 tioning accuracy can be significantly higher in 5G networks relative to legacy LTE net- works due to higher signal bandwidths and dense network deployments, which enables LoS communication with a high probability. Estimation of the relative position between vehicles is directly useful for certain use cases such as platooning, but can also serve as input to the cellular network in order to improve the estimation of vehicles' absolute position [16]. Global ITS spectrum is currently under further study within ITU-R Working Party 5A. This study is considered essential in improving the safety and efficiency of roads and highways. Specifically, the ITU World Radio communication Conference 2015 (WRC-15) adopted a ITU-R Study Group 5 (SG 5) Working Party 5A (WP 5A): Land mobile service excluding IMT; amateur and amateur-satellite service https://www.itu.int/en/ITU-R/study-groups/rsg5/rwp5a 802 Chapter 7 resolution to include a new agenda item in the WRC-194 to conduct studies on technical and operational aspects of evolving ITS implementation using existing mobile service allocations. 7.3 Network Architecture and Protocol Aspects 7.3.1 Reference Architecture LTE-based V2X operates in two modes: direct connection via sidelink and via the network. In the direct communication, the UE uses the LTE PC5 interface, which is based on 3GPP Rel-12 ProSe. The D2D communication in LTE operates over the sidelink channel in two different modes. In the first mode, the resources are allocated by the network (i.e., eNB). Devices that intend to transmit will send a request to the network for resource allocation, and the network subsequently allocates the resources and notifies the device. This mode requires additional signaling for every transmission, thus increasing transmission latency. In the second operation mode, devices select the resource autonomously. The autonomous mode reduces latency, but some issues related to possible collisions and interference may arise. Optimization of resource allocation procedures has been considered wi