On Enhancing Semi-Persistent Scheduling in 5G NR V2X to support Emergency Communication Services in Highly Congested Scenarios

3GPP Rel. 16 introduced 5G New Radio (NR) Vehicle to Everything (V2X) to support advanced use cases in Intelligent Transport Systems (ITS) with stringent Quality of Service (QoS) requirements, which include vehicular safety and traffic management applications. However, in highly congested traffic scenarios (i.e., too many vehicles in a single collision domain than the number of available channel resources), emergency vehicles performing sensing-based Semi-Persistent Scheduling (SPS) suffer from low Packet Reception Rate (PRR) and high Packet Inter-Reception time (PIR) due to scarcity of channel resources and increased collision rates. To address these issues, we propose a Priority-Based Semi-Persistent Scheduling (P-SPS) scheme which helps emergency vehicles by classifying them as high priority (HP) vehicles with higher Reselection Counter (RC) values and letting them to transmit in one of the channel resources whenever they have CAM messages ready for transmission irrespective of whether such resources are available or not. We also propose a complementary Probabilistic Collision Mitigation (PCM) scheme and an Intelligent Grant Removal (IGR) scheme to minimize the chances of collision among HP and low priority (LP) vehicles in the network. We implement the proposed enhancements to SPS in NR V2X module of the system level Network Simulator 3 (NS-3) and demonstrate how these help in improving the performance of HP and LP vehicles as compared to legacy SPS of 5G NR V2X in highly congested scenarios.