# Resolving Persistent Packet Collisions through Broadcast Feedback in Cellular V2X Communication +
## 1 Introduction  
The Third Generation Partnership Project (3 GPP) has recently published Release 16 [1]. The newest release defines the sensing-based semi-persistent scheduling (SPS) as the resource allocation scheme for Sidelink Mode 2 for 5G New Radio (NR) vehicle-to-everything (V2X) communication. SPS allows vehicles to autonomously select a frequency resource through a sensing procedure and periodically use it for a few seconds without further scheduling until the next selection (also known as reselection) [2, 3]. The scheduling scheme well serves the periodic safety-oriented beacons such as the basic safety messages (BSMs) [4] in vehicle-to-vehicle (V2V) scenarios because it reduces the overhead in resource selection. To minimize the chances of multiple vehicles selecting the same frequency resource, which would cause persistent packet collisions, SPS requires that each vehicle signals the time gap to the next packet called the resource reservation interval (RRI) in the Sidelink control information (SCI). Using this information, neighboring vehicles that monitor the SCI in every received packet can exclude the reserved resource from its candidate resource pool if they need to reselect a resource. Although SPS can prevent many potential packet collision events through channel sensing, a well-known issue in Sidelink Mode 2 is that the persistent packet collisions are not completely avoidable [5]. The problem occurs when two vehicles need to select a new frequency resource at nearly identical times. In the current standard [2], vehicles do notadvertise the resource location that they select for the next batch of periodic packets. As a consequence, the vehicles concurrently performing reselection can unknowingly decide on the same resource. Once this occurs, the packet collisions can persist for at least one second because a selected resource is used for one second on average. The problem can be aggravated even further because the current standard prescribes that the previous frequency resource can be kept in the reselection with a non-zero probability [3], producing a geometric distribution for the number of reuses. This condition will be only resolved when one or both of the vehicles reaches the time to reselect again. Unlike in Sidelink Mode 1, where the base station centrally allocates the resources, such resource collisions are inevitable in the distributed resource selection algorithm in Mode 2. The cost of a multi-packet collision event can be high. The event renders the involved vehicles to be unrecognized by neighboring vehicles and by each other during the persistent collision event. Onboard sensors, such as radars, LiDARs, and cameras may still be able to maintain awareness in many situations, but in others like the non-light-of-sight (NLoS) positions, neighboring vehicle movements become difficult to trace under the elongated packet loss event. Therefore, in safety-critical situations, this communication lull may push a vehicle into dangerous driving conditions [5], which might even be worse off than not relying on the C-V2X safety communication at all. Ideally, vehicles that happened to choose the same frequency resource should be able to sense the conflict as soon as it happens and back off. Choosing a resource, either on a different frequency and/or at a different time offset, would immediately terminate the persistent collision event early on. Unfortunately, however, vehicles involved in the event cannot realize the problem by themselves due to the typical half-duplex communication capability of vehicular user equipment, such as onboard units (OBUs). They are both transmitting when a collision occurs so they are unable to hear each other. Even the hybrid automatic repeat request (HARQ) mechanism and the physical sidelink feedback channel (PSFCH) introduced in NR V2X [2] cannot help because they are not for broadcast traffic, such as periodic BSM transmissions. The objective of this paper is to solve the persistent packet collision problem through a novel feedback scheme.