## 1 Introduction
Therefore, if two UEs happen to choose the same time-frequency resource for BSM transmission, collision cannot be avoided. Moreover, it cannot be detected by either party. Worse yet, the sidelink Mode 4 uses Semi-Persistent Scheduling (SPS), the collision episode can persist across multiple messaging intervals without the colliding UEs knowing it. Furthermore, vehicles may choose one of a few standardized messaging intervals (e.g., 10 Hz, 20 Hz, or 50 Hz [20]), raising the possibility of the UEs with the same messaging interval facing the repeated message collision risk. This is one of the reasons that the scheduling is not persistent, but semi-persistent. Each \"streak\" of periodic message transmissions has an average duration of one second, after which a UE moves to other resource with some probability for the next streak [21]. This resource reselection will part the unknowingly colliding UEs. Another reason for the SPS is the topology change. When new vehicles join the group of vehicles that are coordinating the resource use through the sensing-based SPS, a newly joining vehicle may have been using the same time-frequency resource that another vehicle from the existing group uses. In this case, the same sustained message collisions can ensue. Although the current standard has the aforementioned message collision resolution mechanism, we observe that it has not been engineered to its maximum potential to meet the message delivery probability for safety communications. Especially when we envision a high-reliability C-V2X communication for future applications such as remote driving and autonomous driving, we need a very tight resource coordination and control among vehicular UEs. In this paper, we show that by sharing the information regarding their resource reselection to neighboring UEs, each UEs can mutually lower the message collision probability. Specifically, we propose the vehicles broadcast in their safety beacons their planned resource reselection earlier than their actual reselection instance. We show through simulation that the proposed algorithm far exceeds the packet delivery ratio (PDR) performance in various situations. We also discuss what changes should be made in the current standard to carry the planned reselection information. In particular, we discuss how the SCI Format 1 should be modified for better message collision resolution. In this paper, we limit ourselves to the study of the _algorithmic aspect_ of the message collision resolution mechanism in the sensing-based SPS algorithm. Other aspects such as the physical channel models and their impacts to the algorithm are deferred to a future work. Also, note that the message collision resolution itself is not a congestion control mechanism, but it is directly related with the congestion control in C-V2X. When congestion becomes severe, message collisions will become more likely, and an efficient collision resolution algorithm will mitigate the undesirable impact of the high congestion level on the packet delivery performance. However, devising a fully-fledged congestion control algorithm using one or more standard-provided means such as power control, MCS control, retransmission control, or packet dropping, is beyond the scope of this paper. The rest of the paper is organized as follows. In Section 2, we summarize the works that compare C-V2X with the incumbent IEEE 802.11p DSRC/C-ITS. Then we discuss recent related work on radio resource management, network-controlled and autonomous, where our work falls in the second category. In Section 3, we describe the sensing-based SPS algorithm and its collision resolution mechanism employed in the current standard specification. In Section 4, we discuss the proposed scheduling algorithm. In Section 5, we perform simulation experiments to compare the current SPS and the proposed algorithm, showing the performance gap we can obtain through the proposal. In Section 6, we conclude the paper with the summary of our work and the list of future work items.