# Resolving Persistent Packet Collisions through Broadcast Feedback in Cellular V2X Communication +
## 4 Consecutive Error Prevention Performance: Evaluation
### Packet Reception Ratio  
Having explored their effectiveness in preventing consecutive packet collision events, it is necessary to investigate how the compared schemes fare in the overall performance measure of PRR. But before starting this section, it is important to emphasize again that improving the average PRR is not the objective of this paper. Instead, it is necessary to improve the consecutive loss characteristics of the V2V transmission without harming the average PRR. Any gain on PRR is obtained should be considered as not what is intended but a good side effect. Therefore, the objective here is to ascertain that the proposed scheme does not sacrifice the PRR performance to achieve the shorter lengths of consecutive packet collisions. Figure 10: Increased collision probability with SPS + alternating. Figure 11 shows the PRR performance as a function of the Tx-Rx distance under three different traffic densities. In particular, if the transmitter at the relative position 0, the SPS + colFeedback scheme will provide the feedback so that it can prevent further colliding packets. Figure 11: Packet reception ratio comparison. (**a**) \(\rho=100\); (**b**) \(\rho=150\); (**c**) \(\rho=200\). First observe in the figure that in all tested traffic densities, the use of the collision feedback does not deteriorate the PRR, but instead very slightly improves it over SPS especially in the shorter Tx-Rx distances. It owes to the side-effect of spreading the transmission more evenly over the resource plane through prompt reselections upon collision. The reason that the PRR improvement is confined to the shorter distances is that the RSSI threshold is set so that only the feedbacks from approximately within 100 m of the packet-colliding vehicles are allowed (inequality (3)). Therefore, it will mainly resolve the collision-induced losses sensed in the shorter distances from the transmitter. Compared with the SPS + colFeedback scheme, the SPS + alternating scheme and the standard blind retransmission scheme exhibit worse PRR performance than the original SPS. The reason for the former is the significantly increased packet collision probability as illustrated in Figure 10. The blind retransmission scheme is the worst and deteriorates fast as the traffic density increases. It is because it requires double the bandwidth, which becomes increasingly more difficult to afford under higher channel utilization (Table 1, right column). Nevertheless, the blind retransmission scheme cannot be dismissed as useless because retransmissions can be helpful at shorter TX-RX distances, which is the most safety-critical region. For instance, power control can be associated with the blind retransmission so that it can obtain the high PRR without persistent packet collisions by reducing the transmit power so that the spatial footprint is reduced. In that case, the overall PRR may not be the best metric to evaluate the value of the blind retransmission. Unfortunately, congestion control is a major topic in itself and the C-V2X standards are still immature. Therefore, it will be worthwhile to explore in the future topic of blind retransmission as a tool to resolve the persistent packet collisions in association with a power control. In summary, the proposed feedback scheme effectively solves the persistent collision problem while not hurting the overall packet reception probability over the distance. This feature of broadcast feedback to the previous broadcast transmission has not been actively considered in the literature or the standard framework, but if carefully designed and implemented it can be a useful tool in making the broadcast-oriented safety communication more reliable in C-V2X. Packet collisions are the biggest source of packet loss in the most safety-critical distances from the transmitter [25], and the broadcast collision feedback can prove to be highly relevant.