# TARS: A Novel Mechanism for Truly Autonomous Resource Selection in LTE-V2V Mode 4
## 5 Performance Evaluation  
The model for TARS was implemented in MATLAB, and its performance was compared with the LTE Mode 4 standard and a competing autonomous resource selection approach proposed and described in [14]. The simulation parameters are given in Table 2.  
The result in Figure 11(a) shows the resource collision probability with respect to increasing distances. The probabilistic range of values for collision probability is also depicted in the figure at a confidence interval level of 95%. At lower distances, the resource collision probability is negligible. As the distance between the sender and receiver increases, i.e., beyond 50 m, the collision probability starts to increase. This is due to the increase in the number of neighbors of a vehicle, which results in higher contention for resource blocks. At even higher distances, i.e., around 250 m, the collision probability starts to reduce. Observe that at higher distances, the vehicle density remains the same and so does the transmit power for sender. Therefore, the resource collision probability reduces for higher distances. Observe that the standard, i.e., LTE Mode 4, is comparatively more prone to collisions due to the random nature of resource selection. The probability of resource collisions was reduced by [PERSON] et al. due to the piggyback mechanism through SA, which contains information about the reserved RBs for future transmissions. However, continuous collisions are still possible during the initial selection of RBs from multiple vehicles. TARS, on the other hand, reduces resource collisions by effectively predicting the RBs being used by the neighbors, hence reducing the probability of collision. Figure 12(a) reports the increase in the probability of resource collision with the increase in vehicle density and the probabilistic range of values for collision probability at a confidence interval level of 95%. Note that the increase in the probability of resource collisions with respect to density in LTE Mode 4 and He et al. is due to the uniform resource pool and the random selection of RBs by vehicles. TARS improves this performance through its ability to select unique RBs instead of a uniform random resource selection mechanism. Moreover, at lower densities, better TARS performance is due to the unique selection of resources by TARS instead of the uniform random resource selection mechanism of LTE Mode 4 and He et al. However, the at around 300 vehicles and beyond, the performance of TARS is comparable with LTE Mode 4 and He et al. This is because at higher vehicle densities, a unique selection of resources by TARS is not fully applicable because of the limited number of RBs. The Packet Delivery Ratio (PDR) against increasing vehicle densities is depicted in Figures 14 and 15. At lower densities, the PDR is high. It drops with the increase in the number of vehicles in the network because of the interference caused by other vehicles. We also evaluated TARS by employing retransmission mechanism as proposed in LTE Mode 4 and in [PERSON] et al. The PDR of all approaches improves when packets are retransmitted. There are more RBs available for selection; therefore, TARS without retransmission achieves a \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline Vehicular Density & 50 to 350 \\ \hline Sender/Receiver Distance & 50 m to 400 m \\ \hline Vehicular Speed & 55 km/h \\ \hline No. of lanes & 2 lane highway \\ \hline RB Bandwidth & 10 MHz \\ \hline RBs & 50 \\ \hline Tx Power & 26 dBm \\ \hline Pkt Size & 350 B \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters. Figure 12: Resource collision probability; (**a**) of the Markov model and, (**b**) a confidence interval of 95%. better PDR than LTE Mode 4 and [PERSON] et al. Similarly, with retransmissions, the better PDR in TARS is due to the fewer resource collisions owing to its effective local RB prediction and subsequent selection for transmission. LTE Mode 4 and [PERSON] et al. employ retransmissions for redundancy and reliability in terms of PDR.