## 2 Background for Location-Aware Beamforming in 5G Ultra-Dense Networks
First of all, higher carrier frequencies in frequency range 2 (FR2) between 24.25 and 52.6 GHz, and extended from 52.6 to 71 GHz, dedicated to 5G networks, are subject to strong attenuation, thus, in order to achieve coverage, mmWave devices require to use narrower beams with higher AA gain. Narrowing HPBW, in turn, complicates BM procedures during beam sweeping on the initial access [25] and further beam refinement [26] stages. The reason is that narrower beams need largertransceiver codebook size, causing increase of beam sweeping overhead, complexity and latency [61]. One of the recommendations to accommodate a large number of pencil beams for sparse mmWave MIMO channel (with much less beams, than the number of antenna array elements), is to reduce beam search space, utilizing distributed compressed sensing approach [62]. One more solution for codebook design is to use narrower beam search space using prompt estimates of UE direction of arrival (DOA) and direction of departure (DOD) [27]. Another challenge of BM concerns beam alignment for highly mobile VUE and necessitates tradeoff between timing for beam sweeping, expiration time of beam correspondence state and antenna array HPBW. One of the solutions to this problem is also LAB, relying on predictable VUE trajectories. To the best of our knowledge, one of the first experimental demonstrator of location-aided beam alignment technology in mmWave band through field trials is described in [63]. Sufficient link margin, needed to overcome severe pathloss in mmWave, can be provided by high gain directional antennas, which necessitates both transmitter (TX) and receiver (RX) to use beam steering with narrow beamwidth. This challenge facilitates development of effective BM procedures and solutions with low latency and signaling overhead, actual for high-speed trains (HST) [64]. Movement assessment in linear and angular directions with inertial measurement units (IMU) for slow vehicles, utilizing BF in 60 GHz links, is evaluated in [65]. In particular, experiment in 10 \(\times\) 10 \(\times\) 4 m room with IEEE 802.11 ad stations, equipped with 30\({}^{\circ}\) HPBW antenna, validated possibility to quickly realign beams during 10 ms interval, based on IMU prediction, thus substantiating LAB for low-speed vehicles. Authors in [63] implemented LAB testbed at 28 GHz with baseband processing in National Instruments (NI) software-defined radio (SDR) universal software radio peripheral (USRP) board NI USRP-2954R, passband processing in Analog Devices (ADI) radio frequency (RF) modules ADF5356, ADMV1013, ADMV4801 and beamforming in uniform planar array (UPA) antenna, comprising 4 \(\times\) 16 patch antenna elements (AE). The beamsteering is based on analog BF approach through the delay-sum beam codebook, capable of scanning azimuth plane from \(-40^{\circ}\) to \(40^{\circ}\) with \(2^{\circ}\) step. Experiment was performed for LOS scenario with single TX-RX pair, separated by 4.2 m, in anechoic chamber when TX is stationary and RX is linearly moving at speed of 0.1 km/h. LAB realization included following steps: (a) estimation of RX location by means of ultra-wideband (UWB) ranging module; (b) angle of departure (AOD) and angle of arrival (AOA) calculation, based on TX and RX positions; (c) steering TX and RX beams according to calculated AOD and AOA respectively. Described in [63] demo accounted beam tracking interval lower than 200 ms, and proved, that LAB enables tradeoff between beam alignment accuracy and timing even for large-scale AA with pencil HPBW at both TX and RX. However, for high speed VUE in outdoor scenario, using 3 GPP network positioning trilateration technologies, LAB demonstrator had not yet been tested. One of the first experimental investigation of outdoor mmWave propagation, using small form factor steerable antennas for mobile UE, to the best of authors knowledge, was published in [66].