On Medium Access Control for Vehicular Communication over Device-to-Device Links: Radio Resource Management and Network Synchronization

Recent progress in wireless communication technologies along with the development in automotive industry bring new potentials to improve traffic safety, efficiency, and comfort. The enhancement can be achieved by vehicle-to-everything (V2X) communications that enable vehicles to cooperate with other vehicles, devices, and infrastructures. Due to the limitations of the current solutions for vehicular applications, integrating direct device-to-device (D2D) links into cellular systems is deemed a promising technology for V2X communications. However, the investigation of D2D-based V2X communication is still at its infancy and many issues, especially on medium access control (MAC) layer, need to be addressed before its efficient deployment. The aim of the thesis is to motivate the use of direct D2D links in cellular systems for supporting safety-related vehicular communications and to study two important MAC layer issues in this regard: radio resource management (RRM) and network synchronization. In the first part of the thesis, we first give an overview of vehicular communications including its legacy solutions and some future potentials. Then, we motivate the promising usage of D2D links for supporting V2X communications and investigate at a high level the MAC layer in this context, where generally speaking, MAC mechanisms decide how several transmitters share a common medium. Moreover, depending on different coverage scenarios, we distinguish between two MAC modes for D2D-based V2X communication and accordingly present two research questions, which are then studied from various aspects in detail. More specifically, when communicating vehicles are in network coverage, the base station (BS) can coordinate transmissions in a centralized approach, which is referred to as the network scheduled MAC mode. In this case, to mitigate the interference resulting from resource reuse between conventional cellular users (C-UEs) and vehicular users (V-UEs) as well as to satisfy the stringent requirements of vehicular applications, RRM is a key design issue. On the other hand, when some of, or all of, the communicating vehicles are out of network coverage, V-UEs can select the resource to transmit in an autonomous manner, which is referred to as the autonomous MAC mode. In this case, network synchronization is a crucial enabler to implement efficient time-division MAC methods. The second part of the thesis includes six appended papers that investigate the two above research questions specifically. In [Paper A], under the assumption that different V-UEs use orthogonal resource blocks (RBs) among each other, we study an RRM problem for D2D-based V2V communications with strict latency and reliability requirements and with access only to slowly time-varying channel state information. Even though the orthogonality among V-UEs brings simplicity to RRM design, allowing multiple V2X transmissions on the