# Multipath Mitigation for BOC Signals Based on Prompt-Assisted-Offset Correlator
## 1 Introduction
Multipath signals can introduce errors to the subcarrier ranging measurement, thus resulting in pseudo-range bias and positioning errors. Therefore, subcarrier multipath mitigation techniques are very important for BOC signals to improve positioning accuracy [21; 22; 23]. Depending on the loop structure, subcarrier multipath mitigation techniques for BOC signals can be divided into two categories: multi-correlator techniques and the offset correlator (OC) technique. Multi-correlator techniques combine multiple correlator outputs to synthesize a narrower correlation function to reduce the multipath sensitive area, such as the DD and strobe correlator techniques applied in the SLL/SPLL [21; 23]. The OC technique uses forward offset correlators instead of prompt correlators for subcarrier discrimination [21; 22]. Since multipath signals have longer path delays than the direct signal, forward offset correlators are less susceptible to multipath than prompt correlators, so the OC technique can reduce multipath errors. Furthermore, the multipath mitigation performance of OC technique improves with the increase of forward offset. It has been demonstrated that the OC technique with a large forward offset has better subcarrier multipath mitigation performance than that of multi-correlator techniques [22]. However, the signal-to-noise ratio (SNR) loss of the OC technique increases with the forward offset, which may cause the tracking loop to lose lock. As a consequence, the OC technique has to compromise between the thermal noise performance and multipath mitigation performance. To ensure the tracking stability, the forward offset of the OC technique cannot be very large, which limits its multipath mitigation performance in practice. In addition to the code and subcarrier ranging measurements, the BOC signal also provides carrier phase measurements for centimeter-level positioning applications. The carrier multipath error can reach up to several centimeters and is a major error source for high-precision positioning [24; 25]. However, existing multipath mitigation methods for BOC signals generally focus on the code and subcarrier multipath errors, while neglecting the carrier multipath error. Moreover, in DPE and DBT methods, the SPLL is coupled with the PLL, which implies that the carrier and subcarrier multipath errors interact with each other. When using the DPE or DBT method to track BOC signals, the subcarrier multipath error cannot be completely eliminated without eliminating the carrier multipath error. This paper proposes a prompt-assisted-offset correlator (PAOC) technique based on the DBT loop to mitigate multipath for BOC signals. The PAOC technique uses the prompt and offset correlators to significantly improve multipath performance with small noise performance loss. Considering the interaction between carrier and subcarrier errors, this paper further proposes using the PAOC technique in both the PLL and the SPLL of the DBT loop. In addition, the code multipath error is mitigated by the NC technique. The main contributions of this paper are summarized as follows:  
1. A new multipath mitigation technique named PAOC is proposed for BOC signals. The PAOC technique is an improved OC technique with less thermal noise performance loss. Therefore, the PAOC technique can be configured with a larger forward offset to obtain better multipath performance in practice. 2. It is proposed that carrier and subcarrier multipath errors affect each other when using the SPLL to track the subcarrier. Therefore, this paper proposes to apply the PAOC technique in both the PLL and the SPLL to mitigate carrier and subcarrier multipath errors simultaneously. Compared with existing methods that ignore the carrier multipath, the proposed method better mitigates the subcarrier multipath error and additionally mitigates the carrier multipath error. The remainder of this paper is organized as follows. Section 2 introduces the multipath model for BOC signals and analyzes the multipath effects in detail.