毕业设计英文翻译——认知无线电频谱感知算法仿真

1、*仅供参考上 海 电 力 学 院毕业设计（英文翻译）课题名称 认知无线电频谱感知算法仿真 院（系） 专 业 班 级 学 生 学 号 Implementation Issues in Spectrum Sensing for Cognitive Radios认知无线电频谱感知的实现问题Abstract:There are new system implementation challenges involved in the design of cognitive radios, which have both the ability to sense the spectral environm

2、ent and the flexibility to adapt transmission parameters to maximize system capacity while co-existing with legacy wireless networks. The critical design problem is the need to process multi-gigahertz wide bandwidth and reliably detect presence of primary users. This places severe requirements on se

3、nsitivity, linearity, and dynamic range of the circuitry in the RF front-end. To improve radio sensitivity of the sensing function through processing gain we investigated three digital signal processing techniques: matched filtering, energy detection, and cyclostationary feature detection. Our analy

4、sis shows that cyclostationary feature detection has advantages due to its ability to differentiate modulated signals, interference and noise in low signal to noise ratios. In addition, to further improve the sensing reliability, the advantage of a MAC protocol that exploits cooperation among many c

5、ognitive users is investigated. 摘要：出现了一些新系统的实施挑战，其涉及认知无线电具有感知频谱环境下的传输参数既适应能力和灵活性，以最大限度地提高系统容量，同时并存于传统无线网络。关键设计问题是需要处理多千兆赫宽的带宽，可靠地检测主用户的存在。这对灵敏度，线性度和在RF前端电路的动态范围有严格要求。为了通过处理增益而提高无线电灵敏度，我们调查了三种数字信号处理技术的感测功能：匹配滤波，能量检测和循环平稳特征检测。我们的分析表明：由于能够区分调制信号，干扰和噪声在低信噪比，循环平稳特征检测更有优势。此外，为了进一步提高检测的可靠性，MAC协议，它利用在许多认知用户

6、合作的优势已经在这一块研究很久了。I. INTRODUCTION一、介绍1. It is commonly believed that there is a spectrum scarcity at frequencies that can be economically used for wireless communications. This concern has arisen from the intense competition for use of spectra at frequencies below 3 GHz. The Federal Communications Co

7、mmissions (FCC) frequency allocation chart indicates overlapping allocations over all of the frequency bands, which reinforces the scarcity mindset. On the other hand, actual measurements taken in downtown Berkeley are believed to be typical and indicate low utilization, especially in the 3-6 MHz ba

8、nds. Figure 1 shows the power spectral density (PSD) of the received 6 GHz wide signal collected for a span of 50s sampled at 20 GS/s 12. This view is supported by recent studies of the FCCs Spectrum Policy Task Force who reported 1 vast temporal and geographic variations in the usage of allocated s

9、pectrum with utilization ranging from 15% to 85%. In order to utilize these spectrum white spaces, the FCC has issued a Notice of Proposed Rule Making (NPRM FCC 03-322 2) advancing Cognitive Radio (CR) technology as a candidate to implement negotiated or opportunistic spectrum sharing.人们普遍认为，有一个频谱稀缺

10、的频率，可以经济地用于无线通信。从使用3 GHz以下频率光谱的激烈竞争开始，它就越来越受到关注。美国联邦通信委员会（FCC）的频率分配图表显示所有频段都有重叠的分配，这加强了稀缺的现状。另一方面，在伯克利市中心采取的实际测量被认为是典型的实验结果，从中指出频谱利用率低，特别是在3-6 MHz频段。图1显示接收到的6千兆赫的功率谱密度（PSD）宽信号，50s的采样在20 GS / s的的一个跨度。这种观点是最近FCC的频谱政策任务组的研究报告，报告称浩瀚时空和地域变化在分配的频谱利用率介乎15至85的使用。为了利用这些频谱空白，FCC已发出建议规则制定的通知（NPRM - FCC03-3222）

11、推进认知无线电（CR）技术，以实现协商或伺机频谱共享的候选人。2.Wireless systems today are characterized by wasteful static spectrum allocations, fixed radio functions, and limited network coordination. Some systems in unlicensed frequency bands have achieved great spectrum efficiency, but are faced with increasing interference t

12、hat limits network capacity and scalability. Cognitive radio systems offer the opportunity to use dynamic spectrum management techniques to help prevent interference, adapt to immediate local spectrum availability by creating time and location dependent in “virtual unlicensed bands”, i.e. bands that

13、 are shared with primary users. Unique to cognitive radio operation is the requirement that the radio is able to sense the environment over huge swaths of spectrum and adapt to it since the radio does not have primary rights to any pre-assigned frequencies. This new radio functionality will involve

14、the design of various analog, digital, and network processing techniques in order to meet challenging radio sensitivity requirements and wideband frequency agility.现在无线系统的特点是浪费的静态频谱分配，固定无线功能，以及有限的网络协调。在未经许可的频带一些系统已经取得了很大的频谱效率，但都面临着限制的网络容量和可扩展性增加的干扰。认知无线电系统提供使用动态频谱管理技术，以帮助防止干扰，通过创建时间和位置相关的“虚拟免授权频段”，适

15、应当地的即时频谱可用性的机会，例如与主要用户共享频段。独特的认知无线电操作是有需要的，它使无线电能够通过频谱巨大的大片感知环境，并适应它，纵然无线电对任何预指配频率没有基本权利。这种新的无线功能将涉及各种模拟设计，数字化和网络处理技术，以满足具有挑战性的射频灵敏度的要求和宽带频率捷变。3.Spectrum sensing is best addressed as a cross-layer design problem. Cognitive radio sensitivity can be improved by enhancing radio RF front-end sensitivity

16、, exploiting digital signal processing gain for specific primary user signal, and network cooperation where users share their spectrum sensing measurements. 频谱感知在跨层设计问题上是最好的方法。认知无线电的灵敏度可以通过增强无线电射频前端的灵敏度，利用数字信号处理增益为特定主用户信号，以及网络合作，用户共享他们的频谱感测的测量得到改善。4.The paper is organized as follows; Section II defi

17、nes spectrum sensing function and proposes a cross-layer approach for its implementation. Section III considers RF front-end and A/D requirements for spectrum sensing and analog techniques for feasible implementations. In section IV we investigate digital signal processing techniques that can improv

18、e radio sensitivity and detect primary userspresence. Section V presents the results from a cooperative sensing scheme, achievable gains and implementation issues. Finally, conclusions are presented in Section VI.本文结构如下，第二部分定义频谱感知功能，并提出了实施一个跨层的方法。第三部分讨论频谱感知的射频前端，后端和A / D的要求，与可实现的模拟技术。在第四部分中，我们研究，可以提

19、高无线电灵敏度和检测主用户的存在的数字信号处理技术。第五部分提出了从协作感知方案，可实现收益和实施问题的结果。最后，结论载于第六部分。II. SPECTRUM SENSING2、 频谱感知1.A “Cognitive Radio” is a radio that is able to sense the spectral environment over a wide frequency band and exploit this information to opportunistically provide wireless links that best meet the user co

20、mmunications requirements 2. While many other characteristics have also been discussed as possible additional capabilities, we will use this more restricted definition and consider physical (PHY) and medium access control (MAC) functions that are linked to spectrum sensing as illustrated in Figure 2

21、认知无线电是一种无线电，它能够感觉到在很宽的频带内的频谱环境，并利用这些信息来投机提供最能满足用户的通信需求的无线链路。在许多其他特性也已讨论过作为可能的附加功能时，我们将使用这个更受限制的定义，并考虑物理层（PHY）和介质访问控制（MAC）功能链接到频谱感测，如图22.Since cognitive radios are considered lower priority or secondary users of spectrum allocated to a primary user, a fundamental requirement is to avoid interference

22、to potential primary users in their vicinity. On the other hand, primary user networks have no requirement to change their infrastructure for spectrum sharing with cognitive networks. Therefore, cognitive radios should be able to independently detect primary user presence through continuous spectrum

23、 sensing. For example, TV broadcast signals are much easer to detect than GPS signals, since the TV receivers sensitivity is tens of dBs worse than GPS receiver. 由于认知无线电被认为是低优先级或分配给主用户频谱的二级用户，一个基本要求是，避免附近的有潜力的主要用户的干扰。另一方面，主用户网络具有不要求改变他们与认知网络频谱共享的基础设施。因此，认知无线电应该能够通过不断的频谱感知独立检测主用户的存在。例如，广播电视信号更容易比GPS信

24、号来检测，因为电视接收器的灵敏度是几十分贝比GPS接收器差。3.In general, cognitive radio sensitivity should outperform primary user receiver by a large margin in order to prevent what is essentially a hidden terminal problem. This is the key issue that makes spectrum sensing very challenging research problem. Meeting the sensit

25、ivity requirement of each primary receiver with a wideband radio would be difficult enough, but the problem becomes even more challenging if the sensitivity requirement is raised by additional 30-40 dB. This margin is required because cognitive radio does not have a direct measurement of a channel b

26、etween primary user receiver and transmitter and must base its decision on its local channel measurement to a primary user transmitter. This type of detection is referred to as local spectrum sensing and the worst case hidden terminal problem would occur when the cognitive radio is shadowed, in seve

27、re multipath fading, or inside buildings with high penetration loss while in a close neighborhood there is a primary user whose is at the marginal reception, due to its more favorable channel conditions. Even though the probability of this scenario is low, cognitive radio should not cause interferen

28、ce to such primary user. 一般情况下，认知无线电灵敏度应远远胜过主用户接收器，为的是防止出现隐藏终端问题。这是使频谱感知非常具有挑战性的研究课题的关键问题。满足每个主接收器的灵敏度要求，宽带无线电将难以足够了，但如果灵敏度的要求是提出额外的30-40分贝的问题就变得更加具有挑战性。这个利润率是必需的，因为认知无线电没有直接测量主用户接收机和发射机之间的信道，并且必须立足其上的本地信道测量决定主要用户发送。这种类型的检测被称为本地频谱检测和当认知无线电是阴影，在严重的多径衰落会发生隐藏终端问题的最坏的情况下，在严重的多径衰落，或内部具有高穿透损耗的物体，而在一个近邻域有一

29、个主要用户，其是在边缘接收，由于其更有利的信道条件。即使这种情况的概率较低，认知无线电应该不会对这样的主用户造成干扰。4.The implementation of the spectrum sensing function also requires a high degree of flexibility since the radio environment is highly variable, both because of different types of primary user systems, propagation losses, and interference. T

30、he main design challenge is to define RF and analog architecture with right trade-offs between linearity, sampling rate, accuracy and power, so that digital signal processing techniques can be utilized for spectrum sensing, cognition, and adaptation. This also motivates research of signal processing

31、 techniques that can relax challenging requirements for analog, specifically wideband amplification, mixing and A/D conversion of over a GHz or more of bandwidth, and enhance overall radio sensitivity.频谱检测功能的实现也需要高度灵活性，由于无线环境是高度可变，另一个原因是不同类型的主用户系统中存在传播损耗和干扰。主要的设计挑战是定义射频和模拟结构与线性，采样率，精度和功率之间权利权衡，以使数字信

32、号处理技术可被用于频谱感知，认知和适应。这也促使了可以放宽对模拟的，特别宽频带放大，混合和超过带宽的GHz或多个A/ D转换的挑战性的要求，提高整体的无线电灵敏度的信号处理技术的研究。III. COGNITIVE RADIO FRONTEND3、 认知无线电前端1.There are two frequency bands where the cognitive radios might operate in a near future: 400-800 MHz (UHF TV bands) and 3-10 GHz. The FCC has noted that in the lower U

33、HF bands almost every geographical area has several unused 6 MHz wide TV channels. This frequency band is particularly appealing due to good propagation properties for long-range communications. Furthermore, given the static TV channel allocations, the timing requirements for spectrum sensing are ve

34、ry relaxed. The FCC approval of UWB underlay networks in 3-10 GHz indicates that this frequency range might be opened for opportunistic use. Furthermore, this band has very low spectral utilization, as indicated in Figure 1. 有两个频带，其中的认知无线电设备可能在不久的将来操作：400-800MHz的（UHF电视频段）和3-10千兆赫。美国联邦通信委员会已经注意到，在较低的

35、UHF频段几乎每一个地区有几个未使用的6 MHz宽的电视频道。该频段是由于远距离通信良好的传播特性特别有吸引力。此外，鉴于静态电视频道的分配，对时序要求频谱感知是很轻松的。FCC认证表明在3-10 GHz的超宽带底层网络，该频率范围内可能会开始使用。此外，该带具有非常低的频谱利用，如图1所示。2.Regardless of operating frequency range, a wideband front-end for a cognitive radio could have an architecture as depicted in Figure 3. The wideband RF

36、 signal presented at the antenna of a cognitive radio includes signals from close and widely separated transmitters and from transmitters operating at widely different power levels and channel bandwidths. As a result, detection of weak signals must frequently be performed in the presence of very str

37、ong signals. Thus, there will be extremely stringent requirements placed on the linearity of the RF analog circuits as well as their ability to operate over wide bandwidths. In order to keep the requirements on the final analog to digital (A/D) converter at a reasonable level in a mostly digital arc

38、hitecture, front-end design needs a tunable notch analog processing block that would provide a dynamic range control. 不论工作频率范围，宽带前端的认知无线电可以有一个体系结构，如图3所示。在认知无线电的天线提出的宽带射频信号包括从密切和广泛分离发射器和在广泛的不同功率级别和信道带宽操作发射器的信号。其结果，必须频繁地在非常强的信号的存在下进行检测微弱信号。因此，将对RF模拟电路的线性度，以及电路工作在宽的带宽的能力提出极为严格的要求。为了保持对最终模拟的要求，它需要模数（A/

39、D）转换器处在一个多数字结构合理水平，所以前端设计需要一个可调陷波模拟处理模块以提供动态范围控制。3.Reducing the in-band interference to a manageable level is a critical design problem, since the traditional strategy of narrow band analog frequency selective filtering to avoid the wide dynamic range of interfering signals is not viable. The ultima

40、te solution to this problem would involve a combination of techniques, including adaptive notch filtering such as employed in UWB designs, banks of on chip RF filters possibly using MEMS technology such as FBARs, and spatial filtering using RF beam-forming through adaptive antenna arrays. Other more

41、 sophisticated approaches could involve active cancellation, because in the situation in which the interfering signal is extremely strong, it is then possible to decode the signal and provide an active canceling signal before the A/D conversion process. While the active cancellation approach will co

42、nsume significantly more hardware, it has the important advantage of ultimately being more flexible. 减少了带内干扰到一个可管理的水平是一个关键的设计问题，因为窄带模拟频率选择性滤波的传统策略，以避免干扰信号是不可行的宽动态范围。最终解决这一问题将涉及的技术的组合，包括自适应陷波滤波器，如在超宽带设计采用，对芯片射频银行滤波器可能使用MEMS技术，如FBAR的，并使用通过自适应天线阵列射频波束形成空间滤波。其它更复杂的方法可涉及有源取消，因为在干扰信号是非常强的情况下，那么有可能对信号进行解码，

43、并在A / D转换处理前提供一个有源抵消信号。而活性消除方法会消耗显著更多的硬件，它具有最终处理更灵活的重要优势。4.The spatial dimension provides several new opportunities. The sensitivity of the sensing receiver can be increased by the exploitation of multiple antennas through diversity increase and range extension, which in effect could make it much mo

44、re sensitive than the primary users which it is trying to detect.空间维度提供了一些新的机遇。传感接收器的灵敏度的增强可以通过减少多个天线来实现，这是因为多样性的增加和范围扩大，这实际上可能使比它试图检测主用户来说更加敏感。IV. SIGNAL PROCESSING TECHNIQUES FOR SPECTRUM SENSING4、 频谱感知的信号处理技术1.A key advantage of CMOS integration is that digital signal processing can be used to as

45、sist the analog circuits. In case of spectrum sensing the need for signal processing is two-fold: improvement of radio front-end sensitivity by processing gain and primary user identification based on knowledge of the signal characteristics. In this section we discuss advantages and disadvantages of

46、 three techniques that are used in traditional systems: matched filter, energy detector and cyclostationary feature detector. CMOS集成的一个主要优点是，数字信号处理可以用来协助模拟电路。在频谱感知需要的信号处理是双重的情况下：通过处理增益和主用户基于信号特征知识的识别，无线电前端灵敏度有所进步。在本节中，我们讨论用于传统系统的三种技术的优点和缺点：匹配滤波器，能量检测和循环平稳特征检测器。2.A. Matched Filter The optimal way for

47、 any signal detection is a matched filter,since it maximizes received signal-to-noise ratio. However, a matched filter effectively requires demodulation of a primary user signal. This means that cognitive radio has apriori knowledge of primary user signal at both PHY and MAC layers, e.g. modulation

48、type and order, pulse shaping, packet format. Such information might be pre-stored in CR memory, but the cumbersome part is that for demodulation it has to achieve coherency with primary user signal by performing timing and carrier synchronization, even channel equalization. This is still possible s

49、ince most primary users have pilots, preambles, synchronization words or spreading codes that can be used for coherent detection. For examples: TV signal has narrowband pilot for audio and video carriers; CDMA systems have dedicated spreading codes for pilot and synchronization channels; OFDM packet

50、s have preambles for packet acquisition. The main advantage of matched filter is that due to coherency it requires less time to achieve high processing gain since only O(1/SNR) samples are needed to meet a given probability of detection constraint. However, a significant drawback of a matched filter

51、 is that a cognitive radio would need a dedicated receiver for every primary user class. A. 匹配滤波器对于任何信号检测的最佳方法是一个匹配滤波器，因为它最大化接收到的信号噪声比。然而，匹配滤波器十分需要主用户信号的解调。这意味着，认知无线电有主用户信号的先验知识，在两个PHY和MAC层，例如调制类型和顺序，脉冲整形，报文格式。这样的信息可能被预先存储在认知无线电的记忆模块，但是繁琐的部分是，对于解调，它不得不通过执行定时和载波同步，即使信道均衡，来实现与主用户信号的相干性。这仍然是可能的，因为大多数主要

52、用户具有导频，前导码，同步字或扩散可用于相干检测的代码。举例来说：电视信号为了音频和视频运营商装上了窄带试点，CDMA系统为了飞行员和同步信道装上了扩频码; OFDM包为了数据包采集装上了前导码。匹配滤波器的主要优点是，由于一致性它需要更少的时间，以实现高处理增益，因为只需要O（1/SNR）需要的样品，以满足检测条件约束的给定概率。然而，匹配滤波器的显著缺点是认知无线电将需要一个专用的接收器，用于每一级用户类。3.B. Energy Detector One approach to simplify matched filtering approach is to perform non-co

53、herent detection through energy detection. This sub-optimal technique has been extensively used in radiometry. An energy detector can be implemented similar to a spectrum analyzer by averaging frequency bins of a Fast Fourier Transform (FFT), as outlined in Figure 4.Processing gain is proportional t

54、o FFT size N and observation/averaging time T. Increasing N improves frequency resolution which helps narrowband signal detection. Also, longer averaging time reduces the noise power thus improves SNR. However, due to non-coherent processing O(1/SNR2) samples are required to meet a probability of de

55、tection constraint.B.能量检测简化匹配滤波的一种方法是通过能量检测来执行非相干检测。这种次优的技术已被广泛地用于辐射。一个能量检测器可以实现类似于用频谱分析仪的功能，通过平均快速傅立叶变换（FFT）的频率箱，如在图4中列出。处理增益和FFT大小N和观察/平均时间T成正比。增加N可以提高频率分辨率，频率分辨率有助于窄带信号检测。另外，较长的平均时间减少了噪声功率从而提高信噪比。然而，由于非相干处理为O（1/SNR2）样本必须符合检测约束的概率。4.There are several drawbacks of energy detectors that might dimini

56、sh their simplicity in implementation. First, a threshold used for primary user detection is highly susceptible to unknown or changing noise levels. Even if the threshold would be set adaptively, presence of any in-band interference would confuse the energy detector. Furthermore, in frequency select

57、ive fading it is not clear how to set the threshold with respect to channel notches. Second, energy detector does not differentiate between modulated signals, noise and interference. Since, it cannot recognize the interference, it cannot benefit from adaptive signal processing for canceling the inte

58、rferer. Furthermore, spectrum policy for using the band is constrained only to primary users, so a cognitive user should treat noise and other secondary users differently. Lastly, an energy detector does not work for spread spectrum signals: direct sequence and frequency hopping signals, for which more sophisticated signal processing algorithms need to be de