# Hybrid Beamforming for Reduction of Inter-Beam Interference in Millimeter-Wave Cellular Systems
## 1 Introduction  
Mobile traffic has been increasing markedly owing to the growth of various devices such as smart phones and sensor nodes. Especially, the sensor nodes such as complementary metal-oxide semiconductor (CMOS) sensors and charge-coupled device (CCD) cameras require a high data rate transmission. By 2020, the mobile traffic volume is expected to increase by more than 1000 times because of the increase in smart phones and sensor networks [1]. In order to support such a considerable increase in the mobile traffic, the 30-300 GHz millimeter-wave (mm-wave) frequency band supporting a wide bandwidth is considered as a possible candidate for 5G mobile communication systems with antenna arrays for directional beamforming at both the base station (\(BS\)) and the mobile station (\(MS\)) [2, 3]. The array gain obtained with the directional beamforming extends the coverage of the mm-wave systems that suffer from a high path loss owing to a high carrier frequency [4, 5, 6]. Because of the small wavelength of the mm-waves, antenna arrays can be easily installed in the \(MS\)s.  
Owing to significant costs in the radio frequency (\(RF\)) chain and the power consumption, analog beamforming is currently preferred over digital beamforming for mm-wave communication systems. In mm-wave communication systems with analog beamforming, switched beamforming techniques with a set of predefined angles are usually used for transmit-receive (Tx-Rx) beamforming. Analog phase shifters are typically used to implement analog beamformers in the \(RF\) domain. However, the use of analog phase shifters places a constant modulus constraint on the elements of the \(RF\) beamformer. The limitations of analogue beamforming are the difficulty of controlling signal amplitude and the low-resolution signal phase control [7]. In order to overcome the issues faced by digital and analog beamforming, an alternative beamforming architecture, known as hybridbeamforming, has been introduced [8; 9; 10; 11]. In hybrid beamforming, beamforming is partitioned into analog and digital domains. Because of the additional digital processing, more degrees of freedom are available when designing the hybrid precoder compared to analog-only beamformers. In addition, hybrid beamforming allows the support of multi-stream and multiuser transmissions in a flexible way. The Tx-Rx beamforming technique using the mm-wave frequency band has already been standardized in IEEE 802.11 ad and IEEE 802.15c standards to provide a multigigabit-per-second data rate [12; 13; 14]. In addition, the pre-5G specification for 5G mm-wave cellular systems, called KT Pyeongchang 5G Technical Specification (5G-SIG), was released in 2016 [15; 16]. 5G-SIG, which was approved by leading global wireless communication companies, was prepared to demonstrate 5G pilot services for the Pyeongchang Winter Olympic Games in February 2018. Mm-wave communication is also being considered for enhanced mobile broadband (eMBB) in New Radio (NR) currently being standardized for next generation cellular systems [17]. Both analog beamforming and hybrid beamforming are allowed in the pre-5G specification and NR standard. In [18], a cell selection technique was considered for mm-wave cellular systems with hybrid beamforming in the initialization stage. In general, the serving cell and best beam pair are selected based on the measurement results obtained by all possible Tx-Rx beam pairs of candidate cells in the initialization stage. A beam pair with maximum signal-to-noise ratio (SNR) is selected as the best beam pair and serving cell. However, the selected cell and beam pair may not be optimal for hybrid beamforming system because the effect of the multipath channel is not considered in the initialization stage. In the paper, a cell selection technique was proposed for mm-wave cellular systems with hybrid beamforming in the initialization stage, considering the hybrid beamforming structure in data transmission stage.