# A Review on Millimeter-Wave Hybrid Beamforming for Wireless Intelligent Transport Systems
## 3 An In-Depth Analysis of the mmWave Hybrid Beamforming Landscape
### Applications  
The following section will explore various applications considering radar, a WLAN, Railway, security perspectives, satellite, and also UAVs. * **Radar** A subarray architectural technique for developing a multi-carrier dual-function radar communication (DFRC) system is able to accommodate a large number of users. The challenge of hybrid beamforming design for DFRC systems is to provide an optimal transmit beam pattern while reducing power consumption and maximizing the sum rate [103]. Using the transmit beam of a dual-functional radar communication (DFRC) base station (BS), hybrid beamforming aims to approach the objective radar beam pattern as closely as possible, considering the signal-to-interference-plus-noise ratio (SINR) of communication users and the total transmission power of the DFRC BS. By adding a phase vector to the objective beam pattern and applying the alternating minimization method, more freedom is given to the beam design problem. The intended transmit beam is used to locate the analog and digital beamformers. The alternating minimization approach is used, and they are subject to the constant envelop restriction of the phase shifter network in mm-wave MIMO. When the SINR of the communication users stays the same, a larger antenna array increases the radar beam even more [104]. In multiple-input multiple-output (MIMO) systems, optimal utilization of the congested spectrum requires simultaneous sensing and communication [105]. One way to achieve this is by facilitating simultaneous sensing and providing communication users with high-capacity connectivity [106]. * **Wlan** A two-tiered beamforming design is known as hybrid beamforming. Analogue beamforming, the first level, uses inexpensive phase shifters to regulate the signal phases sent by each antenna. Digital beamforming using RF chains constitutes the second tier. In order to achieve spatial multiplexing gain, digital beamforming prevents interference between users, whereas analog beamforming increases the beamforming gain of each RF chain. The beam-user selection hybrid (BUSH) algorithm has few positive aspects. By using several antenna components, the access point may achieve a high digital beamforming gain with phased array antennas. The RF chains may also minimize inter-receiver interference by using the receivers' fine-grained CSI feedbacks. Implementing BUSH in WLANs has its own set of challenges. It is not simple to discover the AoAs of several users simultaneously in 802.11 WLANs. Users do not necessarily dispatch pilots to the access point (AP) in WiFi networks, in contrast to LTE systems where centralized control is present. Additionally, WiFi systems have much longer sounding times. In multi-user multiple-input multiple-output (MU-MIMO) systems that use hybrid beamforming, analog beamforming selects a fixed number of predefined beam patterns, while digital beamforming selects a fixed number of RF chains. When there are an excess of antennas and insufficient RF links, BUSH digital-analog hybrid beamforming can be a useful solution. One way to reduce system latency is to collect AoA data from several users simultaneously using separate subcarriers for each user [107]. * **Railway** For High-Speed Railway Communications (HSRC), pilot workload and hardware complexity are critical factors in the development. Utilizing the high spatial resolution of the array and beam alignment, a new approach for compensating for Doppler frequency offset (DFO) is proposed. To recover the CSI and monitor the beam gain, an angle-domain (AD) tracking approach is suggested using spatial angular information. In order to reduce computational complexity and hardware overhead, hybrid beamforming is used. The AD technique seeks to follow linearly variable beam gain by using the temporal coherence of the channel, as opposed to tracking the nonlinearly varying angle of arrival (AOA). This simplifies the calculation.