# A Packet Collision Reduction Based on Reselection for LTE V2X Mode 4
## 2 LTE V2X Mode 4
### Radio Resources  
Figure 2 shows an example of the resource usage for sidelink communication. A radio resource consists of a subframe (1 ms) and subchannels divided between the time domain and frequency domain. A subchannel consists of multiple Resource Blocks (RBs) of 180 kHz. The figure shows an example when the number of the subchannel is 2, and each UE transmits using the resource represented by the rectangle. The UEs transmit control information and data using adjacent frequencies. The control information is transmitted as Sidelink Control Information (SCI) [36] over Physical Sidelink Control Channel (PSCCH). Although the SCI can notice periodic resource reservations, it cannot be used to reserve reselected resources. The data are transmitted in Transport Blocks (TBs) over Physical Sidelink Shared Channel (PSSCH). The TB is assumed to contain Cooperative Awareness Message (CAM) [37], Decentralized Environmental Notification Message (DENM) [38], and so on.  
The structure of SCI Format 1 used in Mode 3 and 4 is illustrated in Table 1. SCI Format 1 includes the following information:  
* Priority: the priority of the packet set by upper layers;
* Resource Reservation: the time period of the intended next use of the resource;
* Frequency resource location of initial transmission and retransmission: Resource Indication Value (RIV) corresponding to a starting subchannel index and length in terms of contiguously allocated subchannels;  
Figure 2: An example of resource usage for the sidelink communication.  
* Time gap between initial transmission and retransmission: subframe differences in repeated transmissions;
* Modulation and coding scheme: the combination of modulation scheme and code rate;
* Retransmission index: a boolean denoting whether the first transmission or second transmission.  
The resource reservation value is interpreted for the other UEs as \(X\) according to Table 2, where it is equal to RRI divided by 100. The RRI represents the transmission cycle, which is basically 100 ms, and 20, 50, 100, 200,..., 1000 ms can be configured [6].  
In the sidelink communication, the UEs transmit by Single Carrier Frequency Division Multiple Access (SC-FDMA) like LTE uplink. A subframe consists of 14 symbols that are time-multiplexed as shown in Figures 3 and 4, where the last symbol is used for Tx/Rx switching and timing adjustment [39]. Two DeModulation Reference Signals (DM-RS) are contained in a subframe in Release 12 as shown in Figure 3. To improve the demodulation accuracy in a fast-moving environment, the DM-RS is increased to four symbols in Release 14 as shown in Figure 4.  
\begin{table}
\begin{tabular}{c c} \hline Priority & 3 bits \\ \hline Resource Reservation & 4 bits \\ \hline Frequency resource location of initial & Calculation based on the number of \\ transmission and retransmission & subchannels \\ \hline Time gap between initial transmission and & 4 bits \\ retransmission & 5 bits \\ \hline Retransmission index & 1 bit \\ \hline Reserved information & add until 32 bit total \\ \hline \end{tabular}
\end{table}
Table 1: Structure of SCI Format 1.  
\begin{table}
\begin{tabular}{c c c} \hline
**Resource Reservation Field** & **Indicated** & **Condition** \\
**in Value \(X\) SCI Format 1** & & The UE keeps the resources \\ \hline ’0001’, ’0010’,..., ’1010’ & Decimal equivalent of the field & for the next transmission after \\  & & \(100\times X\) ms \\ \hline ’1011’ & 0.5 & The UE keeps the resources \\  & & for the next transmission \\  & & after 50 ms \\ \hline ’1100’ & 0.2 & The UE keeps the resources \\  & & for the next transmission \\  & & after 20 ms \\ \hline ’0000’ & 0 & The UE does not keep the \\  & & resources for the \\  & & next transmission \\ \hline ’1101’, ’1110’, ’1111’ & Reserved \\ \hline \end{tabular}
\end{table}
Table 2: Determination of the Resource Reservation field in SCI Format 1.