## 3 Resource Management in LTE V2X Mode 4  
The way to allocate resources for V2X communications in LTE is based on the use of different communication modes, i.e., mode 3 or mode 4. Mode 3 requires cellular network coverage and the use of the Uu interface is mandatory. On the other hand, through mode 4, UEs can manage resources in time and frequency to establish communications. In that sense, \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multirow{2}{*}{**Use Case Group**} & **Max.** & **Packet** & **Relia-** & **Data** & **Min.** \\  & **Latency** & **Size** & **bility** & **Rate** & **Range** \\  & **(ms)** & **(Bytes)** & **(\%)** & **(Mbps)** & **(Meters)** \\ \hline Vehicle Platooning & 10–500 & 50–6000 & 90–99.99 & 50–65 & 80–350 \\ Advanced Driving & 3–100 & 300–12,000 & 90–99.999 & 10–50 & 360–500 \\ Extended Sensors & 3–100 & 1600 & 90–99.999 & 10–1000 & 50–1000 \\ Remote Driving & 5 & - & 99.999 & UL: 25 & - \\  & & & & DL: 1 & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Potential requirements for NR V2X [5]. one Resource Block (RB) is the basic unit of resources that can be allocated in frequency, i.e., 180 kHz. In addition, the basic unit of time is the duration of a subframe in LTE, i.e., 1 ms. These RBs can be mapped by using channel bandwidths of 10 and 20 MHz. Furthermore, the possible subchannel sizes are defined in [8,12]: 4, 5, 6, 8, 9, 10, 12, 15, 15, 16, 18, 20, 25, 30, 48, or 50 RBs and the possible number of subchannels to 1, 3, 5, 10, 15, or 20. Additionally, the concept of subchannels is used, which represent a set of n RBs in the same subframe. Such subframes can be used for the transmission of both data and control information in V2X communications. Specifically, data is transmitted via Transport Blocks (TBs) and control information via SL Control Information (SCI) messages. TBs are mapped via Physical Sidelink Shared Channels (PSSCHs) and SCIs via Physical Sidelink Control Channels (PSCCHs) [8]. The information carried by an SCI is related to the type of Modulation and Coding Scheme (MCS) used in the TBs, Frequency Resource location of initial and retransmission, resource reservation, retransmission index, etc. Therefore, the transmission of a TB with its corresponding SCI in the same subframe is mandatory [13]. LTE V2X defines two subchannelization schemes: adjacent PSCCH + PSSCH and nonadjacent PSCCH + PSSCH, as shown in Figure 3. In the first one, an SCI, which occupies two RBs in frequency, is transmitted in adjacent RBs with corresponding TBs. The number of RBs occupied by the TBs depends on the type of message to be transmitted, e.g., Cooperative Awareness Message (CAM) messages. In the second subchannelization scheme, different resource pools are used to transmit SCIs and TBs. In both schemes, it is understood that for each TB mapped in a subframe, its corresponding SCI is also transmitted. In addition, it is possible to transmit several sets of TB + SCIs in the same subframe. Other physical layer aspects specified by the standard are [14]: TBs can be transmitted using Quadrature Phase Shift Keying (QPSK) or 16-QAM modulation schemes. On the other hand, SCIs can be transmitted using QPSK. The transmit power is specified at a maximum level of 23 dBm and a receiver sensitivity of \(-90.4\) dBm. When vehicles use mode 4 to establish V2V communications, Semi-Persistent Scheduling (SPS) is used [8,15]. As described in [14,16,17], using SPS vehicles can reserve channels using a set of standardized procedures. Such procedures ensure that vehicles perform time and frequency sensing and reservation of available resources using a technique that decreases the probability of multiple vehicles selecting the same resources. As shown in Figure 4, the sensing and resource reservation procedures are based on the use of a sensing window and a selection window. Through the sensing window a  
Figure 3: Adjacent and noadjacent PSCCH + PSSCH channelization.