??? (13) 基于雙門限的頻譜感知算法在瑞利信道下的虛警概率 fQ ，漏檢概率 mQ 和檢測概率 dQ 分別為： )1)(1(}1,|1{}|1{}|0,1{0000AfPKHuPHKPHKuPQ??????????? (14) mQ = dQHKuP ???? 1}|0,0{ 1 (15) )1)(1(}1,|1{}|1{}|0,1{1111?PaKHuPHKPHKuPQ d?????????? (16) 其中： dA QHKuPP ????? 1}|0,0{ 1 =