atts)] per SC-FDMA symbol observed by the UE only in the configured subchannels over SC-FDMA symbols (1, 2, 6) of the first slot SC-FDMA symbols (0, 5) of the second slot of a subframe. The reference point for the S-RSSI is the antenna connector of the UE. If receive diversity is used by the UE, the reported value should not be lower than the corresponding S-RSSI of any of the individual diversity branches. In the sidelink, there is no HARQ feedback and retransmissions are always performed in a predefined/configured manner. Measurement gaps and sidelink discovery transmission dur- ing a sidelink discovery gap for transmission are of higher priority than HARQ retransmis- sions, and whenever a HARQ retransmission collides with a measurement gap or sidelink discovery transmission during a sidelink discovery gap for transmission, the HARQ retrans- mission does not take place. Vehicles (or the UEs) select their subchannels in mode 4 using the sensing-based SPS scheme specified in Rel-14 [6]. A vehicle reserves the selected subchannel(s) for several consecutive reselection counter packet transmissions. This counter is randomly set between 818 Chapter 7 PSCCH PSSCH PSSCH PSSCH LTE V2X Mode 3 (eNB) Multiple active SPS configurations No UE transmission if no data Different MCS and periods PSSCH LTE V2X Mode 4 (Ad-Hoc) PSSCH Autonomous selection of resource Location and MCS PSSCH Advance resource reservation (SPS) Control data in the same subframe PSSCH D: Data Resource selection window (1s) C: Control PSSCH PSSCH LTE V2X Mode 4 resource allocation 1s Monitoring windows PSSCH Selection of 20% RB with lowest SNR PSSCH Color of resources indicate the SNR (High to low) Figure 7.18 Comparison of LTE V2X modes 3 and 4 [25]. Selection window TCSR-1000 (ms) TCSR-300 (ms) TCSR-200 (ms) TCSR-100 (ms) TCSR (ms) Average sensed RSSI = ( RSSI (Tcsr100 j)) Figure 7.19 Calculation of the average RSSI of a candidate resource [17]. 5 and 15, and the vehicle includes its value in the SCI. After each transmission, the reselec- tion counter i