Impact of the Generation Interval on the Performance of Sidelink C-V2X Autonomous Mode

Cellular-vehicle-to-everything (C-V2X) is gaining an increasing interest among the technolo- gies under consideration for future connected and automated vehicles, especially for sidelink LTE-V2X Mode 4 and sidelink 5G-V2X Mode 2, where nodes autonomously perform resource allocations and transmissions, without relying on any infrastructure. In these cases, the allocation process has been designed based on the assumption of periodic packet generation constrained to a few possible allocation periods. Nevertheless, even assuming that the awareness messages are continuously exchanged, the packet generation might not always be exactly periodical or might come with a periodicity not constrained to the expected values, as for example with the Cooperative Intelligent Transport Systems (C-ITS) standard defined by the European Telecommunications Standards Institute (ETSI). The non-ideal periodicity of packet generation, if not properly taken into account, can significantly impact the operation and performance of C-V2X when vehicles autonomously select their radio resources. This paper provides an analysis of how a misalignment between packet generation and resource allocation affects the system performance and provides insights into how to design the parameter settings to improve the performance. A case study is provided, focusing on LTE-V2X Mode 4 and the cooperative awareness message (CAM) generation in agreement with the the ETSI specifications. Results show that the performance generally improves if the smallest allocation interval is adopted and if the reservation is maintained even when no packets are ready for transmission. It is also shown how the parameter controlling the maximum latency in LTE-V2X, jointly with the misalignment between the allocation and generation intervals, can further affect the packet reception rate.