**3. LTE-V2X standard**

The LTE standard is widely used in public and private mobile radio networks. LTE technology has been identified to support vehicular network services using V2X architecture. The V2X service architecture is shown in **Figure 2**. As mentioned in the previous section, the V2X communication services include four different modes of communication (V2V, V2I, V2P, and V2N). These links are bidirectional. 3GPP study groups in collaboration with transport industries have started standardization activities on LTE-based vehicular networks in the working group 1. After several studies and developing several initial specifications on V2X services based on LTE, Release 14 was published in 2017 [8]. The standard is further developed in Release 15 in 2018 supporting enhanced V2X networking features. The enhancements go beyond the support of CAM and Decentralized Environmental Notification Messages (DENM) transmissions as shown in **Table 1**. The 3GPP specifications did not allocate any specific frequency band to support V2X services. European Telecommunications Standard Institute (ETSI) has allocated a 70 MHz spectrum in the 5.9 GHz band in which there is no overlap between V2X and conventional cellular network services. This separation of operating frequency will enable different operators to provide vehicular network services independent of conventional mobile operators. The 5.9 GHz LTE band will allow the system to coexist with IEEE 802.11p-based systems. However, the mobile operators can also use the licensed band to support the V2X services. The V2X services can use the conventional air interface as well as the newly developed D2D interface using the sidelink channel. The D2D communication architecture is briefly introduced in the following section.
