对载波跟踪混合DS 跳频扩频测控系统的研究.doc
对载波跟踪混合DS 跳频扩频测控系统的研究【中文4565字】
收藏
资源目录
压缩包内文档预览：(预览前20页/共22页)
编号：6099432
类型：共享资源
大小：414.96KB
格式：RAR
上传时间：2017-11-16
上传人：闰***
认证信息
个人认证
冯**（实名认证）
河南
IP属地：河南
13
积分
- 关 键 词：
-
对于
载波
跟踪
混合
ds
跳频扩频
测控
系统
研究
钻研
中文
- 资源描述：
-
对载波跟踪混合DS 跳频扩频测控系统的研究【中文4565字】,对于,载波,跟踪,混合,ds,跳频扩频,测控,系统,研究,钻研,中文
- 内容简介：
-
【中文 4565 字】对载波跟踪混合 DS /跳频扩频测控系统的研究摘要由于载波频率调频的影响，DS/FHSS（直接序列/ 跳频扩频）TT&C（测控）系统的载波跟踪回路的输入信号具有多普勒频移灵活的特征。跟踪环路将转移到频率不断阶跃响应状态并且测量分辨率严重衰退, 即使循环可能是开启的。本文提出一种跟踪环路藉由承运人跳频模式。为了保持这个跟踪环路的稳定性，在下一跳频住的多普勒频移敏捷性事估算的并且根据预设跳频模式和通用航天器速度及时补偿载体的数控振荡器的频率调整器。仿真结果表明该方法有效地清除了由于载波跳频引起的不稳定性，这种回路方法满足测控系统的使用要求。关键字：载体跟踪；DS / FHSS；频率敏捷；辅助；测控01对载波跟踪混合 DS /跳频扩频测控系统介绍测控（跟踪，遥测和系统的指令)系统的主要功能是排列和速度测量。目前，最常用的测控系统是 单位载波系统和 单位扩频系统。对于单位载波系统，排列是通过测量被发送和接收的音调之间的相位差来实现的；对于单位扩频系统，根据自相关性质的伪码, 排列是通过测量这接收和当地pseudonoise(伪)代码之间的相位延迟来实现的。两个测控系统之间的速度测量依赖于提取发送和接收的载体之间的 多普勒现象所产生的多普勒频差。而上述所有的 程序是根据高分辨率的载体跟踪来完成的, 而且锁相环是程序进行测控系统最常见的方法。由于电磁环境的空间变得越来越复杂,对于将来的测控系统具有较好的抗干扰能力是有必要1。所以我们考虑用这种混合DS/跳频扩频技术来建造一种更强健的测控系统。对于许多普通的 混合DS / FHSS通信系统,最重要的功能是解调数据而不测量，所以是没有必要测量出的载波频率。但是，在这个混合DS / FHSS系统里精确地测量和跟踪载体是系统的基础，所以一些特殊的问题是需要解决的。在复合DS / FHSS测控系统, 甚至收到的信号同步模式dehopped模块,由于多普勒效应和载波调频技术，跟踪回路的输入频率严重地含有频率捷变。结果，这个回路可能反复地移动它的频率阶跃响应状态，对于频率测量载波跟踪这似乎是不可能的。摘要组织如下：在第一段,跳频模式同步模块在 DS / FHSS测控系统作了简要介绍。在第二段，我们将分析载体调频技术如何影响载体跟踪回路。在第一段，载体跟踪环路藉由跳频模式并且通用航天器速度是提议的；因为测控系统的实际要求，其仿真模式被建造并且仿真结果表明，对于测控系统这种方法是非常简单又有效的。最后在第一段引入了一些结论。2载体跟踪环路的输入信号像传统的测控和通信系统, 载波跟踪回路的输入信号一定是个单调的中频信号, 所以收到的射频信号应该被跳频模式同步模块dehopped。在FH通信系统里，在跳跃时间内信号是窄带信号并且一般得功率检测器通常被用于检测跳频信号2。但是在复合DS / FHSS测控系统内，这信号是潜伏在噪声中，这就不可能通过功率检波器如跳频通信系统直接获取信号。然而,在这系统内信号在跳的停留时间期间只是直接序列扩频信号, 所以我们可以获得基于获得直接序列扩频信号的信号。这种获取方法，例如连续-搜索获取，并行获取和快速获取在傅立叶1变换的基础上,讨论了大量的文献3 - 5,这样我们就不对这一问题进行详细的讨论。在我们的系统中,因为一个跳跃停留的时间很短,但基于傅里叶变换的快速捕获,可以提取相位延迟和载波频率，在一段时间内对于捕获这将会成为最好方法。跳频的方案,也就是粗同步,可以表现为图1。图1.跳频模式同步的方案跳频模式的同步是由同步跳频模式是由局部频率合成器快速搜索和二维快速获取直接序列伪码相位和载波频率实现的。开始，链接开关在位置1,局部频率合成器的输出信号，比接收信号更高得跳跃速度，与接收信号相混合。然后，通过带通滤波器，混合器的输出信号输入了采集模块的伪码和载波。如果在采集模块中相关器的输出数据低于预设阈值, 在这跳跃停留的时间和局部频率合成器设置的下个频率下，直接序列扩频信号是不能获得的。相比之下，如果发现采集模块变量超过预设阈值,它的意思是,跳频信号以获取并且混合器输出了一个稳定的区域扩频信号。之后，开关在位置2，局部频率合成器根据跳频模式及时改变输出频率。如粗同步所提到的,已经被dehopped的DS / FHSS信号被提供给伪码跟踪环并且在收到的伪码和局部伪码的精准是由一个代码跟踪环路实现的即delay-locked循环。最后，代码跟踪环路的输出信号,即收到的伪码的副本,混合着被粗同步dehopped的中频直接序列扩频信号，用于载体跟踪环路的单调的中频窄带信号获取到了。3. DS / FHSS载波跟踪环路的特征在普通的通信系统中与载波跟踪环路相比, 由于航天器的极高的动态,特别是在着陆时,加减速,混合DS / FHSS测控系统的载波跟踪环路受多普勒效应 (达100KHz)的影响会更严重的。除了那些 ,多普勒频率捷变还由载波频率跳频没有消除跳频载体被dehopping造成的,这是影响DS / FHSS系统载波跟踪环路性能的主要因素。DS / FHSS测控系统下行信号的频率可以描述为:2)(1)()(000 ivfcififiifdi是载波频率的序列号， 是电流载波频率，在电流跳跃停留时间的期间是多普勒频率偏移和 是航天船当前的速度。我们可以假定同步跳频模)(fd )(iv式已经完成,局部频率合成器的输出频率为： , 是接收到)()(ifiifol 的和局部频率的频差，也就是，载波跟踪环路输入信号的中频。通过中频带通滤波器，这个中频信号，就是 ，就这样获取了。)(if根据速度的关系，载波频率和多普勒频移，这个载波跟踪环路的输入频率如下所示： )(1)()(0ivfcififififdn 在ith频率和(i+i)th频率的间隔之间,产生了多普勒频率捷变 ，可以表示为：d)()1(1)(00ivfivfcifd 一般来说，我们假设在两个相邻频率的航天器速率不变，即 ，所以)(1ivi，这个解释了频率捷变在两个相邻跳跃的中频的功能，还有)(1)(0ifvcifd航天器当前的速度。载波跟踪环路的输入信号可以表示为： )()()(122sin)()( 0 tnnTtpfnvctftftRPts ab P是载波频率， 是已调置数据， 是中频， 和 是初始频偏和初始相位tfdf偏移，如果 否则 .T是跳跃停留时间，O是定时误差，n(t) ;1)(,0p0)(tp附加性双边带N/2高斯白噪声的功率谱密度,c是光速。跟踪器的分辨率是这个环路性能基本的描述，我们可以通过误差传递函数获得，如下： )()(1)(0 sKFsHsHF(s)是环路滤波器的传递函数，K是开环增益，我们可以应用极限原理，如 )()(0100lims从而获得稳态跟踪误差。不幸的是，这个推导中的拉普拉斯转移 是不可能3的，所以我们准确的计算测量误差，仅仅通过仿真来分析。对于第二个顺序环路，其采集时间乐意表示为： 320nT是初始频偏， 和 分别是固有频率和阻尼系数，对于复合DS / FHSS测控0n系统 只是频率捷变的对于跳频模块的时间函数，从而有三种讨论情况：情况1：TpTc,即跳跃停留时间小于环路采集时间，在采集循环状态，输入信号的频率可能突然提高,然后环路再一次进入采集循环状态。对这些情况,追踪环路一遍又一遍地进入采集状态。情况3：对于non-ideal 2ed和高度顺序环路，采集带宽 是限制的，跳频捷变p影响环路的性能，当 ，)(ifd )(ifdp)(ifdp这个载波环路永远不会到达闭合状态。这个2ed 顺序跟踪环路的仿真结果通常如图 2所示，虚线表示多普勒捷变，实线表示时间响应。a图表示没有多普勒频偏捷变时的性能；当Tp ，这环路的跟踪)(ifdp能力是无效的。图2. 多普勒偏移敏捷跟踪回路的反应时间:(a) No hopping, (b) TpTc ,(d) )(ifdp44跳跃模式载波跟踪环路的方案跳跃频率模式载波跟踪环路的结构如图3。一般来说,我们可以假定在两个相邻的频率之间速度之间的间隔时间保持定值, 那么在接下来的频率间隔多普勒频率偏移量 与载波频率相结合可以计算出太空船的流速。当下个频率信号进入到环路时， 及时增加载波NCO的调节系数值。所以 NCO的输出频率改变同步，如同输入信号的频率改变的和循环保持稳定。应该提到,在环路走到稳定状态时,在计划中的宇宙飞船速度被用于采集模块。之后,被锁状态,然后流速应该从循环本身中直接提取。通过这种方法,循环能够保持稳定甚至高动态条件。图3.跳频模式载波跟踪环路除了热噪声的抖动,载体跟踪环路的主要误差受到跳频形态频率合成器频率的跳动和由于频率模式同步化的时序错误。前者取决于频率合成器的分辨率和其他的通信，我们只讨论后者。简单地说,对于接收信号当局部频率合成器的局部频率改变是提前或延迟, 在错误的时间里每位模块将会提供一个频率偏移给载波NCO ，循环将一步步走向开启状态,即频率阶跃响应。幸运的是,当输入信号的频率变得确定,循环将会迅速回到稳态。但是随着同步错误的增加,它也有可能变得太严重而不能满足该测控系统的需求。5仿真该混合DS / FHSS系统的载波跟踪环路模型显示在图3,这是建立在Matlab下进行的。这跟踪环路是标准的科斯塔斯环路，通常用于测控领域,就是能消除推理起因极性变化的调制的数据9。适应由于飞船运动而产生多普勒频移变化,循环设计为第二个顺序环路,循环过滤器为第一个顺序的过滤器。仿真参数设置根据实际测控任务如下:载波频率：2.2GHz 2.3GHz；频率总合：128 ；跳频模式:基于m-sequence；5获取后的初始频率偏移:300Hz；载波跟踪环路的中频:4.8MHz；采样频率: 16.3Mbps；环路的噪声带宽:10Hz；A匀速运动和匀加速运动的时间响应我们认为宇宙飞船速度7.9km / s,多普勒频率之间的关系,载波频率和速度、回路的输入信号的频率偏移获得如图4(a)。最大的频率捷变是2.3KHz 。没有辅助的时间响应显示在图4(b)和一个在跳跃模式下有辅助的时间响应显示在图4(c)。结果表明,没有辅助的环路完全被释放,而有辅助的环路可以准确追踪载波。当宇宙飞船在加速运动(初始速度7.9km / s、加速度30g),时间响应如图5。获得了如pre-paragraph同样的结论。图4.匀速运动下的时间响应：(a) 多普勒频率，(b) 没有人工辅助，(c) 有人工辅助图5.匀加速运动下的时间响应：(a) 多普勒频率，(b) 没有人工辅助，(c) 有人工辅助B不同跳跃速度下跟踪器分辨率在仿真里，,通过计算方差得到了载波跟踪环路的分辨率。跟踪器分辨率与跳跃速度之间的关系如图6，在不同的输入的信噪比和在测控系统确保解调正确6的最小值是13分贝。仿真结果证实对于跳跃速度这个分辨率是不敏感的,对于不同的跳跃速度这个方案具有较好的健壮性。图6.Stead-state 跟踪器分辨率与跳跃速度C频率模式同步化下不同的定时误差的跟踪器分辨率对载波跟踪环路受到跳频模式,基于上述讨论可知影响环路稳定性的主要因素是模式同步化定时误差。图7显示在不同的输入信噪比和不同的模式同步化定时误差的stead-state 跟踪精度。测量误差随着定时误差的增加而增加,信噪比造成的测量误差甚至可以被忽略，当定时误差是由一些指定的值。因此,我们可以推断跟踪器准确性不会满足测控系统的所需,以后这些问题需要进一步研究。图7.Stead-state 跟踪器分辨率与模式同步定时误差6. 结论在复合DS / FHSS测控系统中,基本多普勒频率捷变导致载波跟踪环路不断地持有频率阶跃响应, 所以为了测量距离和速度而精确地提取多普勒频率偏移是很困难的。通过分析频率捷变对跟踪器的影响的性能,跳频模式跟踪器和当前的宇宙飞船速度模型是当前的。一个补偿的频率是添加跟踪环路，作为载波跳频,该方法的准确性，通过仿真已经验证了。参考文献7参考文献1L.Simone, N.Salerno,and M. Maffei, “Frequency-Hopping Techniques for Secure Satellite TT&C: System Analysis & Trade- Offs”, Satellite and Space Communications, 2006 International Workshop on , Sept.2006, pp.13-17, dio:10.1109/WSSC.2006.255980. 2Don.Torrieri,Frequency-Hopping Communication Systems,Amy research laboratory, Mar.2003. 3M.K.Simon,J.K.Omura,Robert A.Scholtz and Barry K.Levitt,Spread Spectrum Communication Handbook. Boston: McGraw-Hill, 2003. 4D.Akopian,“Fast FFT based GPS satellite acquisition Methods,” Proc.Inst. Elect. Eng. Radar, Sonar, Navig., vol. 152, no. 4, pp.227-286, Aug.2005. 5S.Yoon, I.Son, and S.Y.Kim,“Code acquisition for DS/SS communications in non-Gaussian impulsive channels,” IEEE Trans. Commun., vol. 52, no.5, pp.909-919, May.2005. 6Roland E.Best, Phase-Locked Loops: Design, Simulation, and Application (5th Edition). Boston: McGraw-Hill, 2003. 7S.Hinedi and B.Shah, “Acquisition Performance of Various QPSK Carrier Tracking Loops,” IEEE Trans.Commun., vol.40, no.9, pp.1426-1429, Sep.1992. 8I.N.Psaromiligkos, S.N Batalama, and M.J.Medley, “Rapid combined synchronization/demodulation structures for DS-CDMA systems.I.algorithmic developments,” IEEE Trans. Commun., vol.51, no.6, pp.983-994,June 2003. 9Elliott D.Kaplan,UNDERSTANDING GPS Principles and Applications. Artech House, 1996.AbstractResearch on Carrier Tracking in Hybrid DS/FH Spread Spectrum TT&C SystemAbstractBecause of the effect of carrier frequency hopping, the input IF signal of carrier tracking loop in DS/FHSS (Direct Sequence/Frequency Hopping Spread Spectrum) TT&C (Telemetry, Tracking & Command) System is characterized by the Doppler frequency agile. The tracking loop will shift to the frequency step response state ceaselessly and the measurement resolution severely decline, even the loop is likely to be unlocked. This paper presents a carrier tracking loop aided by frequency hopping pattern. In order to keep the stability of the tracking loop, the Doppler frequency agility in the next frequency hopping dwell is estimated and timely compensated to the frequency adjustment of carrier NCO according to the preset frequency hopping pattern and current spacecraft velocity. Simulation results show that this method effectively eliminates the instability due to carrier frequency hopping, and the resolution of loop meets the requirement of TT&C system. Keywords：carrier tracking；DS/FHSS；frequency agility；aided；TT&CResearch on Carrier Tracking in Hybrid DS/FH Spread Spectrum TT&C System0IINTRODUCTIONThe main function of TT&C (Telemetry, Tracking and Command) system is ranging and velocity measurement. Presently, the most common used TT&C systems are unit carrier system and unit spread spectrum system. For the unit carrier TT&C system, ranging is realized by measuring the phase difference between transmitted and received tones, and for the unit spread spectrum TT&C system, according to the autocorrelation properties of PN code, ranging is realized by measuring the phase delay between the received and local pseudonoise (PN) code. Velocity measurement in both of TT&C systems depends on extracting the frequency difference resulting from the Doppler phenomena between the transmitted and received carrier. While all the processes mentioned above are finished on the ground of high resolution carrier tracking, and the phase lock loop is the common used method to implement it in TT&C system. As the space electromagnetism environment become more and more complicated, the capability of anti-jamming is required by the future TT&C system 1. So we consider using the hybrid DS/FHSS (Direct Sequence/Frequency Hopping Spread Spectrum) technology to build a more robust TT&C system. For many ordinary hybrid DS/FHSS communication systems, the most important function is demodulating data but not measuring, so it is not necessary to measure the carrier frequency precisely. However, in hybrid DS/FHSS TT&C system, measuring and tracking the carrier precisely is the foundation of system, so some special problem needs to be solved. In the hybrid DS/FHSS TT&C system, even the received signal has been dehopped by the pattern synchronization module, due to the Doppler Effect and carrier frequency hopping, the input frequency of tracking loop contains frequency agility severely. As a result, the loop is likely to shift to the frequency step responses state again and again, and it seems to be impossible for frequency measurement and carrier tracking. The paper is organized as follows. In section I, the frequency hopping pattern synchronization module in the DS/FHSS TT&C system is introduced. In section II, we analyze how the carrier frequency hopping influences the performance of the carrier tracking loop. In section III, a carrier tracking loop aided by frequency hopping pattern and current spacecraft velocity is proposed. In section IV, a simulation mode on the ground of actual requirement of TT&C system is built and the results of simulation show that this method is very simple and effective Research on Carrier Tracking in Hybrid DS/FH Spread Spectrum TT&C System1for DS/FHSS TT&C system. Finally, some conclusions are drawn in section V.II INPUT SIGNAL OF CARRIER TRACKING LOOPAs the traditional TT&C and communication system, the input signal of carrier tracking loop must be a monotonous intermediate frequency signal, so the received RF signal should be dehopped by the frequency hopping patternsynchronization module. In FH communication system, the signal during a hop dwell time is a narrowband signal and the general power detector is commonly used to detect the frequency hopping signal 2. But in the hybrid DS/FHSS TT&C system, the signal is submerged in the noise, it is impossible to acquire signal directly by power detector such as FH communication system. However, the signal during a hop dwell time in the system just is a direct sequence spread spectrum signal, so we can acquire it based on the acquisition of direct sequence spread spectrum signal. The acquisition methods, such as serial-search acquisition, parallel acquisition and rapid acquisition based on FFT have been discussed in a lot of papers 3-5, so we wont discuss the problem detailedly in this paper. In our system, since one hop dwell time is very short, the rapid acquisition based on FFT which can extract the phase delay and carrier frequency at one time will be the best way for acquisition. The scheme of the frequency hopping patters acquisition, i.e., coarse synchronization, could be shown as Fig 1.Figure 1. Scheme of frequency hopping pattern synchronizationThe synchronization of frequency hopping pattern is realized by the local frequency synthesizer rapid searching and the two dimension rapid acquisition of Direct Sequence PN code phase and carrier frequency. At the beginning, the link switch is on the location 1, and the output signal of local frequency synthesizer with higher hop speed than the received one is mixed with the received signal. Then, via the band pass filter, the output signal of mixer is fed into the acquisition module of Research on Carrier Tracking in Hybrid DS/FH Spread Spectrum TT&C System2PN code and carrier. If the output of correlator in acquisition module is less than the preset threshold, the direct sequence spread spectrum signal is not acquired during this hop dwell time and the local frequency synthesizer steps the next frequency. By contrast, if detection variable of acquisition module is more than the preset threshold, it means that the frequency hopping signal is acquired and the mixer outputs a stable district spread spectrum signal. After that, the switch is on the location 2 and the local frequency synthesizer will timely change the output frequency according to the frequency hopping pattern. After the coarse synchronization mentioned above, the DS/FHSS signal have being dehopped is fed to PN code tracking loop and a fine alignment between the received PN code and local PN code is achieved by a code tracking loop namely the delay-locked loop. Then, the output of code tracking loop, i.e., a duplicate of received PN code, is mixed to the IF direct sequence spread spectrum signal dehopped by coarse synchronization, and a monotonous intermediate frequency narrowband signal which will be fed to carrier tracking loop is obtained.III. CHARACTERISTIC OF DS/FHSS CARRIER TRACKING LOOPCompared with the carrier tracking loop in ordinarycommunication system, because of the high dynamic of the spacecraft, especially during the landing, accelerating and decelerating, the carrier tracking loop of hybrid DS/FHSS TT&C system will be influenced more severely by the Doppler Effect (up to 100KHz). Addition to that, a Doppler frequency agility resulted from the carrier frequency hopping wont be eliminated by dehopping the frequency hopping carrier, and which becomes the main factor influencing the performance of carrier tracking loop in DS/FHSS TT&C system. The frequency of downlink signal of DS/FHSS TT&C system may be described as: )(1)()(000 ivfcififiifdwhere i is the sequence number of carrier frequency, is the ith carrier frequency , is the Doppler frequency offset during the ith hop dwell time and )(fdis the current speed of spacecraft. We can assume that the synchronization of )(ivfrequency hopping pattern has been completed, and the output frequency of local Research on Carrier Tracking in Hybrid DS/FH Spread Spectrum TT&C System3frequency synthesizer is , where is the frequency difference )()(ifiifol )(ifbetween the received and local frequency, i.e., the intermediate frequency of input signal of carrier tracking loop. Passing a IF band pass filter, a IF signal, the frequency of which is , is obtained. )(ifAccording to the relation among the velocity, carrier frequency and Doppler frequency offset, the input frequency of carrier tracking loop is derived easily as follow: )(1)()(0ivfcififififdn Then, between the interval of the ith frequency and the (i+i)th frequency, the Doppler frequency agility is generated, and can be expressed as: )(ifd )()1(100ivfivfcGenerally speaking, we assume that the velocity of spacecraft during two adjacent frequency wont change, i.e. , so , which )(ii )(1)(0ifcifdshows that the frequency agility is a function of the frequency difference of two adjacent hop and the current speed of spacecraft. Then, the input signal of the carrier tracking loop can be expressed as: )()()(122sin)()( 0 tnnTtpfnvctftftRPts ab where P is the carrier power after the synchronization of frequency hopping pattern, is the modulated data, is the intermediate frequency, and are the )(tRf dfrudimental frequency offset and rudimental phase offset brought from acquisition module respective. otherwise , T is one hop dwell time, ;1)(,0tpt 0)(tpis the timing error of the synchronization of frequency hopping patterns, n(t) is the additive white Gaussian noise with two-side power spectral density W/Hz and c is 2Nthe velocity of light. The tracking resolution is the basic description of the loop performance, and we can obtain it by the error transfer function as follow: Research on Carrier Tracking in Hybrid DS/FH Spread Spectrum TT&C System4)()(1)(0 sKFsHsHwhere, F(s) is the transfer function of loop filter, K is the gain of open loop. Then we can apply the limit theorem, which is expressed as ,to )()(0100limsHsderive the steady-state tracking error. Unfortunately, the derivation of Laplacian transfer of is seen to be impossible, so we cant calculate the measuring error precisely and only analyze it by simulation. For the 2edorder loop, the acquisition time can be expressed as: 320nTwhere, is the initial frequency offset, and are the natural frequency and 0damping factor of the tracking loop. In the hybrid DS/FHSS TT&C system, just is 0the frequency agility which is a function of time according to the frequency hopping pattern. Thereby, three cases are discussed. Case 1: TpTc, i.e., hop dwell time is less than the loop acquisition time. During the acquisition state of loop, the frequency of input signal is likely to step up suddenly, and then the loop steps to the acquisition state once again. For the case, the tracking loop will step to acquisition state again and again for all time. Case 3: For the non-ideal 2ed or high-degree order loop, the acquisition band is limited, and the hopping frequency agility also influences the p )(ifdperformance of loop. When , the tracking loop wont locked the signal forever.The simulation result of 2ed order tracking loop used commonly in TT&C field Research on Carrier Tracking in Hybrid DS/FH Spread Spectrum TT&C System5is shown in Fig 2. The Doppler agility is plotted by broken line and the time response is denoted by real line. Fig. 2(a) shows the tracking performance without Doppler offset agility; the time response as Tp , the tracking )(ifdpcapability of the loop is invalid entirely.Figure 2. Time response of tracking loop with Doppler offset agility: (a) No hopping, (b) TpTc ,(d) )(ifdpResearch on Carrier Tracking in Hybrid DS/FH Spread Spectrum TT&C System6IV. THE SCHEME OF CARRIER TRACKING LOOP AIDED BY HOPPING PATTERNThe structure of the carrier track loop aided by the hopping frequency pattern is shown in Fig 3. Generally speaking, we can assume that the velocity during the interval time between two adjacent frequency will keep a fixed value, then the doppler frequency offset in the next frequency interval can be calculated by the current velocity of spacecraft combined with carrier frequency. The is added timely to the adjustment value of the carrier NCO when the new frequency signal is fed to the loop. So the output frequency of NCO also changes synchronal as the frequency changing of input signal, and the loop keeps stable. Deserve to mentioned, before the loop stepped to steady state, the spacecraft velocity used by the scheme is given from the acquisition module. After having being locked state, then the velocity should be extracted from the loop itself directly. By this way, the loop is able to keep stable even on the high dynamic condition. Figure 3. Carrier tracking loop aided by frequency hopping patternBesides the thermal noise jitter, the main error of carrier tracking loop aided by the frequency hopping pattern is the frequency jitter of the frequency synthesizer and timing error due to frequency pattern synchronization. The former one depends on the resolution of frequency synthesizer as other communication and we only discuss the latter one. Briefly, when the local frequency changing of the local frequency synthesizer is advanced or retarded to the one of receive signal, the aiding module will provide a frequency offset to the carrier NCO at the wrong time and the loop will step to the unlocked state at once, i.e., response of frequency step. Fortunately, when the frequency of input signal changes actually, the loop will return to the steady state rapidly. But as the increase of synchronization error, it also be likely to become too severe to meet the resolution requirement of the TT&C system.Research on Carrier Tracking in Hybrid DS/FH Spread Spectrum TT&C System7V. SIMULATIOMThe model of carrier tracking loop of hybrid DS/FHSS system is shown in Fig 3, which is built in the simulink of Matlab. The tracking loop is the standard costas loop commonly used in the TT&C field, which is able to eliminate the inference resulted form the polarity change of the modulated data 9. To adapt the Doppler frequency change due to the spacecraft movement, the loop is designed as a 2ed order loop, and the loop filter is a 1st order filter. The simulation parameter is set according to the actual TT&C task as follows: Carrier frequency: 2.2GHz2.3GHz Amount of frequencies: 128 Frequency hopping pattern: based on m-sequence Rudimental frequency offset after acquisition: 300Hz Intermediate frequency of the carrier tracking loop: 4.8MHz Sampling frequency: 16.3Mbps Noise Bandwidth of the loop: 10Hz A. The time response on uniform motion and uniformly accelerated motionWe assume the spacecraft speed is 7.9km/s, by the relation among the Doppler frequency, carrier frequency and velocity, the frequency offset of the input IF signal of loop is obtained as Fig 4(a). The max frequency agility is up to 2.3KHz. The time response without aid is shown in the Fig 4(b) and the one with aid by hopping pattern is shown in Fig4(c). The results show that the loop without aid is unlocked completely, while the one with aid can track the carrier accurately. When the spacecraft is on the uniformly accelerated motion (the initial speed is 7.9km/s, and speed accelerator is 30g), the time response is shown in Fig 5. The same conclusion is obtained as pre-paragraph. Research on Carrier Tracking in Hybrid DS/FH Spread Spectrum TT&C System8Figure 4. The time response on uniform motion: (a) doppler frequency,(b)without aid, (c) with aid.Figure 5. Time response on uniformly accelerated motion:(a) doppler frequency,(b)without aid (c) with aidB. Tracking resolution on different hopping speedIn this simulation, the resolution of carrier tracking loop is obtained by calculating variance. The relation between tracking resolution and hopping speed is shown in Fig 6 on different input SNR and the minimum value insuring the demodulating correctly in TT&C system is 13 dB. The result of simulation testified that the resolution is not sensitive to the hopping speed and the scheme is very robust for different hopping speed.Research on Carrier Tracking in Hybrid DS/FH Spread Spectrum TT&C System9Figure 6. Stead-state tracking resolution vs hopping speedC. Tracking resolution on different timing error of frequency pattern synchronization For carrier tracking loop aided by the frequency hopping pattern, according to the above discussion the main factor impacting the stability of loop is the timing error caused by the patterns synchronization. Fig 7 shows the stead-state tracking accuracies on different timing error of synchronization pattern on different input SNR. The measuring error is increase as increasing of timing error and the measurement error resulted from the SNR even can be ignored when the time error is up to some specified value. Consequently, we can infer that the track accuracy wont meet the requirement of TT&C system finally, and the problem needs to be researched in the future.Figure 7. Stead-state tracking resolution vs timing error of pattern synchronizationResearch on Carrier Tracking in Hybrid DS/FH Spr
- 温馨提示:
1: 本站所有资源如无特殊说明，都需要本地电脑安装OFFICE2007和PDF阅读器。图纸软件为CAD,CAXA,PROE,UG,SolidWorks等.压缩文件请下载最新的WinRAR软件解压。
2: 本站的文档不包含任何第三方提供的附件图纸等，如果需要附件，请联系上传者。文件的所有权益归上传用户所有。
3.本站RAR压缩包中若带图纸，网页内容里面会有图纸预览，若没有图纸预览就没有图纸。
4. 未经权益所有人同意不得将文件中的内容挪作商业或盈利用途。
5. 人人文库网仅提供信息存储空间，仅对用户上传内容的表现方式做保护处理，对用户上传分享的文档内容本身不做任何修改或编辑，并不能对任何下载内容负责。
6. 下载文件中如有侵权或不适当内容，请与我们联系，我们立即纠正。
7. 本站不保证下载资源的准确性、安全性和完整性, 同时也不承担用户因使用这些下载资源对自己和他人造成任何形式的伤害或损失。
人人文库网所有资源均是用户自行上传分享，仅供网友学习交流，未经上传用户书面授权，请勿作他用。
|
2:不支持迅雷下载,请使用浏览器下载
3:不支持QQ浏览器下载,请用其他浏览器
4:下载后的文档和图纸-无水印
5:文档经过压缩，下载后原文更清晰
|
川公网安备: 51019002004831号