# Resolving Persistent Packet Collisions through Broadcast Feedback in Cellular V2X Communication +
## 4 Consecutive Error Prevention Performance: Evaluation
### Duration of Consecutive Collisions  
The main objective of the feedback scheme is to shorten the duration of the consecutive collision events through the broadcast feedback, so that vehicles can avoid the loss of awareness among its neighbors. Evaluating the impact of the proposed scheme in terms of the duration of such events needs a careful definition of the metric because the duration obviously depends on the Tx-Rx distance. Namely, the farther a receiver is from the broadcast transmitter, the longer the average duration of packet losses will be. Therefore, the packet \"collision\" is defined as an event in which two vehicles physically positioned within a pre-defined, application-specific distance simultaneously use the same frequency resource to transmit their BSMs. In this paper, the distance is set to 100 m. In NR V2X, a \begin{table}
\begin{tabular}{c c c} \hline \hline \(\rho\) & CBR (SPS) & CBR (SPS + Blind ReTx) \\ \hline
100 & 45.94\% & 69.45\% \\
150 & 62.85\% & 84.30\% \\
200 & 74.44\% & 92.68\% \\
250 & 83.03\% & 96.45\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Channel busy ratio (CBR) for various traffic densities. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Parameter** & **Default Value** \\ \hline \multirow{8}{*}{PHY} & Channel model & Winner+ B1 \\  & Channel bandwidth & 10 MHz \\  & MCS level & 7 \\  & No. of subchannels & 2 \\  & Transmit power & 23 dBm \\  & IBE ratio (\(\alpha\)) & 0.0035 \\ \hline \multirow{2}{*}{Application} & BSM size & 300 bytes \\  & BSM rate & 10 Hz \\ \hline \multirow{4}{*}{SPS} & Sensing window size & 1 s \\  & Selection window size & 100 ms \\  & Resource reselection counter & [5:15] \\  & Resource keep prob. (\(\mathrm{P_{k}}\)) & 0.8 \\  & Initial RSRP threshold & \(-\)110 dBm \\ \hline \multirow{4}{*}{Road} & Road topology & Highway, straight \\  & Road length & 3 km \\ \cline{1-1}  & Vehicle density & [100:250] veh./km \\ \cline{1-1}  & Vehicle distribution & 1-D Poisson point process \\ \hline \multirow{2}{*}{Feedback algorithm} & RSSI threshold & \(-\)85.2 dBm \\ \cline{1-1}  & SINR threshold & 7.3 dB \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Simulation parameters and their default values. transmitter can specify the required communication range in SCI Format 2B [6]. Thus, in practice the feedback providers may instead apply application-specific thresholds for the three checking conditions in the previous section. Figure 8 shows the average consecutive packet collision duration once a collision event takes place, assuming the 10 Hz beaconing by vehicles. Note that the y-axis is in log scale. First observe in the figure that regardless of the traffic density, the average duration of the consecutive collisions in the standard SPS ranges between 2000 ms and 2500 ms. They are an order of magnitude longer than in the proposed scheme (\"SPS + colFeedback\") or in the alternating transmission scheme (\"SPS + alternating\"). The standard blind retransmission is expected to have a similar duration as the alternating scheme, but it has an excessive bandwidth requirement that it cannot be used when the channel utilization is as high as assumed in Table 1 (i.e., higher than 50%). So, the discussion of the blind retransmission scheme is deferred until Section 4.4. Analyzing the SPS performance itself is a major undertaking that has not been done so far in the literature. Moreover, analyzing SPS is not the focus of this paper so this paper does not attempt a precise analysis of the long consecutive packet collision durations of SPS. However, an explanation through a first-order approximation can be readily given as follows. Let us call the consecutive packet transmissions by a vehicle using the same frequency resource until the resource reselection counter (RC) reaches zero by a \"packet run,\" having a uniform random variable \(X_{i}\) as the run length where \(i\) is the index of the run. The RC for the \(i\)-th run is initialized to a standard-specified value upon a resource reselection.