# A Review on Millimeter-Wave Hybrid Beamforming for Wireless Intelligent Transport Systems
###### Abstract  
As the world braces for an era of ubiquitous and seamless connectivity, hybrid beamforming stands out as a beacon guiding the evolutionary path of wireless communication technologies. Several hybrid beamforming technologies are explored for millimeter-wave multiple-input multi-output (MIMO) communication. The aim is to provide a roadmap for hybrid beamforming that enhances wireless fidelity. In this systematic review, a detailed literature review of algorithms/techniques used in hybrid beamforming along with performance metrics, characteristics, limitations, as well as performance evaluations are provided to enable communication compatible with modern Wireless Intelligent Transport Systems (WITSs). Further, an in-depth analysis of the mmWave hybrid beamforming landscape is provided based on user, link, band, scattering, structure, duplex, carrier, network, applications, codebook, and reflecting intelligent surfaces to optimize system design and performance across diversified user scenarios. Furthermore, the current research trends for hybrid beamforming are provided to enable the development of advanced wireless communication systems with optimized performance and efficiency. Finally, challenges, solutions, and future research directions are provided so that this systematic review can serve as a touchstone for academics and industry professionals alike. The systematic review aims to equip researchers with a deep understanding of the current state of the art and thereby enable the development of next-generation communication in WITSs that are not only adept at coping with contemporary demands but are also future-proofed to assimilate upcoming trends and innovations. MIMO; wireless intelligent transport systems; hybrid beamforming; millimeter-wave; next-generation communication system +
Footnote †: journal: _Future Internet_  
Hybrid beamforming has emerged as a pivotal technology that achieves a compromise between fully digital and fully analog beamforming techniques, offering both flexibility and power efficiency. This approach utilizes a reduced number of radio-frequency (RF) chains connected to a larger antenna array through phase shifters, switches, or attenuators. It is particularly well suited to mmWave MIMO systems where large antenna arrays can be packed into a compact form factor due to the small wavelength; yet, the cost and power consumption of fully digital beamforming with one RF chain per antenna element would be prohibitive [2]. The efficacy of hybrid beamforming is reliant on its design, crafted around multiple dimensions: link characteristics, user demands, band-specific propagation properties, scattering scenarios, structural constraints, duplexing modes, carrier frequencies, network topologies, as well as the applications envisioned ranging from cellular communication to satellite links. Each facet introduces distinct considerations, demanding a classification framework that systematically encapsulates the hybrid beamforming technologies [3]. Research in this sphere has produced a myriad of designs, algorithms, and methodologies that grapple with the intrinsic complexity of hybrid beamforming. A substantial volume of literature addresses algorithmic strategies for beamformer design, such as phase-only control, subspace methods, and sparse optimization. These techniques aim to tailor the beam patterns for enhanced throughput, reduce interference, and improve overall system capacity [4]. Moreover, recent advancements have expanded the application scope of hybrid beamforming, integrating it into diverse systems. These include but are not limited to Intelligent Transportation Systems (ITSs), unmanned aerial vehicles (UAVs) for robust control and communication, satellite communications for improved link reliability, (RADAR) for enhanced resolution and target detection, and a wireless local area network (WLAN) for higher throughput and coverage.