Davide Colombi

Implications of EMF Exposure Limits on Output Power Levels for 5G Devices Above 6 GHz

Spectrum is a scarce resource, and the interest for utilizing frequency bands above 6 GHz for future radio communication systems is increasing. The possible use of higher frequency bands implies new challenges in terms of electromagnetic field (EMF) exposure assessments since the fundamental exposure metric (basic restriction) is changing from specific absorption rate (SAR) to power density. In this study, the implication of this change is investigated in terms of the maximum possible radiated power (Pmax) from a device used in close proximity to the human body. The results show that the existing exposure limits will lead to a non-physical discontinuity of several dB in Pmax as the transition is made from SAR to power density based basic restrictions. As a consequence, to be compliant with applicable exposure limits at frequencies above 6 GHz, Pmax might have to be several dB below the power levels used for current cellular technologies. Since the available power in uplink has a direct impact on the system capacity and coverage, such an inconsistency, if not resolved, might have a large effect on the development of the next generation cellular networks (5G).

DOWNLINK POWER DISTRIBUTIONS FOR 2G AND 3G MOBILE COMMUNICATION NETWORKS

Knowledge of realistic power levels is key when conducting accurate EMF exposure assessments. In this study, downlink output power distributions for radio base stations in 2G and 3G mobile communication networks have been assessed. The distributions were obtained from network measurement data collected from the Operations Support System, which normally is used for network monitoring and management. Significant amounts of data were gathered simultaneously for large sets of radio base stations covering wide geographical areas and different environments. The method was validated with in situ measurements. For the 3G network, the 90th percentile of the averaged output power during high traffic hours was found to be 43 % of the maximum available power. The corresponding number for 2G, with two or more transceivers installed, was 65 % or below.

Exposure to RF EMF From Array Antennas in 5G Mobile Communication Equipment

In this paper, radio-frequency (RF) electromagnetic field (EMF) exposure evaluations are conducted in the frequency range 10-60 GHz for array antennas intended for user equipment (UE) and low-power radio base stations in 5G mobile communication systems. A systematic study based on numerical power density simulations considering effects of frequency, array size, array topology, distance to exposed part of human body, and beam steering range is presented whereby the maximum transmitted power to comply with RF EMF exposure limits specified by the International Commission on Non-Ionizing Radiation Protection, the US Federal Communications Commission, and the Institute of Electrical and Electronics Engineers is determined. The maximum transmitted power is related to the maximum equivalent isotropically radiated power to highlight the relevance of the output power restrictions for a communication channel. A comparison between the simulation and measurement data is provided for a canonical monopole antenna. For small distances, with the antennas transmitting directly toward the human body, it is found that the maximum transmitted power is significantly below the UE power levels used in existing third and fourth generation mobile communication systems. Results for other conceivable exposure scenarios based on technical solutions that could allow for larger output power levels are also discussed. The obtained results constitute valuable information for the design of future mobile communication systems and for the standardization of EMF compliance assessment procedures of 5G devices and equipment.

Antenna Current Optimization for Lossy Media With Near-Field Constraints

Optimization of the current distribution is used to analyze how small antennas are affected by amplitude constraints on the near field and by lossy background media. The optimal antenna current that minimizes the stored energy, for a prescribed radiated field in a given direction, and with limited near field in a set of control points, is formulated as a convex optimization problem. The analysis is also extended to antennas in lossy media by using a frequency-derivative approximation of the stored energy. The results suggest that many fundamental antenna problems involving near-field constraints and lossy background media can be analyzed using convex optimization.

Radio frequency electromagnetic field compliance assessment of multi‐band and MIMO equipped radio base stations

In this paper, different methods for practical numerical radio frequency exposure compliance assessments of radio base station products were investigated. Both multi-band base station antennas and antennas designed for multiple input multiple output (MIMO) transmission schemes were considered. For the multi-band case, various standardized assessment methods were evaluated in terms of resulting compliance distance with respect to the reference levels and basic restrictions of the International Commission on Non-Ionizing Radiation Protection. Both single frequency and multiple frequency (cumulative) compliance distances were determined using numerical simulations for a mobile communication base station antenna transmitting in four frequency bands between 800 and 2600 MHz. The assessments were conducted in terms of root-mean-squared electromagnetic fields, whole-body averaged specific absorption rate (SAR) and peak 10 g averaged SAR. In general, assessments based on peak field strengths were found to be less computationally intensive, but lead to larger compliance distances than spatial averaging of electromagnetic fields used in combination with localized SAR assessments. For adult exposure, the results indicated that even shorter compliance distances were obtained by using assessments based on localized and whole-body SAR. Numerical simulations, using base station products employing MIMO transmission schemes, were performed as well and were in agreement with reference measurements. The applicability of various field combination methods for correlated exposure was investigated, and best estimate methods were proposed. Our results showed that field combining methods generally considered as conservative could be used to efficiently assess compliance boundary dimensions of single- and dual-polarized multicolumn base station antennas with only minor increases in compliance distances.

Measurements of downlink power level distributions in LTE networks

Human exposure to the radio frequency electromagnetic fields (EMF) emitted by radio base stations (RBS) is proportional to the transmitted power. Thus, knowledge of actual power levels is key for a correct estimate of the actual EMF exposure. In this study, downlink output power distributions for RBSs in a 4G LTE mobile communication network have been determined. By extracting data using the operations support system (OSS) of the network, statistics were obtained based on 24 hours measurements for more than 5000 RBSs. The network measurement approach was verified with in-situ power density measurements. It was found that the actual output power levels are significantly below the theoretical maximum. For high-traffic periods, the 90th percentile transmitted power was found to be about 12% of the theoretical maximum.

Power Level Distributions of Radio Base Station Equipment and User Devices in a 3G Mobile Communication Network in India and the Impact on Assessments of Realistic RF EMF Exposure

The aim of this paper is to present results on output power level distributions of radio base stations (RBSs) and user devices connected to a wideband code division multiple access-based third generation (3G) mobile communication network in India and relate the results to realistic human exposure to radio frequency (RF) electromagnetic field (EMF) emitted by the corresponding RBSs and the devices. The output power level distributions have been obtained through network-based measurements. In downlink, data from 868 RBSs were gathered during seven days. The RBSs were connected to five different radio network controllers (RNCs) located in different regions of India. The mean, the median, and the 95th percentile RBS output power values were found to be 24%, 21%, and 53%, respectively, of the maximum available power. In the uplink direction, output power levels of 3G devices connected to 1256 RBSs and the same five RNCs as in the downlink were assessed separately for voice, data, voice + data, and video applications. In total, more than 1 million hours of data traffic and more than 700 000 h of voice calls were measured in the uplink. The mean output power for the voice, data, the voice + data, and the video were found to be around 1%, 3%, 2%, and 4%, respectively, of the maximum available power for the 3G user devices. The findings are in line with previously published results obtained in other networks in Europe, and demonstrate that knowledge on realistic power levels is important for accurate assessments of the RF EMF exposure.

Time-Averaged Realistic Maximum Power Levels for the Assessment of Radio Frequency Exposure for 5G Radio Base Stations Using Massive MIMO

In this paper, a model for time-averaged realistic maximum power levels for the assessment of radio frequency (RF) electromagnetic field (EMF) exposure for the fifth generation (5G) radio base stations (RBS) employing massive MIMO is presented. The model is based on a statistical approach and developed to provide a realistic conservative RF exposure assessment for a significant proportion of all possible downlink exposure scenarios (95th percentile) in-line with requirements in a recently developed International Electrotechnical Commission standard for RF EMF exposure assessments of RBS. Factors, such as RBS utilization, time-division duplex, scheduling time, and spatial distribution of users within a cell are considered. The model is presented in terms of a closed-form equation. For an example scenario corresponding to an expected 5G RBS product, the largest realistic maximum power level was found to be less than 15% of the corresponding theoretical maximum. For far-field exposure scenarios, this corresponds to a reduction in RF EMF limit compliance distance with a factor of about 2.6. Results are given for antenna arrays of different sizes and for scenarios with beamforming in both azimuth and elevation.

Output Power Levels of 4G User Equipment and Implications on Realistic RF EMF Exposure Assessments

The aim of this paper is to present results on output power level distributions of 4G user equipment (UE) using data applications based on a very large number of samples collected over seven days in a long-term evolution (LTE) operating network. The output power data have been obtained through network-based measurements conducted for about 7000 UE connected to 41 LTE radio base stations located in rural, suburban, urban, and indoor environments in Sweden. More than 300 000 power samples were collected. In rural environments, the 95th percentile time-averaged output power values were found to be 2.2% of the maximum available power for LTE UE, while the corresponding values were less than 1% in other environments. The mean output powers in all the environments were found to be less than 1% of the maximum available output power. These values are in line with results obtained for 3G UE despite an almost tenfold increase in the achievable peak data throughput. The findings show that knowledge on realistic power levels is important for accurate assessments of the radio frequency electromagnetic field exposure from mobile communication equipment.

Implication of RF EMF Exposure Limitations on 5G Data Rates above 6 GHz

For future 5G devices operating above 6 GHz, a consequence of existing electromagnetic field (EMF) exposure limits is that the maximum transmit power may have to be reduced several dB compared with what is used for the current cellular bands. To understand the consequences of such limitations on the system capacity, an indoor geometry is used to assess the achievable data rates for low power indoor nodes and UEs. The evaluation is performed as a function of the carrier frequency, the Tx power, and the amount of Tx and Rx beamforming. Ray tracing in combination with physical and experimental models or reasonable assumptions of diffraction-, reflection- and penetration-losses are used for modeling. The paper shows that the current exposure limitations might reduce the achievable data rates in non-line-of-sight regions to levels that may be inconsistent with 5G performance expectations. For the currently given maximum transmit power, high order beamforming seems necessary to achieve a good coverage already for carrier frequencies well below 15 GHz.