A novel BeiDou weak signal acquisition scheme based on modified differential correlation

BeiDou signals are modulated with a secondary code of NH (Neumann–Hoffman) code to obtain a better positioning performance. The data bit rate increases to 1？kbps and bit transitions can occur in every 1 millisecond (ms) random sampling signal. As a result, the frequent bit transitions lead to an acquisition-sensitivity attenuation of classic integration algorithms. In order to improve acquisition sensitivity for BeiDou weak signals and resolve the problem that frequent bit transitions limit integration time, a novel BeiDou weak signal acquisition scheme based on modified differential correlation (M-DC) is proposed. First, conventional differential combination method is modified. The differential operation is moved from the post-correlator stage to the IF (intermediate frequency) samples to weaken the influence of data bit reversions. Second, an acquisition scheme implemented via fast Fourier transform is provided. Code phase and carrier frequency are estimated by one-dimensional search using fast Fourier transform. Third, power consumption and computation complexity of the proposed method are analyzed. Finally, Monte Carlo simulations and real data tests are conducted to analyze the performance of the proposed acquisition scheme. The results show that the proposed acquisition scheme can weaken the influence of data bit transitions to extend the integration time and enhance signal-to-noise ratio in weak signal environment. With the increase of accumulation time, the detection probability and acquisition sensitivity increase as well. The detection probability is 0.97 at CN0 (carrier-to-noise ratio) of 25？dB-Hz with an accumulation of 20？ms. This method is applicable to all kinds of BeiDou satellites and is also applicable to GPS, Galileo and other satellites navigation systems with or without secondary codes