# A Review on Millimeter-Wave Hybrid Beamforming for Wireless Intelligent Transport Systems
## 3 An In-Depth Analysis of the mmWave Hybrid Beamforming Landscape
### Reconfigurable Intelligent Surface  
Due to the substantial signal loss associated with mm-wave frequency bands, hybrid beamforming has been considered an integral component of mmWave communications. A novel technique called an intelligent reflecting surface (IRS) has recently been proposed as a way to improve the performance of mm-wave communication systems. This technology makes use of inexpensive passive reflecting components. * **Fewer RF Chains** Signals are sent from the base station (BS) to the users in a downlink hybrid beamforming RIS-based multi-user system with few discrete phase shifts. The digital beamforming is handled by the BS, while the analog beamforming is handled by the RIS. Two side issues arise from a RIS-based HBF mixed-integer sum rate maximization problem; for the RIS-based analog beamforming problem, outer approximation is the method of choice. The digital beamforming subproblem is addressed by using zero-force beamforming (ZF) in conjunction with power allocation [99]. * **Less Power Demand** Each RF chain is only linked to a distinct set of antenna components in a subconnected hybrid architecture that takes into account RIS-assisted multi-user downlink hybrid beamforming at the base station and reflection coefficients at the receiver-injector station. Using a subconnected architecture is preferable due to its reduced power consumption and hardware expenses. It is also necessary to fix the quality of service (QoS) issue, i.e., identify the base station (BS) that uses the least amount of power during transmission while still meeting the SINR constraints of all users. The issue is not convex at all due to the deeply coupled variables and unit-modulus phase shifts; so, a two-layer penalty-based technique is used to resolve it. In the inner layer, the penalized issue is solved using the BCD approach; in the outer layer, the penalty factor is varied until convergence [100]. * **More Safety** Transmission solutions that are both safer and more energy efficient are crucial components of an ITAN. This is for the simple reason that RIS technology may significantly enhance data security while also reducing system power consumption. For reduced power consumption and attack protection, a multi-layer RIS-assisted secure hybrid beamforming ITAN architecture is used. The block coordinate descent (BCD) architecture uses incomplete angular channel state information (CSI) to improve the user's received decoder, the digital precoder for terrestrial and aerial usage, and the multi-layer RIS analog precoder, all with the goal of maximizing the system's energy efficiency (EE). For the purpose of the received decoder, a heuristic beamforming method is created. A technique known as iterative sequential convex approximation, which makes use of the auxiliary variables and first-order Taylor series expansion, provides the basis for the digital precode [100]. * **Limited-Resolution Phase Shifters** Multi-user communications based on a RIS that have reasonable propagation models and restricted phase shifts face significant hurdles. First, the RIS components are very close together, which means that data rates may be affected by signal correlation. This is particularly true when the RIS is the only channel by which the BS and users may transmit LoS data. Additionally, the RIS necessitates discrete phase changes, which leads to a sum rate maximization or mixed-integer programming issue that is particularly challenging to resolve in complicated domains. One solution to these issues is the HBF method, which combines digital beamforming at the base station with analog beamforming at the receiver input sub-system (RIS) using limited-resolution phase shifters. Digital beamforming and RIS-based analog beamforming are two subproblems that are assessed in the mixed-integer sum rate maximization problem. To resolve these issues, an iterative technique is used. When compared to conventional HBF systems, this one based on a RIS can cut down on radio frequency (RF) chains by half.