# A Review on Millimeter-Wave Hybrid Beamforming for Wireless Intelligent Transport Systems
## 3 An In-Depth Analysis of the mmWave Hybrid Beamforming Landscape
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#### 3.2.1 Downlink  
In a single-user downlink scenario, increasing the number of base station antennas enhances system speed because of the larger diversity. In a scenario with multiple users, the number of cell users grows as the bit error rate (BER) decreases. This loss is more pronounced with conjugate beamforming precoding as compared to zero-forcing precoding [38].  
* **Joint Two-stage Beamforming** Hybrid beamforming can send out numerous streams to each user's device when there is perfect channel information. Unlike the conventional (functions independently) dual-stage design, this combined two-stage beamforming uses both stages to reduce data loss at the analog and digital stages. It employs the block-diagonalization method and water-filling approach to find the asymptotic solution in multi-user downlink MIMO [39].
* **Hardware Constraint Beamforming** A blockage-dependent channel model is used for hybrid beamforming in multi-user MIMO in the presence of hardware constraints, showing a trade-off in power demand, achievable rate and coverage in a downlink cellular network. With more channel knowledge, the round-robin method of scheduling is better. This technique ignores computational complexity and channel overhead to provide better performance results [40].
* **Capacity Maximization and Interference Minimization Beamforming** For downlink multi-user MIMO, a step hybrid beamforming method is used to enhance capacity in the first step and mitigate interference in the second step. The capacity is maximized by finding an equal response of the baseband channel by using radio frequency processing in analog. The interference is decreased by the block-diagonalization method. These two steps perform better than conventional block-diagonalization with a smaller quantity of RF chains in terms of capacity [41].
* **Inter-group Interference Minimization Beamforming** Consider the scenario where the base station does not know the information of inter-group interference at any instant of time in downlink multi-user MIMO; this is a challenging issue to solve. Therefore, inter-group mitigation-based hybrid beamforming which consists of analog and digital beamforming is used. In the presence of a knocker channel model, an analog beamformer with a second-order statics problem, decoupling is required to optimize the analog precoder/combiner that mitigates that interference (inter-group) and noise, enhances the intra-group signal [42].
* **Efficient Transmission Beamforming** Traditional mm-wave MIMO for multi-users with a large quantity of antenna are energy demanding when using high-resolution analog-to-digital converters and vice versa. To minimize the energy utilization, either the number of converters needs to be reduced or low resolution needs to be installed. A hybrid framework for multi-user downlink with a low-resolution converter is used at both the transmitter and receiver side. To minimize interference, a new data transmission technique utilizes a light-weight phased array to incorporate the beamforming matrix in the analog domain [43].
* **Joint Sensing Beamforming** To reduce the components' complexity and hardware costs, a beamforming framework is utilized in systems which demand fewer radio frequency chains. Dual-functional radar communication (DFRC) uses radar and hybrid beamforming in the downlink [44].