direct communication, mode 1 [18,19]. Vehicle-to-Everything (V2X) Communications 813 Mode 3: eNB Controlled resource selection ((HH)) ((HH)) Mode 4: UE Autonomous resource selection Geo-Area ID1 Geo-Area ID2 Geo-Area ID3 Resource Resource Pool 2 Pool 1 Resource Pool 3 Figure 7.15 Illustration of LTE V2X modes 3 and 4 operation [25]. on the availability of cellular coverage. Mode 4 includes a distributed scheduling scheme for vehicles to select their radio resources and includes the support for distributed conges- tion control. In mode 3, the vehicle reports its location and coordinates to assist the eNB in scheduling, whereas in mode 4, the vehicle location information can be used to restrict side- link resource selection (to enable spatial reuse in a distributed system). The C-V2X utilizes LTE uplink multiple access scheme (SC-FDMA), and supports 10 and 20 MHz channels. Each channel is divided into subframes, resource blocks (RBs), and sub- channels. Subframes are 1 ms long. The resource block is the smallest unit of frequency resources that can be allocated to a user. It is 180 kHz wide in frequency (12 subcarriers with 15 kHz subcarrier spacing). The C-V2X defines subchannels as a group of RBs in the same subframe, and the number of RBs per subchannel can vary. Subchannels are used to transmit data and control information. The data is transmitted in the units of transport blocks (TBs) over PSSCH, and the SCI messages are transmitted over PSCCH. A TB con- tains a full packet to be transmitted; for example, a beacon or cooperative awareness mes- sage. A node that wants to transmit a TB must also transmit its associated SCI, which is also referred to as a scheduling assignment. The SCI includes information such as the MCS that is used to transmit the TB, the number of RBs, and the resource reservation interval for SPS. This information is critical for other nodes to be able to receive and decode the TB; thus the SCI must be correctly received. A TB and its associated SCI must always be trans- mitted in the same s