# A Review on Millimeter-Wave Hybrid Beamforming for Wireless Intelligent Transport Systems
## 3 An In-Depth Analysis of the mmWave Hybrid Beamforming Landscape
### Band
#### 3.3.2 Wideband  
The discrepancy in the extra transmission distance of each subcarrier signal arriving at various antennas in the array, known as beam squint, is the cause of the frequency-dependent beam phase problem. Wide-spectrum MIMO-OFDM communication with hybrid beamforming with the beam squint technique begins by solving the frequency-dependent beam phase dilemma. The precoder design is the matrix factorization and the wide-spectrum hybrid precoding approach based on the manifold optimization, which seeks to identify the best precoder for the wide-spectrum communication [48].  
The subarray structure used in Figure 6 addresses the issue of the hybrid beamforming design in MIMO-OFDM communication. It lowers the need for phase shifters because each RF chain should only be coupled to a single subarray. An adaptive hybrid analog-digital beamforming design based on a singular value thresholding technique is used [49]. The spectral efficiency of this design outperforms the semidefinite relaxation algorithm and offers a significant decrease in computational cost [50]. In Figure 6, it is illustrated that the system is equipped with N quantity of antennas and NRF radio frequency chains.  
* **Low-Resolution ADC Beamforming**  
There are two primary ways to lower power consumption: digital beamforming using low-resolution analog-to-digital converters (ADCs) and hybrid beamforming. For a wideband multipath model, the impacts of transmitter impairments, numerous simultaneous users, and channel estimates have taken into account devices set at 60 GHz in the power consumption framework. The correlation of the quantization error is modeled to include various quantizers at each antenna and non-uniform quantizers. The outcome  
Figure 6: Subarray architecture [49; 50].  
demonstrates that the ADC resolution reaches the ideal energy efficiency along with the signal-to-noise ratio (SNR). Furthermore, it is demonstrated that any desired trade-off between power usage and rate can be accomplished with mixed ADC resolutions, which are comparable to those obtained with a single ADC resolution [51].
* **Low-Resolution Phase Shifters** In [52], a hardware-efficient, low-resolution phase shifter (PS) and adaptive antenna subarray hybrid beamforming design for a wideband mm-wave MIMO orthogonal frequency-division multiplexing (MIMO-OFDM) technology is discussed. Through the use of switch networks and PSs, each RF chain is dynamically connected to a non-overlapping antenna subarray, allowing numerous antenna diversity to be utilized to reduce the performance loss caused by the use of realistic low-resolution PSs. Next, an effective iterative hybrid beamformer algorithm is developed on the foundation of traditional block coordination descent (BCD) techniques. Additionally, a convergence and complexity evaluation for the algorithm is given. The superiority of this hybrid beamforming technique with dynamic subarrays and low-resolution phase shifters (PSs) is illustrated by extensive simulation data.
* **Wideband** Alternating direction method of multipliers (ADMM)-technique-based hybrid beamforming is used for channel estimation in the uplink MIMO system for wideband. This channel estimation depends on precise pilot estimation in the channel with small complexity. The prediction of the channel depends on the received channel matrix. Its performance is impressive, with a smaller quantity of radio frequency chains in the presence of high-noise environment [53].