# A Review on Millimeter-Wave Hybrid Beamforming for Wireless Intelligent Transport Systems
## 3 An In-Depth Analysis of the mmWave Hybrid Beamforming Landscape
### Communication  
Handling large volumes of signals in the air is challenging due to pilot contamination and the critical role of channel state information in hybrid beamforming. Additionally, beamforming enhances connectivity in indoor communications. * **Air Compass Communication** To lower the processing mean square error (MSE) in the hybrid beamforming system of the air compass, it is essential to simultaneously enhance both the analog and digital beamforming at the access point, as well as the digital beamforming of the transmittingdevices, especially given the large volume of signals from numerous devices [91]. A kind of digital predistortion called BO-DPD is used to make the large-scale hybrid beamforming array more linear by using over-the-air diversity feedbacks, or OTA DFBs. By estimating and linearizing the main beam signal without using complex feedback settings, the DPD interacts with hardware in the HBF array [92]. * **Pilot Contamination** In order to locate several user data streams, the hybrid beamforming design makes use of numerous RF chains. The hybrid beamforming system incorporates both low-dimensional digital beamformers and high-dimensional analog beamformers. Improving data signals and making the Kronecker decomposition technique more generic for a single-user system are the goals of the analog beamforming matrix. In order to aid with spatial multiplexing, the digital version employs the widely used digital minimum mean square error (MMSE) beamforming algorithm for signal separation. A closed-form analysis of the AoA spectrum reveals that as the array size, the square root of the pilot sequence length, or the minimum distance between data and interference increases, the inter-cell interference (pilot contamination level) decreases. Consequently, various approaches can be employed to address pilot contamination. The next step is to determine the gain of the associated path. This can be achieved by directing an analog beam along the data's intended route, known as coherent-combining (CC), or by minimizing interference between paths, known as zero-forcing (ZF) [93]. * **Channel State Information** For 5G mm-wave MIMO hybrid beamforming to work, downlink channel state information (CSI) is required. This approach is based on frequency-division duplexing (FDD). Because of the high-level input data required for massive MIMO communication and the short wavelength of the mm-wave band, channel response may vary very quickly. To reduce the feedback overhead, a phase beamforming method is used to take advantage of the improved phase stability of the channels. Simply being aware of which beam combinations provide the most robust link between the sender and receiver is sufficient; this is known as implicit channel knowledge. The suggested methodology is both simpler and easier to implement than hybrid beamforming algorithms that rely on explicit CSI [94]. The fundamental idea behind this approach is to efficiently resolve critical parameters such as the channel response's Frobenius norm, beamforming gain, or absolute value. One limitation of all compressed sensing systems is that they cannot tolerate deviations from certain ideal beam patterns [95]. To solve the sum rate maximization issue with limited feedback, a joint scheduling and hybrid beamforming method with a two-stage channel state information (CSI) feedback system is used. Using channel quality feedback, the base station (BS) first narrows down the potential RF beamforming vectors [96]. This hybrid beamforming approach, when combined with multi-user scheduling, makes the achievable rate performance more attractive by considering the benefits of random hybrid beamforming and zero-forcing. This method outperforms the RF beamforming scheme and the ZF-based beamforming system, respectively [97]. * **Indoor Channels** Indoor 60 GHz spatial-division multiple access (SDMA) communication makes use of a hybrid architecture that employs phased antenna arrays (PAAs) for downlink beamforming (BF).