Research On Acquisition For CPM Signals At Low SNR

At present satellite communication technology is relatively mature, and the binary phase shift keying is commonly used, however, dual limited power and bandwidth phenomenon makes new signal modulation technology become a research direction. In order to improve the validity and reliability of satellite communication, we should adopt efficient modulation technology. The continuous phase modulation(CPM) technique has excellent characteristics of constant envelope and phase continuity, which can guarantee the spectrum of CPM signals is not be extended when signals are amplified by the nonlinear power amplifiers. CPM technology is applied to satellite communication systems for its high power and bandwidth efficiency, such as in CPM signals, SOQPSK signal is recommended to be used in the military standard MIL-STD 188-181 of UHF satellite communication and standard IRIG 106- 04 of aviation remote measurement system. How to implement CPM signals acquisition under high-dynamic condition at low Signal-to-Noise Ratio(SNR) is a problem to be solved.This paper focuses mainly on satellite signal acquisition, which based on SOQPSK modulation and m sequence spread spectrum. Firstly, we introduce the format of the satellite signal, the CPM modulation technology with emphasis on the formation process and properties of SOQPSK signal and the theoretical basis of the spread spectrum that focuses on the direct sequence spread spectrum technology, as well as the generator and characteristic of m sequence. Secondly, acquisition principles based on maximum likelihood estimation is deduced, and its essence is search in two dimensions of Doppler frequency and code phase, based on which FFT-based acquisition method is been derived. The expressions of statistics for the COH, NCH and DFC are derivated mathematically. Through simulation, the performance of them is compared pointing out the effect of COH is best in improving the SNR gain. Then, to reduce the complexity of the system, a simplified computation method is proposed that can reduce the number of FFT operations significantly and the computation of the acquisition algorithm effectively. Doppler frequency and code phase are finely estimated, which makes the estimated frequency and code phase closer to the true value. Finally, it is present a dual domain acquisition method that combining circular correlated Double-Block Zero-Padding algorithm with improved differential accumulation method, which is not only able to overcome the impact of navigation data bits, but also able to work in low SNR. At the same time, the algorithm in this paper is simulated and compared with the algorithm in a literature. Under the same conditions, the acquisition time of this algorithm is seventh of which in literature and when the signal to noise ratio is less than-22 d B, acquisition probability of this algorithm gradually higher than that of literature method.