## 4 Overview of DSRC, LTE C-V2X, and 5G NR V2X
### DSRC vs. C-V2x  
There has been a heated debate regarding the performance of DSRC and C-V2X technologies, leading to extensive studies comparing their performance [9; 35; 36; 37; 38; 39]. The commonly used key performance indicators (KPIs) include: packet delivery ratio (PDR) [38; 40], packet reception ratio (PRR) [9; 36; 37], block error ratio (BLER) [34], packet inter-reception time (PIR), and packet loss [38]. In the literature, certain studies [13; 38; 40; 41] have reported that DSRC outperforms C-V2X, while others have observed similar performance [42] in the specific scenarios that they examined. [PERSON] et al. [38] demonstrated that variations in message size and pattern significantly affect the medium access control (MAC) operation and performance of both technologies and that DSRC handles variations in the message size and time interval more effectively than LTE-V2X's sensing-based semi-persistent scheduling, except under very low channel loads. [PERSON] et al. [40] showcased that only the DSRC technology is able to support the V2I communication scenarios without any major limitations and achieves an average end-to-end delay of less than 100 ms and a PDR above 95% in all of their investigated simulation scenarios. However, the majority of research suggests that C-V2X performs better [36; 43]. [PERSON] et al. [35] proposed an analytical model for assessing the reliability of DSRC control channels in safety applications. The model considers factors such as vehicle follow-on safety rules, mobility, transmitter-receiver speeds, channel fading, hidden terminals, and collisions. Simulation results using realistic vehicular traces validate the modeling and analysis, showcasing that DSRC may lead to significant performance degradation in terms of delay and packet reception probability in dense and high-mobility conditions. [PERSON] et al. [34] conducted link-level simulations comparing LTE V2X and DSRC for urban and freeway scenarios with and without line of sight (LOS). Their study demonstrated that LTE V2X achieves the same block error ratio (BLER) at a lower signal-to-noise ratio (SNR)or with reduced receiving power compared to DSRC, resulting in a more reliable link. [PERSON] et al. [9] conducted simulations to compare the average PRR between LTE and DSRC at different vehicle densities in both urban and freeway scenarios with varying distances. Their study illustrates that LTE-based alternatives, including LTE multicast and LTE sidelink, outperform IEEE 802.11p in all the studied cases. This improved performance is credited to LTE's more robust PHY layer signaling scheme and more efficient resource allocation mechanism. [PERSON] et al. [36] compared DSRC and cellular V2X, with cellular V2X consistently outperforming or matching DSRC in various aspects. Notably, cellular V2X offers an extended communication range, crucial for vehicular safety. The evaluation favors DSRC despite using an advanced DSRC receiver and a standard cellular V2X receiver with conservative AGC loop training assumptions. [PERSON] et al. [37] compared LTE-V2X and DSRC in terms of the average PRR at different distances in freeway and urban scenarios. Their findings indicate that LTE-V2X offers longer, more reliable communication ranges compared to DSRC. This advantage is attributed to LTE-V2X's efficient sensing and SPS resource allocation scheme, confirming its superiority over DSRC. It is worth noting that recent research also investigates solutions to leverage the advantages of DSRC and C-V2X [26; 44; 45; 46; 47] and mitigate limitations for the joint use of DSRC and C-V2X. [PERSON] et al. [26] reviewed the background and technical aspects of both technologies and elaborated on V2X platform models allowing for the concurrent and simultaneous propagation of DSRC and C-V2X messages for a hybrid V2X environment.