LTEV2X Mode 4: Increasing Robustness and DCC Compatibility with Reservation Splitting IEEE 802.11p (ITSG5 in Europe) and LTEV2X are the dominant wireless access technologies for vehicletovehicle communication. While the former has been extensively tested and evaluated for years, the cellular approach is relatively new. Previous research reported safetycritical issues of the distributed MAC mode such as reoccurring packet collisions, caused by repeated resource reuse due to the semipersistent scheduling (SPS). While the evaluation of LTEV2X 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 randomaccess 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 reoccurring 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 LTEV2X. The modifications and DCC variant are evaluated in a systemlevel simulation and show significant benefits compared to the standardcompliant versions.

Evaluation and optimization of Decentralized Congestion Control Algorithms for Vehicular Networks Nowadays, road traffic management is becoming a major challenge for realistic society. As a result, reliable communication between vehicles is the key point to this challenge. Currently, IEEE802.11p which is considered as de facto standard for road communication is designed to solve this challenge. However, the communication channel medium is still expected to get congested when a large number of vehicles exist. Target to solve this, European Telecommunication Standards Institute (ETSI) has standardized a set of Decentralized Congestion Control (DCC) mechanisms to control channel load. One of the main topics is achieving channel load control to guarantee reliable communication for platooning systems. In this paper, we focus on investigating on DCC reactive control approaches, aiming to provide comprehensive insights of how DCC framework transmission parameters, i.e. message generation rate, transmission power and data rate, will impact the stability of platooning systems. Besides, for each instance of the transmission parameter, we target to optimize the parameter and propose more stable control algorithms by running repetitive simulations.