# Robust Hybrid Beamforming Scheme for Millimeter-Wave Massive-MIMO 5G Wireless Networks
## 1 Introduction  
In recent years, with the development of science and technology, the emergence of new technologies such as virtual reality and autonomous driving is gradually changing the way people live and work. At the same time, the emergence of these new technologies is accompanied by an increase in the traffic of mobile-communication data services, which poses a severe challenge to the bandwidth and transmission rate of wireless-communication systems [1; 2; 3]. The future 5G communication system aims to provide users with Gbit/s transmission rates to meet increasing user demands. High transmission rates mean that a sufficient amount of transmission bandwidth is required, and existing spectrum resources are far from meeting Gbit/s transmission requirements. So, researchers turned their attention to millimeter-wave (mmWave) communication technology, and 5G networks are being powered by this new technology. mmWave refers to electromagnetic waves with a frequency of 30-300 GHz. It has rich spectrum resources and is favored by many researchers. However, since mmWaves have serious path loss, rain attenuation, and penetration loss [4; 5; 6], propagation in THz bands is limited by the severe path loss and molecular absorption [7]. These, in turn, make communications in THz bands more sensitive to blockages compared to those in mmW bands. Other important limiting factors are as follows. First, they suffer much higher penetration losses when passing through many common materials (including concrete, tinted glass, and water), owing to their smaller wavelength. Second, mmWave frequencies do not diffract well in terrestrial environments because the wavelength is much smaller than the objects it would preferably bend around. This makes blocking objects effectively larger. Third, because of the aforementioned required directionality, both transmitter- and receiver-beam patterns are focused over a narrower beamwidth, which affords millimeter-wave signals fewer chances to avoid strong blocking than in a nearly omnidirectional transmit/receive scenario, where energy is radiated and collected over much wider angles [8; 9]. MmW communications are characterized by higher path loss and stronger sensitivity to blockage than microwave communications [10]. To face these impairments, the small wavelengths of mmW frequencies allow for the implementation of a large number of antenna elements in a small space to make narrow beams (pencilbeams) [11]. How to overcome the path loss in the mmWave communication process has become an urgent problem to be solved. MmWave wavelength is relatively short, which allows it to integrate more antennas in the same physical size, which provides conditions for large-scale spatial multiplexing and high directional beamforming. Researchers have proposed to use large-scale antennas through beamforming methods to overcome the signal attenuation problem of mmWave transmission, which makes mmWave beamforming technology a key 5G technology [12]. Traditional beamforming is implemented in the digital domain. It is necessary to configure a dedicated RF link for each antenna. However, due to the high cost and high-power consumption of the RF link, it is not suitable for a large-scale system. So, traditional beamforming is only suitable for small-scale antenna systems. However, since the mmWave MIMO system integrates a large-scale antenna array, the conventional method is obviously no longer suitable for an mmWave MIMO system. In order to solve the problem caused by RF limitation, this paper studies the hybrid beamforming problem in mmWave MIMO systems by placing most of the beamforming in the analog domain and using only small-dimensional digital beamforming in the digital baseband section. In this way, signal beamforming can be completed with a small number of RF links, which greatly reduces the cost and power consumption of the system [13; 14; 15; 16]. According to different RF link-to-antenna mapping modes, the hybrid beamforming structure is divided into fully and partially connected types [14; 15; 17].