Neighbor Discovery for ProSe and V2X Communications

Device-to-Device (D2D) communication underlying cellular networks was first introduced in the 3GPP Rel. 12 specifications, and was initially referred to as proximity services (ProSe). Its primary aim was to serve billions of Internet of Things (IoT) devices for 5G and beyond-5G networks. To enable D2D link establishment between various user equipments (UEs), the neighbor discovery process became crucial. This article investigates neighbor discovery for ProSe and Vehicle-to-Everything (V2X) communications through a SideLink interface, which is specifically introduced to support D2D communications over cellular networks. A single-user scenario is first considered to derive the probability of discovery in its closed-form and compare it with simulation results to validate its theoretical analysis. This scenario employs the demodulation reference signal (DMRS), where a power-normalized-correlation (PNC)-based metric is performed to determine the presence of active peers in the vicinity. Moreover, a multiuser scenario is considered to assess the impact of interference on the discovery probability in the cases of low and high vehicular mobility channel models. Then, group discovery is investigated using two strategies: 1) a distributed scheme incorporating out-of-coverage communications (modes 2 and 4) or 2) a network-assisted scheme applicable for supervised communications (modes 1 and 3). In this study, discovery periods are modeled as an Aloha-like protocol in the first case and a Polling-like protocol in the second case with either MAC layer or PHY layer collision models. Simulations are performed to evaluate the time required for group discovery completion as well as the collision rate in both low-mobility and high-mobility channels.