# An Optimized Vector Tracking Architecture for Pseudo-Random Pulsing CDMA Signals
###### Abstract  
The vector tracking loop (VTL) has high tracking accuracy and a superior ability to track weak signals in GNSS. However, traditional VTL architecture is established on continuous Code Division Multiple Access (CDMA) signal and is incompatible with pseudolite positioning systems (PLPS) because PLPS generally adopts a pseudo-random pulsing CDMA signal structure to mitigate the near-far effect. Therefore, this paper proposes an optimized VTL architecture for pseudo-random pulsing CDMA signals. To avoid estimation biases in PLPS, the proposed VTL adopts irregular update periods (IUP) pre-filters which adjust the update cycles according to the active timeslot intervals. Meanwhile, as the active timeslots of different pseudolites do not overlap, the sampling time of the navigation filter inputs is inconsistent and time-varying, causing jitter degradation. Thus, the proposed VTL predicts the measurements so that they can be sampled at the same time, which improves tracking accuracy. Simulation is carried out to evaluate the performance of the proposed VTL. The results suggest that the proposed VTL outperforms the traditional pre-filter-based VTL and IUP pre-filter-based VTL. p 2021 27 27 2021 27  
failure to track and synchronize the distant pseudolites signal, referred to as the near-far effect [16]. Various techniques have been proposed for alleviating the near-far effect, including frequency hopping [17], new spreading codes [15] pulse modulation [18], and optimizing pseudolite antenna [19]. However, the preferred one is to pulse the pseudolite signals in pseudo-random because of the low resource cost and good resistance to the near-far effect. As the name indicates, the pulse signals could be regarded as gated direct sequence spread spectrum (DSSS) signals. Each pseudolite transmits signals at different timeslots, preventing mutual interference. Although the pulse signal with equal intervals is easier to realize, it may result in sub-peaks in the frequency domain, causing false frequency-lock in the receiver [20; 21]. Thus, the pseudo-random pulse scheme should be adopted in PLPS. Some literature [18; 22; 23] offer different designs of the pulse scheme. The cornerstone of these schemes is to use a pseudo-random pulse sequence to control pseudolite pulse signal transmission. The pseudo-random pulse signal (PRPS) brings some challenges in tracking compared to GNSS signals. It would increase the measurement noise and decrease the tracking convergence speed if the receiver tracks the PRPS according to the traditional tracking architecture. A tracking loop architecture of conventional PLL and delay lock loop (DLL) pre-cascaded with a Noise Silencer Module (NSM) is proposed in Reference [24]. This tracking loop is turned on only when the valid timeslot of the signal arrives. It mitigates the interference of other pseudolite signals and reduces noise accumulation when the signal is silent. Because the intervals between two adjacent valid timeslots are pseudo-random, the discriminator of the NSM-PLL/DLL tracking loop has a bias in estimating the carrier phase and code phase, reducing the tracking accuracy of the loop. Therefore, [PERSON] et al. propose a variable update rate carrier tracking loop based on the Kalman filter (KF-VURL) [25], which effectively solves the estimation bias problem of the discriminator. In estimating the carrier phase and doppler, the KF-VURL shows higher tracking accuracy and faster convergence. Though much research on pseudolite tracking focuses on improving and optimizing the scalar tracking loop (STL) [24; 25; 26; 27], they ignore the abundant information of other channels. In contrast, the VTL uses a big loop to filter the observations of all channels, which makes full use of the information of each channel and improves the tracking performance of the signals. Initially, the VTL was introduced by [PERSON] [28] and popularly developed in recent years, mainly applied in GNSS.