## 2 Overview of 5G-NR-V2X Mode 2  
In 5G-NR-V2X Mode 2, communication systems utilize the physical (PHY) layer for signal transmission and the medium access control (MAC) layer for managing access to the communication medium. The 3 GPP also devoted significant efforts to the MAC sublayer to develop a new resource allocation Mode, known as 5G-NR-V2X Mode 2. The MAC layer employs two main resource allocation schemes: SB-SPS and DS. SB-SPS operates by allowing vehicles to periodically reserve subchannels for a fixed number of transmissions, defined by the reseelection counter \(C_{resel}\) and the resource reservation interval (RRI). Vehicles using the SB-SPS scheme periodically reserve the selected subchannel(s) for a number of \(C_{resel}\) consecutive transmissions. The time between consecutive reservations is called the resource reservation interval (RRI), and 5G-NR-V2X Mode 2 supports any RRI value in the {0, [1:1:99], [100:100:1000]} ms range, where in the [x:y:z] notation, x denotes the minimum allowed value; z, the maximum; and y, the incremental step between consecutive values. Vehicles employ the SCI to broadcast the adopted RRI configuration and to inform neighboring vehicles about their next reservation. Depending on the selected RRI, the value of the \(C_{resel}\) is set as follows: if RRI \(\geq\) 100 ms, \(C_{resel}\) is randomly set between the [5; 15] interval [1,5]. After every transmission, the value of the \(C_{resel}\) is decremented by one, and when \(C_{resel}=0\), the ego vehicle selects new subchannels with probability \(1-\mathrm{P}\), where \(\mathrm{P}\) is the keep probability; \(\mathrm{P}\in[0,\,0.8]\). On the other hand, vehicles using the DS scheme select new subchannels every time a new message is generated, and they are not allowed to place any reservations. In other words, DS is the reservation-less variant of the SB-SPS scheme, which sets \(C_{resel}=1\) and \(\mathrm{P}=0\). Despite being characterized by a totally different reservation policy, the SB-SPS and DS schemes employ the same resource reselection process (illustrated in Figure 1). The ego vehicle's subchannel selection process involves two main phases: the sensing window and the selection window, as shown in Figure 1. During the sensing window, which spans from \(n-T_{o}\) to \(n-T_{proc}\) depending on the subcarrier spacing (SCS), the ego vehicle identifies and excludes subchannels reserved by neighboring vehicles if the reservation signal exceeds a certain Reference Signal Received Power (RSRP) threshold. Additionally, subchannels previously used by the ego vehicle during this period are excluded due to half-duplex limitations. In the selection window, which extends from \(n-T_{1}\) to \(n-T_{2}\) based on the SCS and the PDB, the ego vehicle compiles a list of candidate subchannels. If the list does not cover at least a set percentage (\(\beta\)%) of the window, the RSRP threshold is increased by 3 dB, and the process is repeated. Depending on the priority of the TB, \(\beta\) can be set to 20, 35, or 50. Once an adequate list is determined, the ego vehicle randomly selects adjacent subchannels to accommodate its transmission needs. However, SB-SPS has notable limitations:  
* Persistent collisions: Periodic reservations can result in repeated collisions if multiple vehicles reserve the same resources;
* Inefficiency with variable traffic: SB-SPS struggles to accommodate aperiodic or variable-sized data, leading to unutilized reservations and higher collision probabilities;
* Latency issues: The fixed periodic nature of SB-SPS does not adapt well to varying latency requirements, causing delays for latency-sensitive applications. Figure 2 illustrates these problems in the SB-SPS resource selection process, showing how  
Figure 1: Resource selection procedure in 5G-NR-V2X Mode 2.  
issues like size reselection, latency reselection, and packet interval reselection lead to inefficiencies.