umption, the standard provides an option to only sense a fraction of the 1000 subframes (1000 ms) before Treservation. The mobile devices can only select CSRs in the sensed sub- frames using the sensing-based SPS scheme. LTE V2X supports two types of sidelink trans- mission: (1) single shot without resource reservation and (2) multi-shot with resource reservation. Both types of transmission follow sensing and resource selection procedure. The timing diagram for LTE V2X mode 4, dedicated sensing and resource selection, is illustrated in Fig. 7.20 with sensing (1000 ms) and resource (re)-selection (up to 100 ms) windows. The concept of sensing and resource reservation is further depicted in Fig. 7.21. Sensing window (1000 ms) Resource selection interval PSCCH PSCCH PSSCH PSSCH (n-a), a = 1000 Resource reselection trigger Resource reservation period at time n (20, 50, 100, 200, 1000) Sensing window (1000 ms) PSCCH and PSSCH Resource Resource reselection reselection trigger n- T2 20 <T2<100 Figure 7.20 Mode 4 UE autonomous sensing and resource selection [25]. 820 Chapter 7 Resource reselection trigger Next packet transmission Selection Control Sensing Sensing window Selection window Figure 7.21 Sensing and resource reservation for collision avoidance in LTE V2X [6]. Vehicles also communicate using sidelink or V2V communications in mode 3. However, the selection of subchannels is managed by the base station and not by each vehicle as opposed to mode 4 (see Fig. 7.18). Mode 3 is only available when vehicles are within the network coverage. 3GPP has defined network architecture enhancements to support V2X. One of these enhancements is the V2X control function that is used by the network in mode 3 to manage radio resources and to provide vehicles with the sidelink V2X configurable parameters. Mode 3 utilizes the same subchannel arrangements as defined for mode 4. Vehicles using mode 3 must also transmit an associated SCI/TB, and the transmission of the SCI/TB must take place in the same subframe. In contrast to mode 4, t