# 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
A typical hybrid MIMO system may obtain a large MI gain with little complexity and expense by simply adding passive phase shifters, since the DFT matrix is channel-independent. The ergodic rate of the MRC combiner reaches the same ideal scaling law as that of the two-stage analog combiner. A decrease in quantization error with increasing RF chain density accomplishes this. Figure 8 decpits the unit cell architecture in which the base station is connected with N quantity of antennas and NRF radio frequency chains such that the quantity of radio frequencies chains is less than number of receiving antennas. There are two steps for an analog beamforming combiner; the disadvantage of this design is that it needs more phase shifters. Each radio frequency chain is connected with two low-resolution analog-to-digital converters (ADCs); therefore, less digital processing is required [62]. For a mixed-structure stepped antenna array, the optimal beam pattern is obtained using a hybrid beamforming design. The first step is to determine the beamforming vector bases that will maximize power distribution throughout the specified angle range by  
Figure 8: Double analog combiner with low-resolution ADCs [62]. Figure 7: Muti-group hybrid beamforming design [60]. referring to the radiation power ratio. The next step is to use the bases to solve a max-min optimization problem using the semidefinite relaxation (SDR) technique. This will flatten the response of the passband, which is the required angle range. The desired beam pattern may be found in the reference beamforming vector that this produces. Finally, given a phased array antenna with a hybrid structure, the beamforming vector can be determined using hybrid decomposition. In Figure 9, the hybrid antenna array architecture is connected with N quantity of antennas and NRF radio frequency chains such that the quantity of radio frequencies chains is less than number of transmitting antennas. Each power splitter is connected with a single radio frequency chain; the splitter divides the power in order to provide more phase shifting in the analog portion. Then, power is added again by using the adder connected to the antenna. This architecture provides improved beam concentration [63]. Massive MIMO hybrid beamforming with PS selection may significantly enhance energy efficiency while preserving almost all of the spectral efficiency. The benefits of energy savings are more apparent for fully connected hybrid beamformers. In Figure 10, it is shown that the system is equipped with N quantity of antennas and NRF radio frequency chains. The beamforming operation is carried out through analog and digital beamforming. The analog beamformer is composed of N \(\times\) NRF analog radio frequency beamformers. The digital part consists of N\({}_{\text{s}}\)\(\times\) NRF digital baseband beamformers. In this subconnected architecture, each radio frequency is attached with N\({}_{\text{p}}\) quantity of phase shifters attached through the switches. The total number of switches open is represented by N\({}_{\text{so}}\). The number of working antennas N\({}_{\text{w}}\) or the antenna which dissipates power is equal to N-N\({}_{\text{so}}\). This N\({}_{\text{p}}\) is equal to the ratio of N\({}_{\text{w}}\) to N\({}_{\text{RF}}\). The advantage of this design is that the energy efficiency is further improved due to switches, although the beamforming gain is reduced. In Figure 11, it is shown that the system is equipped with N quantity of antennas, NRF radio frequency chains, and a quantity of data streams N\({}_{\text{s}}\). The beamforming operation is carried out through analog and digital beamforming. The analog beamformer is composed of N \(\times\) NRF analog radio frequency beamformers. The digital part consists of N\({}_{\text{s}}\)\(\times\) NRF digital baseband beamformers. In this architecture, each radio frequency is attached to N\({}_{\text{p}}\) quantity of phase shifters equal to N. The advantage of this design is beamforming gain with huge hardware complexity [64].