In this paper, we present advances of sidelink (SL) device-to-device (D2D) communications as a key-enabling technology for 5G enhanced vehicular-to-everything (eV2X) communications. We provide an overview about the resource allocation and scheduling of different SL D2D modes under incoverage and out-of-coverage application scenarios. Moreover, we present the scheduling for SL D2D V2X communications, which relies on semi-persistent scheduling (SPS) as proposed within the 3GPP specification. Simulations were carried out to evaluate the performance in terms of collision probability assuming different values of the key parameters such as resource reselection interval (RRI) and resource selection window. We finally discuss about the open technical challenges for ultrareliable and low-latency communications as distilled from the 5G V2X use cases introduced in 3GPP Rel.15. A cooperative resource allocation and scheduling solution is also given, where a more detailed version is considered our future work on this topic.

In this paper, a cooperative solution for vehicle-to-everything (V2X) communications is proposed and presented, which can guarantee the reliability and latency requirements for the 5G enhanced V2X (eV2X) services. Cooperation is useful for both in-coverage and out-of-coverage vehicular communications' scenarios. The proposed solution relies on the sidelink (SL) device-to-device (D2D) communications for the V2X communications. In this paper, we first provide a performance evaluation of the SL D2D V2X communications in terms of resource allocation and scheduling. The resource allocation is known as modes 3 and 4 SL D2D communications, and the scheduling is using a semi-persistent scheduling (SPS) approach. Simulation results are obtained in order to identify and highlight the reliability tradeoffs considering different payload sizes and SPS parameters. In the sequel, a cooperative solution that decreases the transmission collision probability is devised and presented, which is able to significantly improve the reliability of the future 5G eV2X communications. Different application scenarios are simulated to obtain the results that can guarantee the latency requirements per 5G eV2X use case as specified in 3GPP Rel.16 toward ultrareliable and low latency communications. INDEX TERMS Sidelink device-to-device, 5G enhanced V2X (eV2X) services, V2V communications, resource allocation, scheduling, cooperative ultra reliable and low latency communications.

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