## 2 Background for Location-Aware Beamforming in 5G Ultra-Dense Networks
Field trials for 38 GHz radio propagation in urban scenario revealed, that transmitters, elevated at heights from two to eight stories, require 60\({}^{\circ}\) scanning range (up to \(\pm 30^{\circ}\) off-boresight) in the azimuth, while receivers would benefit from larger azimuth scan range. Study [66] indicates, that steerable antenna architectures at mmWave should work best in dense urban environment with ISD up to 200 m, where non-line-of-sight (NLOS) are rarely preferred to LOS or partially obstructed LOS links, since NLOS have from 10 to 50 dB higher pathloss. Also [66] forecasts usefulness of site-specific ray-tracing models due to environment dependency of TX/RX AOD/AOA distribution. Work [67], devoted to full-dimension MIMO (FD-MIMO) with antenna array elements, placed in horizontal and vertical domains, draws attention to the additional degree of freedom in beam steering, and the need for accounting not only azimuth, but also elevation direction in three-dimensional (3D) spatial channel models (SCM). In particular, it considerssimulation in 3D SCM azimuth, elevation AOD \(\varphi_{k,AOD},~{}\theta_{k,AOD}\) and azimuth, elevation AOA \(\varphi_{k,AOA},~{}\theta_{k,AOA}\) for each multipath component (MPC) \(k=1,\ldots,K\). Investigation [68] shifts focus from point-to-point MIMO links to the state-of-the-art signal processing and information theoretic overview of MU mMIMO with large-scale antenna systems, consisting of dozens and more AEs, being typically significantly larger, than the number of UE, and thus providing additional degrees of freedom for simultaneous individual beamsteering for signal-of-interest (SOI) and null interference for signal-not-of-interest (SNOI). It also presents one example about feasibility of CSI acquisition in frequency division duplex (FDD) systems: channel coherence interval of 1 ms \(\times\) 100 kHz in time-frequency grid can support transmission of 100 orthogonal pilot waveforms, and if there are 100 AE at the gNB, the whole coherence interval should be used only for training, leaving no symbols for data transmission. One of the solutions to mitigate non-orthogonal pilot contamination is to use AOA-based methods for each UE. This circumstance serves as an additional justification for the use of LAB instead of traditional CSI acquisition both in uplink (UL) and downlink (DL), especially in LOS conditions. Three-dimensional beamforming (3 DBF) [69] promises to substantially improve capability of intracell interference management, comparing with only two-dimensional (2 DBF) beamforming. 3 DBF allows per user beam tracking due to electronically adapting ARP orientation both in azimuth and elevation planes according to specific UE current location. Applying additional coordination between neighboring cells, it becomes possible to also alleviate intercell interference. Monte Carlo simulations of dynamic 3 DBF [69], when vertical beam pattern is adapted according to UE location, results in 109% throughput gain over the 2 DBF at the cell edge with signal-to-noise ratio (SNR) is 0 dB; elevation HPBW decrease from 24\({}^{\circ}\) to 8\({}^{\circ}\) doubles throughput when the cell edge SNR is 10 dB. Experimental prototype of mmWave adaptive BF algorithm realization is described in [70]. It uses 28 GHz carrier frequency, 500 MHz bandwidth and hybrid BF scheme: 32 AEs comprise UPA with 8 horizontal and 4 vertical elements; 8 antenna elements are grouped into a sub-array, thus requiring only 4 RF units per analog channel with a set of phase shifters to form a desired beam pattern. Resulting HPBW is approximately 10\({}^{\circ}\) in azimuth and 20\({}^{\circ}\) in elevation plane with BF gain of 18 dBi. To reduce CSI exchange, codebook contains a set of predefined overlapping beams, each with a unique identifier, further supplied to baseband modem. Developed prototype performed beam alignment within 45 ms.  
Work [71] points out challenges of spatial multiplexing processing, inherent to massive MIMO and concerning CSI acquisition in both UL and DL.