# Resolving Persistent Packet Collisions through Broadcast Feedback in Cellular V2X Communication +
## 2 Related Work  
### Avoiding Persistent Packet Collisions  
[PERSON] et al. [5] proposed to curtail semi-persistent resource use at a fixed length, overriding repeated reselection of the same resource over a certain limit, even when it is prescribed by a high probability of keeping the same frequency resource [7]. As a side-effect, any persistent collisions are limited to a deterministic length. One concerning aspect with this proposal is that a single pre-configured length may not satisfy various application requirements. On the one hand, if the resource use time limit is set for too long, the protection from persistent packet collisions may be insufficient for a more safety-critical application. On the other hand, if the limit is set for too short, most vehicles that are not suffering from the persistent collision problem will be unnecessarily forced to frequently reselect resources, which tend to raise packet collision probability in SPS due to decreased resource use predictability [8].  
[PERSON] et al. [9] proposed selecting two resources instead of one and use them alternatingly for each packet transmission. Even if the packets transmitted on one of them persistently collide with other vehicle transmissions, it is less likely for the other vehicle to choose the same two resources and collide on the second as well. Therefore, it can break a persistent packet collision spurt into non-contiguous collisions. However, the alternation decreases the resource-use predictability, which has a detrimental effect on SPS performance [8]. In addition, the proposed method violates the cellular V2X standard framework in terms of the RRI specification. If two resources are used alternatingly, much more diverse resource reservation intervals (RRIs) will be required than are allowed in the standard, which are 20, 50, and \(100\cdot n\) (\(1\leq n\leq 10\)) ms in LTE V2X. NR V2X can have more diverse RRIs, but the newer ones are all below 100 ms [7]. Therefore, the issue persists if the original RRI exceeds 50 ms in NR V2X.  
[PERSON] et al. [10] proposed to explicitly publicize the next resource coordinate that will be used by the first packet after the reselection. In case the explicit reservation by a neighbor points to the same resource, the host vehicle can change it before publicizing it and avoid the collision. For example, a vehicle can change its resource selection to avoid a resource collision because the other vehicle already publicized that it was reserving it. However, even this cannot be the solution if the two vehicles currently use the same time slot, hence in the half-duplex relation. As they cannot hear each other, the vehicles reserving the same resources will be subject to the persistent packet collision problem.  
The blind retransmission scheme is a standard feature in both LTE [11] and NR [2]. If turned on, it always transmits one additional copy of the original packet at a random time displacement and at a different subchannel. Even if two vehicles coincide in the initial transmission, it is much less probable that they also coincide in the retransmission. Therefore, it can alleviate the consecutive packet collisions problem. The biggest problem with this, however, is that it consumes twice the bandwidth. For all vehicles to use it, the channel utilization must be under 50%. In this vein, NR additionally provides HARQ-based retransmission [2], which is much more efficient. Unfortunately, however, the HARQ-based retransmission is not applied to broadcast traffic. The blind retransmission may be usable when the channel load is not high. When the channel load is high, however, it may have to be coupled with congestion control (e.g., power control) that reduces its channel use because doubling the transmission requires a higher bandwidth.