LTE-V2X Mode 4: Increasing Robustness and DCC Compatibility with Reservation Splitting

IEEE 802.11p (ITS-G5 in Europe) and LTE-V2X are the dominant wireless access technologies for vehicle-to-vehicle communication. While the former has been extensively tested and evaluated for years, the cellular approach is relatively new. Previous research reported safety-critical issues of the distributed MAC mode such as re-occurring packet collisions, caused by repeated resource reuse due to the semi-persistent scheduling (SPS). While the evaluation of LTE-V2X is in its early stages, modifications and additions are simultaneously being released. The ETSI recently published a distributed congestion control (DCC) mechanism to limit the channel load and ensure that the random-access based MAC protocol can work as intended. By dropping packets and reducing the predictability of resource allocations achieved by SPS, this DCC mechanism raises concerns about the compatibility with the MAC protocol. In this paper, we present a modification to the MAC protocol that limits the impact of re-occurring collisions while retaining the benefits of SPS. Additionally, the proposed modification allows to implement an improved DCC version, which is better compatible with the MAC protocol of LTE-V2X. The modifications and DCC variant are evaluated in a system-level simulation and show significant benefits compared to the standard-compliant versions.