# A Review on Millimeter-Wave Hybrid Beamforming for Wireless Intelligent Transport Systems
###### Abstract
Furthermore, reflective intelligent surfaces (RISs) have emerged as a new frontier, leveraging programmable metasurfaces to modify the propagation environment for improved communication performance [5]. In the context of Intelligent Transportation Systems (ITSs), the integration of satellite communication, mmWave roadside units, and connected vehicles present a multifaceted communication scenario, as illustrated by Figure 1. Satellites play a crucial role in providing wide-area coverage and connectivity for ITS applications, offering a reliable communication link that can support data exchange over vast geographical areas [6]. When combined with mmWave roadside units, which enable high-speed and low-latency communication at close proximity to the vehicles, the system enhances real-time data transmission, such as traffic updates, navigation information, and safety alerts [7]. Connected vehicles within this framework benefit from seamless connectivity and information exchange, enabling them to receive and transmit critical data for optimal navigation, collision avoidance, and overall traffic management in the intelligent transportation ecosystem. This integrated setup leveraging satellites, mmWave roadside units, and connected vehicles showcases a promising communication infrastructure for enhancing the efficiency, safety, and intelligence of modern transportation systems [8]. MmWave technology, with its wide bandwidth and precise beamforming capabilities, is increasingly being utilized to enhance communication in unmanned aerial vehicle (UAV) systems. This advancement offers a range of applications for UAVs, as illustrated in Figure 2, across various scenarios such as aerial photography, surveillance, and remote sensing. In these contexts, UAVs frequently generate substantial uplink data traffic by sending visual imagery, sensor readings, and other critical data to centralized control centers for analysis and decision-making. The high data transfer requirements inherent in aerial photography, surveillance, and remote sensing applications necessitate efficient and reliable communication links between the UAVs and the control centers. To address this challenge, mmWave access links are established to establish seamless connectivity between the UAVs and the core networks. These links can be established through ground-based base stations or satellite connections, depending on the specific operational requirements and coverage area [9]. Figure 1: Intelligent Transportation Systems (ITSs): the integration of satellite communication, millimeter-wave (mmWave) roadside units, and connected vehicles. Figure 2: MmWave-unmanned aerial vehicle (UAV) communications with scenarios such as aerial photography, surveillance, and remote sensing. For instance, in scenarios where UAVs are utilized for real-time surveillance or monitoring activities, the ability to transmit high-quality video streams and sensor data in a timely manner is paramount for effective decision-making and response. By leveraging mmWave access links, UAVs can establish robust connections with ground-based infrastructure or satellites, enabling the rapid and reliable transmission of data to control centers for analysis and situational awareness. Furthermore, in the realm of aerial photography, mmWave technology facilitates the efficient transfer of high-definition images and videos captured by UAVs, enabling photographers, filmmakers, and surveyors to obtain detailed and accurate visual data from aerial perspectives. This seamless connectivity enabled by mmWave access links ensures that visual and sensor data collected by UAVs can be transmitted in real time to control centers or storage facilities for processing, storage, and subsequent retrieval [10]. In the domain of remote sensing, mmWave-enabled UAV communications play a crucial role in collecting environmental data, conducting scientific research, and supporting precision agriculture applications.