  In the latest years, 3GPP has added short range cellular-vehicle-to-anything
(C-V2X) to the features of LTE and 5G in order to make vehicles, roadside
devices, and vulnerable users 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 defined to manage
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-V2V Mode 4, in this work
we analyze 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.
