Research On Carrier Tracking For DSSS Signal In Low SNR And High Dynamic Environment

With good anti-jamming and security performances, Spread-spectrum communication technology plays an important role in the modern Telemetry, Track And Command(TT&C) communication systems and is applied widely in the field of spaceflight. Concerning the research on the Spread-spectrum communication, tracking is a key technique to realize accurate synchronization. Since the high orbital maneuver flight at the high frequency band will causes severe Doppler effect, which mainly embodies in three aspects including great Doppler frequency shift, the first- and second-order derivatives. Meanwhile, the Signal to Noise Ratio(SNR) is very low at the receiver because the emission power is limited and the antenna gain is wasted. The coexistence of high dynamic and low SNR will greatly increase the difficulty of realizing the accurate carrier tracking.Traditional carrier tracking algorithms are mainly the trade-off between the dynamic tracking performance and anti-noise performance, and the case that larger tracking errors may exist or dynamically changed frequency shifts can not be tracked will arise in the condition of the coexistence of high dynamic and low SNR. This thesis puts a research on the carrier tracking technology of the Direct Spread Spectrum based on the synchronization in high dynamic and low SNR conditions. The specific research work and achievements are as follows:Firstly, according to the high accuracy of Phase Locked Loop(PLL) and strong dynamic ability of Frequency Locked Loop(FLL), traditional FLL-assisted-PLL algorithms just combines the two loops simply. To improve the tracking accuracy, this thesis adjusts the new loop with the weight of PLL and FLL by use of tracking error of each time, which is the FLL-assisted-PLL algorithm. The new loop, which adopts the structure of mixed loop filter, can adjust the weight dynamically according to the dynamic change of weights on real-time error. Simulation results show that compared with the conventional FLL-assisted-PLL algorithm, the new loop has a better performance on the tracking sensitivity in lower SNR and tracking accuracy in the same SNR.Secondly, through the research on the signal parameter theory, an adaptive tracking loop based on the square root Unscented Kalman Filter(UKF) in view of the kalman filter is come up with. Abandoning the discriminator module of the traditional loop, the new loop directly treats the coherent integration results among the received signal and the local signal as the measurement of the filter, and introduces the adaptive fading factor ultilizing the innovation sequence in the filter recursive estimation process and updating the noise matrix iteratively, so it can achieve the best estimation performance. In the simulation part, the new loop has a better performance than the FLL-assisted-PLL algorithm based on the weight dynamically adjustment and the square root UKF on the tracking sensitivity and accuracy, which indicates its good performance at the scenario of high dynamic and low SNR.