# Review of Communication Technologies for Electric Vehicle Charging Management and Coordination
## 4 Cellular Networks
### Cellular V2X  
Both cellular and DSRC technologies have their benefits and limitations when used for the implementation of vehicular networks. Although DSRC-based vehicular networks are widely used and are proven to be effective, they suffer from certain limitations such as channel access delay, limited coverage, and limited QoS guarantee [71]. On the other hand, existing cellular networks (UMTS and LTE) are not optimized for vehicular communication and, hence, may result in cellular network congestion in dense areas [72]. Accordingly, to  
Figure 4: LTE sub-frame structure: (**a**) sub-frame with 1 ms TTI for conventional cellular systems, (**b**) sub-frame with shortened TTI (1/14 ms) for low latency systems.  
overcome these limitations, the Third Generation Partnership Project (3 GPP) published Release 14 to support vehicle-to-everything (V2X) communication with low latency and a high data rate, and referred to this evolution as LTE-V, or cellular V2X (C-V2X) [73].  
The C-V2X specifications in Rel. 14 are based on existing LTE standards and, hence, can be rapidly deployed with minimum additional infrastructure costs. C-V2X supports maximum vehicle speed of 250 km/h with a maximum relative speed of 500 km/h [38]. It can operate using the ITS-dedicated 5.9 GHz band with a coverage range of 450 m as well as in the licensed LTE frequency bands with typical LTE coverage. Nevertheless, the high relative speeds at 5.9 GHz introduce a high Doppler shift of \(\approx\)2700 Hz, leading to channel variations within the sub-frame and introducing challenges in channel estimation [74]. Accordingly, some advanced link layer enhancements are introduced in Release 14 to address these challenges, improve channel scheduling and effectively manage the resources used in V2X communications [75; 76; 77].  
Two communication modes of C-V2X are introduced in Release 14: mode 3 and mode 4. In mode 3, the cellular base stations, i.e., LTE e-Node-B (eNB), allocate and manage resources used by the vehicles for V2X communications [75; 76]. In mode 4, on the other hand, distributed allocation is performed and vehicles select and manage the required resources autonomously. The authors in [78] present an analytical model for the performance of V2X in mode 4, and formulate the packet delivery ratio (PDR) as a function of the distance between the transmitter and the receiver. Furthermore, resource allocation in C-V2X where vehicles share resources with cellular users is discussed in [79; 80; 81] considering the latency constraints of V2V links.  
Despite offering better resource management than DSRC, C-V2X is still quite intolerant to high density networks [73], particularly in mode 4 where interference increases with higher traffic density as the frequency re-use distance decreases. The authors in [73] compare the performance of LTE cellular V2X operating in mode 4 to DSRC technology in a highway scenario. The obtained results show that when DSRC is set to the default data rate of 6 Mbps, C-V2X provides better packet delivery ratio (PDR), whereas increasing the data rate to 18 Mbps allows DSRC to improve its performance and achieve a higher PDR. Furthermore, the results illustrate that, similar to DSRC, as the number of transmitted packets increases, the performance of C-V2X shows significant degradation. Hence, the same problem of inefficient operation in high density charging scenarios is expected to take place using C-V2X communications, and further enhancements to the link capacity are required.