## 5 Obstacles and Challenges in Evolving V2X Communication
### RAN Obstacles and Challenges
#### 5.1.2 Subcarriers or CSMA/CA--The Perks and Drawbacks  
As shown in Table 2, 5G (and 4G) uses semi-persistent sensing of the least-occupied resource technique, contrary to the 802.11 family's CSMA/CA. The latter technique divides the available frequency band into so-called radio resource blocks. The allocation of these resource blocks is performed distributively. This technique has pros and cons as well. One of the key advantages is the enhanced range. The reason for the enhanced range is that C-V2X uses relatively higher transmission power. The 5.9 GHz band is allocated for ITS safety purposes, and the communication on this channel is essentially distributed. Thus, the law specifies the maximum allowed transmission power for the devices in order to avoid congestion on the channel. The difference between the DSRC and the C-V2X is that the latter uses a tighter frequency band with the same transmission power to transmit the message. The tighter band results in less noise, eventually resulting in a higher range. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Project Name** & **Project Life** & **Sites** & **Focus** \\ \hline
5 GCAR [129; 130; 131] & 2017–2019 & France & - 5G V2X architecture with IoT platform \\ \hline
5 GCroCo [85; 88] & 2018–2021 & France, Germany, and Luxembourg & - CCAM in the cross-border corridor - Predictive QoS \\ \hline
5G-DRIVE [132] & 2018–2021 & Finland, Italy, and UK (new) & - EU–China collaboration development based on 5G - Enabling self-driving vehicles - Multi-tenant platform \\ \hline
5G-CARMEN [89] & 2018–2021 & Germany, Austria, and Italy & - Enabling self-driving vehicles - Multi-tenant platform \\ \hline
5G-MOBIX [90] & 2018–2022 & Spain–Portugal & - Evaluating connected and automated mobility applications \\ \hline \multicolumn{3}{c}{Motorway Measurement Campaign [84]} & 2020 & Hungary & - Road geometry mapping and digital twin \\ \hline \multicolumn{3}{c}{Central system for supporting automated vehicle testing and operation [91]} & 2021–2024 & Hungary–Austria & - Autonomous vehicles in cooperation - Autonomous with infrastructure \\ \hline
5G-ROUTES [92] & 2020–2023 & Latvia–Estonia–Finland & - Deployment of 5G end-to-end interoperable CAM services \\ \hline
5G-Blueprint [93] & 2020–2023 & Belgium–Netherlands & - Evaluating cross-border teleoperated transport - \\ \hline
5G-MED [94] & 2020–2023 & Spain–France & - 5G infrastructure architecture for roads and railways - \\ \hline Vital-5G [95] & 2021–2024 & Romania–Belgium–Greece & - Network applications for the T and L industry - \\ \hline \multicolumn{3}{c}{Automated valet parking over 5G Network [96]} & 2022 & Germany & - Automated valet parking \\ \hline \multicolumn{3}{c}{Real-time racing experience on 5G network [97]} & 2022 & Hungary & - Real-time digital twin on race track \\ \hline \multicolumn{3}{c}{Live trial of 5G-connected car} & 2022 & Italy & - Connected services to the digital transformation of smart cities \\ \hline \multicolumn{3}{c}{Live trial of 5G-connected car} & & \\ \multicolumn{3}{c}{Live trial to launch in Blacksburg, Virginia [99]} & 2022 & USA & - Connected mobility to enhance safety on roads. \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison table for cellular V2X projects from the recent years. The other perk of the usage of radio resource blocks is that the channel allocation can be performed by an external entity, which increases the channel utilization. However, this method would introduce a single point of failure, which is not desired in such systems [134]. This approach has some drawbacks as well. The tight resource block alignment introduces a dependency on GNSS signals (in Modes 2/4), which can unnecessarily limit communication in urban canyons or tunnels. The simultaneous transmission of the packets can increase the probability of simultaneous transmission, which results in packet loss. There is a little additional latency added to the system as well due to the scheduling.