An Unaided Scheme for BeiDou Weak Signal Acquisition

NH code modulation in BeiDou signals increases the difficulty and calculation of acquisition as the data bit transitions are more frequent. And for weak or weak signal acquisition, NH code phase need to be determined to complete the long coherent and non-coherent integration to improve the acquisition sensitivity, which results in undesirable calculation burden. Double block zero padding (DBZP) method, which combines the superiority of parallel code phase search and parallel frequency space search together with Fast Flourier Transform (FFT), has great potential in simplifying the calculation for conventional GNSS weak signal acquisition. However, traditional DBZP is very sensitive to bit transition and the misalignment of bit edge can cause serious power loss. A DBZP-based acquisition approach is proposed in this paper to deal with the NH code in BeiDou weak signal acquisition and further improve the efficiency. Firstly zero pad operators and Viterbi algorithm are applied to eliminate interference caused by NH code and navigation data bit transitions, which results in a long coherent integration time to contribute to the acquisition sensitivity. Secondly, a computational improvement at block level is implemented to alleviate the conflict between block sampling points and FFT input points. The baseband signal and local pseudo-random code are reconfigurated assisted by matrix reconfiguration, which can reduce the calculation and increase the running speed at the same time. Theoretic performance analysis and simulation results show that the proposed scheme can acquire signal modulated with secondary code as weak as 15 dBHz. The block operation is not only more suitable for FFT, but also reduces the calculation time more than 30%. It has the potential for current and new applications in weak signal conditions especially for portable devices where computation capability is limited.