## 4 Overview of DSRC, LTE C-V2X, and 5G NR V2X
### 5g Nr V2x  
The development of the 3 GPP sidelink transmission protocol played a crucial role in the long-term evolution-advanced (LTE-A) communication technology for public safety and V2X services [17]. However, recent advancements in data-driven applications require more sidelink functions, such as feedback channels, grant-free access, and enhanced channel sensing procedures, which are not provided by the LTE-A technology. Advanced applications that deal with an overwhelming degree of data packets usually demand higher bandwidth, more frequent message passing, and lower latency, which C-V2X cannot provide [8]. Moreover, these advanced applications also use larger and variable-sized packets transmitted aperiodically, unlike the basic safety applications. As a result, a new communication technology, 5G new radio (NR) V2X, has been developed to supplement C-V2X in supporting those particular use cases. More specifically, 5G NR V2X provides advanced functionalities in addition to the 5G NR air interface that meet stringent requirements of users and/or services in connected and autonomous vehicles.  
Among the most significant parts of the 5G NR V2X communication technology is the sidelink transmission. A thorough analysis of 5G NR V2X communications is presented in [49] and some references therein. It is important to note, however, that the 5G NR V2X communication technology is not meant to replace C-V2X communication; rather, it can operate in cooperation with the existing C-V2X technology to meet the requirements of complex services in recent data-driven connected and autonomous vehicles. It is also worth noting that an individual application may be operated and supported with either its corresponding communication technology or a combination of different communication technologies. For instance, advanced V2X services provided by the NR sidelink transmission may coexist with those provided by the LTE sidelink transmission in different channels [50].  
Sidelink communication is the key enabler of direct V2X communication without the support of a base station in 5G NR. According to [51], the NR V2X supports four different physical channels for enabling unicast, groupcast, and broadcast communications among the vehicles: (1) the PSBCH (physical sidelink broadcast channel) is used to disseminate broadcast information; (2) the PSECH (physical sidelink control channel) communicates control information; (3) the PSSCH (physical sidelink shared channel) is used for exchanging control, data, and CSI (channel state information) for unicast communication; and (4) the PSFCH (physical sidelink feedback channel) is used for exchanging HARQ (hybrid automatic repeat request) feedback for unicast and groupcast communication. For enabling these channels at the sub 6 GHz frequency band, three numerologies (\(\mu\)) are used with different sub-carrier spacing (SCS): (1) \(\mu=0\) with SCS = 15 KHz; (2) \(\mu=1\) with SCS = 30 KHz; and (3) \(\mu=3\) with SCS = 60 KHz. One numerology differs from another by varying (1) the number of available sub-channels/resource blocks in communication, and (2) the resource selection window length. For instance, a higher numerology has a lower number of sub-channel availability, but it also has a shorter slot length, which results in a lower resource selection window in ms.