# Advanced Hybrid Beamforming Technique in MU-MIMO Systems
## 1 Introduction  
The mmWave is capable of wide-band transmission and is used for various purposes such as satellite, mobile communication. The mmWave communication technology is attracting attention for high data rate in 5G NR [1; 2; 3; 4; 5; 6]. Although signal absorption and path loss occur due to the propagation characteristics in mmWave frequencies, the data rate can be increased by using mmWave frequencies. In addition, due to the propagation characteristics of the short wavelength of the mmWave frequencies, the antenna size can be reduced in a hardware system to form a plurality of antenna arrays. Also, the antenna arrays can provide large antenna gains to compensate for the high path loss of the mmWave. Therefore, mmWave communication technology is selected as an important technology for 5G NR.  
The fully digital beamforming system requires a separate radio frequency (RF) chain for each antenna. However, as the number of antennas increases, more RF chains are required. Therefore, the cost and the power consumption are increased. To reduce the cost and the power consumption, hybrid beamforming is constructed by concatenation of a low-dimensional digital beamformer and RF beamformer using phase shifters [7; 8; 9; 10] is issued. Hybrid beamforming systems employ a fewer RF chains than conventional fully digital beamforming system. Therefore, efficient communication withlow hardware complexity is possible. Hybrid beamforming systems include fully connected structure (FCS) and partially connected structure (PCS). FCS hybrid beamforming system is a structure which each RF chain is connected to all antennas to obtain high channel capacity. However, FCS hybrid beamforming system requires high complexity in a hardware system. To reduce the complexity of the FCS hybrid beamforming system, there is a PCS hybrid beamforming system with the same number of antennas connected to each RF chain. PCS hybrid beamforming system has low hardware complexity and performance degradation. Also, hybrid beamforming system based on codebook is considered in 5G NR. The hybrid beamforming system based on codebook selects the most suitable analog beam pattern with the channel status of each user in the codebook storing the predesigned analog beam patterns. To reduce the channel feedback overhead required in designing a hybrid beamformer using multiple antennas, the methods of selecting an analog beam within a predesigned codebook were proposed [11; 12]. The hybrid beamforming system based on codebook is a realistic method and can be applied to 5G NR.  
This paper proposes the improved PCS hybrid beamforming increasing the average sum rate, and the proposed technique can be used in any bands containing mmWave.  
The technique of hybrid beamforming is not limited to only mmWave band which is larger than 6 GHz, and can be used at other bands in addition to mmWave band. However, the hybrid beamforming is suited well in the mmWave band compared to below 6 GHz since the hybrid beamforming was originally developed to use for realizing massive MIMO system. For practical massive MIMO system, mmWave band is required for various reasons such as small antenna size and less RF chains, etc. [13]. Therefore, all studies about the hybrid beamforming are based on mmWave band since massive systems are required for improving data rate or reliability.  
Section 2 introduces the system models. Section 3 describes the conventional hybrid beamforming system. The proposed hybrid beamforming system is described in Section 4. In Section 5, the simulation results are illustrated to compare the error performance and the average sum rate. Section 6 concludes the paper.