# TARS: A Novel Mechanism for Truly Autonomous Resource Selection in LTE-V2V Mode 4
###### Abstract  
Effective communication in vehicular networks depends on the scheduling of wireless channel resources. There are two types of channel resource scheduling in Release 14 of the 3 GPP, i.e., (1) controlled by eNodeB and (2) a distributed scheduling carried out by every vehicle, known as Autonomous Resource Selection (ARS). The most suitable resource scheduling for vehicle safety applications is the ARS mechanism. ARS includes (a) counter selection (i.e., specifying the number of subsequent transmissions) and (b) resource reselection (specifying the reuse of the same resource after counter expiry). ARS is a decentralized approach for resource selection. Therefore, resource collisions can occur during the initial selection, where multiple vehicles might select the same resource, hence resulting in packet loss. ARS is not adaptive towards vehicle density and employs a uniform random selection probability approach for counter selection and reselection. As a result, it can prevent some vehicles from transmitting in a congested vehicular network. To this end, the paper presents Truly Autonomous Resource Selection (TARS) for vehicular networks. TARS considers resource allocation as a problem of locally detecting the selected resources at neighbor vehicles to avoid resource collisions. The paper also models the behavior of counter selection and resource block reselection on resource collisions using the Discrete Time Markov Chain (DTMC). Observation of the model is used to propose a fair policy of counter selection and resource reselection in ARS. The simulation of the proposed TARS mechanism showed better performance in terms of resource collision probability and the packet delivery ratio when compared with the LTE Mode 4 standard and with a competing approach proposed by [PERSON] et al. LTE-V2V Mode 4;3 GPP; eNodeB; resource collision; vehicular network; road safety applications +
Footnote †: journal: _Sensors_  
0000-0001-9993-3300-3  
shared wireless channel before transmission. The downside of shared wireless resources in the WAVE comes from the limited channel capacity, which is subject to frequent saturation, which causes packet loss and inaccurate traffic information [3]. More recently, the Long-Term Evolution (LTE)-based Vehicle-to-Everything (V2X) proposal has shown potential to improve upon the channel saturation experienced in the WAVE [4; 5; 6; 7; 8; 9; 10; 11]. Explicitly, Release 14 of the Third Generation Partnership Project (3 GPP) includes support for Vehicle-to-Vehicle (V2V) services [12; 13]. There are two types of channel resource scheduling, i.e., (1) controlled by Evolved Node B (eNodeB), i.e., LTE-V2V Mode 3, and (2) a distributed scheduling carried out by every vehicle, also known as the Autonomous Resource Selection (ARS), i.e., LTE-V2V Mode 4, shown in Figure 1. Resource scheduling via eNodeB provides a centralized mechanism of allocating resources to vehicles, and it involves a slight delay before communication occurs. vehicle safety applications require more spontaneous transmission of messages; therefore, the distributed scheduling approach in Release 14 of the 3 GPP is preferred for safety applications [14]. Vehicles in distributed scheduling autonomously select resources from a set of periodically reoccurring resource pools. Each vehicle then broadcasts its selection through a message called Scheduled Assignment (SA). This broadcast helps prevent other vehicles from selecting the same resource. However, resource collisions may still occur in the initial selection, where multiple vehicles can select the same resource at a time [15]. Vehicles involved in resource collisions might not cooperate with other vehicles, making the performance of the network questionable and bringing Release 14 of the 3 GPP away from its design objectives for vehicle safety communication. It is also worth noting that counter selection specifies the time for which allocated resources can be used by a vehicle. Upon expiry of the counter value, the resources must be released and reselected.