# A Robust Timing Synchronization Algorithm Based on PSSS for LTE-V2X
## 1 Introduction  
Cellular Vehicle-to-Everything (C-V2X) [1] is a V2X communication technology developed for cellular systems, serving as a crucial enabler for autonomous driving and intelligent transportation systems. It seamlessly integrates cellular communication technology with short-range direct communication, facilitating low-latency and high-reliability communication among various nodes in the vehicular network, including Vehicle-to-Vehicle (V2V), Vehicle-to-Pedestrian (V2P), Vehicle-to-Infrastructure (V2I) and Vehicle-to-Network (V2N) communication [2, 3]. This integration enables broader and more accurate information perception, as well as robust networked intelligence, compared to single-vehicle intelligent technologies relying solely on onboard sensors and computing modules. As a result, vehicles equipped with C-V2X technology can efficiently navigate, enhancing traffic efficiency and safety [4]. As cellular systems evolve from 4G LTE to 5G New Radio (NR), C-V2X transitions from LTE-V2X to NR-V2X [5]. The development of C-V2X technology within the 3 rd Generation Partnership Project (3 GPP) standards can be divided into two main versions: LTE-V2X and NR-V2X. LTE-V2X, as the initial version, is responsible for providing basic road safety services. With the evolution of standards and technology, NR-V2X is introduced to support advanced applications such as autonomous driving and vehicle platooning. NR-V2X is not intended to replace LTE-V2X but rather serves as an enhancement and complement to LTE-V2X technology. Both versions are expected to coexist in the long term, jointly supporting various applications and services in vehicular networks [6]. Therefore, research on LTE-V2X scenarios remains crucial for advancing C-V2X and the development of intelligent transportation systems. As the first C-V2X solution proposed by China Datang [4; 7], LTE-V2X incorporates several enhancement technologies to address the challenges posed by high-speed node movement, low latency and high-reliability transmission requirements in V2X communication. Enhanced synchronization technology stands out as a key component of these enhancements [8; 9]. As illustrated in Figure 1, synchronization enhancement technology enables nodes equipped with Global Navigation Satellite System (GNSS) modules to achieve direct time-frequency synchronization through the GNSS. However, since V2X scenarios can also include tunnels and densely populated high-rise urban areas where GNSS signals may be obstructed, a significant portion of nodes still relies on synchronization signals broadcasted by the upper synchronization source to achieve precise time-frequency synchronization. In such scenarios, the dependence on synchronization signals becomes crucial for ensuring accurate time-frequency synchronization. In contrast to the traditional LTE cellular network system, the movement of communication nodes within the LTE-V2X system leads to changes in the channel state between these nodes over time. Regarding time synchronization in LTE-V2X, three main issues need to be addressed:  
1. V2X communication equipment utilizes Single Carrier Frequency Division Multiple Access (SC-FDMA) modulation, accompanied by low-transmission power. This necessitates the symbol timing synchronization algorithm at the receiver to set a low detection threshold, preventing the missed detection of the Primary Sidelink Synchronization Signal (SPSS). The On-Board Unit (OBU) antenna is typically mounted atop the vehicle, and the signal may be blocked by large vehicles or other obstacles during relative movement, leading to a deep fading of the SNR of the received signal. 2. The high speed of relative movement between nodes poses significant challenges to timing synchronization. The frequency of the LTE-V2X system is considerably higher than that of the LTE cellular network, with relative speeds between nodes potentially reaching up to 240 km/h, resulting in a substantial Doppler frequency offset.