## 2 3 GPP Vehicular Communication Standards  
### LTE and NR V2X Modes  
The 3 GPP, in order to develop new services and applications that can be offered through the LTE platform, has defined new standards for vehicular communications. Thus, the first specifications for C-V2X arise from Release 14 (Rel-14), which was completed in the fourth quarter of 2016 [6]. The V2V communications included in the standard are based on D2D communications that were defined as part of Proximity Service (ProSe) services in Release 12 (Rel-12) and Release 13 (Rel-13). In this sense, the specific Device-to-Device (D2D) interface, known as PC5, is reused for V2V, V2I and V2P communications. At the present, as part of 3 GPP standards defined for vehicular communications, there are two technologies based on LTE and NR networks: LTE C-V2X and NR C-V2X. In the 3 GPP technical specifications, four types of vehicular communications have been defined: V2V, V2P, V2I and V2N [4]. These four types of vehicular communications are intended to provide intelligent transportation services in order to improve road safety, better traffic management, pollution reduction, etc. It can be noted that, unlike technologies based on IEEE 802.11p standard, C-V2X technologies include V2P and V2N connection possibilities, so new use cases can be defined. In addition, such types of communications enable the inclusion of new internetworking mechanisms through the MR-DC approach, cross border/cross MNO communications and the use of hybrid communication interfaces to add redundancy to communications. Regarding modes of operation, for LTE C-V2X, two modes of operation are introduced for V2X communications, called mode 3 and mode 4 [8]. Basically, modes 3 and 4 are an extension of D2D modes 1 and 2, standardized in Release 12. In mode 3 (managed mode), like mode 1 for D2D communications, the UE requests resources for the V2X service from the evolved Node B (eNB). Thus, the LTE-Uu interface is used to communicate the UEs with the eNBs, and the PC5 interface for V2V, as shown in Figure 1. On the other hand, mode 4 (unmanaged mode), like mode 2 for D2D communications, consists of an autonomous mode in which no communication from the UEs to the network is required, i.e., communication is independent and is performed through the PC5 interface, also known as Sidelink (SL). Section 4 of this paper shows the performance evaluation results of LTE modes 3 and 4 obtained through simulations in an end-to-end simulator and an analytical model. Thus, different simulation scenarios are configured according to the parameters and specifications of the standard. The 3 GPP Rel-14 specifications for C-V2X are the basis for the development of the new NR-based technology. Thus, interoperability with previous technologies is ensured and, on the other hand, service performance for V2X communications is improved thanks to new techniques developed in the new generations of mobile networks. This first phase, known as V2X Phase 1, was completed in March 2017. Phase 2 V2X is introduced through 3 GPP Release 15 (Rel-15), where some enhancements are incorporated to 4G LTE networks oriented to the development of 5G NR. The improvements added, related to the V2X service, consist of providing improvements such as carrier aggregation, latency reduction, high transmission rates, etc. Additionally, improvements are added in direct communications over PC5, transmission diversity and use of higher order modulations, e.g., Quadrature Amplitude Modulation (QAM) with 64 symbols. As already reviewed, for LTE C-V2X, two communication modes are defined, called mode 3 and mode 4. Similarly, for NR C-V2X two transmission modes are defined: SL mode 1 and SL mode 2. Such communication modes are known as SL, since it allows establishing a communication link between two devices, without having to carry traffic, in this case, through a Next Generation Node B (gNB). In SL mode 1, similar to LTE C-V2X mode 3, the gNB is responsible for allocating resources to UEs.