# Effective PSCCH Searching for 5G-NR V2X Sidelink Communications
## 1 Introduction  
Vehicle networks are seen as a key enabler for connected and autonomous driving, but have been limited to increase the safety of road users and improve the traffic efficiency of roads [1]. However, the Fifth-Generation (5G) New Radio (NR) V2X comes to support new applications, where ultra-low latency and very high reliability are required, and can meet an end-to-end latency of 3 ms with a high reliability up to 99.999% [2]. The 5G-NR V2X will not replace its Long-Term Evolution (LTE) version, i.e., the cellular V2X (C-V2X), but they might coexist, giving support to the use cases that cannot be supported by C-V2X [3]. The requirements for both periodic and aperiodic traffics, as well as the half-duplex and the hidden node problem, are the main aspects for 5G-NR V2X physical layer design [4]. One of the main new features of 5G-NR V2X physical layer is to support the different subcarrier spacing associated to different frequency range, which enables robustness against Doppler [5]. However, there are studies on the impact of numerology on V2X Communications which claim that increasing subcarrier spacing involves a trade-off between the Inter Carrier Interference (ICI) and the Inter Symbol Interference (ISI) effects over the system performance [6]. In 5G-NR V2X, user equipment's (UEs) are vehicles, Road Side Units (RSUs), or mobile devices that are carried by pedestrians [7], which brings us to four different types of V2X communications: Vehicle-to-Vehicle (V2V), Vehicle-to-Infrastructure (V2I), Vehicle-to-Pedestrian (V2P), and Vehicle-to-Network (V2N) [8]. One or a combination of V2V,V2I, V2P, and V2N may be applied to each case, which are divided in the following four groups [9]:  
* Vehicles Platooning: It supports the formation of an interconnected group of vehicles which exchange information to get shorter distances and fuel saving for the platoon;
* Advanced Driving: It enables the semi-automated or fully-automated driving, where each vehicle and/or RSU shares data obtained from its local sensors with surrounding vehicles in proximity, to coordinate their trajectories or maneuvers, which increases the safety, avoids collisions, and improves the traffic efficiency;
* Extended Sensors: It allows to improve the perception of the environment beyond the perception capabilities of the drivers, through the collection of information by local sensors present in vehicles, RSUs, mobile devices of pedestrians, or V2X application servers;
* Remote Driving: It allows to control vehicle driving remotely, for dangerous and severe conditions, or for passengers who cannot drive themselves, whether by humans or cloud and edge computing applications. The 3 rd Generation Partnership Project (3 GPP) standard defines two frequency bands as the operating bands for V2X: 5.9 GHz band (n47) and 2.5 GHz band (n38), where the supported channel bandwidths in both frequencies are 10, 20, 30, and 40 MHz [10]. Both frequency bands belong to Frequency Range 1 (FR1) ranging from 410 MHz to 7125 MHz, and therefore none belongs to the millimeter Wave (mmWave) Frequency Range 2 (FR2) yet, ranging from 24.25 GHz to 52.6 GHz [10]. A new performance metric is needed, because other technologies may also work in the 5.9 GHz band in some regions, which may affect the 5G-NR V2X system performance, namely the safety-critical applications [11]. Resource allocation in these bands for 5G-NR V2X is defined in two modes: Mode 1 and Mode 2 [12]. In Mode 1, the vehicles are covered by one or more base stations, and they are the ones which proceed to configured or dynamic scheduling of resources. In configured scheduling, resource allocation is based on pre-defined bitmap, while dynamic scheduling is performed every millisecond based on the varying channel conditions. In Mode 2, the UE is out-of-coverage and autonomously selects the resources from a preconfigured resource pool.