無線網絡外文翻譯

1、摘要該文提出了一種在認知無線網絡控制信道帶寬受限條件下基于信任度的雙門限協同頻譜感知算法。首先每個認知用戶基于雙檢測門限獨立進行頻譜感知，但只有部分可靠的認知用戶通過控制信道向認知無線網絡基站發(fā)送本地感知結果。當所有的用戶都不可靠時，選取信任度最高的認知用戶發(fā)送本地感知結果進行判決。理論分析和仿真表明，同常規(guī)能量檢測算法相比較，該算法能夠在控制信道帶寬受限條件下，以較少的網絡開銷獲得更好的頻譜感知性能。關鍵詞：認知無線電；頻譜感知；信任度；雙門限1引言隨著無線通信技術的飛速發(fā)展，有限的頻譜資源與不斷增長的無線通信需求的矛盾越來越突出。然而根據現有的固定分配頻譜資源策略，絕大多數頻譜資源得不到有

2、效利用。據fcc 的調查統(tǒng)計，70%的已分配頻譜資源沒有得到有效利用。為了提高頻譜資源的利用率，認知無線電技術由joseph mitola 提出并得到了廣泛的關注。頻譜感知技術是認知無線電網絡的支撐技術之一。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。能量檢測算法因為應用簡單且無需知道任何授權用戶信號的先驗知識成為研究熱點。認知用戶在接入授權頻帶之前，必須首先感知該頻帶空閑即授權用戶沒有工作，否則會對授權用戶造成干擾。一旦授權用戶重新工作，認知用戶必須退避，實現在不對授權用戶產生干擾的情況下對頻譜資源的共享。由于實際信道中的多徑和陰影效應，單個認知用戶頻譜感知的性能并不樂觀，針

3、對這個問題d. cabric等人提出了協同頻譜感知算法5-6。協同頻譜感知算法性能較好，但是當認知用戶數量很大的時候，控制信道的帶寬將不夠用。文獻7中提出了一種在控制信道帶寬受限條件下的基于雙檢測門限的頻譜感知算法，該算法能夠以較小的網絡開銷，獲得接近普通單門限頻譜檢測算法的性能。針對認知無線電頻譜感知的需要，本文提出了認知無線電環(huán)境下一種基于信任度的雙門限協同頻譜感知算法。該算法中每個認知用戶基于雙檢測門限獨立進行頻譜感知，但只有部分可靠的認知用戶通過控制信道向認知無線網絡基站發(fā)射感知報告。當所有的用戶都不可靠時，選取信任度最高的認知用戶發(fā)射感知報告進行判決。本文對該算法進行了性能分析并通過

4、仿真表明，本文方法比較常規(guī)能量檢測算法，在減小網絡開銷的同時提高了檢測性能。2系統(tǒng)模型假設一個認知無線電網絡有n個認知用戶和一個認知無線網絡基站，如圖1 所示。認知無線網絡基站負責管理和聯系n個認知用戶，在收到認知用戶的檢測報告后做出最終判決。圖1. 認知無線電網絡示意圖頻譜感知的實質是一個二元假設問題，即 （1）其中x(t)代表認知用戶接收到的信號，s(t)表示授權用戶的發(fā)送信號，h(t)代表授權用戶與認知用戶之間信道的衰落因子。代表授權用戶沒有工作，代表授權用戶正在工作。設是認知用戶接收信號的能量，根據能量檢測理論8，服從以下分布： （2）其中表示瞬時信噪比，并且其服從均值為的指數分布，表

5、自由度為2m的中心卡方分布，代表自由度為非中心參數為的卡方分布，表示時間帶寬積。在能量檢測算法本地判決中，每個認知用戶把接收到的能量跟預設的門限進行比較，如圖2（a）所示。當時，本地能量檢測器做出本地判決，表示授權用戶在工作，否則判決 d 為 0。而雙門限能量檢測算法本地判決如圖3(b)所示，本地能量檢測器判決規(guī)則如下： （3）其中nd表示認知用戶接受到的能量值不可靠，認知用戶不作出任何判決，發(fā)送感知報告給認知無線電網絡基站。如果出現所有認知用戶都不作出判決的情況，則選擇信用度最高的認知用戶依據單門限能量檢測算法作出本地判決。并發(fā)送感知報告給認知無線電網絡基站。本地判決d=0本地判決d=1(a

6、)(b)0本地判決d=0本地判決d=1nd0 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9的方法：在最開始階段，認知無線電網絡基站把每個認知用戶數目的可信度設為0，當某認知用戶本地判決結果與認知無線電網絡基站的最終判決結果一致時，該認知用戶可信度加1，否則減1。假設認知用戶i的可信度是，則其更新過程如（4）： （4）其中是認知用戶傳送給認知無線電網絡基站的判決結果，是認知無線電網絡基站的最終判決結果。據文獻8可知，認知用戶在高斯信道下的平均檢測概率、平均漏檢概率和平均虛警概率如下所示： (5) (6) (7)出于對授權用戶的保護，認

7、知無線電網絡基站最終采用or準則作出判決。3頻譜感知性能分析3.1網絡開銷在1bit量化條件下，代表歸一化平均感知位數，和分別代表k個已向認知無線電網絡基站發(fā)送數據和n-k個未向認知無線電網絡基站發(fā)送報告。則：，。設和，則劃歸一劃平均感知位數如式8所示： (8)定義：， 則： (9)由9式可得：可知：基于雙門限的協同頻譜檢測算法的網絡開銷始終小于常規(guī)的能量檢測算法。3.2檢測性能分析設和別表示 在假設和下的概率分布，則根據文獻10可知: (10)= (11)顯然，。假設，分別代表在授權用戶在工作和授權用戶未工作情況下沒有認知用戶發(fā)送感知報告，即當k=0時，則： (12) (13) 基于雙門限的

8、頻譜感知算法在瑞利信道下的虛警概率，漏檢概率和檢測概率分別為： (14)= (15) (16)其中： = = (17) (18)則： (19) (20) 由上式可知當=0時，此算法與常規(guī)算法相同。當參與協同的認知用戶數目n較大時，則基于雙門限的頻譜檢測算法的檢測性能與常規(guī)能量算法的檢測性能近似，可知在控制信道帶寬受限制的情況下以較小的性能損失大大降低了網絡開銷。4 仿真及分析本節(jié)通過計算機仿真來評估所提出的基于信任度的雙門限協同頻譜感知算法的性能。仿真參數設置如表1 所示。表1 仿真參數設置參數數值認知用戶數目平均信噪比時間帶寬積授權用戶占用信道概率授權用戶不占用信道概率圖3 給出了在的情況下

9、算法的檢測性能?？梢钥闯鐾Ｒ?guī)能量檢測算法相比較，本文所提出算法的檢測性能得到了明顯的改善。例如當時，基于信任度的雙門限協同頻譜感知算法的檢測概率比常規(guī)能量檢測算法高出0.019。圖3檢測性能示意圖圖4 描述了在不同的條件下，基于信任度的雙門限協同頻譜感知算法對網絡開銷的影響。同常規(guī)能量檢測算法即=0時相比較，本文所提出算法的歸一化平均感知位數急劇下降，控制信道帶寬與認知用戶數量之間的矛盾得到了緩解。例如當，= 0.01 時，基于信任度的雙門限協同頻譜感知算法的歸一化平均感知位數下降了38%。當，=0.001時，歸一化平均感知位數則下降了44%圖4 不同條件下算法對網絡開銷的影響5結束語頻譜感

10、知技術是認知無線電網絡的支撐技術之一。當認知用戶數量很大的時候，控制信道的帶寬將不夠用。本文提出了認知無線電環(huán)境下一種基于信任度的雙門限協同頻譜感知算法。每個認知用戶基于雙檢測門限獨立進行頻譜感知，但只有部分可靠的認知用戶通過控制信道向認知無線網絡基站發(fā)射感知報告。當所有的用戶都不可靠時，選取信任度最高的認知用戶發(fā)射感知報告進行判決。本文對該算法進行了性能分析并通過仿真表明，本文方法比較常規(guī)能量檢測算法，在減小網絡開銷的同時提高了檢測性能。參考文獻1 federal communications commission. spectrum policy task force, rep. et d

11、ocket no. 02-135 r. nov. 2002.2 j. mitola and g. q. maguire. cognitive radio: making software radios more personalc,ieee personal communication. vol. 6, pp. 1318, aug. 1999.3 s. haykin. cognitive radio: brain-empowered wireless communications j. ieee j. sel. areas communication. vol. 23, pp. 201220,

12、 feb. 2005.4 akyldiz if. next generation/dynamic spectrum access/cognitive radio wireless networks: a survey j. elsevier computer networks, 2006(50):2127-2159.5 d. cabric, s. m. mishra, and r. w. brodersen. implementation issues in spectrum sensing for cognitive radiosc/ in proc. of a silomar conf.

13、on signals, systems, and computers, pacific grove,ca, usa, nov. 7-10, 2004, pp. 772 - 776.6 a.ghasemi and e. s. sousa. collaborative spectrum sensing for opportunistic access in fading environmentsc/ in proc. 1st ieees ymp. new frontiers in dynamic spectrum access networks, baltimore, usa, nov. 811,

14、 2005, pp. 131136.7 chunhua sun, wei zhang, letaief k.b. cooperative spectrum sensing for cognitive radios under bandwidth constraintsc/ in proc. ieee wcnc, march 11-15, 2007, pp. 1-5.8 h. urkowitz. energy detection of unknown deterministic signals c. proceedings of ieee, vol.55, pp. 523-531, april

15、1967.9 ruiliang chen, jung-min park, kaigui bian. robust distributed spectrum sensing in cognitive radio networksc. in proc. ieeeinfocom, april 2008, pp. 1876-1884.10 f. f. digham, m. -s. alouini, and m. k. simon. on the energy detection of unknown signals over fading channelsc. in proc. ieee icc, a

16、nchorage, ak, usa, may 11-15, 2003, pp. 35753579.附原文：a new cooperative spectrum sensing algorithmfor cognitive radio networksabstractspectrum sensing is a critical phase in building a cognitive radio network. however, the bandwidth for reporting secondary users sensing results will be insufficient,

17、when the number of secondary user is very large. in this paper, we propose a new cooperative spectrum sensing algorithm to alleviate the bandwidth problem of reporting channel. compared with conventional method, only the secondary users with reliable information are allowed to report their sensing r

18、esults. when no user with reliable information, only the secondary user with highest reputation will report its sensing result. simulation results show that our algorithm achieves better sensing performance and the average number of sensing bits decrease greatly.keywordscognitive radio; cooperative

19、spectrum sensing; double threshold; reputation. 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

20、 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

21、 secondary users can use the licensed spectrum as long as the primary user is absent. however, when the primary user comes 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 bu

22、ilding 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 been studied to impro

23、ve the spectrum sensing performance 4, 5. in6, 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 common receiver. by censoring the colle

24、cted local observations, only the secondary users with enough information will send their local decisions to the common 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 observati

25、on independently and only the users with reliable information send their local decisions to the common 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

26、 performance under awgn channels is improved and the communication traffic is also reduced as opposed to the conventional method. the rest of the paper is organized as follows. in section , system model is briefly introduced. sensing performance is analyzed in section . in section , we present the s

27、imulation results of our cooperative spectrum sensing method. finally, we draw our conclusions in section .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. cooper

28、ative 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 hypothesis-testing problem:primary user is absent;:primary user is present. for implementation simplicity, we restrict ourselves to energy detection in

29、 the spectrum sensing. the local spectrum sensing is to decide between the following two hypotheses: (1) where is the signal received by secondary user, is primary users transmitted signal,is awgn, and is the temporary amplitude gain of the channel. according to energy detection theory 7, we have th

30、e following distribution: (2) where is the energy value collected by secondary user, is instantaneous snr and follows exponentially distribution with the mean value , is the time bandwidth product of the energy detector,represents a central chi-square distribution with 2m degrees of freedom and. rep

31、resents a non-central chi-square distribution with degrees of freedom and a non-centrality parameter . in conventional energy detection method, the local decision is made by comparing the observation with a pre-fixed threshold as fig.1 (a). when the collected energy exceeds the threshold , decision

32、will be made. otherwise decision will be made. in contrast, the system model which has two thresholds of our interest is shown infig.1 (b). where “ decision ” and “decision ” represent the absence and the presence of licensed user, respectively.“no decision” means that the observation is not reliabl

33、e enough and the th cognitive user will send nothing to the common receiver. but when all the secondary users dont send their local decisions, only the cognitive user with the highest reputation is selected to sense spectrum based on conventional energy detection method, and send its local decision

34、to the common receiver. reputation is obtained based on the accuracy of cognitive users sensing results. the reputation value is set to zero at the beginning. whenever its local spectrum sensing report is consistent with the final sensing decision, its reputation is incremented by one; otherwise it

35、is decremented by one. under this rule, assuming the th cognitive users reputation value is 1, the last sensing report of cognitive user send to common receiver is , and the final decision is ,then is updated according to the following relation: for the cognitive radio users with the energy detector

36、, the average probabilities of detection, the average probabilities of missed detection, and the average probabilities of false alarm over awgn channels are given, respectively, by 7: (3) (4) (5) where , are complete and incomplete gamma function respectively, and is the generalized marcum function.

37、 in this paper, we consider cooperative spectrum sensing with 1bit quantization. let represent the normalized d=0 d=1(a)(b)0 d=0 d=1nd0fig1. (a)conventional detection method (b)double threshold energy detection methodaverage number of sensing bit. let and represent he event that there are k unlicens

38、ed users reporting 1-bit decision and n-k users not reporting to the common receiver, respectively. the , .and then the average number of sensing bits for our method can be derived as: （6）for simplicity, we define: ， （7）let denote the normalized average number of sensing bits, then, we obtain as fol

39、lows： （8）from (8), it can be seen that, the normalized average number of sensing bits is always smaller than 1. the communication traffic of our method is are deduced as opposed to the conventional energy detection method.iii. the performance analysis of spectrum sensingin this section, the spectrum

40、 sensing performance of the proposed method will be analyzed. assume the control channel between the unlicensed users and the common receiver is perfect, the local decisions are reported without any error. let and denote the cumulative distribution function (cdf) of the local test statistic under th

41、e hypothesis and , respectively. then, we have 10: (9) (10)obviously,，.if no any local decision is reported to the common receiver, i.e., k=0 , we call that fail sensing. for this case, the common receiver will request the user which has the highest reputation to send its local decision based on con

42、ventional energy detection method. let and denote the probability of fail sensing under hypothesis and , respectively. here we have: (11) (12)apparently, and .in our scheme, the false alarm probability ,the detection probability,and the missing probability : (13) = (14) (15)for simplicity, we assume

43、 the channel between the unlicensed users and the base station are ideal, the local decision will be reported without any error. so stand for the probability of the event that under hypothesis , all the k users claim and other n-k users make no local decisions. = = （16） （17） （18） （19）iv. simulation

44、resultsin this section, some simulation results are presented to illustrate the system performance of our cooperative spectrum sensing algorithm based on reputation. the results of the conventional one threshold energy detection method are also shown for a comparison. in our simulation, the common s

45、imulation parameters are given as follows: table 1. simulation parametersfig.2 depicts the performance of cooperative spectrum sensing and .it can be observed that, compared it with the conventional method, the detection performance has improved significantly. for example, while = 0.001, our method

46、achieves extra 0.019 detection probability. fig.3 shows the decrease of the normalized transmission bits for different values of fail sensing, i.e. = 0, 0.001, 0.01, 0.1. compared with conventional method, i.e., when = 0, the normalized average number of sensing bits is dramatically decreased and ba

47、ndwidth limited problem of the reporting channel is relieved. for example, when = 0.01, almost 44% and 38% reduction of the normalized average number of sensing bits can be obtained for = 0.001 and = 0.01, respectively. in our algorithm, is upper bounded and lower bounded because of the probability

48、of fail sensing and the false alarm probability are based on (7), (13).fig 2.vs., fig 3.vs.,=00,0.001,0.01,0.1v. conclusionin this paper, a new scheme in cooperative spectrum sensing for cognitive radio networks under bandwidth constraints was proposed. in our method, only the secondary users with r

49、eliable information are allowed to report their sensing results. when no user has reliable information, only he secondary user with highest reputation will report its sensing result. we analyzed the closed expression for the probability of the detection and the false-alarm. from the preliminary simulation results, we demonstrated the average number of sensing bits decrease greatly and the sensing performance is a