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1、Four short words sum up what has lifted most successful individuals above the crowd: a little bit more.-author-date通信专业的外文及外文翻译Research on Carrier Tracking in Hybrid DS/FH Spread Spectrum TT&C SystemResearch on Carrier Tracking in Hybrid DS/FH Spread Spectrum TT&C SystemAbstractBecause of the effect
2、 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 ce
3、aselessly 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
4、 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
5、 meets the requirement of TT&C system. Keywords：carrier tracking；DS/FHSS；frequency agility；aided；TT&C-IINTRODUCTIONThe 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 spr
6、ead 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
7、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 h
8、igh 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/FH
9、SS (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
10、, 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 ca
11、rrier 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 f
12、ollows. 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
13、 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 for DS/FHSS TT&C system. Finally, some conclusions are drawn in section
14、V.IIINPUT 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 communic
15、ation 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
16、 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
17、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
18、 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
19、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 recei
20、ved signal. Then, via the band pass filter, the output signal of mixer is fed into the acquisition module of PN 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
21、 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
22、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
23、 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 freq
24、uency 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 d
25、ecelerating, 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 whi
26、ch 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:where i is the sequence number of carrier frequency, is the ith carrier frequency , is the Doppler frequency offset duri
27、ng 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, and the output frequency of local frequency synthesizer is , where is the frequency difference between the received and local frequency, i.e., the
28、 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. According to the relation among the velocity, carrier frequency and Doppler frequency offset, the input frequency of carrier tracking loop is derived
29、 easily as follow: 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: Generally speaking, we assume that the velocity of spacecraft during two adjacent frequency wont change, i.e., so, which shows that the f
30、requency 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 is the carrier power after the synchronization of frequency hopping pattern, is the modulated data, is
31、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 of the synchronization of frequency hopping patterns, n(t) is the additive white Gaussian noise with tw
32、o-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 follow: where, F(s) is the transfer function of loop filter, K is the gain of open loop. Then we can app
33、ly the limit theorem, which is expressed as ,to derive 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 c
34、an be expressed as: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 discusse
35、d. 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
36、 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 performance of loop. When , the tracking loop wont locked the signal forever.The simulation result of 2ed order tracking loop
37、 used commonly in TT&C field is 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 capability of the loop is invalid entirely.Figur
38、e 2. Time response of tracking loop with Doppler offset agility: (a) No hopping, (b) TpTc ,(d) IV. 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
39、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 carri
40、er 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 fro
41、m 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
42、 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
43、 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.
44、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.V. SIMULATIOM
45、The 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.
46、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 S