# Cell Selection Technique for Millimeter-Wave Cellular Systems with Hybrid Beamforming
## 1 Introduction  
Owing to the wide use of smartphones and sensor nodes for various applications and services, mobile data traffic is growing rapidly. In particular, sensor nodes such as CMOS sensors and charge-coupled device (CCD) cameras require a high data rate transmission. Very large arrays and/or higher frequency spectrum are recognized to accommodate the increase in mobile traffic. Massive multiple-input multiple-output (MIMO) is expected to bring large gains in terms of spectral and energy efficiency and robustness to hardware failures and impairments [1; 2; 3]. Further, millimeter wave (mm-wave) communication is a promising technology that allows the use of a wide bandwidth for the high data rates required for next-generation outdoor cellular systems [4; 5]. However, mm-wave signals experience severe large-scale path losses compared to lower-frequency band signals [5; 6]. To compensate for the large path losses, and to increase the transmission range in the mm-wave frequency band, highly directional beamforming antennas are necessary at both the base station (BS) and the mobile station (MS). Fortunately, a decrease in wavelength enables a large number of antenna arrays to be packed into small form factors, making it feasible to realize the large arrays needed for high beamforming gains [7; 8]. Traditionally, MIMO processing is often performed digitally in baseband to support multi-stream service in multipath environments. To perform digital beamforming at mm-wave frequencies, each antenna element requires dedicated baseband and radio frequency (RF) hardware, resulting in high costs and a substantial amount of power consumption. This has constrained mm-wave systems to rely heavily on analog or RF processing [8; 9]. Analog beamforming has hitherto been used in the mm-wave systems; however, it is suboptimal compared to digital beamforming. As analogbeamforming relies entirely on RF domain processing, the performance is limited by quantization of phase angles and the support of only single stream transmissions [10; 11]. In order to overcome the issues faced by digital and analog beamforming, an alternative beamforming architecture, known as hybrid beamforming, has been introduced [12; 13]. In hybrid beamforming, beamforming is partitioned into analog and digital domains. Because of the additional digital processing, more degrees of freedom are available in designing the hybrid precoder compared to analog-only beamformers. In addition, hybrid beamforming allows for the supporting of multi-stream and multi-user transmissions. However, in mm-wave systems, analog beamforming has been used to date for beam training and cell selection in the initialization stage. In the standards of wireless LAN and wireless PAN (IEEE 802.11 ad and IEEE 802.15c) widely used for wireless sensor networks (WSNs), analog beamforming is used for both beam training and data transmission using the 60 GHz unlicensed spectrum [14; 15; 16; 17]. In the pre-5G specification for 5G mm-wave cellular systems, called KT Pyeongchang 5G Technical Specification (5G-SIG), analog beamforming is used for beam training and cell selection in the initialization stage. However, both analog beamforming and hybrid beamforming are allowed in the data transmission stage [18; 19]. The 5G-SIG, approved by leading world companies in wireless communication, was released in 2016 to demonstrate 5G pilot services for the Pyeongchang Winter Olympic Games in February 2018. In 5G-SIG directional beamforming, antennas are used to maintain the link budget during the initialization stage. The serving cell is found by time-division beam switching (TDBS) in which transmit (Tx) beams of candidate BSs, and receive (Rx) beams of MS, are swept to measure signal-to-noise ratios (SNRs) for all possible links. The serving cell and Tx-Rx beam pair with the highest SNR in the initialization stage are selected.