23 dBm with mandatory receiver diversity. However, regional differences can exist-for example, Korean deployments with 100 mW maximum transmit power or 2 W Effective Isotropic Radiated Power (EIRP). Vehicle-to-anything standards and deployment timeline Today, two main approaches for V2X communication are competing against each other: 802.11p (IEEE-based standard) and C-V2X (3GPP-based standard². Both standards operate in the 5.9 GHz band described earlier. Within C-V2X, there is a 4G/LTE and a 5G version of the stan- dard. Given relative slow adoption of technology, a 5G version for V2X communication is 356 Open Radio Access Network (O-RAN) Systems Architecture and Design Reserved Ch.172 Ch.174 Ch.176 Ch.178 Ch.180 Ch.182 Ch.184 10MHz 10MHz 10MHz 10MHz 10MHz 10MHz 10MHz U-NII-4 (WiFi) Service Service Non-Safety Control Service Safety Safety ITS (trial) Non-Safety Safety Future FIGURE 11-4 V2X spectrum allocation. V2X, vehicle-to-anything. considered the most likely candidate of the two. This chapter summarizes 802.11p and C-V2X technologies before explaining the technology behind 5G sidelink communication. 802.11p The 802.11p standard is related to 802.11a. It defines an orthogonal frequency division multi- plexing waveform with 64 subcarriers with convolutional coding and a Carrier Sense Multiple Access with Collision Avoidance channel access method. Note that there are stricter spectrum mask and adjacent and nonadjacent channel rejection conditions. 4G C-V2X (PC5) LTE C-V2X was introduced in 3GPP Rel-14 and Rel-15 (2017 and 2018) as an extension of 3GPP device-to-device Rel-12. It defines a "sidelink" direct communication between devices (e.g., V2V), also known as the PC5 interface. At the same time, the previously known device- to-infrastructure link (Uu) was adapted to support V2I communication. C-V2X refers to both the LTE C-V2X sidelink and the LTE C-V2X Uu interface. These different communication links (V2V VS V2I) are supported through C-V2X "modes." Mode 1 and mode 2 are standard- ized from Rel-12. Both 