Parallel Redundancy Protocol anupcoming standard for hard-realtime redundancy in IndustrialEthernetApplication to IEC61850Hubert Kirrmann, ABB Corporate Research,SwitzerlandIEC SC65C MT9HAAbstract: description of a method to implementhighly available automation networks by usingparallel redundant networks,currently standardized in IEC 62439Highly Available Automation Networks2 Highly available automation networksAutomation networks require a high availability to ensure continuous plant operation.Beyond high quality elements and good maintenance, availability is increased by redundant elements (links, switches, interfaces or devices) activated automatically in case of failure.The most important factor of the recovery process is the disruption time, during which the network is unavailable.IEC SC65C MT9 is standardizing as IEC 62439 (CDV) several methods to implement high availability networks, divided into two main categories:-“redundancy in the network”, e.g. redundant rings, with devices attached to a single switch only (singly attached devices)-“redundancy in the devices”, using devices with two network interfaces attached to redundant networks (doubly attached devices)This presentation describes a “redundancy in the devices” method, the Parallel Redundancy Protocol(PRP) that provides bumpless switchover in case of failure or reintegration. Application to IEC 61850 (Substation Automation) Ethernet network is appended.Highly Available Automation Networks3PRP Network Architecturenode node node node node node nodeswitchswitchswitchswitchswitchswitchLAN ALAN BTwo separate, similar Ethernet networks (LANs) operate in parallel, their topologies may differ. Each end node (doubly attached node) has two network interfaces, one for each LAN A and B.Each end node sends simultaneously on both LANs and receives from both LANs.Highly Available Automation Networks4 PRP characteristicsPRP allows to attach nodes to a network by redundant network interfaces and links:•Addresses high availability networks tolerating any single network component failure •Provides bumpless switchover (0 nanoseconds) in case of network failure.•Is transparent to the application•Can be used with any Industrial Ethernet on layer 2 or above•Does not rely on higher layer protocols to operate or be configured•Allows nodes not equipped for redundancy to communicate with redundant nodes •Uses off-the shelf components (network interfaces, controllers, switches and links)•Can be used with any topology (tree, ring,…)Highly Available Automation Networks5Basic IdeaA Bsend TxRxTxTx RxTx Rxreceivesend on both LANs: the stack sends each frame simultaneously on LAN A and LAN B.Frames over LAN A and B have different transmission delays (or may not arrive at all)receive from both LANs:the stack receives both frames, the entity between the link and the network layer handles the frames and can filter duplicates. Both lines are treated equal.network layertransport layer send Rxreceivenetwork interfaces linkredundancy entity transceivers A B A B publisher/subscriber upper layers LAN A LAN Bapplicationsnetwork layertransport layer publisher/subscriber applicationsHighly Available Automation Networks6 Why a layer 2 redundancy ?Many Industrial Ethernet networks operate with a layer 2 (Link Layer) protocol.One motivation for this is the use of the publisher-subscriber method, that relies on broadcast of source-addressed data within the MAC broadcast domain.This excludes a redundancy scheme based on routers (Layer 3).Each node in PRP has the same MAC address on both network interfaces. Therefore, management protocols such as ARP operate as usual and assign that MAC address to the IP address(es) of that node.Highly Available Automation Networks7Typical Industrial Ethernet stackMAC/PHYEthernet 802.3TCPUDPIPClient / Server servicesAPIPT=0800Link Layer application application applicationSNTP ,PTP ,(SNMP)ARP802.p1 / 802.1QPriority tag0806PTID=8100802.2spanning tree (802.1d)Hard Real-Time stackoptional priority taggingSoft Real-Time stackICMPLayer 7publisher/subscriber (producer/consumer)Layer 2Publisher /Subscribervoidredundancy methods must operate on Layer 2 (Logical Link) to address IndustrialHighly Available Automation Networks8LAN B What happens in case of fault ?LAN AA receiver always processes both frames it receives.In case of failure, it just ceases to receive frames from one port. When traffic is reestablished, it resumes processing both frames For diagnostics purposes, it supervises the network errorsLAN BDANP DANPDANPDANP DANP DANP DANPDANP = Doubly Attached Node with PRPHighly Available Automation Networks9PRP supports any network topologycentralized wiringswitch BStar topologyParty-Line topologyboth cables can run in the same conduct if common mode failure unlikelyswitch Acommon mode failures cannot be excluded since wiring switches to be separately poweredLAN A LAN BHighly Available Automation Networks10Attachment of non-redundant nodesinter-switch link edge portsswitchDANP switchlocal area network Bleaf linkDANP switchswitchinter-switchportswitchSAN leaf linkswitchDANPDANP local area network ASANSANSANred boxSANSingly attached nodes are preferably all attached to the same network.Network traffic is asymmetrical, but this does not affect redundant operation.Singly attached nodes can also be attached through a “redundancy box” (red box )DANPHighly Available Automation Networks11 Filtering: duplicate discard and duplicate acceptA node receives twice the same frame if both redundant LANs are fault-free. It couldforward both frames to the higher layer of communication.There is in principle no need to discard duplicates at the link layer.Any communication or application software must be able to deal with duplicates, since switching networks (e.g. 802.1D RSTP) could generate duplicates:•Most applications work on top of TCP, which was designed to discard duplicates •Applications on top of UDP or Layer 2 protocols (publisher/subscriber) must be able to ignore duplicates because they rely on a connectionless communication. PRP can operate without duplicate filtering (“Duplicate Accept” mode).However, PRP also offers a “Duplicate Discard” mode, helpful:1) to offload the processors, especially when communication controllers are used.2) to supervises redundancy, bus errors, partner nodes and topology.Highly Available Automation Networks12 Discarding duplicates: solutions evaluatedDiscarding duplicates means that the link layer can identify that two frames received on both LANs are duplicates.Several methods were evaluated:1) Compare frames: a buffer with recent frames must be kept. At 20'000 frames per second and skews of several seconds, this is not bearable.2) Keep a checksum for each frame. Costs more processing power than it saves.3) Use IEEE 802.2 Type 2 which has a link layer sequence control. Unfortunately, few use 802.2.4) introduce a sequence counter in each frame. Problem: where to locate this sequence counter so that nodes not aware of redundancy can interoperate with doubly attached nodes ?Solution 4 was retained and will be explained.Highly AvailableAutomation Networks 13Discarding duplicates: sequence counter -each frame carries a sequence counter , a line indicator and a size field , inserted after the payload to remain transparent to normal network traffic.-the send inserts the same sequence counter in both frames of a pair, andincrements it by one for each sending from this node to that other node.-the receiver keeps track of the sequence counter for each for each source MACaddress it receives frames from. Frames with the same source andcounter value coming from different lines are ignored.-to this purpose, each node keeps a table of all other nodes in the network,based on observation of the network. This allows to detect nodes absence and bus errors at the same time.timedestination source LLC FCS payload preamble size sequencecounter l i n e standard Ethernet frameHighly Available Automation Networks14Discarding duplicates: Drop Window Algorithm startSeq(A)expectedSeq(A)LAN A currentSeq(A)c es LAN A dropWindowkeepBdropB keepB eLAN A s LAN B c if B dropped expectedSeq(B)eLAN B LAN B e if B kept c c scurrentSeq(B)startSeq(B)sstartSeq(A)currentSeq(B)initial:A is leading late earlyc e expectedSeq(A)sstartSeq(B)Highly AvailableAutomation Networks 15Distinguishing redundant from non-redundant frames timedestination source LLC FCS payload preamble size sequencel i n e In a frame sent by a singly attached node, assuming random data in the payload,there is a probability of 2-12that the size field matches the payload.However, this frame will not be discarded, unless a second frame arrives from theother line from that same source and with that same sequence numberand with the correct line identifier, which is not possible since a singlyattached node is attached only on one half network.The receiver detects if a frame is sent by a doubly attached node running PRP oris sent by a singly attached node with no redundancy trailer.To this purpose, a receiver identifies the PRP frames using the size field. If thelast 12 bits of the frame match the size of the payload, the receiverassumes that this frame is a PRP frame and a candidate for discarding.Highly Available Automation Networks16 Is appending a redundancy trailer in line with 802.3 ?PRP’s Duplicate Discard mode requires that frames be stretched by 4 octets.All protocols must be able to deal with data in the frames after the payload,since padding is part of the Ethernet protocol.E.g., IP has its own length control and checksum –data after the IP payload are ignored.The size of the payload must however be reduced by four octets to match the size of 1498 octets foreseen by 802.3.This would not be necessary for the switches, since all commercial switches permit longer frames to allow double tagging.However, some Ethernet Network Interfaces do not accept longer frames. Therefore, frames must be reduced by acting on the IP protocol.Network analyzers can deal with PRP, the corresponding extension already exists.Highly AvailableAutomation Networks 17Frame formats in Industrial Ethernet …LPDU = 1500 octetsLPDU = 1500 octets(TCP , UDP , ICMP)length < x0600destination source DSAP LLC (802.2)SSAPprotocol type >x0600destination sourcechecksum destinationsource protocol type checksumETPID = x8100TCI, CFI 1518octets 1518octets 1522 octets801.d (VLAN)Ethernet II 802.2 / 802.3Most Industrial Ethernet allow a mix of 802.2, 802.3, etc…resulting in 3 different frame types... IP traffic STP trafficPrioritized traffic checksumLPDU = 1494 octetsHighly AvailableAutomation Networks 18PRP frames: redundancy control after the payload length <x0600destination source DSAP LLC SSAP protocol type >x0600destination source destinationsourceprotocol typechecksumETPID = x8100TCI, CFI = x81001518 octets 1518 octets 1522 octets801.d802.3/802.2Ethernet II LPDU = 1492 octetsLPDU = 1496 octetsLPDU = 1492 octetssequencechecksum sequence checksum sequence size line size line sizeline The additional four bytes could generate oversize frames of more than 1522 octets.Although these frames are accepted by all certified switches (up to 1536 octets), some older bus controllers do not accept oversize frames, and therefore the sender must reduce the payload byplaying on the LSDU size in the network layer.appendedredundancy control trailerHighly AvailableAutomation Networks 19Redundancy Control Trailer and Padding timedestination source LLC FCS LSDU preamble padding size sequence l i n e Small frames use padding to meet the minimum frame size of 64 octets.Since padding can be introduced by switches, the sender should always insert the padding itself to reduce the decoding burden (otherwise, the receiver has to search backwards for a matching field)64 octets timedestination source LLC FCS LSDUpreamble padding size sequence l i n e 64 octets Padding inserted by the senderPadding introduced automatically:Highly AvailableAutomation Networks 20Why a size field is also useful timetime truncated destination source PT LPDU FCS ?destination source LLC FCS LPDU 01010101size sequencedestination source PT FCSLPDU LPDU truncation error: the last 32 bits are treated as CRCpreambleEthernet frames have, contrary to IEC 8802.3 frames, only a Hamming Distance of 1 against synchronization errors (frames truncation), since they do not contain a length field.To improve protection of the frames, a size field is appended to the frames.A node that receives a PRP frame from a known partner with incorrect size can flag an error. 01010101preamble01010101FCS l i n e only the application (if it is aware of it) can detect the problem...."We detected a mismatch between TCP and Ethernet checksum in up to one frame in 400“(Siggcom 2000, J. Stone & C. Patridge @standford, bbn)Highly Available Automation Networks21 Network supervisionThe duplicate discard mode allows to keep track of all nodes in the network.In order not to rely on application cyclic data for this, each DANP sends periodically a supervision frame (beacon) that indicates its state.The period is relatively long (some seconds) since the supervision frame is not needed for switchover, but only to check redundancy.All nodes keep a node table of all detected partners and registers the last time a node was seen as well as missing duplicates and out-of-sequence frames.Changes to the topology are communicated over SNMP or to the Link Management Entity.Highly Available Automation Networks22Network managementsingle-attachednodes 172.16.1.14172.16.1.66node 1node 2node 4node 65node 66switch A"red box"LAN ALAN Bswitch B2172.16.1.11172.16.1.12172.16.1.129172.16.1.192dual-attached nodesswitch B3172.16.1.195COTS NM tool172.16.1.14switch B1172.16.1.193Switches are single-attached devices and have different IP addresses on each LAN, although their function can be the same as that of a corresponding switch on the other network.Each PRP node has an SNMP agent to keep track of redundancy.Highly Available Automation Networks23Extension: Serial Redundancy Protocol modeworkstation1gatewayPRP nodes can also operate in networks that are not fully duplicatedThis mode is called SRP (Serial Redundancy Protocol) –it uses a subset of RSTP ,in which the nodes never become root bridges.SAN switch 1SANSAN bay 1switch 2SAN SANSAN bay 2SANSANSAN bay N. . .switch 10switch 20switch 30DANP nodes operating in SRPmodeHighly Available Automation Networks24 State of standardizationThe “Highly Available Automation Networks” maintenance group MT9 selected PRP is one of the three redundancy methods, along with Siemens-Hirschmann’s Hyperring (MRP) and Fieldbus Foundation’s (CRP).Siemens-Hischmann’s MRP implements “redundancy in the network” with singly attached devices attached to a ring, with moderate increase in availability and disruption delay of 200ms-500ms.Fieldbus Foundation’s CRP implements –like PRP –“redundancy in the devices”, offers the same availability increase as PRP, but has disruption times of 200 ms, compared with 0 ns for PRP. Its advantage is to allow to connect singly attached devices to both network halves, incurring the cost of aggregated links in the root switches to sustain performance.IEC 62439 is being circulated as a CDV and should become an IEC standardin 2007.Highly Available Automation Networks25 State of implementationPRP is supported by ABB and the Swiss Technical University of Wintherthur, which built an independent Linux implementation based only on the standard document.An interoperability test was performed between VxWorks, Linux and Windows implementations without problems.Highly Available Automation Networks26 PRP: Pros & Cons+ suits any Industrial Ethernet network in IEC 61784-2 (Profinet IRT has its own redundancy). + deterministic, seamless switchover+ uses standard switches and protocols+ supervises constantly the redundancy in duplicate discard mode (both LANs are active)+ monitors actual topography (over network management / SNMP)+ application-independent-requires a complete doubling of the network (twice the costs)-is limited to a layer 2 broadcast domain-requires two Ethernet controllers per node and special drivers (link redundancy entity)-request that singly attached nodes are connected to the same LAN (or use a red box).Highly Available Automation Networks27 Application to IEC 61850PRP can be readily applied to IEC 61850.It supports the layer 2 communication of GOOSE in 61850-8It offers the seamless switchover necessary for Sampled Values in 61850-9-2It provides the same redundancy scheme and hardware for the station bus and the process bus It offers a higher Hamming distance than plain Ethernet due to the length check.It can expose the link layer redundancy objects through the management interface directly as IEC 61850 objects rather than using SNMP.PRP has been tested with a real IEC 61850 installation.Highly Available Automation Networks28Redundant IEC 61850 network with PRPworkstation1loggerprinterCOM NCCDANPDANP SANduplex bay COMNCClegacylegacyduplex bayback-up workstation2GatewaylegacySAN non-redundant bayDANP gatewayDANP gatewayMixing redundant (DANP), non-redundant (SAN) and legacy devicesswitch A1switch B1SANHighly Available Automation Networks29PRP layer in IEC 61850link redundancy entityEthernet BEthernet A TCP RFC 793UDPIPISO TransportRFC 1006ISO SessionACSEISO/IEC 8649:1996MMSISO 9506-1:2003Client/ServerservicesACSIPT=0800Link Layerapplication application applicationSNTP ,PTPARPclock802.p1 / 802.1QTagging PT=0806PTID=8100802.2STP(802.1d)MAC Layerx88B9x88B8x88BAGSEmgtGOOSESVhard real-time-soft real timeHighly Available Automation Networks30 ConclusionPRP (IEC 62439 CDV)-is a redundancy method independent of IEC 61850 and used in automation networks.-provides bumpless switchover and reintegration for real-time data-permits to supervise redundancy continuously for asset management-suitable for hot swap -24h/365d operation in substations-allows to mix devices with double and single network attachment for cost optimization-allows to connect laptops and workstations to the network with standard Ethernet adapters -supports any topology-is well suited for IEC 61850。
组播OVERLAY网络动态分布式路由算法
刘克俭;余镇危;程忠庆
【期刊名称】《计算机工程与应用》
【年(卷),期】2004(040)005
【摘要】给出了组播覆盖网络MON动态路由的定义,并在此基础上提出了MON 动态组播路由计算所应考虑的问题,给出了基于Prüfer编码的覆盖组播树核的生成算法及基于分布式触发重组的MON动态组播路由算法NPPR-N,该文最后对算法的复杂度进行了推证,对算法的有效性进行了以EAD模型为基础平台的网络模拟.【总页数】5页(P28-31,75)
【作者】刘克俭;余镇危;程忠庆
【作者单位】中国矿业大学研究生院,北京,100083;中国矿业大学研究生院,北京,100083;海军后勤学院,天津,300450
【正文语种】中文
【中图分类】TP393
【相关文献】
1.基于分布式网络编码的机会网络组播路由算法 [J], 孙建飞;高媛;王淑敏
2.组播Overlay网络分布式动态路由的研究 [J], 刘克俭;余镇危;程忠庆
3.动态优化的分布式组播路由算法 [J], 李岩龙;王华;童永安
4.应用层组播分布式动态路由算法 [J], 崔新伟;王德志;张瑾;薛阳
5.Overlay组播网络上带QoS约束的路由算法 [J], 潘耘;张丽;王励成;余镇危
因版权原因，仅展示原文概要，查看原文内容请购买。
认知无线电中的频谱共享技术 杨金星;高月红;杨大成 【摘 要】由于认知无线电网络的一些独有的特性,比如,可用频谱资源的动态变化以及与第一类用户的共存,使得认知无线电网络中的频谱共享技术面临着许多新的挑战。当前关于频谱共享技术的研究旨在应对这些挑战,并且主要从以下四个方面着手：网络架构,频谱分配行为,频谱接入技术和共享频谱的范围。%Due to the unique characteristics of cognitive radio networks,such as dynamic changes of available spectrum resources and co-existence with primary users,the spectrum sharing policy in cognitive radio networks faces many new challenges.The existing work i
【期刊名称】《现代电信科技》 【年(卷),期】2011(041)007 【总页数】4页(P33-36) 【关键词】认知无线电;频谱共享;第二类用户;第一类用户 【作 者】杨金星;高月红;杨大成 【作者单位】北京邮电大学信息与通信工程学院;北京邮电大学信息与通信工程学院;北京邮电大学信息与通信工程学院
【正文语种】中 文 【中图分类】TN92 1 引言 随着电信技术的快速发展,频谱资源匮乏问题日益突出。传统的无线网络普遍采用固定式频谱分配方案，导致大量的许可频谱（Licensed Spectrum）利用率低下，频谱分配极不平衡，更加剧了频谱资源不足和用户需求之间的矛盾。动态频谱接入技术可以充分利用有限的频谱资源，极大提高频谱利用率，有效缓解当前固定频谱分配政策导致的对频谱资源不合理利用的问题。人们把利用这一技术的网络称为下一代网络或者认知无线电网络（CR Network）。在CR技术中，通常称工作频段的授权用户为第一类用户，而认知用户（CR Users）为第二类用户。认知用户可以感知、学习、自适应周围无线频谱环境的变化。通过与周围无线频谱环境的相互作用，认知用户可以通过频谱感知、频谱管理、频谱移动性和频谱共享四个功能动态接入最适宜的空闲频谱，实现有效通信。本文主要介绍频谱共享技术，重点从以下四个方面着手：网络架构频谱分配行为，频谱接入技术和共享频谱的范围，并可依次将频谱共享技术分为如图1所示的八种类型。 图1 频谱共享技术的分类 2 频谱共享技术 频谱共享，就是通过无线场景感知建立频谱空穴检测，根据感知结果调节各个发射机功率输出，以适当的调制编码策略选择最合适的频段进行可靠的通信，自适应时变的无线射频环境，实现认知用户与第一类用户之间以及认知用户与认知用户间的频谱共享[1]。 2.1 集中式与分布式频谱共享 基于网络架构的不同，频谱共享技术被分为集中式和分布式两种类别[2]。 集中式网络架构中，设有一个中央控制单元，它负责动态收集整个网络的可用频谱信息、控制频谱资源的分配和接入。在这种架构中，频谱感知仍然可以是分布式的，网络中的每个分布式节点都把自己探测感知的频谱信息汇集到中央控制单元，中央控制单元负责各个认知用户的频谱配置，兼顾不用用户的不同需求，尽可能地避免认知用户间的相互干扰。如图2所示，图中箭头表示中央控制单元与认知用户间的信令和控制信息的流动，基于收集到的可用频谱信息，中央控制单元建立频谱分配映射图。基于建立起的频谱分配映射图，频谱资源被合理配置给相应的认知用户，同时频谱效率和用户接入公平性也得到很好的平衡。 图2 集中式频谱共享网络架构 分布式解决方案主要应用在不能构建集中式结构的场合，在这种情况下，每个分布式节点都参与频谱分配，这可能会大大降低系统部署的复杂度，提高网络的可扩展性，但同时也可能引起认知用户间的相互干扰，进而造成频谱利用率下降，影响整个网络的性能。近年来的相关研究表明，分布式频谱共享方案可以逼近集中式频谱共享的性能，而网络部署的复杂度和节点间需要交换的信息量都可以极大地降低。 2.2 协作式与非协作式频谱共享 基于频谱分配行为的不同，频谱共享技术被分为协作式与非协作式两种类别。 协作式频谱共享方案考虑节点通信对其他节点产生的影响，在节点间共享干扰信息，因为每个节点的感知能力有限，协作行为可以为它们提供必要的信息以解决全局性的问题。协作行为考虑了公平性，提高了频谱的利用率和网络的吞吐量，但要求用户间信息的共享，会引入额外的通信开销。集中式解决方案可以看作是协作式的，同时也存在着分布式的协作方案。一个更通用的方案是，网络中若干相邻认知用户组成认知用户群，每个认知用户群内部共享本地干扰信息，而用户群与用户群之间是相互独立的，这样，既可以获得一定的协作性能，又可以把系统复杂度控制在一定范围内。 与协作式频谱共享方案相反，非协作式方案仅考虑节点自己的行为，因而这种方案也被称作是“自私”的，频谱分配和接入基于本地的规则执行，因此能大大减少通信的开销，但会导致频谱利用率下降。 研究表明，协作式频谱共享方案性能优于非协作式方案，甚至可以近似逼近全局优化方案性能[2]。此外，协作式频谱共享方案还能获得更好的用户接入公平性能，以及更高的系统吞吐量性能。另一方面，非协作式频谱共享方案需要更少的节点间信息交互，相应的也就需要消耗更少的功率，从这个角度来看，在吞吐量和公平性性能等方面的劣势又可以得到一定程度的弥补。 2.3 Underlay和Overlay式频谱共享 基于频谱接入技术的不同，频谱共享技术被分为Underlay和Overlay两种类别[3]。 Overlay频谱共享本质上是一种基于认知无线电的窄带通信技术，它使用授权用户没有使用的一部分频谱接入网络，这样可以最小化对第一类用户系统的干扰。认知用户选择频谱资源时遵循以下两个原则：第一，该部分频谱资源没有被第一类用户占用；第二，认知用户占用该部分频谱资源后对第一类用户的干扰尽可能小。认知用户一旦发现在自己的干扰范围内有第一类用户要占用自己正在使用的频谱资源，它会立即释放该部分频谱，因此，认知用户需要通过感知功能获得第一类用户的具体位置以及是否进行数据的传输。 图3 Overlay频谱共享资源配置 如图3所示，深灰色代表的是第一类用户正在使用的资源块，白色代表的是频谱空洞，也就是没有被第一类用户占用的那部分频谱资源，而浅灰色代表的则是正在被认知用户使用的频谱资源块。 Underlay频谱共享使用已开发的用于蜂窝网络的扩频技术，认知用户占用系统的整个带宽。这样，任何单个认知用户的干扰信号在第一类用户看来就近似于高斯白噪声，但多个认知用户的干扰叠加仍然能对第一类用户造成严重干扰。此时，认知用户和第一类用户占用的频谱资源以及相应的发射功率关系如图4所示,其中高的实线白色立方体表示第一类用户占用的频谱和相应的发射功率P1,低的虚线红色立方体表示认知用户占用的频谱和相应的发射功率P2，从图中可以看出，认知用户将自己信号扩展到了整个带宽，包括正在被第一类用户使用的那部分频谱资源。 图4 Underlay频谱共享资源配置 文献[3]和[4]对Overlay和Underlay方法进行了比较。当用户之间的干扰非常大或在假设有完全的系统知识情况下，Overlay比Underlay的性能更好。但在更现实的情况下，认知用户感知能力有限，只能获得部分系统知识，Overlay由于在频谱共享上的不足，导致很差的性能。因此文献[4]中提出了基于扩频的干扰避免Underlay技术（混合技术），认知用户仍然将信号扩展到整个系统带宽，但避开那些正在被第一类用户使用的频谱资源，同时带给授权用户的干扰可达最小化。该混合技术方案比纯粹的Overlay技术允许更多的节点接入。认知用户和第一类用户占用的频谱资源以及相应的发射功率关系如图5所示，相应的立方体所表达的含义同Underlay方式。 2.4 网内和网际频谱共享 图5 混合方式频谱共享资源配置 基于频谱共享范围的不同，频谱共享技术被分为网内和网际两种类别[2]。网际频谱共享技术允许一个网络内的认知用户共享其它网络内没有使用的频谱资源，它是一种更广泛意义上的频谱共享行为，网络结构如图6所示。相应地，频谱共享行为仅局限于单个网络内的认知用户间时，称之为网内频谱共享技术，网络结构如图7所示。 图6 网际频谱共享 图7 网内频谱共享 3 结语 本文中，我们分别从网络架构、频谱分配行为、频谱接入技术和共享频谱的范围角度介绍了频谱共享技术。集中式方案可以更好地兼顾到不同用户的不同需求，以一种全局的观点优化频谱分配和接入，分布式方案可能会引起系统性能的下降，但系统部署的灵活性更大；合作式方案的优势在于它能够提高频谱利用率和公平性，然而，如果考虑到诸如用户之间频繁的信息交换这些成本开销的话，它的优势就不是很明显了；Overlay技术利用频谱空洞进行数据传输，理论上可以最小化对第一类用户系统的干扰，Underlay技术采用了扩频技术来降低对主用户系统的干扰，考虑到认知用户感知能力有限，只能获得部分系统知识，混合技术应当优先考虑。总之，我们很难用一句话去总结哪种频谱共享技术最优，只能综合考虑性能与代价，进行合理评估，选择一种最合适的方案。MS TT 参考文献 [1]田峰.认知无线电频谱共享技术的研究 [D].南京：南京邮电大学，2007. [2]Akyildiz I F,Won-Yeol Lee.A survey on spectrum management in cognitive radio networks.IEEE Communications Magazine,2008,46(4):40-48. [3]Menon R,Buehrer R M,Reed J H.Outage probability based comparison of underlay and overlay spectrum sharing techniques[C].in:Proc.IEEE DySPAN2005,November 2005:101-109. [4]Etkin R,Parekh A,Tse D.Spectrum sharing for unlicensed bands[C].In:Proc.IEEE DySPAN2005,November 2005:251-258. [5]Salami G,Durowoju O.A Comparison Between the Centralized and Distributed Approaches for Spectrum Management.IEEE Communications Surveys&Tutorials,2010,(99):1-17. [6]Bany Salameh H A,Krunz M.Cooperative Adaptive Spectrum Sharing in Cognitive Radio Networks.IEEE/ACM Transactions on Networking,2010,18(4):1181-1194.
