The revolution of cooperative connected and automated vehicles is about to begin and a key milestone is the introduction of short range wireless communications between cars. Given the tremendous expected market growth, two different technologies have been standardized by international companies and consortia, namely IEEE 802.11p, out for nearly a decade, and short range cellular-vehicle-to-anything (C-V2X), of recent definition. In both cases, evolutions are under discussion. The former is only decentralized and based on a sensing before transmitting access, while the latter is based on orthogonal resources that can be also managed by an infrastructure. Although studies have been conducted to highlight advantages and drawbacks of both, doubts still remain. In this work, with a reference to the literature and the aid of large scale simulations in realistic urban and highway scenarios, we provide an insight in such a comparison, also trying to isolate the contribution of the physical and medium access control layers.

In the latest years, 3GPP has added short-range cellular-vehicle-to-anything (C-V2X) to the features of LTE and 5G to allow vehicles, roadside devices, and vulnerable users to directly exchange information using the same chipset as for classical long-range connections. C-V2X is based on the use of advanced physical layer techniques and orthogonal resources, and one of the main aspects affecting its performance is the way resources are allocated. Allocations can be either managed by the network or in a distributed way, directly by the nodes. The latter case, called Mode 4, is required in those situations where the network cannot be involved in the scheduling process, for example due to a lack of coverage, but could also be adopted in order to reduce the processing burden of eNodeB. An algorithm, defined in the standards, makes nodes sense the medium and identify the best time-frequency combination to allocate their messages. Focusing on C-V2X Mode 4, in this work we analyse the parameters of the algorithm designed by 3GPP and their impact on the system performance. Through simulations in different large-scale scenarios, we show that modifying some parameters have negligible effect, that the proper choice of others can indeed improve the quality of service, and that a group of parameters allows to trade-off reliability with update delay. The provided results can also be exploited to guide future work. INDEX TERMS C-V2X, Intelligent vehicles, Vehicular and wireless technologies, Wireless networks

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1 With the increasing needs of the automotive industry, 3GPP is developing new functionalities in the long term evolution (LTE) cellular technology to specifically address vehicleto-everything (V2X) connectivity, including in particular vehicleto-vehicle (V2V) direct communications. At the basis of most enabled applications, either dealing with safety, traffic management or infotainment, it is of particular importance the cooperative awareness service, corresponding to messages sent periodically in broadcast by all vehicles to inform about their status and intentions. To optimize the usage of radio resources, allocations of packets are required to be smartly performed, following algorithms that currently represent a challenging open issue. Having in mind to face the design and validation of resource allocation in LTE-V2V networks when cooperative awareness is addressed, we implemented a simulator, called LTEV2Vsim, which will be shared with all the interested researchers. LTEV2Vsim is written in MATLAB and manages vehicles mobility either following simple models or more complex and realistic input files, and performs allocations following different implemented algorithms, both network controlled and autonomous. Finally, to demonstrate the possibilities of LTEV2Vsim, some example results are provided.