LPR-DBT: A Low-Complexity Unambiguous Tracking Method for BOC and BOC-like Modulated Signals
Zhenyu Tian, Tsinghua University
LPR-DBT: A Low-Complexity Unambiguous Tracking Method for BOC and BOC-like Modulated Signals
With the rapid development of navigation applications, global navigation satellite systems (GNSSs) have begun to broadcast modernized navigation signals. Due to the limited spectrum resources, most of the modernized GNSS signals adopt different variants of the binary offset carrier (BOC) modulation. In addition to providing the backward compatibility with traditional binary phase shift keying (BPSK) modulated signals, the split spectrum of BOC modulated signal can improve the ranging accuracy and anti-interference capabilities. Moreover, the combination of modernized GNSS signals and legacy GNSS signals provides a great potential for improving ranging performance. For example, the B1I signal and B1C signal broadcast by the third generation Beidou navigation satellite system (BDS-3) can be combined to form a composite signal, which could be regarded as a high-order BOC-like signal.
However, the multi-peak auto-correlation function (ACF) of the BOC signal can cause the tracking loop to easily false lock on a secondary peak rather than the primary peak, which is well known as the ambiguous problem. Many tracking methods have been proposed to solve the ambiguity problem, including the Bump-Jumping, Double Estimator Technique (DET), and Dual BPSK Tracking (DBT), etc. However, since the bandwidth of the entire BOC signal is much larger than that of either sideband, all the existing wideband tracking methods require a high sampling rate and a high processing rate, resulting in great computational burden.
To solve these problems mentioned above, we propose a low-processing-rate dual band tracking (LPR-DBT) method for BOC modulated signals, and further extend it for BOC-like modulated signals. In the LPR-DBT method, we propose to add a preprocessing unit between the wideband analog-to-digital converter (ADC) and correlator channels. It converts both the upper and lower sidebands of the received BOC signal to near-zero intermediate frequency (IF) respectively, and then decimates the two sidebands to reduce the sampling rate. This significantly reduces the processing rate of the correlator channel, especially for high-order BOC signals. It is worth pointing out that the spectrum shifting is not carried out rashly, but is conducted under the constraint of retaining the subcarrier ranging information, which is hidden in the carrier phase relationship between the upper and lower sidebands of the BOC signal. Therefore, the delays and phase shifts introduced by the preprocessing unit to the upper and lower sidebands have been carefully analyzed and dealt with to ensure that the phase relationship between the two sidebands is not destroyed. Both theoretical analysis and experimental results reveal that the proposed LPR-DBT method can fully exploit the ranging potential of BOC and BOC-like modulated signals to achieve high ranging performance with low computational complexity.