# Resolving Persistent Packet Collisions through Broadcast Feedback in Cellular V2X Communication +
## 3 Broadcast Feedback on Packet Collisions  
### Problem of Resource Collision in SPS  
Most packet collisions are prevented in SPS through channel sensing that informs a vehicle of the resource reservations made by its neighboring vehicles. However, there are collisions that the current standard SPS cannot resolve. Figure 1 illustrates when such a collision occurs. Here, vehicle X reselects a frequency resource \(k\) for the next batch of periodic beacons to begin at slot n' when its resource reselection counter (\(RC_{X}\)) decrements to zero at slot \(n_{X}\). Unfortunately, the resource at the frequency-time coordinate <\(k\),\(n\)? (henceforth, denoted by \(R_{k}^{n^{\prime}}\)) has also been selected by another vehicle Y when \(RC_{Y}=0\) at slot \(n_{Y}\). Since these vehicles do not communicate their choices to other vehicles, there is no way they can change their decisions to evade the resource collision. If the message transmission period is the same for \(X\) and \(Y\), the vehicles will experience repeated packet collisions for up to a few seconds. In general, this problem occurs when the re-selecting vehicles have overlapping resource selection windows.  
The root cause of the persistent packet collision problem in the current SPS is the lack of communication on the selected resources between the vehicles that have overlapping selection windows. Even if vehicles could signal the choice to other vehicles [10], it cannot completely prevent the problem if the two vehicles currently use the same time slot, hence in the half-duplex relation. An alternative solution approach that does not have such problem is letting a third vehicle that can observe the collision event provide a feedback to the vehicles involved in the packet collisions. This is the approach taken by the present paper.  
Unlike LTE V2X, NR V2X supports all cast types in 3 GPP Release 16 [3; 6], namely unicast, groupcast, and broadcast. For reliability, the standard specifies the receiver feedback mechanisms, such as HARQ and PSFCH [2], for unicast and groupcast. Although relying on broadcast, however, the vehicular safety communication needs as much reliability as can be arranged. Providing broadcast feedback on the failed broadcast delivery can improve the reliability aspect. A difficulty is that because broadcast receivers can lie in various distances and channel conditions, they can provide conflicting feedbacks as to whether a packet collision occurred. Nevertheless, a careful and purpose-oriented design can overcome it. Specifically, defining the region of interest (ROI) helps reduce the chances of conflicting reports as far as the packet collision feedback is concerned. Then, the following aspects should also be addressed in the design of a broadcast feedback scheme:  
* Accurate identifying conditions of a packet collision within the ROI;
* Reliable method of selecting adequate feedback provider(s) among collision-witnessing vehicles;
* Mechanism to convey the packet collision notification;
* Procedure that the feedback receiver should follow.  
In this section, these aspects as well as how the scheme can be implemented in the current standard framework are discussed.