# A Review on Millimeter-Wave Hybrid Beamforming for Wireless Intelligent Transport Systems
## 3 An In-Depth Analysis of the mmWave Hybrid Beamforming Landscape  
In a comprehensive analysis of the mmWave hybrid beamforming landscape, it is imperative to consider a myriad of factors that influence the deployment and optimization of hybrid beamforming techniques in wireless communication systems. The distinction between single-user and multi-user scenarios is critical in determining the design and effectiveness of hybrid beamforming algorithms, as the spatial multiplexing capabilities and interference management strategies vary significantly between these contexts. Moreover, the choice of link--whether downlink or uplink--introduces distinct challenges and opportunities in hybrid beamforming, necessitating tailored solutions for optimizing signal transmission and reception in each direction. The bandwidth considerations in mmWave hybrid beamforming are pivotal in determining the achievable data rates and spectral efficiency of communication systems. Narrowband mmWave applications may benefit from precise beamforming techniques that focus energy within limited frequency bands, while wideband mmWave systems require adaptive beamforming strategies to encompass broader frequency ranges and support high data throughput. The presence of rich scattering environments versus isotropic propagation characteristics further influences the design and performance of hybrid beamforming schemes, with rich scattering scenarios allowing for enhanced spatial diversity and more effective beamforming mechanisms. The structuring of hybrid beamforming systems as fully connected or hybrid configurations has implications for system complexity, power consumption, and beamforming granularity. Fully connected architectures involve a one-to-one mapping between analog and digital components, while hybrid structures combine analog precoding and digital baseband processing to alleviate hardware constraints and enhance adaptability. Similarly, the choice between time-division duplexing (TDD) and frequency-division duplexing (FDD) in mmWave hybrid beamforming impacts the system's flexibility, interference management capabilities, and spectral efficiency, with implications for dynamic channel reciprocity and resource allocation. Carriers in mmWave hybrid beamforming systems, such as orthogonal frequency-division multiplexing (OFDM) and non-orthogonal multiple access (NOMA), dictate the modulation and multiple access strategies employed in communication networks. The selection of carrier schemes influences system capacity, interference tolerance, and spectral efficiency, with implications for beamforming design and algorithm optimization. Furthermore, the network architecture, whether cellular or clustered, plays a key role in determining the deployment scenarios, coverage area, and interference coordination strategies in mmWave hybrid beamforming systems. Communication aspects such as channel state information (CSI) estimation, pilot signaling, and reconfigurable intelligent surfaces for power demand management and quality of service (QoS) optimization are integral to the performance and reliability of mmWave hybrid beamforming solutions. Leveraging reconfigurable intelligent surfaces allows for dynamic control of signal reflection and propagation paths, enabling efficient power allocation and QoS provisioning in mmWave communication environments. In addition, the application of mmWave hybrid beamforming extends beyond traditional wireless communication use cases, encompassing applications in radar systems for target detection and tracking, as well as in UAV communications for high-altitude and high-speed data transmission. The utilization of codebooks in mmWave hybrid beamforming systems, whether predefined or codebook-free, affects beam training overhead, adaptability to channel variations, and beam alignment accuracy, emphasizing the importance of codebook design and management in optimizing system performance and spectral efficiency.