Ray-Tracing-Based Numerical Assessment of the Spatiotemporal Duty Cycle of 5G Massive MIMO in an Outdoor Urban Environment  
[PERSON]  
1 Department of Information Technology, Ghent University/IMEC, 9052 Ghent, Belgium;  
1 [EMAIL_ADDRESS] (A.T.); [EMAIL_ADDRESS] (S.A.); [EMAIL_ADDRESS] (L.V. );  
1 [EMAIL_ADDRESS] (G.T. ); [EMAIL_ADDRESS] (L.M. ); [EMAIL_ADDRESS] (P.D. );  
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[PERSON]  
2 Department of Information Technology, Ghent University/IMEC, 9052 Ghent, Belgium;  
1 [EMAIL_ADDRESS] (A.T.); [EMAIL_ADDRESS] (S.A.); [EMAIL_ADDRESS] (L.V. );  
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referred to as beamforming. Precoding is another transmission technique that dynamically sets the antenna element coefficients to fulfill a predefined optimization objective. The more elements the antenna array has (up to a certain limit), the more narrow the beams it is capable of forming. A narrower beam means a higher maximum gain for equal total transmit power, as the EMF energy gets focused more tightly in the desired direction. Several different beamforming and precoding techniques are discussed below. Multi-User Multiple-Input Multiple-Output (MU-MIMO) with codebook-based beamforming simultaneously generates a subset of a predefined set of beams (the \"codebook\") at a time. A user equipment (UE) uplink (UL) signaling is processed at the BS to choose the beam that best reaches the UE location. In line-of-sight (LOS) conditions, this typically is the beam which has the closest direction-of-departure (DOD) to the true direction to the UE. However, in non-LOS (NLOS), the largest portion of the EMF power might reach the UE through interactions with the propagation environment (e.g., reflections and diffractions), and thus the DOD of the beam chosen by codebook-based precoding does not necessarily correlate with the path of the shortest distance to the UE. The Massive MIMO technology utilizes full channel state information (CSI) to multiplex multiple UEs in the same time-frequency resource, that is, to simultaneously transmit multiple data streams through separate spatial channels at a shared frequency. Massive MIMO leverages the knowledge of a continuously updated channel to _precode_ the transmitted signal. The Maximum Ratio Transmission (MRT) precoding scheme aims at maximizing the signal to noise ratio (SNR) at the target UEs [2]. If the UE in focus has a dominant LOS path, this results in forming a precisely directed single beam. If the UE is in a deep shadow region with multiple scattering paths, the BS distributes the available power over all these paths. This results in a less directive array pattern that forms a compact region with an elevated EMF around the target UE, a so-called _hot-spot_. The Zero Forcing (ZF) on the other hand, aims to maximize the signal to interference ratio (SIR) at the target UEs. If the target UEs' channels correlate considerably, a large portion of the BS resources is spent for interference mitigation. By doing so, unintended hot-spots might be created in the regions of the environment without active UEs. The effect of these precoding schemes on the power distributions in the angular domain serviced by the BS will be investigated in the following sections. Human EMF exposure in the far-field is directly related to the incident electric field (E-field) strength. The EMF values are typically averaged over time. The International Commission on Non-Ionizing Radiation (ICNIRP) specifies a time duration of 6 min to be used for the EMF averaging in compliance assessment [3; 4]. The averaging interval of 30 min is specified for the whole-body surface area of the E-field averaging in the most recent version of the guidelines [3]. In free space the E-field can be directly derived from the antenna gain, which is used in practice to establish the BS compliance boundary [5]. Actual E-field values observed in an environment are influenced by propagation or blockage of the transmitted signals.