用于多跳认知无线电网络的分布式网络编码控制信道通信工程毕业论文外文翻译.doc

1、多次反射罐方案 DSA第 4 页 (共 13 页)用于多跳认知无线电网络的分布式网络编码控制信道Alfred Asterjadhi 等 著 1 前言大多数电磁频谱由政府机构长期指定给公司或机构专门用于区域或国家地区。由于这种资源的静态分配，许可频谱的许多部分在许多时间和/或位置未使用或未被充分利用。另一方面，几种最近的无线技术在诸如 IEEE802.11，蓝牙，Zigbee 之类的非许可频段中运行，并且在一定程度上对 WiMAX 进行操作;这些技术已经看到这样的成功和扩散，他们正在访问的频谱 - 主要是 2.4 GHz ISM 频段 - 已经过度拥挤。为了为这些现有技术提供更多的频谱资源，并且

2、允许替代和创新技术的潜在开发，最近已经提出允许被许可的设备（称为次要用户）访问那些许可的频谱资源，主要用户未被使用或零星地使用。这种方法通常被称为动态频谱接入（DSA） ，无线电设备发现和机会性利用未使用或未充分利用的频谱带的能力通常称为认知无线电（CR）技术。DSA 和 CR 最近都引起了无线通信和网络界的极大关注。通常设想两种主要应用。第一个是认知无线接入（CWA ） ，根据该认知接入点，认知接入点负责识别未使用的许可频谱，并使用它来提供对次用户的接入。第二个应用是我们在这个技术中研究的应用，它是认知自组织网络（CAN） ，也就是使用用于二级用户本身之间通信的无许可频谱，用于诸如点对点内容

3、分发，环境监控，安全性等目的，灾难恢复情景通信，军事通信等等。设计 CAN 系统比 CWA 有更多困难，主要有两个原因。第一是识别未使用的频谱。在 CWA 中，接入点的作用是连接到互联网，因此可以使用简单的策略来推断频谱可用性，例如查询频谱调节器在其地理位置的频谱可用性或直接与主用户协商频谱可用性或一些中间频谱经纪人另一方面，在 CAN 中，与频谱调节器或主要用户的缺乏直接通信需要二级用户能够使用检测技术自己识别未使用的频谱。第二个困难是辅助用户协调媒体访问目的。在 CWA 中存在接入点和通常所有二级用户直接与之通信（即，网络是单跳）的事实使得直接使用集中式媒体接入控制（MAC）解决方案，如时

4、分多址（TDMA）或正交频分多址（OFDMA） 。相反，预计 CAN 将跨越多跳，缺少集中控制器 ;而对于传统的单通道多跳自组织网络而言，这个问题的几个解决方案是已知的，因为假设我们处理允许设备访问的具结论第 5 页 (共 13 页)有成本效益的最先进技术的状态，因此将它们重用于 CAN 是不直接的一次只能限制频谱的一部分，中间访问将在多个信道上执行，而且可用于二次通信的实际信道可能会随着位置和时间而变化。由于刚刚描述的两个问题，CAN 中出现了几个实际的设计挑战，如实现控制信道，辅助用户对媒体接入的协调，实现用于检测未使用频谱的可靠方案等。在这篇文章中，我们将讨论这些挑战，我们显示，在以前的

5、文献中，有几个很好的解决方案可以有效地解决一个或者一些这些问题。在讨论之后，我们提出了我们设计的方案，以克服 CAN 缺乏完整的解决方案。我们的方案是基于一个虚拟控制通道，利用用户以伪随机方式访问信道，并在任何频道遇到任何情况时交换控制信息。通过网络编码实现对所有用户的控制信息的高效传播。用户交换的控制信息包括根据预定义的确定性算法确定信道切换模式以及数据通信的资源分配所需的所有信息（带宽要求，主要用户存在和位置等） 。我们通过提出和讨论模拟结果来讨论所提出的方案的性能，表明它是 CAN 实际实现的有效解决方案。2 多功能 CAN 中的技术挑战我们在 CAN 中遇到的第一个问题是鸡蛋问题：二次

6、设备需要彼此协调来执行频谱接入，但是它们还需要访问频谱以便通信和实现协调。这个问题通常被称为控制频道问题，不幸的是，在与 DSA 相关的工作中往往被忽视。事实上，大多数 DSA 相关出版物更侧重于主要用户检测和/或高效频谱分配的问题，并且在这样做时，假设某些控制信道实现对于次要用户是可用的。为了实际实现控制通道，一些作者提出静态分配一些频谱带。这个实际提出了两个主要问题：一是需要静态频谱调节，这正是 DSA 旨在避免的一个问题。第二，选择的控制带可能很容易成为瓶颈。这在多跳场景中尤其如此，其中对控制信息交换的需求潜在地非常高（例如，不仅对于媒体访问，而且用于路由目的） 。已经提出了一些其他解决

7、方案，其尝试通过动态地选择未使用的许可频带来执行次要用户控制来解决第一个问题沟通;然而，这些建议没有解决控制瓶颈问题。结论第 5 页 (共 13 页)当然，CAN 的理想解决方案不仅需要解决控制信息交换的问题，而且还要有效地实现对可用频谱资源的有效利用。在这方面，应该注意的是，先前讨论的多重会合策略最初被提出作为单通道技术的扩展，最着名的是 IEEE 802.11;特别地，在这些解决方案中看到的优点是仅仅通过使用多个通道，可以在单通道情况下实现网络容量的显着增加。然而，要注意的是，多通道网络的容量限制还远远没有达到多重交会方案，这更是解决问题的实际方法，而不采取系统的方法来最大限度地提高信道利

8、用效率。应该考虑到频谱有效使用的一个方面是在多跳网络中，通常只有一部分用户处于给定用户的干扰范围内。这通过频率重用来提高频谱利用率的可能性。不幸的是，在实践中，这需要更复杂的频谱分配策略，以及更多信息的可用性（例如每个用户的位置知识） 。以分布式的方式是非常具有挑战性的。与此相关的问题是链路调度和路由问题：传统的自组织网络路由策略在多信道网络中是无效的，主要是因为给定的链路在任何时候都不能被激活，因为要求发送方和接收器在同一个通道上。理想情况下，应共同执行信道分配，链路调度和路由，以最大化频谱利用效率和网络性能。在这方面，已经提出了一些有趣的解决方案，但是它们具有要求集中式调度器的缺点。鉴于

9、CAN 的性质，需要一种分布式解决方案来实现实施。到目前为止，我们还没有处理可能最具特色的 CAN 的特征：适用于二次频谱接入的频谱的这些部分的识别必须由次要用户自己使用感测技术来执行。从最近的文献中已经深入研究了从单个二次用户的角度进行感测的主题，并且已经提出了从简单的能量或匹配滤波器检测到复杂的循环平稳特征检测技术的几种解决方案。然而，如对于无线电接入频谱的情况所讨论的，对主要用户的二次干扰维持在一定阈值以下的要求转化为对单用户检测策略的灵敏度要求高到不符合成本效益，或者甚至完全不切实际，用现有技术实现这种检测器。3 多功能 CAN 中的 DSA 方案我们考虑每个次要用户具有单个收发器的情

10、况，因此可以在任何给定时间仅在单个信道上进行调谐。我们有一套次要用户和一组可用于无牌访问的渠道。为了设计在这种情况下有效的频谱接入方案，我们需要解决以下两个问题：如何使二级用户彼此协调，以及如何以有效的方式为这些用户分配频谱资源。结论第 5 页 (共 13 页)如上一节所述，这一领域的大多数以前的工作只解决了其中一个问题;相反，我们的方法旨在同时解决这两个问题。直观地，频谱分配和传输调度最好使用关于特定通信需求（例如，服务质量QoS要求）和频谱可用性的知识来执行（例如，由主用户检测信息）。将这些知识称为控制信息，通过收集所有用户生成的控制包获得。在文献中，当完整的控制信息用于资源分配时，通常假

11、定集中式方案。这意味着有一个集中控制器收集所有用户生成的控制包，确定全球资源分配，然后告诉每个用户什么资源用于数据通信。为了得出分布式方法，我们选择不同的策略：每个用户收集完整的控制信息，并为整个网络独立地确定资源分配。关键在于，如果相同的控制信息成功传播给所有用户，并且资源分配算法是确定性的，则每个用户将能够确定相同的资源分配，而无需用户之间的任何进一步的交互。这是我们首先提出的单跳多通道网络的多通道方案的基本原理，并在此讨论在多跳 CAN 中的使用。在本节的其余部分，我们提供更多关于我们的计划如何工作的细节;本文的其余部分更侧重于多机场和机场频谱接入问题。控制信息的确切性质由所选择的特定调

12、度算法确定。作为一个例子，在我们讨论了一种相对简单的单跳网络统一资源分配算法。该算法仅需要参与参与分配的用户组的知识以及用于确定伪随机信道切换模式的随机数发生器的种子。因此，由每个用户生成的控制信息分组仅包括用户的唯一标识符（例如，其 MAC 地址）和使用的随机比特串以及所有其他用户的比特串来确定公共种子为随机数发生器。我们的方案正常工作的一个重要要求是控制信息的传播到达所有用户。每当特定用户在分配周期结束时无法检索控制信息时，该用户将潜在地确定用于后续分配周期的错误的信道切换模式和传输调度，可能开始使用资源（某些信道中的传输时隙）的传输将其分配给其他用户。在本文的其余部分中，我们将此事件称为

13、频谱冲突，并参考无法将控制信息检索为误传用户的用户。一般来说，频谱冲突的机会，因此频谱资源浪费的平均数量随着用户数量的错误而增加。因此，我们想要一种传播方案，其中定义为普通用户从所有其他用户成功检索控制信息的概率的检索成功概率很高。结论第 5 页 (共 13 页)我们建议使用网络编码，以便为控制信息实施可靠而有效的传播方案。网络编码是最近推出的用于数据传播的范例，根据该模式，由多个源产生的分组在中间节点处共同编码并在最终目的地解码。该编码策略可以在增加吞吐量，减少延迟和提高鲁棒性方面非常有效。为了实现网络编码的实际，我们提到，作者提出了一种网络编码分布式方案，消除了对编码和解码功能的集中化知识

14、的需要，同时允许节点间的异步数据交换。根据该方法，每个节点将所有传入的分组存储在内部缓冲器中，并且在其自己的缓冲器中发送包含所有分组的随机线性组合的编码分组。在传输时间，该分组被转发到位于传输范围内的所有节点。现在，如果编码矢量是随机生成的，并且符号位于足够大小的有限伽罗瓦域，则信息将以高概率传播给所有用户。基于这种方法，每当节点接收到编码的分组时，它必须知道用于执行编码的系数，以便恢复原始信息分组。一个简单的解决方案包括在每个编码包中附加对应的编码矢量，该编码矢量描述了其包含的信息包的哪个线性组合。这样，解码存储在编码包中的信息所需的编码系数可以在编码包本身内找到。任何节点都可以恢复信息包由

15、所有节点简单地通过反转存储在数据传播期间接收的分组的所有系数的矩阵来产生。将编码向量追加到数据包引起额外的开销，这将需要在确定我们的 DSA 解决方案的总体控制开销时予以考虑;有关这个问题的详细讨论，请参阅读者。最后，为了实现网络编码的实际，我们采用缓冲模型。如我们以前的工作中所讨论的，网络编码大大优于其他策略，以便在单跳多通道网络中传播控制信息。换句话说，使用网络编码与伪随机信道切换模式相结合，为我们提供了一个虚拟控制信道，允许用户有效地共享控制信息。该网络编码的虚拟控制信道对于分组丢失和链路故障是鲁棒的，并且最重要的是不需要存在专用于交换控制信息的静态频谱资源。对于适用于二次接入的未使用频

16、谱资源的检测，我们注意到，网络编码控制信道自然适合实施协同主用户检测解决方案。4。结论在本文中，我们讨论了 CAN 中出现的主要挑战，并提出了基于虚拟网络编码控制通道的这些挑战的实际解决方案。我们提出模拟结果，证明在几种结论第 5 页 (共 13 页)情况下如何实现控制信息的有效分散和有效的频谱利用。我们的解决方案显示出对主用户活动的鲁棒性，并且可以针对次级用户的数量进行扩展。未来的研究方向包括在提出的解决方案中整合更精细的频谱分配，传输调度和路由策略。A Distributed Network Coded Control Channel for Multihop Cognitive Radi

17、o NetworksAlfred Asterjadhi waiting Zhang bowen translation1 Prefacemost of the electromagnetic spectrum is assigned by government agencies to companies or insti-tutions for exclusive use over regional or national areas on a long-term basis. As a result of this static allocation of resources, severa

18、l portions of the licensed spectrum are unused or underused at many times and/or locations . On the other hand, several recent wire-less technologies operate in unlicensed bands, such as IEEE 802.11, Bluetooth, Zigbee, and to some extent WiMAX; these technologies have seen such success and prolifera

19、tion that the spectrum they are accessing mostly the 2.4 GHz ISM band has become overcrowded. In an effort to provide further spectrum resources for these existing technologies, as well as to allow the potential development of alternative and innova-tive ones, recently it has been A Distributed Netw

20、ork Coded Control Channel for Multihop Cognitive Radio Networks第 6 页 (共 13 页)proposed to allow unlicensed devices, called secondary users, to access those licensed spectrum resources that are unused or sporadically used by their owners, called primary users. This approach is normally referred to as

21、dynamic spectrum access (DSA), and the ability of radio devices to find and opportunistically exploit unused or underused spectrum bands is normally called cognitive radio (CR) technology .Both DSA and CR have recently attracted significant atten-tion from the wireless communications and networking

22、community. Two main applications are commonly envisioned. The first is cognitive wireless access (CWA), according to which a cognitive access point takes care of identifying unused licensed spectrum and uses it to provide access to secondary users. The second application, which is the one we investi

23、gate in this arti-cle, is cognitive ad hoc networks (CANs), that is, the use ofunlicensed spectrum for communications among the secondary users themselves, for purposes such as peer-to-peer content distribution, environmental monitoring, safetycommunications in disaster recovery scenarios, military

24、communi-cations, and many others.Designing a system for CANs presents more difficulties than for CWA, for two main reasons. The first is the identification of unused spectrum. In CWA the access point is by its role connected to the Internet, and therefore can infer spectrum availability using simple

25、 strategies, such as querying the spectrum regulator for spectrum availability at its geographic location or directly negotiating spectrum availability with the primary user or some intermediary spectrum broker . On the other hand, in CANs the lack of direct communication with the spectrum regulator

26、 or primary users requires secondary users to be able to identify unused spectrum by them-selves using detection techniques. The second difficulty is the coordination of secondary users for medium access purposes.In CWA the presence of an access point and the fact that commonly all secondary users c

27、ommunicate directly with it (i.e., the network is single-hop) makes it straightforward to use centralized medium access control (MAC) solutions, such as time-division multiple access (TDMA) or orthogonal frequency-division multiple access (OFDMA). On the contrary,CANs are A Distributed Network Coded

28、 Control Channel for Multihop Cognitive Radio Networks第 8 页 (共 13 页)expected to span over multiple hops and to lack a centralized controller; while several solutions to this problem are known for traditional single-channel multihop ad hoc networks, it is not straightforward to reuse them for CANs du

29、e to the fact that, assuming we deal with cost-effective state of the art technology that allows devices to access only a limited portion of the spectrum at a time, medium access is to be performed across several channels, and moreover the actual channels that can be used for secondary communication

30、s might vary with respect to location as well as time.Due to the two issues just described, several practical design challenges arise in CANs, such as the realization of the control channel, the coordination of secondary users for medium access, the implementation of a reliable scheme for the detect

31、ion of unused spectrum, and so on. In this article we discuss these challenges, and we show that, while in the prior literature there are several good solutions that can effectively solve one or some of these issues.After this discussion we present the scheme we have designed in an effort to overcom

32、e this lack of a complete solution for CANs. Our scheme is based on a virtual control channel which exploits the fact that users visit channels in a pseudo-random fashion and exchange control information whenever they happen to meet in any channel. Efficient dissemination of the control information

33、to all users is achieved by means of network coding . The control information exchanged by users consists of all the information (bandwidth requirements, primary user presence and location, etc.) that is needed to determine channel switch patterns as well as resource allocation for data communicatio

34、n according to a predefined deterministic algorithm. We discuss the performance of the proposed scheme by presenting and discussing simulation results which show that it is an effective solution for the practical realization of CANs.2 Technical Challenges in Multihop CANs The first issue we encounte

35、r in CANs is a chicken-egg problem: secondary devices need to coordinate among themselves to perform spectrum access, but they also need to access the spectrum in order to communicate and achieve A Distributed Network Coded Control Channel for Multihop Cognitive Radio Networks第 8 页 (共 13 页)coordinat

36、ion.This issue is often referred to as the control channel problem,and unfortunately it is often neglected in work related to DSA. The fact is that most DSA related publications focus more on the problem of primary user detection and/or efficient spectrum allocation, and in doing so assume that some

37、 control channel implementation is available to secondary users.For the practical realization of the control channel, some authors propose to statically allocate some spectrum band.This practice presents two major issues: first, it requires static spectrum regulation, which is exactly what DSA aims

38、at avoiding; second, the chosen control band could easily become the bottleneck. This is especially true in multihop scenarios,where the need for control information exchange is potentially very high (e.g., not only for medium access, but also for routing purposes).Some other solutions have been pro

39、posed that attempt to solve the first issue by dynamically choosing an unused licensed band to perform secondary user controlcommunications; however, the control bottleneck issue is not addressed by these proposals.Of course, the ideal solution for CANs needs not only to address the issue of the exc

40、hange of control information, but also to effectively enable efficient usage of the available spectrum resources. In this respect, it is to be noted that the multiple-rendezvous strategies discussed earlier were originally proposed as an extension to single-channel technologies, most notably IEEE 80

41、2.11; in particular, the advantage seen in these solutions was that just by enabling the use of multiple channels, a significant increase in network capacity could be achieved over the single-channel case. However, it is to be noted that the capacity limit of multichannel networks is still far from

42、being reached by multiple-rendezvous schemes,which are more of a practical solution to the problem and do not take a systematic approach to maximizing the channel utilization efficiency.One of the aspects that should be taken into account for an efficient usage of the spectrum is that in a multihop

43、network typically only a subset of the users are in the interference range of a given user. This opens up the possibility of higher spectrum A Distributed Network Coded Control Channel for Multihop Cognitive Radio Networks第 10 页 (共 13 页)utilization efficiency by means of frequency reuse.Unfortunatel

44、y, in practice this requires more complex spectrum allocation strategies, as well as the availability of more information (e.g., knowledge of the location of each user).Doing this in a distributed fashion is very challenging. Coupled with this problem is the issue of link scheduling and routing: tra

45、ditional ad hoc network routing strategies are not effective in multichannel networks, due primarily to the fact that a given link cannot be activated at all times because of the requirement that both the sender and the receiver are on the same channel. Ideally, channel allocation, link scheduling,a

46、nd routing should be jointly performed in order to maximize spectrum utilization efficiency as well as network performance. In this respect some interesting solutions have been proposed , but these have the drawback of requiring a centralized scheduler. Given the nature of CANs, a distributed soluti

47、on would be needed in order to allow practical implementation.So far, we still have not dealt with what is possibly the most peculiar trait of CANs: the identification of those parts of the spectrum that are suitable for secondary spectrum access must be performed by the secondary users themselves u

48、sing sensing techniques. The topic of sensing from the point of view of a single secondary user has been intensively investigated in the recent literature, and several solutions have been proposed,from simple energy or matched filter detection to complex cyclostationary feature detection techniques.

49、 However, as discussed in for the case of unlicensed access of TV spectrum, the requirement of maintaining secondary interference to primary users below a certain threshold translates into a sensitivity requirement for single-user detection strategies so high that it is not cost-effective, or even completely impractical, to implement such detectors with current technology.3 A Scheme for DSA in Multihop CANs We consider the case in which each secondary user has a single transceiver, and thus can be tuned only on a single channel at any given time. We have a set of secondary users and