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1、河北建筑工程學(xué)院畢業(yè)設(shè)計(jì)（論文）外文資料翻譯 系別： 電氣系 專業(yè)： 電子信息工程 班級： 電子092班 姓名： 學(xué)號： 2009315213 外文出處： Wireless.Sensor.Networks： A.Networking.Perspective 附 件：1、外文原文；2、外文資料翻譯譯文。指導(dǎo)教師評語：簽字： 年 月 日1、 外文原文（復(fù)印件）8.3 RANGING TECHNIQUES FOR WIRELESS SENSOR NETWORKSThe RF location sensors operating in different environments can measur

2、e the RSS, AOA, phase of arrival (POA), TOA, and signature of the delay - power profile as location metrics to estimate the ranging distance 4,7 . The deployment environment (i.e., wireless RF channel) will constrain the accuracy and the performance of each technique. In outdoor open areas, these ra

3、nging techniques perform very well. However, as the wireless medium becomes more complex, for example, dense urban or indoor environments, the channel suffers from severe multipath propagation and heavy shadow fading conditions. This finding in turn impacts the accuracy and performance in estimating

4、 the range between a pair of nodes. For this reason, this chapter will focus its ranging and localization discussion on indoor environments. This is important because many of the WSN applications are envisioned for deployment in rough terrain and cluttered environments and understanding of the impac

5、t of the channel on the performance of ranging and localization is important. In addition, range measurements using POA and AOA in indoor and urban areas are unreliable. Therefore, we will focus our discussion on two practical techniques,TOA and RSS.These two ranging techniques, which have been used

6、 traditionally in wireless networks, have a great potential for use in WSN localization. The TOA based ranging is suitable for accurate indoor localization because it only needs a few references and no prior training. By using this technique, however, the hardware is complex and the accuracy is sens

7、itive to the multipath condition and the system bandwidth. This technique has been implemented in GPS, PinPoint, WearNet, IEEE 802.15.3, and IEEE 802.15.4 systems. The RSS based ranging, on the other hand, is simple to implement and is insensitive to the multipath condition and the bandwidth of the

8、system. In addition, it does not need any synchronization and can work with any existing wireless system that can measure the RSS. For accurate ranging, however, a high density of anchors or reference points is needed and extensive training and computationally expensive algorithms are required.The R

9、SS ranging has been used for WiFi positioning in systems, for example, Ekahau, Newbury Networks, PanGo, and Skyhook. This section first introduces TOA based ranging and the limitations imposed by the wireless channel. Then it will be compared with the RSS counterpart focusing on the performance as a

10、 function of the channel behavior. What is introduced here is important to the understanding of the underlying issues in distance estimation, which is an important fundamental building block in WSN localization.8.3.1 TOA Based Ranging In TOA based ranging, a sensor node measures the distance to anot

11、her node by estimating the signal propagation delay in free space, where radio signals travel at the constant speed of light. Figure 8.3 shows an example of TOA based ranging between two sensors. The performance of TOA based ranging depends on the availability of the direct path (DP) signal 4,14 . I

12、n its presence, for example, short distance line - of - sight (LOS) conditions, accurate estimates are feasible 14 . The challenge, however, is ranging in non - LOS (NLOS) conditions, which can be characterized as site - specific and dense multipath environments 14,22 . These environments introduce

13、several challenges. The first corrupts the TOA estimates due to the multipath components (MPCs), which are delayed and attenuated replicas of the original signal, arriving and combining at the receiver shifting the estimate. The second is the propagation delay caused by the signal traveling through

14、obstacles, which adds a positive bias to the TOA estimates. The third is the absence of the DP due to blockage, also known as undetected direct path (UDP) 14 . The bias imposed by this type of error is usually much larger than the first two and has a significant probability of occurrence due to cabi

15、nets, elevator shafts, or doors that are usually cluttering the indoor environment. In order to analyze the behavior of the TOA based ranging, it is best to resort to a popular model used to describe the wireless channel. In a typical indoor environment, the transmitted signal will be scattered and

16、the receiver node will receive replicas of the original signal with different amplitudes, phases, and delays. At the receiver, the signals from all these paths combine and this phenomenon is known as multipath. In order to understand the impact of the channel on the TOA accuracy, we resort to a mode

17、l typically used to characterize multipath arrivals. For multipath channels, the impulse respons characterizes the arrival paths, their respective amplitudes, and delays. Mathematically, it can be represented as a summation of all the arriving multipath components or ，（8.1）where Lp is the number of

18、MPCs, and , , and are amplitude, phase, and propagation delay of the kth path, respectively 7,23 . Let and denote the DP amplitude and propagation delay, respectively. The distance between the sensor node and the RP or anchor is , where v is the speed of signal propagation. In the absence of the DP,

19、 ranging can be achieved using the amplitude and propagation delay of the non - direct path (NDP) component given by and, respectively; resulting in a longer distance, where. For the receiver to identify the DP, the ratio of the strongest MPC to that of the DP given by ，（8.2）must be less than the re

20、ceiver dynamic range k and the power of the DP must be greater than the receiver sensitivity . These constraints are given by ，（8.3a）, （8.3b）where. In general, ranging and localization accuracy is constrained by the ranging error, which is defined as the difference between the estimated and the actu

21、al distance; that is, .（8.4） In an indoor environment, the node/MT will experience a varying error behavior depending on the availability of the DP and in the case of its absence on the characteristics of the DP blockage. It is possible to categorize the error based on the following ranging states 2

22、4 . In the presence of the DP, both (8.3a) and (8.3b) are met and the distance estimate is very accurate, yielding ,（8.5a）where the random bias induced by the multipath, is the bias corresponding to the propagation delay caused by NLOS conditions, and z is a zero - mean additive measurement noise. I

23、t has been shown that is indeed a function of the bandwidth and the signal to noise ratio (SNR) 14 , while bpd is dependant on the medium of the obstacles.When the node experiences sudden blockage of the DP, Eq. (8.3a) is not met and the DP is shadowed by some obstacle, burying its power under the d

24、ynamic range of the receiver. In this situation, the ranging estimate experiences a larger error compared to Eq. (8.5a) . Emphasizing that ranging is achieved through the NDP component, the estimate is then given by ,（8.6a）,（8.6b）where is a deterministic additive bias representing the nature of the

25、blockage. Unlike the multipath biases, but similar to the biases induced by the propagation delay, the dependence of on the system bandwidth and SNR has its own limitations as reported in Ref. 14 . Formally, these ranging states can be defi ned as,（8.7a）,（8.7b）Figures 8.4 and 8.5 provide sample chan

26、nel profiles of these two ranging situations 24 . The performance of TOA based ranging can be determined by the Cramer-Rao lower bound (CRLB), which has been studied extensively for existing systems. The variance of TOA estimation is bounded by the CRLB 25 ,（8.8）where T is the signal observation tim

27、e, is the SNR, is the frequency of operation, and w is the system bandwidth. In practice, TOA can be obtained by measuring the arrival time of a wide-band narrow pulse, which can be obtained either by using spread spectrum technology or directly. 8.3.1.1 Direct Spread Spectrum. One TOA estimation te

28、chnique based on the direct spread spectrum (DSS) wideband signal has been used in GPS and other ranging systems for many years. In such a system, a signal coded by a known pseudorandom (PN) sequence is transmitted and a receiver cross - correlates the received signal with a locally generated PN seq

29、uence using a sliding correlator or a matched filter. The distance between the transmitter and the receiver is determined from the arrival time of the first correlation peak. Because of the processing gain of the correlation at the receiver, DSS ranging systems perform much better than competing sys

30、tems in suppressing interference from other radio systems operating in the same frequency band. In these band - limited systems, super- resolution techniques for TOA estimation have been applied successfully. Results have shown that these high - resolution algorithms can provide improved accuracy 25

31、 . 8.3.1.2 Ultra - Wideband Ranging.A promising alternative to DSS systems is ultra - wideband (UWB) ranging 26 . According to Eq. (8.8) , it is clear that in multipath propagation environments, the performance of TOA estimation is inversely related to the system bandwidth. Increasing the system ban

32、dwidth (i.e., narrower time - domain pulse) results in higher time resolution and thus better ranging accuracy. As a result, these systems have attracted considerable attention in recent years 16,22,26 . For UWB applications, the FCC regulation allocated an unlicensed flat frequency band 3.1 10.6 GH

33、z for which there are two proposals: direct sequence (DS) UWB and multiband orthogonal frequency division multiplexing (MB OFDM). The former is pulse based, which utilizes large bandwidths, for example, 3 GHz, while the latter occupies a bandwidth of 528 MHz. The accuracy of these systems can be eva

34、luated by examining their behaviors in the multipath channel. Sample measurements in indoor office environments are provided in Fig. 8.6 a for 500 - MHz systems, resembling the MB OFDM channels and Fig. 8.6 b for 3 - GHz bandwidth, resembling the wider channel of the DS UWB.The expected TOA between

35、the transmitter and the receiver is 40.5 ns and the estimated arrival with 500 - MHz and 3 - GHz bands are 45.5 and 40.7 ns, respectively. The 5 - and 0.2 - ns errors in TOA estimation results in 1.67 - m and 7 - cm errors, respectively, clearly illustrating the impact of a higher system bandwidth o

36、n accuracy. One important observation from these measurement results is that higher bandwidths improve time - domain resolution, which resolves the pulse into respective components, resulting in improved accuracy. The trade - off, however, is that higher resolution implies lower energy per MPC, whic

37、h means a higher probability of DP blockage. This means that the ranging coverage of 500 - MHz systems is larger than that of the 3 - GHz counterpart. Although UWB can reduce multipath significantly, combating the excess propagation delay and UDP becomes challenging because the amount of delay and t

38、he type of blocking material are not known in advance and cannot be mitigated through large bandwidths alone. Understanding of the error behavior in light of these major error contributors is necessary to enable effective UWB ranging. Specifi cally, WSN localization algorithms must analyze the chann

39、el statistics and attempt to identify and mitigate DP blockage 27,28 . 2、外文資料翻譯譯文8.3無線傳感器網(wǎng)絡(luò)的測距技術(shù)射頻位置傳感器在不同的環(huán)境中運(yùn)行可測量RSS，AOA，階段的到來（POA），TOA，和作為位置的度量估計(jì)距離延遲功率譜 4,7。這種部署環(huán)境（例如，無線射頻信道）將限制精度和每種技術(shù)的性能。在戶外空曠地區(qū)，這些測距技術(shù)執(zhí)行得很好。然而，隨著無線介質(zhì)而變得更加復(fù)雜，例如，密集的城市或室內(nèi)環(huán)境中，信道存在嚴(yán)重的多徑傳播和嚴(yán)重的陰影衰落環(huán)境。這一發(fā)現(xiàn)反過來說明了在一對節(jié)點(diǎn)之間的距離估計(jì)對精度和性能的影響。為此，

40、本章將重點(diǎn)討論在室內(nèi)環(huán)境中的測距和定位。這點(diǎn)很重要，因?yàn)樵S多WSN應(yīng)用程序設(shè)想在崎嶇的地形和雜亂的環(huán)境中部署傳感器，因此，對測距和定位性能的信道的影響的理解是很重要的。此外，采用POA和AOA在室內(nèi)和城市地區(qū)進(jìn)行測距是不可靠的。因此，我們將重點(diǎn)討論兩個(gè)實(shí)用技術(shù)，TOA和RSS。這兩種測距技術(shù)，已經(jīng)有在無線網(wǎng)絡(luò)中使用的傳統(tǒng)，它們對于在無線傳感器網(wǎng)絡(luò)定位有著很大的潛力。TOA測距適合于精確的室內(nèi)定位是因?yàn)樗恍枰苌俚奈墨I(xiàn)并且不需要事先訓(xùn)練。但是，通過使用這種技術(shù)，硬件會變得復(fù)雜、精度的多徑條件和系統(tǒng)帶寬會敏感。這種技術(shù)已經(jīng)被實(shí)施在GPS，PinPoint，wearnet，IEEE 802.15.

41、3，和IEEE 802.15.4系統(tǒng)應(yīng)用上。另一方面，RSS測量實(shí)現(xiàn)簡單，對多徑條件和系統(tǒng)的帶寬不敏感。此外，它不需要任何同步，可以與任何現(xiàn)有的無線系統(tǒng)協(xié)同工作，可以測量RSS。然而，對于準(zhǔn)確的測量，錨或參考點(diǎn)的高密度是必要的，并且廣泛的培訓(xùn)和昂貴的算法也是必需的。RSS測距已被用于在WiFi定位系統(tǒng)中，比如Ekahau，Newbury Networks，Pango和Skyhook。本章首先介紹了基于測距的TOA和所施加在無線通道的局限性。然后它與專注于信道行為函數(shù)的RSS的性能進(jìn)行比較。這里所介紹的在測距基本問題上的認(rèn)識很重要，這是研究無線傳感器網(wǎng)絡(luò)定位的重要基礎(chǔ)。8.3.1 TOA測距在T

42、OA測距中，傳感器節(jié)點(diǎn)到另一個(gè)節(jié)點(diǎn)間距離的測量是通過自由空間中的信號傳播時(shí)延來估計(jì)的，信號傳播在無線信號以光速為恒定速度。圖8.3展示了兩個(gè)節(jié)點(diǎn)間的TOA測距。 TOA測距的性能取決于直接路徑的可用性（DP）信號 14 。例如，在DP信號中，短距離的線的視線（LOS）的條件下，準(zhǔn)確的估計(jì)是可行的 14 。然而，我們面臨的挑戰(zhàn)是，在非LOS（NLOS）表現(xiàn)為網(wǎng)站的特異性和密集多徑環(huán)境的條件下。這些環(huán)境提出了一些挑戰(zhàn)。圖8.3 傳感器間的TOA測距第一個(gè)由于多徑分量（MPC）所引起的腐化的TOA估計(jì)，這是原始信號延遲和衰減的復(fù)制品，到達(dá)和合并接收器的移動估計(jì)。第二個(gè)是由信號穿過障礙物引起的傳播延遲

43、，這增加了一個(gè)正向偏置的TOA估計(jì)。第三是由于堵塞的DP的缺失，也被稱為未發(fā)現(xiàn)的直接路徑（UDP） 14 。這種類型的錯(cuò)誤引起的偏壓通常是比前兩大得多，同時(shí)由于櫥柜，電梯，或通常在室內(nèi)門附近，也會引起更大出錯(cuò)的概率。為了分析基于TOA測距的行為,最好采取一個(gè)受歡迎的模型用來描述無線信道。在一個(gè)典型的室內(nèi)環(huán)境中,傳輸信號將被分散，接收者節(jié)點(diǎn)將收到與原始信號不同振幅、階段和延誤的副本信號。在接收機(jī),信號從所有這些路徑結(jié)合,這種現(xiàn)象稱為多徑。為了了解影響精度的渠道,我們常常借助于一個(gè)用于描述多路徑到達(dá)的模型。這個(gè)模型描述了多路徑通道,脈沖響應(yīng)特征路徑,到達(dá)各自的振幅和延誤。在數(shù)學(xué)上,它可以表示為一個(gè)

44、求和的多路徑組件或到達(dá)，（8.1）其中，Lp代表MPCs的數(shù)量，分別是振幅，相位以及傳播延遲的路徑。讓和分別表示DP振幅和傳播延遲。傳感器節(jié)點(diǎn)之間的距離和RP或錨是,v是信號傳播的速度。在DP的缺席中,測距可以通過,分別由和給出的使用振幅和傳播延遲的非直接的路徑(NDP)組件來達(dá)到;這導(dǎo)致了長的距離,其中。為使接收機(jī)識別DP,最大的MPC與DP信號的比例如下，（8.2）它必須低于接收機(jī)動態(tài)范圍k的能力并且DP必須大于接收機(jī)靈敏度。這些約束條件如下，（8.3a）,（8.3b）其中。一般來說,測距和定位精度受到測距誤差的限制,其被定義為估計(jì)和實(shí)際的距離的差異;那就是（8.4）在室內(nèi)環(huán)境中,節(jié)點(diǎn)/M

45、T將會體驗(yàn)一種取決于可用性的DP不同的錯(cuò)誤行為和具有DP堵塞特征對于的缺席。它可能是基于以下測距狀態(tài)24 的錯(cuò)誤分類。在DP下, (8.3a)和(8.3 b)得到滿足和距離的估計(jì)是非常準(zhǔn)確的。，（8.5a）其中，是在隨機(jī)偏差引起的多路徑, 是由NLOS引起的傳播延時(shí)的偏置, z是一個(gè)零,意味著添加劑測量噪聲。它已被證明的確是一個(gè)函數(shù)的帶寬和信號噪聲比(信噪比)14,而bpd是依賴于介質(zhì)的障礙。當(dāng)節(jié)點(diǎn)經(jīng)歷突然DP,Eq阻塞,(8.3 a)不滿足和DP被一些障礙所阻擋,它將它的能量放在在動態(tài)范圍的接收機(jī)。在這種情況下,同Eq(8.5 a)相比，測距估計(jì)將會有一個(gè)更大的誤差范圍。其中值得強(qiáng)調(diào)的是，測

46、距是通過NDP組件來實(shí)現(xiàn)的,然后由以下給出圖8.4 寬帶在200MHz范圍的TOA估計(jì)，（8.6a），（8.6b）是一個(gè)堵塞性質(zhì)的確定性偏置。與多路徑偏置不同,但類似于由于傳播延遲引起的偏置，取決于系統(tǒng)帶寬的，并且信噪比都有自己的局限性上報(bào)信息14。一般來說，這些測距狀態(tài)可以被定義為，（8.7a），（8.7b）圖8.4和8.5提供樣品通道配置文件的這兩個(gè)測距情況24?；赥OA測距的性能范圍可以由最大下界(CRLB)確定,它已廣泛地用于研究現(xiàn)有系統(tǒng)。TOA測距中的估計(jì)由CRLB25確定，（8.8）圖8.5 在寬帶為200MHz范圍內(nèi)的NDP的TOA測距其中T是信號的觀測時(shí)間, 是信噪比、是運(yùn)作

47、的頻率,w是系統(tǒng)帶寬。在實(shí)踐中,可以通過測量獲得長遠(yuǎn)的到達(dá)時(shí)間一個(gè)寬帶窄脈沖來獲得TOA,也可以通過使用或直接擴(kuò)頻技術(shù)。8.3.1.1直接擴(kuò)頻一種基于直接擴(kuò)頻(DSS)寬帶信號的TOA測距技術(shù)已經(jīng)應(yīng)用于GPS和其他測距系統(tǒng)許多年了。在這樣一個(gè)系統(tǒng),一個(gè)由已知的偽隨機(jī)(PN)序列編碼的信號是用來傳播的和一個(gè)交叉關(guān)聯(lián)的接收器接收信號的與本地PN序列生成使用滑動相關(guān)器或一個(gè)匹配濾波器。發(fā)射機(jī)和接收機(jī)之間的距離是由到達(dá)時(shí)間的第一個(gè)相關(guān)峰確定。因?yàn)樘幚碓鲆娴南嚓P(guān)性在接收機(jī)、DSS測距系統(tǒng)在同一頻帶的性能遠(yuǎn)遠(yuǎn)好于競爭的系統(tǒng)抑制干擾其他無線電系統(tǒng)操作。在這些有限的系統(tǒng)中,超級分辨率技術(shù)已經(jīng)成功應(yīng)用于TOA測

48、距。結(jié)果表明,這些高分辨率算法可以提供改善的準(zhǔn)確性25。8.3.1.2超寬頻帶范圍。一個(gè)有前途的可以用來替代DSS系統(tǒng)是超寬帶(UWB)測距26系統(tǒng)。很明顯,根據(jù)Eq. (8.8)，在多徑傳播環(huán)境,TOA測距的性能估計(jì)是逆相關(guān)系統(tǒng)帶寬。通過增加系統(tǒng)帶寬(即更窄的時(shí)間-域脈沖)導(dǎo)致更高的時(shí)間分辨率,從而有更好的測距精度。因此, 近年來這些系統(tǒng)已經(jīng)引起了相當(dāng)大的關(guān)注。對于超寬頻應(yīng)用,FCC規(guī)定分配一個(gè)無照平頻帶3.1 - 10.6 GHz,有兩個(gè)建議:直接序列(DS)超寬頻和多頻帶正交頻分復(fù)用(MB - OFDM)。前者是基于脈沖,利用大帶寬,例如,3 GHz,而后者占有帶寬為528 MHz。這些

49、系統(tǒng)的準(zhǔn)確性的評估可以通過他們在多徑信道的行為來檢查。在室內(nèi)辦公環(huán)境的測量由圖8.6a的500 - MHz系統(tǒng)提供,類似于MB - OFDM渠道。8.6 b為3 GHz帶寬,類似于更廣泛的DS - UWB信道。在發(fā)射機(jī)和接收機(jī)預(yù)計(jì)到達(dá)時(shí)間是40.5 ns，500 - MHz和3 GHz的估計(jì)到達(dá)分別是45.5和40.7 ns。5 和0.2 - ns錯(cuò)誤在TOA估計(jì)中分別導(dǎo)致1.67米和7 -厘米誤差,這清晰說明的影響系統(tǒng)精度更高的帶寬。一個(gè)重要的發(fā)現(xiàn)是這些測量結(jié)果是高帶寬的改善時(shí)間-域分辨率,它解決了到各自組件的脈沖,從而提高精度。然而,反過來看，高分辨率便意味著較低的能量,這意味著MPC更高機(jī)率的DP堵塞。這意味著范圍覆蓋500 - MHz系統(tǒng)比3 GHz更廣。雖然超寬頻可以顯著減少過度傳播延遲引起的多徑,，但是UDP因?yàn)檠舆t變得更加具有挑戰(zhàn)性和屏蔽材料的類型是無法提前知道的，并且它不能獨(dú)自通過大帶寬來減輕。根據(jù)這些主要的誤差貢獻(xiàn)來理解錯(cuò)誤的行為,對于實(shí)現(xiàn)有效的超寬頻測距是很有必要的。特別指出的是，無線傳感器網(wǎng)絡(luò)定位算法必須分析信道統(tǒng)計(jì)和試著識別、減輕DP堵塞(27、28。