Unified Aerial and Terrestrial 5G NR Sidelink Multi-hop Transmission for Enhanced V2X Communication

New Radio (NR) Sidelink was first introduced by 3GPP in Rel. 16 to meet service requirements of advanced Vehicle-to-Everything (V2X) use cases. The mode-2 of NR-V2X enables direct vehicle-to-vehicle communication through distributed resource scheduling, specifically Semi-Persistent Scheduling (SPS). The focus of Intelligent Transport Systems (ITS) till now has been predominantly on ground vehicles and infrastructure, however the anticipated future scenario where airspace becomes as crowded as ground space by the mid-2030s necessitates a robust and efficient cooperation between aerial vehicles (AVs) and terrestrial/ground vehicles (GVs). There is also an ongoing discussion in the 3GPP Technical Specification Group Radio Access Network (TSG RAN) on supporting aerial communications in AVs by using NR-sidelink. This paper is the first to study extensively the performance of Cooperative Awareness Messages (CAM) and Decentralized Environmental Notification Messages (DENM) in a unified setup, involving systems of aerial vehicles (AVs) and systems of ground vehicles (GVs) in mode-2 using NR Sidelink. The SPS at Medium Access Control (MAC) layer schedules CAM and DENM packets. We conduct a comparative analysis of CAM performance for AVs versus GVs using a 3D geographical dataset of Washington DC, evaluating Packet Reception Ratio (PRR) and Packet Inter-Reception (PIR). Furthermore, we enhance the reachability of DENM among GVs by integrating AVs using the NR sidelink UE (User Equipment)-to-UE relay feature introduced in Rel. 17/18. Finally, we propose a congestion-aware multi-hop DENM dissemination protocol with a novel relay selection mechanism to keep congestion below the maximum Channel Busy Ratio (CBR) limit, thereby improving reception node coverage by 17% compared to state-of-the-art NR-V2X DENM dissemination mechanisms.