# Resolving Persistent Packet Collisions through Broadcast Feedback in Cellular V2X Communication +
## 3 Broadcast Feedback on Packet Collisions
### Feedback Implementation  
The feedback information is not separately transmitted, but piggybacked in an already scheduled broadcast message (e.g., BSM) transmission. For this purpose, the SCI in the PSCCH can be utilized. For example, if the first entry in the collision feedback queue points to the resource location \(\varphi=<m,\,k+1>\), the location is included in the SCI of the first transmitted packet, in slot \(n>m\) (Figure 6).  
In NR V2X, up to \(N_{max}-1\) additional resource reservation pointers can be used in the SCI Stage I, where \(N_{max}=3\)[6]. Among \(N_{max}\), one points to the initial transmission, whereas the others point to the retransmission resources associated with the SCI. In the NR SCI Format 1 shown in Figure 7a, the Frequency resource assignment field includes the frequency coordinates of the pointers. The size of the frequency resource field \(L\) depends on the number of resource pointers to use. If \(N_{max}=2,\,L\,=\,log_{2}\left(NsubCH\left(NsubCH+1\right)/2\right)\), where \(N_{subCH}\) is the number of subchannels. If \(N_{max}=3\), \(L=log_{2}\left(NsubCH\left(NsubCH+1\right)/6\right)\). The particular \(N_{max}\) value to be used is configured by the higher layers. Paired with the frequency resource assignments, the Time resource assignment field specifies the time coordinates of the reserved resources. It is \(M=5\) bits for \(N_{max}=2\), and \(M=9\) bits for \(N_{max}=3\)[2].  
In the current standard specification for NR V2X, the additional pointers are specified to point to the reserved resources for potential future HARQ-based retransmissions within 31 slots or approximately 15 ms. However, there have also been proposals to use the two pointers more ingeniously. For instance, a recent proposal in 3 GPP emphasizes that the reservation of a sidelink resource for an initial transmission of a different TB should be applicable [20]. Extending this line of thought, the proposed scheme utilizes one of these pointers, but to point to a collision resource location in the past. When the pointer to the retransmission resource is not used, it can be used for the resource pointer by setting \(N_{max}=2\). If the retransmission resource is reserved by default, the third pointer can be used for the same purpose by setting \(N_{max}=3\). In addition, one more bit is needed to signal that it is a feedback on a collision event, most likely from the Reserved field. In  
Figure 6: Feedback information in SCI pointing to the resource location of the suspected packet collision.  
LTE V2X, there is one pointer that can be used for a blind retransmission resource [21] (Figure 7b), which can be utilized for the collision pointer when the blind retransmission is not used. If a pointer cannot be made available for the collision reporting purpose at all, however, another implementation strategy could be to use the Reserved field. As the C-V2X standard is still evolving, exploring this possibility is left for a future work. However, note that the scheme is an opportunistic enhancement that aims at improving the baseline SPS performance. Whenever there is a space for a single collision pointer left unused by the employed retransmission, the proposed scheme can utilize it to provide the feedback. It does not need to take precedence over retransmissions to rob the pointer space from it.