# A Review on Millimeter-Wave Hybrid Beamforming for Wireless Intelligent Transport Systems
## 3 An In-Depth Analysis of the mmWave Hybrid Beamforming Landscape
### Network  
To provide quality of service, hybrid beamforming is used in cellular and heterogeneous networks. * **Cellular Network** There has been a lot of focus on physical layer and signal processing issues in mm-wave hybrid beamforming. An easily implementable mathematical model is created to depict the core concepts of hybrid beamforming in mm-wave cellular networks. As far as hybrid beamforming techniques go, Turbo-Hybrid Beamforming is the first to achieve the 1 ms real-time benchmark [80]. Many Resource Blocks compatible with multi-MIMO will be required by next-gen cellular phone networks, which will use solely commercial GPU devices [81]. Also, adopting mm-wave and massive multiple-input multiple-output (MIMO) technology into wireless networks is a crucial step towards enhancing network capacity, range, and quality-of-service (QoS) for future communications. A large number of symmetric antennas are used to increase the data flow and system speed. Power consumption and computational complexity are both increased when the number of antennas and radio frequency (RF) connections increases. Alternating minimal mean square error (Alt-MMSE) is a hybrid beamforming method that is used to address these concerns [82]. Further, directed broadcast across unlicensed mm-wave bands is required for sharing mm-wave bandwidth with an eye on frequency and space allocation. The total rate for all cellular users is improved with the least amount of disturbance to WiGig and cellular networks using an iterative channel distribution and hybrid beamforming algorithm [83]. * **Clustering Network** A matrix factorization approach is used to decompose the challenge of hybrid precoding/combining design. Both completely linked and partially linked structures are compatible with it. The user clustering approach is straightforward and simple to implement. Making a massive MIMO system that can support several users at mm-wave frequencies requires certain techniques. It furthermore offers services at frequencies lower than 6 GHz. In order to construct the analog RF precoders, clusters are formed using the AoDs of the users' beams or tracks. The algorithm's performance is enhanced by incorporating the properties of the unconstrained fully digital solution into the design of the digital baseband precoders for the fully connected structure, therefore adding an additional orthogonality criterion. The partially connected structure uses a distinct set of angle bins for each set of RF chains, which are in turn allocated to various user clusters. The digital baseband precoders are constructed using semi-definite programming (SDP) relaxation [84]. * **Multiple Cells Network** Within and between cells, multi-cell wireless systems experience interference. One way to lessen this interference is to use coordinated multipoint (CoMP) techniques [85]. Four mixed beamforming schemes--analog and digital--are proposed for mmWave communication with many cells, users, and streams using CoMP. Spectrum efficiency can be improved compared to the no-coordination situation using CoMP-based maximizing of the signal-to-leakage-plus-noise ratio. The amount of interference, which is impacted by cell size and user count, determines the variations in spectral efficiency among beamforming methods [86]. * **Heterogeneous Network** Every mobile user has their own antenna, which is built on a heterogeneous network for beamforming. Big hybrid antenna arrays that are completely coupled are used by both macro-cell base stations (MBSs) and small-cell base stations (SBSs). A simple variant of hybrid beamforming relies only on Eigenbeamforming. Both MBSs and SBSs use the Eigendecomposition of backhaul channels to locate fixed multi-beams [87]. Heterogeneous networks employ a distributed hybrid analog-digital architecture in tiny cells. The central unit handles the digital processing, while the tiny-cell base stations handle the analog processing. The access points may reduce the amount of data sent to the central unit by using fewer RF chains instead of antennas.