# A Review on Millimeter-Wave Hybrid Beamforming for Wireless Intelligent Transport Systems
## 3 An In-Depth Analysis of the mmWave Hybrid Beamforming Landscape
### Codebook  
Hybrid beamforming can be broadly categorized into two main types: codebook-based and codebook-free.  
* **Codebook-Based** Mm-wave bands have huge bandwidths, enabling high data rates and thereby mm-wave communications in small-cell networks boost system speed [114]. Due to the poor quality of the mm-wave channel, mm-wave communication systems rely on beamforming, which requires a multitude of sensors to generate powerful and focused beams. In this setup, the codes known by the transmitter and the receiver are identical. For the selected beamforming vector, the receiver then transmits this codebook back [115]. A large codebook size allows for the fabrication of thin beams with a sharp rolloff. A smaller codebook is preferable for practical reasons, such as the time and effort required to search for codewords and provide responses. When creating the codebook, it is important to consider how mm-wave connections function as channels in order to obtain the greatest possible total speed [116]. The transmitter employs hybrid beamforming for the channel-link-based codebook. The first step is to identify the optimal beam configuration for every codeword [117]. With the right beam pattern, you can make a codebook design guideline that everyone can follow. Next, the codebook is designed to minimize the mean square error (MSE) between the ideal beam pattern of each code vector and its actual beam pattern. Orthogonal matching pursuit (OMP) allows for this MSE decrease. Beamforming in the mm-wave system is enhanced by the proposed codebook, as shown [118].  
The codebook-based MIMO system employs a hybrid beamforming technique with partly interfering beam feedback, and it serves many users [119]. The analog portion of the precoding makes use of a non-uniform amplitude beamforming vector Taylor codebook and a typical discrete Fourier transform (DFT) codebook with uniform amplitude beamforming vectors [120]. In order to obtain a general notion of the effective channel matrix, the digital precoding phase employs zero-forcing (ZF) [121]. In multiple-user multiple-input multiple-output (MU-MIMO), a beam pairing technique facilitates beam selection and mitigates inter-beam interference [122; 123].  
Codebook-based beamforming [124] has six major advantages. (1) It is compatible with RIS technology, providing backwards compatibility. (2) The pilot overhead is adaptable and scalable according to the situation. (3) The reduced complexity makes it a more attractive technology. (4) This technology mitigates the propagation error. (5) The control link requiresless capacity, making it more durable. (6) The channel imperfections are considered in the codebook, which becomes more robust.
* **Codebook-Free** Hybrid beamforming that automatically turns on the right amount of radio frequency (RF) chains uses fractional programming to enhance energy efficiency and find the optimum number of active RF chains and data streams. The solution changes rapidly based on the current channel conditions. The combined analog/digital (A/D) precoder and combiner matrices at the transmission and receiver are developed using gradient pursuit (GP). These codebook-free methods consisting of the alternating direction method of multipliers (ADMMs) and the singular-value decomposition (SVD) build the precoders and combiners [125].