A fast acquisition method of DSSS signals using differential decoding and fast Fourier transform

YANG Wei-jun(杨伟君)¹, ZHANG Chao-jie(张朝杰)^{1, 2}, JIN Xiao-jun(金小军)^{1, 2}, JIN Zhong-he(金仲和)^{1, 2}, XU Zhao-bin(徐兆斌)¹

(1. Department of Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310027, China;
2. Micro-Satellite Research Center, Zhejiang University, Hangzhou 310027, China)

Abstract:In low earth orbit (LEO) satellite or missile communication scenarios, signals may experience extremely large Doppler shifts and have short visual time. Thus, direct sequence spread spectrum (DSSS) systems should be able to achieve acquisition in a very short time in spite of large Doppler frequencies. However, the traditional methods cannot solve it well. This work describes a new method that uses a differential decoding technique for Doppler mitigation and a batch process of FFT (fast Fourier transform) and IFFT (invert FFT) for the purpose of parallel code phase search by frequency domain correlation. After the code phase is estimated, another FFT process is carried out to search the Doppler frequency. Since both code phase and Doppler frequency domains are searched in parallel, this architecture can provide acquisition fifty times faster than conventional FFT methods. The performance in terms of the probability of detection and false alarm are also analyzed and simulated, showing that a signal-to-noise ratio (SNR) loss of 3 dB is introduced by the differential decoding. The proposed method is an efficient way to shorten the acquisition time with slightly hardware increasing.

Key words:low earth orbit (LEO) satellite; spread spectrum; fast acquisition; fast Fourier transform (FFT); Doppler mitigation