通信与电子信息科技相关的论文

1、 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 Dop

2、pler 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

3、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 effective

4、ly 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&CINTRODUCTIONThe main function of TT&C (Telemetry, Tracking and Command) system is ranging and velocity measur

5、ement. 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

6、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 r

7、eceived 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 ant

8、i-jamming is required by the future TT&C system. 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

9、 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

10、 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

11、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 car

12、rier 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

13、and effective for DS/FHSS TT&C system. Finally, some conclusions are drawn in section V.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 shoul

14、d 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 s

15、ubmerged 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 sp

16、ectrum 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 acq

17、uisition based on FFT which can extract the phase delay and carrier frequency at one time will be the best way for acquisition.CHARACTERISTIC OF DS/FHSS CARRIER TRACKING LOOPCompared with the carrier tracking loop in ordinarycommunication system, because of the high dynamic of the spacecraft, especi

18、ally 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 dehop

19、ping 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:(f i is equal to f i sub 0 plus f i sub d and f i sub d is equal to f i sub 0

20、 multiplied by v i over c)where 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 is the current speed of spacecraft. We can assume that the synchronization of frequency hopping pattern has been completed,

21、and the output frequency of local frequency synthesizer is , where is the frequency difference between 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. Acc

22、ording to the relation among the velocity, carrier frequency and Doppler frequency offset, the input frequency of carrier tracking loop is derived easily as follow: (f i sub in is equal to f i sub plus f i sub d and f i sub d is equal to f i sub 0 multiplied by v i over c)Then, between the interval

23、of the ith frequency and the (i+i)th frequency, the Doppler frequency agility is generated, and can be expressed as: (f i sub delta d is equal to square brackets f i plus one sub 0 multiplied by v i plus one minus f i sub 0 multiplied by v i square brackets over c)Generally speaking, we assume that

24、the velocity of spacecraft during two adjacent frequency wont change, i.e., so, which shows 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: where P i

25、s the carrier power after the synchronization of frequency hopping pattern, is the modulated data, is the intermediate frequency, and are the rudimental frequency offset and rudimental phase offset brought from acquisition module respective. otherwise , T is one hop dwell time, is the timing error o

26、f 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 the 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 fo

27、llow: (H s sub is equal to s sub 0 over s and is equal to one minus H s and is equal to s over s plus K multiplied F s)where, 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 derive the steady-state tracking erro

28、r. 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:(T sub is equal to delta 0 all squared over two multiplied by mult

29、iplied by cubed)where, is the initial frequency offset, and are the natural frequency and damping factor of the tracking loop. In the hybrid DS/FHSS TT&C system, just is the frequency agility which is a function of time according to the frequency hopping pattern. Thereby, three cases are discussed.

30、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 ag

31、ain 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 performance of loop. When , the tracking loop wont locked the signal forever.THE SCHEME OF CARRIER TRACKING LOOP AIDED BY HOPPIN

32、G 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 inte

33、rval 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 sig

34、nal, 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

35、is able to keep stable even on the high dynamic condition. Besides 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 d

36、epends 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 ca

37、rrier 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 b

38、ecome too severe to meet the resolution requirement of the TT&C system.载波跟踪混合DS /跳频扩频测控系统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

39、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

40、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

41、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。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

42、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。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