# On the Impact of Multiple Access Interference in LTE-V2X and NR-V2X Sidelink Communications +
## 3 Cellular-V2X Technologies
### Lte-V2x  
The C-V2X was initially created by the 3 GPP based on the V2X specifications in Rel-14 as LTE-V2X (also named LTE-V), which gives top priority to radio access improvements appropriate for V2X. Until then, IEEE 802.11-based solutions mainly covered V2X scenarios. By employing already-existing cell towers, the LTE-V2X promised minimal costs, quick development, and installation to improve the accessibility and efficiency of the public transportation system. LTE-V2X can use a licensed cellular network for commercial purposes such as voice or data access. Moreover, LTE-V2X enables sidelink direct V2V connections via the so-called PC5 interface. The vehicular nodes communicate with other nodes directly over the same channel at the PC5, with no intervention from the infrastructure.  
LTE-V2X uses the 5.9 GHz band, which has been reserved in several regions (such as the United States, Europe, and China) for ITS services. LTE-V2X supports cooperative active traffic safety, traffic management, and telematics applications. Similar services are supported by LTE-V2X as they are by IEEE 802.11-based V2X or its equivalent, ETSI ITS-G5. These services depend on the broadcast transmission of brief awareness messages called CAMs (Cooperative Awareness Messages) in ETSI ITS-G5 [7] or SAE BSMs (Basic Safety Messages) in IEEE 802.11 V2X [32] to provide essential data such as the location, direction, speed, and acceleration of the transmitting vehicle regularly. LTE-V2X reuses the higher V2X layers and protocols provided by ETSI and SAE while defining new Physical (PHY) and Medium Access Control (MAC) layers for V2X.  
Sidelink LTE-V2X supports 10 and 20 MHz channels in the 5.9 GHz band. At the physical and medium access control (MAC) levels, single-carrier frequency-division multiple access (SC-FDMA) is used. The channel is divided into resource blocks (RBs) that occupy the 180 kHz bandwidth with 12 sub-carriers. In the time domain, the duration of each RB is 1 ms, which is the minimum allocation time (MAT) and can carry 14 orthogonal frequency-division multiplexing (OFDM) symbols. Nine symbols carry data, and four symbols are reserved for channel estimation. One symbol is used to adjust the time and may also indicate a switch between the transmitter and receiver. RBs can be grouped into sub-channels of variable size, and a data packet is carried over sub-channels in a transport block (TB).  
As mentioned above, to accommodate low latency V2X communications, 3 GPP Rel-14 introduced two specific transmission modes (Modes 3 and 4); see [12]. Vehicles can employ LTE-V2X Mode 4, which allows vehicles to autonomously reserve resources using the resource reservation algorithm, while they are outside cellular coverage. The standard defines a sensing-based scheduling approach for resource allocation among neighboring vehicles. The time frame in which resources are reserved by a vehicle is known as the resource reservation interval (RRI). LTE-V2X Mode 4 uses RRI values of 0, 20, 50 ms, and multiples of 100 ms, up to 1 s. The resource reservation algorithm requires each vehicle to sense the channel for 1 s to identify candidate resources to be excluded, therefore decreasing packet collisions.