# A Packet Collision Reduction Based on Reselection for LTE V2X Mode 4
## 2 LTE V2X Mode 4
### Sensing-Based SPS  
In Mode 4, the UEs select a resource using the sensing-based SPS. The SPS is performed by sensing for the past one second, and resource reservation information is notified by PSCCH. The UEs set Reselection Counter (RC) depending on the RRI from a uniform distribution in the range of \([5,15]\) for \(\text{RRI}\geq 100\), \([10,30]\) for \(\text{RRI}=50\), and \([25,75]\) for \(\text{RRI}=20\). The RC is decremented every transmission. The UEs use the same subchannel resources until the RC reaches 0. In addition, when it is equal to 0, they decide whether to reselect the resource based on the probability \(p_{k}\) of keeping their resource where \(p_{k}\) can be set in the range \([0,0.8]\). If the UE decides to reselect the resource, a selection window is set in the time interval \([n+T_{1},n+T_{2}]\) for subframe \(n\) where \(T_{1}\leq 4\) and \(20\leq T_{2}\leq 100\). A candidate resource in selection window \(R_{x,y}\) is defined with subchannel \(x\) and subframe \(y\) where \(0\leq x\leq\text{thenumber of subchannels}-1\) and \(n+T_{1}\leq y\leq n+T_{2}\). Then, the UE initializes the set \(\mathbb{S}_{A}\) with all candidate resources and excludes them according to the following steps a, b, and c.  
In Step a, the UE excludes unmonitored resources due to the constraint of half-duplex transmission. In Step b, the UE excludes any resources reserved by other UEs. When the packets are received with PSCCH Reference Signal Receive Power (PSSCH-RSRP) of the TB corresponding to the SCI greater than or equal to threshold \(P_{th}\) and satisfy \(m+P_{rspp}=y\) where parameter \(m\) is a subframe in sensing window defined by \([n-1000,n-1]\) and \(P_{rspp}\) is the RRI notified over PSCCH, they represent the reserved resources. When the resource is reselected, \(P_{rspp}\) is set to 0 because the UE does not make a reservation. \(P_{th}\) is set by upper layers depending on priority \(\alpha\) of the transmitter and \(\beta\) of the receiver (both within 0 and 7). It can be in the range \([-128,-2]\) dB and can be expressed as [6,40]  
\[P_{th}=-128+2(\alpha\times 8+\beta) \tag{1}\]  
After the exclusion by Steps a and b, the UE checks whether the remainder of \(\mathbb{S}_{A}\) contains at least 20% of the total number of candidate resources. If this condition is not satisfied, then Step b is repeated with \(P_{th}\) increased by 3 dB. In Step c, the UE excludes any resources whose received power is highly evaluated. Metric \(E_{x,y}\) is evaluated as the linear average of Received Signal Strength Indicator (RSSI) that for RRI \(\geq 100\) can be expressed as  
Figure 4: Subframe structure for sidelink for Release 14.  
Figure 3: Subframe structure for sidelink for Release 12.  
\[E_{x,y}=\frac{1}{10}\sum_{i=1}^{10}\text{RSSI}\big{\{}R_{x,y-100\times i}\big{\}} \tag{2}\]  
The UE moves \(R_{x,y}\) with the smallest \(E_{x,y}\) from \(\mathbb{S}_{A}\) to the set \(\mathbb{S}_{B}\) initialized to an empty set. This step is repeated until the number of candidate resources in \(\mathbb{S}_{B}\) reaches greater than or equal to 20 % of the total number of candidate resources. After these steps, the UE randomly selects one of the resources in \(\mathbb{S}_{B}\).  
An example of the exclusion process by Steps a, b, and c is demonstrated in Figure 5. In this figure, the UE excludes the candidate resources in subframe 1020 by Step a because the UE has transmitted in subframe 420. In addition, the UE excludes it in subframe 1050 and subchannel 0 because the UE receives a packet with \(P_{rsvp}=100\) in subframe 950 and subchannel 0. Similarly, the UE excludes it in subframe 1080 and subchannel 1 because the UE largely evaluates its RSSI.