# Asymmetric Dual-Band Tracking Technique for Optimal Joint Processing of B1I and B1C Signals
## 1 Introduction
First, compared to GPS L5 data/pilot channels or L1C/A and L1C signals, B1I and B1C signals are modulated at different center frequencies in the B1 frequency band [6; 14], which brings about a bandwidth gain in addition to power gain. In addition, the center frequencies of B1I and B1C signals are separated by only 14.322 MHz, which is much closer than the \(\geq\)300 MHz separation between the center frequencies of the GPS L1C/A and L2P(Y) signals. As a result, both signals can be received by one broadband radio channel simultaneously, and the ionosphere delays experienced by these two signals are almost identical. To take full advantage of such unique characteristics of B1I and B1C, a better joint processing approach is clearly desired. In this paper we first propose to view B1I and B1C signals as one virtual navigation signal with Asymmetric Dual Sidebands, named B1-ADS. The theoretical analysis in the following shows that the Gabor bandwidth of this virtual asymmetric signal is much wider than those of B1I and B1C, and even wider than those of signals modulated by Binary Phase Shift Keying (BPSK) (10) such as L1P(Y) and L5. This indicates that the ranging accuracy of this virtual signal is much higher than that of B1I or B1C alone. What needs to be emphasized here is that the performance improvement is mainly due to bandwidth gain rather than power gain. Although the impressive potential for improved ranging accuracy is indicated by the result of theoretical analysis, it still remains a big challenge in current receiver design to implement optimal reception processing for B1-ADS signals. On one hand, the traditional joint processing methods mentioned above, such as tracking loop parameter aiding or combining discriminator outputs of different signals, fail to take advantage of the bandwidth gain for the B1-ADS signal. On the other hand, although this B1-ADS signal has the frequency separation characteristic similar to the BOC-modulated signal, the complexity of its ACF caused by the asymmetry between the upper and lower sidebands renders those classical reception techniques of the BOC signal, such as Bump-Jump [15] and the Double Estimator Technique (DET) [16], impossible to apply directly So, in this paper, we further present a novel ASYMmetric Dual-Band Tracking technique to achieve the optimal reception processing of B1-ADS signals through harvesting both power gain and bandwidth gain, which is abbreviated as ASYM-DBT. The key of ASYM-DBT is to construct discriminators of code delay, subcarrier, and carrier by coherently combining the correlator outputs of dual sideband signals and to track these three components by delay lock loop (DLL), subcarrier lock loop (SLL) and phase lock loop (PLL), respectively. The much-improved ranging accuracy committed by the B1-ADS signal is obtained by combining the outputs of the code delay and the subcarrier tracking loops. In fact, ASYM-DBT is an extension of our team's previously proposed Dual BPSK Tracking method (DBT) [17] designed for optimal reception of an alternative BOC (AltBOC) signal. The DBT method adopts the ideas of DET and double phase estimator (DPE) [18] to treat the subcarrier of BOC signals independently and to track it by an additional loop, but the characteristic of no need to generate a subcarrier makes this method have lower hardware complexity and more generality. A similar method to DBT is also mentioned in [19]. As an extension of the original DBT method, ASYM-DBT has been improved to cope with the more complicated asymmetric case of dual-sideband signals with different modulation schemes, power, and initial phase relations. The remainder of this paper is organized as follows: in Section 2, the BDS BII and BIC signal properties are described and the power spectral density (PSD), ACF, and Gabor bandwidth of the whole B1-ADS signal are given and analyzed. In Section 3, we propose a reception model for the B1-ADS signal.