# A Review on Millimeter-Wave Hybrid Beamforming for Wireless Intelligent Transport Systems
## 3 An In-Depth Analysis of the mmWave Hybrid Beamforming Landscape
### Structure
#### 3.5.2 Hybrid Connected  
In mmWave massive multiple-input multiple-output (MIMO) systems, the performance of the hybrid beamforming system based on the full-connected structure is comparable to that of the full-digital one. But it is severely limited because of the complicated circuit wiring in real-world applications [57]. Subconnected  
The subconnected structures in hybrid beamforming are used to mitigate interference and enhance the capacity of a system. In multi-user MIMO, a subconnected architecture is necessary when many streams are needed; this is to ensure quality of service. This design calls for a base station with extra antenna arrays for downlink communication. For analog-digital beamforming, a piece-wise approach is ideal, supposing complete channel information. In order to maximize capacity while minimizing data loss, hybrid beamforming is used [57]. The capability of analog/digital approaches fully digital in stages, according to the rationale of information theory. The effect transmitter and receiver are integrated into the analog beamformer using a piece-wise dual joint iterative approximation (PDJIA) technique. During internal iteration, the optimal phase selected during external iteration of the analog combiner is used as the initial value. In order to obtain the optimal analog matrix for beamforming, this method reduces the number of iterations needed. The performance of single-user and multi-user MIMO can be improved by using successive interference cancelation (SIC) techniques. This approach converges rapidly [58]. There are partially connected hybrid beamforming structures, which are discussed one by one in this section. The cellular communication spectrum is inadequate for mobile traffic. Consequently, hybrid beamforming using an antenna selection approach makes use of massive antenna structures. Given an overall number of antennas, the primary goal of this approach is to choose the optimal number of transceivers, and secondarily, the optimal number of antennas per transceiver. The second goal is to look at the optimal combination of N (transceivers) and M (active antennas per transceiver). When it comes to channel estimate, it performs better [59]. For a multicasting network to provide effective power transmission, a combination of beamforming and antenna selection methods is used. When the number of phase shifters is smaller than the number of radio frequency chains, the antenna selection method works better. In Figure 7, the hybrid multi-group architecture is connected with N quantity of antennas and NRF radio frequency chains. In this design, each radio frequency chain is attached to a power splitter and then connected with group of phase shifters. Each phase shifter is connected with an adder and finally aligned with a transmitting antenna. The multi-grouping of the phase shifters provides better analog beamforming and decreases the burden of beamforming operation on the digital beamformer [60]. For point-to-point and multi-user situations in high and scattering channels, a hybrid beamforming architecture with closed-form spectral efficiency approaches is proposed. Second, when high scattering channels are considered, a novel approach of phase shifter selection is required that simultaneously reduces power consumption in the phase shifter system and improves spectral efficiency. Also, the phase shifters of the RF beamformer need to be adjusted in order for the singular vectors, which are formed by the channel matrix components, to coincide with one another when there are several antennas. As a result, the performance is improved. In addition to facilitating asymptotically optimum behavior, the method also reduces computation latency and is easy to implement. Incorporating digital phase shifters into a hybrid beamforming approach results in a reduced performance limit, and the system is otherwise straightforward and successful [61].