Inner-Outer Synchronization Analysis of Two Complex Networks ...

Inner-Outer Synchronization Analysis of Two Complex Networks ...
Inner-Outer Synchronization Analysis of Two Complex Networks ...

Thread Synchronization Java 线程同步

Thread Synchronization https://www.360docs.net/doc/7b14528124.html,/insidejvm/ed2/threadsynch.html One of the strengths of the Java programming language is its support for multithreading at the language level. Much of this support centers on synchronization: coordinating activities and data access among multiple threads. The mechanism that Java uses to support synchronization is the monitor. This chapter describes monitors and shows how they are used by the Java virtual machine. It describes how one aspect of monitors, the locking and unlocking of data, is supported in the instruction set. Monitors Java's monitor supports two kinds of thread synchronization: mutual exclusion and cooperation. Mutual exclusion, which is supported in the Java virtual machine via object locks, enables multiple threads to independently work on shared data without interfering with each other. Cooperation, which is supported in the Java virtual machine via the wait and notify methods of class Object, enables threads to work together towards a common goal. A monitor is like a building that contains one special room that can be occupied by only one thread at a time. The room usually contains some data. From the time a thread enters this room to the time it leaves, it has exclusive access to any data in the room. Entering the monitor building is called "entering the monitor." Entering the special room inside the building is called "acquiring the monitor." Occupying the room is called "owning the monitor," and leaving the room is called "releasing the monitor." Leaving the entire building is called "exiting the monitor." In addition to being associated with a bit of data, a monitor is associated with one or more bits of code, which in this book will be called monitor regions. A monitor region is code that needs to be executed as one indivisible operation with respect to a particular monitor. In other words, one thread must be able to execute a monitor region from beginning to end without another thread concurrently executing a monitor region of the same monitor. A monitor enforces this one-thread-at-a-time execution of its monitor regions. The only way a thread can enter a monitor is by arriving at the beginning of one of the monitor regions associated with that monitor. The only way a thread can move forward and execute the monitor region is by acquiring the monitor. When a thread arrives at the beginning of a monitor region, it is placed into an entry set for the associated monitor. The entry set is like the front hallway of the monitor building. If no other thread is waiting in the entry set and no other thread

robust frequency and timing synchronization for OFDM

Robust Frequency and Timing Synchronization for OFDM Timothy M.Schmidl and Donald C.Cox,Fellow,IEEE Abstract—A rapid synchronization method is presented for an orthogonal frequency-division multiplexing(OFDM)system using either a continuous transmission or a burst operation over a frequency-selective channel.The presence of a signal can be detected upon the receipt of just one training sequence of two symbols.The start of the frame and the beginning of the symbol can be found,and carrier frequency offsets of many subchannels spacings can be corrected.The algorithms operate near the Cram′e r–Rao lower bound for the variance of the frequency offset estimate,and the inherent averaging over many subcarriers allows acquisition at very low signal-to-noise ratios(SNR’s). Index Terms—Carrier frequency,orthogonal frequency-division multiplexing,symbol timing estimation. I.I NTRODUCTION I N AN orthogonal frequency-division multiplexing(OFDM) system,synchronization at the receiver is one important step that must be performed.This paper describes a method to acquire synchronization for either a continuous stream of data as in a broadcast application or for bursty data as in a wireless local area network(WLAN).In both cases the receiver must continuously scan for incoming data,and rapid acquisition is needed.The ratio of the number of overhead bits for synchronization to the number of message bits must be kept to a minimum,and low-complexity algorithms are needed. Synchronization of an OFDM signal requires?nding the symbol timing and carrier frequency offset.Symbol timing for an OFDM signal is signi?cantly different than for a single carrier signal since there is not an“eye opening”where a best sampling time can be found.Rather there are hundreds or thousands of samples per OFDM symbol since the number of samples necessary is proportional to the number of subcarriers. Finding the symbol timing for OFDM means?nding an estimate of where the symbol starts.There is usually some tolerance for symbol timing errors when a cyclic pre?x is used to extend the symbol.Synchronization of the carrier frequency at the receiver must be performed very accurately, or there will be loss of orthogonality between the subsymbols. OFDM systems are very sensitive to carrier frequency offsets since they can only tolerate offsets which are a fraction of the Paper approved by M.Luise,the Editor for Synchronization of the IEEE Communications Society.Manuscript received April16,1996;revised February11,1997.This work was supported in part by a National Science Foundation Graduate Fellowship.This work was presented in part at the IEEE International Conference on Communications(ICC),Dallas,TX,June1996. T.M.Schmidl is with DSP Research and Development Center at Texas Instruments Incorporated,Dallas,TX75243USA(e-mail:schmidl@https://www.360docs.net/doc/7b14528124.html,). D. C.Cox is with the STAR Laboratory,Department of Electrical Engineering,Stanford University,Stanford,CA94305-4055USA(e-mail: dcox@https://www.360docs.net/doc/7b14528124.html,). Publisher Item Identi?er S0090-6778(97)09083-1.spacing between the subcarriers without a large degradation in system performance[1]. There have been several papers on the subject of synchro-nization for OFDM in recent years.Moose gives the maximum likelihood estimator for the carrier frequency offset which is calculated in the frequency domain after taking the FFT[2]. He assumes that the symbol timing is known,so he just has to ?nd the carrier frequency offset.The limit of the acquisition range for the carrier frequency offset is

解决noserversuitableforsynchronizationfound

解决 no server suitable for synchroni zation found rdate解决方案: 使用ntpdate的遇到这样的错误提示: no server suitable for synchronization found 很可能是防火墙封锁了udp的123端口, 如果关闭的防火墙问题依旧, 很可能是上层路由的设置有问题, 如果这种情况, 我们就只能通过tcp来更新时间啦 那肯定是rdate <!–more–> 查看时间服务器的时间: # rdate https://www.360docs.net/doc/7b14528124.html, 设置时间和时间服务器同步:

# rdate -s https://www.360docs.net/doc/7b14528124.html, 有什么不明白的, 请# man rdate 下面附送系列时间服务器的列表, 有的不一定可以使用哈 https://www.360docs.net/doc/7b14528124.html, https://www.360docs.net/doc/7b14528124.html, 216.118.116.105 https://www.360docs.net/doc/7b14528124.html, 202.106.196.19 https://www.360docs.net/doc/7b14528124.html,ntpdate-debian解决方案: 执行/usr/sbin/ntpdate https://www.360docs.net/doc/7b14528124.html,.tw 出现下述错误: 28 Dec 10:44:25 ntpdate[20348]: no server suitable for synchronization found 注: 于Ubuntu / Debian Linux / FreeBSD 都有看到类似状况.

NTPDate Debug sudo /usr/sbin/ntpdate -d https://www.360docs.net/doc/7b14528124.html,.tw 28 Dec 10:44:20 ntpdate[20348]: ntpdate 4.2.4p8@1.1612-o Tue Dec 8 22:21:54 UTC 2009 (1) transmit(11.11.11.11) transmit(11.11.11.11) transmit(11.11.11.11) transmit(11.11.11.11) transmit(11.11.11.11) 11.11.11.11: Server dropped: no data server 11.11.11.11, port 123 stratum 0, precision 0, leap 00, trust 000

同步网基本概念

一、基本概念 1)同步网(Synchronization Network)是一个提供同步参考信号的网络。是通过同步链路将同步网节点连接起来而形成的物理网。同步网节点由各级时钟构成。 2)网同步(Network Synchronization)是一个广义上的概念,用来描述在网络中将公共频率信号或时间信号传送到所有网元的方法。 3)同步的网(Synchronous Network)指这样一个网络,它的所有时钟在正常工作状态下,都具有相同的长期频率准确度。 4)同步单元(Synchronization Element):指为所连接的网络单元提供定时服务的时钟。包括符合G.811、G.812、G.813建议的时钟。这是一种广义上的定义,包括:基准时钟,即性能满足G.811建议的时钟。定时供给单元,即性能满足G.812建议的时钟,包括独立型和混合型定时供给单元。设备时钟,即各种设备中的时钟或同步单元,其性能满足G.812建议或G.813建议,例如交换机时钟和SDH设备时钟。 5)定时供给单元(SSU,Synchronization Supply Unit):一个逻辑功能单元,能够对参考信号进行选择、处理和分配,并且符合建议G.812规定的性能。定时供给单元可分为独立型定时供给单元和混合型定时供给单元。 欲进一步了解SSU相关要求的请进入。 6)独立型定时供给单元(SASE,Stand Alone Synchronization Equipment):是指能够完成对定时信号选择、处理和分配,并且具有自己的管理功能的独立设备。在北美,独立型定时供给单元又被称为通信楼定时供给系统(BITS,Building Integrated Timing System)。目前人们常说的同步网设备一般指SASE(即BITS)。由于我国同步网起步时主要参照北美标准,因此一直简称同步网设备为BITS。 7)混合型定时供给单元:是指能够完成对定时信号选择、处理和分配等功能,但是这些功能与其他功能结合在一套设备中。例如DXC设备时钟,具有G.812功能,但不是一套独立设备。但它可以做同步网设备使用。 8)同步网设备时钟:一般包括基准时钟和定时供给单元。其中,多数定时供给单元是独立型的,但有些定时供给单元是由设备时钟构成的,即混合型定时供给单元。 9)SDH设备时钟(SEC,SDH Equipment Clock):一般由晶体钟构成,其性能由G.813规范规定。当SDH设备处在通信枢纽或承载的业务量较大时,也可能采用G.812时钟。G.813规定了两类时钟,即I类为2.048Mbit/s系列(我国选用此类)和II类为1.544Mbit/s系列。G.783中规定了SDH设备的时钟功能,G.803中规定了SDH网定时参考链模型。

跳频通信系统中同步技术研究

跳频通信系统中同步技术研究 李娜 【期刊名称】《现代电子技术》 【年(卷),期】2011(034)001 【摘要】Synchronization is one of the key technologies of FH communication.The synchronization of frequency hopping is achieved by adopting synchronization head and time of day to meet the requirement of practical development of FH communication system.The method of frequency-hopping synchronization, the format of synchronization information and the capture of synchronization are studied, and the performance of synchronization is analyzed.The results show that the FH communication system has characteristics of short synchronization time, high capture probability and low false probability.%同步技术是跳频通信系统关键技术之一.针对跳频通信系统中同步的要求,采用同步字头与时间信息相结合的方法实现跳频同步.首先研究了跳频同步方法、同步信息格式和初始同步等问题,最后对同步性能进行了分析.结果表明,该跳频通信系统的同步时间短、捕获概率高、虚警概率低. 【总页数】3页(95-96,100) 【关键词】跳频通信;同步字头;时间信息TOD;同步方案;同步性能 【作者】李娜 【作者单位】广州海格通信集团北京海格神舟通信科技有限公司,北京,100070【正文语种】中文

On the Synchronization Techniques for Wireless OFDM Systems

On the Synchronization Techniques for Wireless OFDM Systems Bo Ai,Member,IEEE,Zhi-xing Yang,Chang-yong Pan,Jian-hua Ge,Yong Wang,Member,IEEE,and Zhen Lu Abstract—The latest research works on the synchronization scheme for either continuous transmission mode or burst packet transmission mode for the wireless OFDM communications are overviewed in this paper.The typical algorithms dealing with the symbol timing synchronization,the carrier frequency syn-chronization as well as the sampling clock synchronization are brie?y introduced and analyzed.Three improved methods for the ?ne symbol timing synchronization in frequency domain are also proposed,with several key issues on the synchronization for the OFDM systems discussed. Index Terms—Carrier frequency synchronization,continuous mode and burst packet mode transmission systems,OFDM, sampling clock synchronization,symbol timing synchronization. I.I NTRODUCTION O FDM,associated with other related technologies have found its wide applications in many scienti?c areas due to its high spectrum ef?ciency,its robustness against both multi-path and pulse noises,its highly reliable transmission speed under serious channel conditions,adaptive modulation for each sub-carrier according to the channel conditions, and etc.It has become fundamental technology in the future 4G-multimedia mobile communications systems[1]. Many digital transmission systems have adopted OFDM as the modulation technique such as digital video broadcasting terrestrial TV(DVB-T)[2],digital audio broadcasting(DAB), terrestrial integrated services digital broadcasting(ISDB-T), digital subscriber line(xDSL),WLAN systems based on the IEEE802.11(a)[3]or Hiperlan2,multimedia mobile access communications(MMAC),and the?xed wireless access(FW A) system in IEEE802.16.3standard.OFDM has also found its application in Cable TV systems.Technologies fundamentally based on OFDM,such as vector OFDM(V-OFDM),wide-band OFDM(W-OFDM),?ash OFDM(F-OFDM)have also shown their great advantages in certain application areas. There are some disadvantages,however,appeared in the OFDM systems,for example,the large Peak-to Average Power Ratio(PAPR)as well as high sensitivity to the synchronization errors.Synchronization issues are of great importance in all Manuscript received April26,2005;revised October27,2005.This work was supported in part by the National Natural Science Funds in China(Nos. 50177001,60372007,and60332030)and by the Ministry of Information Industry Foundation under Grant no.2002291. B.Ai is with the Dept.of E&E Tsinghua University,State Key Lab.on Microwave and Digital Communications,China(100084).He is also with the Engineering College of Armed Police Force,Xi’an,China(710086)(e-mail: abeffort_apple@https://www.360docs.net/doc/7b14528124.html,). Z.Yang and C.Pan are with the Dept.of E&E Tsinghua University,State Key Lab.on Microwave and Digital Communications,China(100084). J.Ge and Y.Wang are with the National key Lab.of Integrated Service Net-works,Xidian Univ.,Xi’an,China(710071). Z.Lu is with the Dept.of Electronic Engineering in Shanghai Jiaotong Uni-versity,China(200052). Digital Object Identi?er10.1109/TBC.2006.872990digital communications systems,especially in the OFDM systems.Synchronization errors not only cause inter-symbol interference(ISI)but also introduce inter-carrier interference (ICI)due to the loss of orthogonality among all sub-carriers. In this paper,we focus on the synchronization schemes in the OFDM systems.Fundamental theory for the synchronization is brie?y described in Section II and in Section III,the symbol timing scheme and three improved methods for the?ne symbol timing in frequency domain are proposed.We then conduct the analysis on the carrier frequency recovery as well as the sampling clock synchronization methods in Sections IV and V respectively.In Section VI,joint estimation of all the synchro-nization errors including timing,frequency and phase offsets is simply described.Technical forecast is made in Section VII with conclusions drawn in Section VIII. II.O VERVIEW FOR THE S YNCHRONIZATION IN OFDM S YSTEMS Synchronization is of great importance for all digital com-munication systems.OFDM systems are very sensitive to both timing and carrier frequency offset,especially,when combined with other multi-access techniques such as FDMA,TDMA,and CDMA.Therefore,synchronization is extremely crucial to the OFDM systems. A.Three Synchronization Issues in the OFDM Systems There are three major synchronization issues in the OFDM systems: a.The symbol timing synchronization,which is to deter- mine the correct symbol start position before the FFT de-modulation at the receiver end. b.The carrier frequency synchronization(i.e.,carrier fre- quency recovery technique),which is utilized to eliminate the carrier frequency offset caused by the mismatch from the local oscillators between the transmitter and the re-ceiver,nonlinear characteristic of the wireless channel as well as the Doppler shift. c.The sampling clock synchronization,which is to miti- gate the sampling clock errors due to the mismatch of the crystal oscillators. All these synchronization errors will signi?cantly degrade system performance[4],[5]. B.Synchronization Technologies in the Continuous Mode and Burst Packet Mode Transmission Systems Accurate synchronization is indispensable to suppress the negative impact of the synchronization errors in the commu-nication systems no matter,whether it is in continuous or burst packet mode transmission systems.However,these two different modes require different synchronization schemes: 0018-9316/$20.00?2006IEEE

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