This paper presents some theoretical considerations and experimental results regarding the problem of maximum power extrapolation for the assessment of the exposure to electromagnetic fields radiated by 5G base stations. In particular the results of an extensive experimental campaign using an extrapolation procedure recently proposed for 5G signal is discussed and experimentally checked on a SU-MIMO signal. The results confirm the effectiveness of the extrapolation technique. Starting from an analysis (that represents a further novel contribution of this paper) on the impact of Spatial Division Multiple Access techniques used in 5G on the measurement of EMF level, some indications of possible extension of the technique to the highly complex MU-MIMO case are also given.

Maximum-Power Extrapolation (MPE) for mobile telecommunication sources follows an established paradigm based on the identification and measurement of a channel that acts as a power reference. Prior to the 5G era, the role of reference channel has been played by always-on broadcast signals since they had the great advantage of being always transmitted at the maximum power level allowed for a generic signal channel. However, the beamforming implemented by 5G sources obliges us to rethink this approach. In fact, with beamforming the 5G source can transmit data traffic streams through a beam characterized by a much higher gain than the broadcast one. This implies that the detected power for traffic beams could be much higher than the corresponding power of broadcast beams. In this paper, a novel approach for 5G MPE procedure is presented, where the direct measurement of the received power of a traffic beam is used to assess the maximum exposure generated by a 5G system. An innovative specific experimental setup is also proposed, with the use of a User Equipment (UE) with the aim of forcing the traffic beam toward the measurement positions. In this way, it is possible to directly measure the power of each Resource Element (RE) transmitted by the traffic beam. As opposed to other MPE proposals for 5G, the discussed technique does not require any correction of the measured data since it relies only on the traffic beam pointing toward the measurement position, simplifying the overall MPE procedure and thus reducing the uncertainty of the MPE estimated field strength.

The fifth-generation (5G) New Radio (NR) cellular network has been launched recently. The assignment of new spectrum bands and the widespread use of Massive MIMO (MaMIMO) and beamforming techniques for better radio coverage are two major features of the new architecture. They imply both opportunities and challenges, one of the most daring one among the latter ones is the research for methods to assess human exposure to electromagnetic fields radiated by the base stations. The long-term time-varying behavior and spatial multiplexing feature of the MaMIMO antennas, along with the radio resource utilization and adoption of Time-Division Duplexing (TDD), requires that the assessment of exposure to electromagnetic fields radiated by 5G systems is based on a statistical approach that relies on the space and time distribution of the radiated power. That, in turn, is determined through simulations based on the actual maximum transmitted power – defined as the 95th percentile of the empirical distribution obtained from historical data of radiated power – rather than on the nominal one. To ensure that exposure limits are never exceeded, a monitoring and control system (usually referred to as Power Lock (PL)) that limits the transmitted power can be used. In this paper we propose a methodology, independent from the specific technical solution implemented by the manufacturer, to characterize such control systems and determine their capability to limit the average power transmitted over a given time interval to a value that keeps the corresponding average exposure to electromagnetic fields below a specified value. Experimental results show the effectiveness of the methodology and that it can also be used to identify when the PL interacts with the higher levels of the MaMIMO system architecture.

Future networks will allow to reach high level of connectivity and capacity in order to respond to the growing traffic needs of the incoming years. It means that, future fifth generation of mobile network (5G) and existing fourth (4G) Long Term Evolution (LTE) network, will be characterized by a very large installations deployment in terms of macro/micro/femto cells. In addition, the introduction of active antenna technologies will permit to enhance performance in terms of capacity. This new paradigm has suggested to international standard bodies to revise RF EMF (Radio Frequency Electromagnetic Field) compliance assessment methodologies for the regulatory requirements on human exposure by using new metrics measurements, no longer based on deterministic maximum transmitted power evaluations. In this work we consider a possible application of a statistical model to calculate real emission from a Massive MIMO antenna, in the specific case of the Italian regulation on EMF, which is based on calculating the average transmitted power over a 24-hour period. From the data collected on a real 4G network, we have seen that the average transmitted power over 24 hours from base stations is approximately the 20% of the maximum transmitted power, which is typically used in the compliance assessment. By evaluations, we found that the statistical model for Massive MIMO antenna can be applied on top of the 24-hour average transmitted power evaluation obtained through network counters. As an example, the real power density @20 m with an antenna Gain of 15 dBi is always below the Italian limit for all possible transmitted power (i.e. 25 ÷35 W). The new combined model of statistical MIMO power model with an EMF evaluation averaged over time, as used in Italy, is an example of how to apply new metrics for EMF emission analysis of future networks.

Multi-User MIMO (MU-MIMO) is widely used in the fifth generation (5G) cellular network to improve network performance at high traffic loads. When demonstrating the compliance of 5G antennas with the radio frequency (RF) electromagnetic fields standards, beamforming and MU-MIMO must be considered, since a 5G MU-MIMO antenna radiation pattern changes in both the space and time domain, adapting to changes of the user’s position. To cope with the time and space variability of the power radiated by 5G systems, mobile operators can activate automatic tools to monitor and control the transmitted power (Power Lock (PL) systems) to ensure that a given radiated power threshold is not exceeded. This paper extends and enhances the methodology introduced by the authors in previous work, by testing and verifying PL systems in an MU-MIMO and beamforming scenario (3D Power Lock systems, 3DPL) through field measurements, controlling the power transmitted by each beam, i.e. in each direction. Measurement results confirm that the 3DPL feature limits the average power of the traffic channels transmitted in each direction, regardless of the configured total maximum power. More specifically, it is shown that the 3DPL reduces the maximum average transmitted power by the expected 2 dB when only one user is active (i.e., in a Single-User MIMO scenario), while it does not operate any reduction when two users are active (Multi-User MIMO scenario) because the power transmitted towards each user is already half (i.e., -3 dB) the maximum power that can be transmitted.

We present a methodology for on-site zero-span (ZS) measurement of incident power generated by 5G base stations that operate at millimeter-waves (mm-waves), analyzing the challenges offered by the new frequency band and Non-Line-of-Sight (NLoS) propagation, which is the most common situation for mobile network users. We also investigate the impact of omnidirectional and directional measurement antennas. This is the first research on this topic available in scientific literature. Our results reveal that the measurement method designed for frequencies below 6 GHz in Line-of-Sight (LoS) conditions is also applicable in the mm-wave range. At the same time, in NLoS, mitigation strategies are suggested to measure signals accurately. That is, an omnidirectional antenna is recommended since a directional one requires an initial assessment to align the probe with the direction of the strongest signal. Additionally, directional antennas under NLoS conditions don’t guarantee that the power density coming from different directions due to reflections is measured entirely. This study holds significant importance for international standardization bodies focusing on establishing measurement methods to evaluate exposure originating from mobile network base stations.