Exploring NR-V2X Dynamic Grant Limitations for Aperiodic Traffic

The recent 3GPP NR-V2X standard (Rel. 16) has largely built upon its precursor Cellular-V2X (Rels. 14 & 15) but has introduced new approaches for dealing with application traffic exhibiting aperiodic arrival rates in the sidelink. This is vital as safety services based on ETSI Cooperative Awareness Messages (CAMs) and Decentralised Environmental Notification Messages (DENM) exhibit such characteristics. It is further envisaged that future vehicular services will also exhibit high aperiodicity to support increased autonomy. In this paper we quantitatively evaluate the reasons why the Sensing based Semi-Peristent Scheduling (SB-SPS) mechanism performs poorly when scheduling aperiodic traffic. We then provide the first in-depth evaluation of the NR-V2X Dynamic Grant mechanism in contrast to schemes that parameterise the existing C-V2X SPS algorithm and evaluate the performance of alternative dedicated scheduling mechanisms specifically designed for aperiodicity. This paper highlights that the level of aperiodicity exhibited by the application model greatly impacts scheduling performance, both for the default SB-SPS and dedicated approaches. As such we conclude that a novel aperiodic scheduling mechanism must be devised, or more promisingly, an approach to enable application traffic to mimic periodic characteristics allowing it to co-exist with the existing scheduling approach.