![Distance of maximum exposure as function of frequency and array size for the 1 cm 2 spatial averaging area employed in FCC 15-138 [19].](https://www.researchgate.net/profile/Bjoern-Thors-2/publication/306266058/figure/fig2/AS:396436828114947@1471529313853/Distance-of-maximum-exposure-as-function-of-frequency-and-array-size-for-the-1-cm-2_Q320.jpg)
![Maximum transmitted power to be compliant with the ICNIRP RF EMF exposure limit [7] for í µí± = 0.5 cm.](https://www.researchgate.net/profile/Bjoern-Thors-2/publication/306266058/figure/fig3/AS:396436828114948@1471529313951/Maximum-transmitted-power-to-be-compliant-with-the-ICNIRP-RF-EMF-exposure-limit-7-for-i_Q320.jpg)
![Maximum transmitted power to be compliant with the FCC RF EMF exposure limits [8, 9, 19] for í µí± = 0.5 cm.](https://www.researchgate.net/profile/Bjoern-Thors-2/publication/306266058/figure/fig11/AS:668650327511074@1536430069690/Maximum-transmitted-power-to-be-compliant-with-the-FCC-RF-EMF-exposure-limits-8-9-19_Q320.jpg)
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Abstract—In this paper, radio-frequency (RF) electromagnetic
field (EMF) exposure evaluations are conducted in the frequency
range 10 GHz – 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 (ICNIRP), the US Federal Communications
Commission (FCC) and the Institute of Electrical and Electronics
Engineers (IEEE) is determined. The maximum transmitted
power is related to the maximum equivalent isotropically
radiated power (EIRP) to highlight the relevance of the output
power restrictions for a communication channel. A comparison
between simulation and measurement data is provided for a
canonical monopole antenna. For small distances, with the
antennas transmitting directly towards 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.
Index Terms— 5G mobile communication, antenna arrays,
beam steering, mobile device, mobile user equipment, radio base
station, RF EMF exposure.
I. INTRODUCTION
The total amount of mobile traffic is expected to increase
dramatically in the coming years [1]. The next generation of
wireless access systems (5G), set for commercial availability
around 2020 [2], is expected to constitute a key enabler for the
larger system capacity and higher data rates of the future.
Various research activities are currently ongoing to lay the
foundation for this new technology, see e.g. [3, 4], which apart
from mobile broadband will involve a range of different use
cases and challenging requirements on latency, security,
B. Thors, D. Colombi, and C. Törnevik are with Ericsson Research,
Ericsson AB, Stockholm, SE-16480, Sweden (e-mail:
bjorn.thors@ericsson.com).
Z. Ying and T. Bolin are with Network Technology Lab., Research and
Technology, Sony Mobile Communications AB, Lund, SE-221 88, Sweden..
reliability, availability, energy performance, and device cost
[5]. In terms of spectrum, 5G systems will need to be able to
operate over a very wide frequency range from below 1 GHz
up to and including millimeter wave (mmW) frequencies [1].
The available spectrum above 10 GHz will be a key
component to fulfill long-term traffic demands and to enable
the very wide transmission bandwidths needed to provide the
desired multi-Gbps data rates in an efficient manner [5].
Products emitting radio-frequency (RF) electromagnetic
fields (EMF) need to be designed and tested to comply with
relevant regulatory requirements and limits on human
exposure to EMF [6-9]. The most widely adopted exposure
limits worldwide are the guidelines specified by the
International Commission on Non-Ionizing Radiation
(ICNIRP) [7] in 1998. In the US, exposure limits specified by
the Federal Communications Commission (FCC) are
applicable [9]. The exposure limits published by the IEEE [10,
11] are of a more recent date but has so far not been adopted
in any national regulations.
For the frequencies used by existing second, third, and
fourth generation (2G, 3G, and 4G) mobile communication
systems, basic restrictions on RF EMF exposure are specified
in terms of the specific absorption rate () to prevent, with
wide safety margins, from established adverse health effects
associated with excessive localized tissue heating and whole-
body heat stress [7, 9, 10]. At higher frequencies, the
absorption in the human tissue becomes more superficial and
the basic restrictions changes from SAR to incident power
density (). The transition frequency where this change in
exposure metric takes place is 3 GHz, 6 GHz, and 10 GHz for
the IEEE, FCC and ICNIRP exposure guidelines, respectively.
A literature review of what is required to ensure safety of
emerging 5G technologies with respect to RF EMF exposure
was presented in [12].
A fundamental property to consider when designing a
mobile communication system is the transmit power to be
used by the base station and user equipment (UE). For
frequencies below 3 GHz, research on RF EMF exposure from
base stations and UEs has been going on for more than 20
years resulting in a solid scientific understanding and well-
defined and standardized exposure assessment procedures see
e.g. [13-15]. Until recently, less attention has been paid to
frequencies above 6 GHz. With the upcoming standardization
of 5G radio access technologies this has started to change as
there is a clear need to define the corresponding system
boundaries and develop RF EMF exposure assessment
methods.
Exposure to RF EMF from Array Antennas in
5G Mobile Communication Equipment
Björn Thors, Davide Colombi, Zhinong Ying, Senior Member, IEEE, Thomas Bolin, and Christer
Törnevik, Member, IEEE
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In [16], the implication of the changing basic restriction
from SAR to power density was investigated in terms of the
maximum possible transmitted power () from a device
(canonical dipole) used in close proximity to the human body.
It was shown that the existing exposure limits will lead to a
non-physical discontinuity of several dB in 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,
might have to be several dB below the power levels used for
current cellular technologies [16]. In a follow-up letter [17],
the increase in skin temperature due to RF exposure from the
same source, when transmitting at the maximum allowable
power to be compliant with these limits, was investigated. The
maximum steady state temperature increase was found to
display a similar discrepancy. Among the relevant U.S. (FCC)
and international exposure limits, IEEE C95.1-2005 [10] was
found to provide the most consistent level of protection
against thermal hazards of exposure over the frequency range
6 - 60 GHz [17].
To improve the link budget and compensate for the
worsened propagation conditions with an increased free space
path loss at the higher frequencies it is desirable to make use
of array antennas for both UEs and base stations. In [18], the
RF EMF exposure for phased arrays intended for mobile
devices and transmitting at 15 GHz and 28 GHz was
investigated. The study considered effects of a progressive
phase shift between the antenna elements but was restricted to
the FCC exposure limits [9]. Until now, no systematic study
on EMF exposure for phased arrays transmitting above
10 GHz has been presented where effects of frequency range,
array size, scan angle, distance to human body, and all major
exposure standards are considered. It is the aim of this paper
to fill this gap and provide valuable information for the design
and standardization of future mobile communication systems.
A method description including the considered RF EMF
exposure limits is provided in Section II. The results are
presented and discussed in Section III and Section IV,
respectively. Finally, some conclusions are provided in
Section V.
II. METHOD
A. RF EMF exposure limits and limits on maximum
equivalent isotropically radiated power (EIRP)
Between 10 GHz – 300 GHz, the ICNIRP exposure
guidelines [7] specify a maximum power density of 10 W/m2
for the general public taken as an average over any 20 cm2 of
exposed area. In addition, the spatial maximum power density
averaged over any 1 cm2 shall not exceed 200 W/m2.
The uncontrolled power density exposure limit for FCC
between 6 GHz to 100 GHz is also 10 W/m2, which in general
is to be considered as a spatial peak value [8, 9]. Spatial peak
is not a well-defined quantity, however, and the obtained
result will depend on the exposure assessment method. For
measurements, an average over the probe dimensions will be
obtained and for computations a sufficient sampling density is
required. In a recent Notice of Proposed Rulemaking (FCC
15-138) [19], the FCC stipulates that spatial peak is to be
interpreted as an average over any 1 cm2 in the shape of a
square for frequencies above 6 GHz. Although this
interpretation of spatial peak power density has not yet
formally made its way into the FCC regulations, in this work a
1 cm2 averaging area was assumed to be consistent with FCC
15-138 [19]. In [19], the FCC also stipulates a maximum peak
EIRP, , of 20 W for mobile
1
devices.
Between 3 GHz to 100 GHz, the IEEE general public basic
restriction on power density is 10 W/m2 [10]. In the frequency
range between 3 GHz to 30 GHz, the power density is to be
spatially averaged over any contiguous area corresponding to
100 where is the free space wavelength of the RF field.
Above 30 GHz, the averaging is to be conducted over any
contiguous area of 100 cm2 [11]. In addition, IEEE also
specifies maximum spatial peak power densities of
W/m2 and 200 W/m2 at frequencies between
3 GHz and 30 GHz and above 30 GHz, respectively, where
shall be taken as the frequency in GHz. No averaging area or
spatial sampling density is specified by the IEEE for the
spatial peak power density limits. In this work the spatial peak
power density was assessed using a minimum spatial sampling
density of four samples per wavelength.
A summary of the RF EMF exposure limits, , is
provided in Table I. For convenience, the spatially averaged
power densities were for all RF exposure limits determined
assuming square-shaped averaging areas.
TABLE I
GENERAL PUBLIC/UNCONTROLLED BASIC RESTRICTIONS ON POWER DENSITY,
, AS DEFINED BY ICNIRP [7], FCC [8, 9, 19] AND IEEE [10, 11]. THE
PARENTHESES BEHIND THE POWER DENSITY LIMITS INDICATE THE APPLICABLE
AVERAGING AREA. ABSENCE OF AVERAGING AREA IMPLIES SPATIAL PEAK
POWER DENSITY.
( = WAVELENGTH IN FREE SPACE, = FREQUENCY IN GHZ).
ICNIRP
FCC
IEEE
(GHz)
10 – 300
6 - 100
3 - 30
30 - 100
(W/m2)
10 (20 cm2)
200 (1 cm2)
10 (1 cm2)
10 100)
18.56
10 100 cm2)
200
B. Power density and maximum transmitted power
For each distance from the face of the array, the maximum
spatially averaged power density over any square-shaped
averaging area, , is determined as the real power, ,
flowing through according to
where and denote the real part, the electric and
magnetic fields and the complex conjugate, respectively. The
1
The U.S. FCC distinguishes between mobile and portable devices [9]. A
mobile device is defined as a transmitter designed to be used in other than
fixed locations and to generally be used in such a way that a separation
distance of at least 20 centimeters is normally maintained between the
radiating structures and the body of the user or nearby persons. A portable
device is defined as a transmitter whose radiating structures are designed to be
used within 20 centimeters of the body of the user. For certification of
medium range RBS, local area RBS, and home RBS [25], RF EMF exposure
assessments are normally conducted according to the requirements for mobile
exposure.
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definition of the coordinate system employed and the
averaging area is illustrated in Figure 1 together with a square-
shaped array antenna.
Fig. 1. Square-shaped array antenna and definition of area over which the
power density is averaged.
For large array antennas, a characteristic feature is that the
maximum exposure may occur at some distance away from
the face of the array due to the focusing of energy, see
Section III. From an EMF compliance assessment point of
view, this implies that the maximum transmitted power must
be determined considering a range of distances In this work,
compliance with the exposure limits has been assessed
considering distances in the range 0.5 cm – 50 cm. In
Section III, when results are presented for a certain distance, it
is understood that compliance with the exposure limits is
ensured for all distances larger than or equal to this distance.
In particular, results are presented for distances
and . The smallest distance is of relevance for
portable devices, i.e. UEs used in close proximity of the body.
The larger distance is of relevance for mobile devices (e.g.
laptops or wireless routers) and low-power RBS
2
.
During the simulations and measurements, the power
density was scaled to a transmitted power, , of 1 W. The
maximum transmitted power, , and the maximum EIRP,
, to comply with the limits in Table I for distances
were determined according to
(3)
2
For low-power RBS, compliance with RF EMF exposure limits is
normally achieved during installation by making sure that the general public
do not have access to a region in the vicinity of the antenna where the
exposure limits may be exceeded (i.e. within the compliance boundary). The
results on maximum transmitted power presented in this paper shall for these
type of products be interpreted as the power level that would result in the
specified compliance distance . Larger power levels are possible to use if
larger compliance boundaries are considered during installation of the low-
power RBS.
where and denote the maximum antenna gain, the
number of antenna elements in the considered square-shaped
arrays, and the progressive phase shift to consider effects of
beam scanning in the -plane, see Section II-C.
C. Numerical simulations
Numerical simulations were conducted using the
commercial electromagnetic solver FEKO (Altair,
Stellenbosch, South Africa) based on the Method of Moments
(MoM) [20]. Square-shaped ground-plane backed dipole
arrays with an inter-element distance of were considered.
The dipoles were rotated 45° from the -axis as indicated in
Figure 1 and the length of the dipoles was. Around the
edge of the ground plane, a wall of height was situated.
All parts of the antenna were made of metal and simulated as
perfectly electrically conducting objects. Arrays with
elements, with , were considered for frequencies
. To investigate the effects of beam
scanning, a progressive phase shift was used.
For the selected inter-element distance and the ideal case of no
coupling among the antenna elements this would correspond
to an azimuthal scan range of .
The antenna ground plane was meshed using triangles with
a maximum edge length of. The dipoles were modeled
as thin wires with a maximum wire segment length of
and an equivalent radius of 00 [21].
To assess the maximum exposure, with the arrays
transmitting directly towards a human body, the electric and
magnetic fields were determined in a volume in front of the
antenna from to with the extent in the -
and -directions chosen to circumscribe both the face of the
array and the location of maximum power density for the
maximum considered scan angle. A minimum sampling
density of four samples per wavelength was used
The array sizes investigated were chosen arbitrarily to span a
large parameter space of relevance for several different
applications. For a particular application, however, only a
subset of the considered domain may be relevant.
D. Reference antenna and measurements set-up
To verify the numerical simulations, measurements were
conducted on a 15 GHz monopole prototype antenna
manufactured by Sony Mobile Communications (Lund,
Sweden). The prototype antenna had a gain of 2 dBi and a
vertical half-power beamwidth of 80 degrees. The
measurements were conducted using a DASY 5 near-field
scanner together with an isotropic E-field probe EF3DV3
(SPEAG, Zurich, Switzerland). The probe was calibrated for
measurements at 15 GHz using the free-space standard-field
method [22] and a PE9854/SF-10 horn antenna (Pasternack,
Irvine, CA) with a nominal gain of 10 dBi. Eccosorb AN-79
RF absorbing material (Randolph, MA) was used to minimize
reflections towards the measurement area from all relevant
mechanical structures present in the anechoic measurement
chamber. Measured results on plane-wave equivalent power
density, , were compared with FEKO simulations based
on a CAD-file of the monopole antenna.
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III. RESULTS
A. Reference antenna measurements
A comparison between simulated and measured plane-wave
equivalent power density results is presented in Figure 2 in
terms of the maximum transmitted power to comply with the
ICNIRP and FCC limits. Shown also are results obtained
using the spherical far-field formula [15] to comply with a
spatial peak power density of 10 W/m2. The agreement
between the simulations and measurements is excellent. The
spatially averaged results over 1 cm2 also agree very well with
the spatial peak results obtained using the far-field formula.
Fig. 2. Comparison between simulated and measured results in terms of
maximum transmitted power to comply with the ICNIRP and FCC limits in
Table 1 for a monopole antenna transmitting at 15 GHz.
B. Power density simulation results
In Figure 3, a plot of the distance of maximum exposure is
provided as function of frequency and array size for the 1 cm2
spatial averaging area employed in FCC 15-138 [19]. The
obtained results illustrate that for large array antennas the
maximum exposure may occur some distance away from the
face of the array. Similar behaviors are also observed for
other averaging areas (not shown) with the largest distance of
maximum exposure obtained for the lowest frequency and
largest antenna arrays considered.
The maximum transmitted power to be compliant with the
ICNIRP, FCC, and IEEE limits in Table 1 with
has been determined and the results are provided in Figure 4 –
Figure 6. The white dashed lines in Figure 4 and Figure 5
correspond to antenna arrays with areas, , equal to the main
averaging areas, for the ICNIRP (20 cm2) and FCC
(1 cm2) exposure limits. Due to the short distance, ,
compared with the extent of the averaging area, in the region
to the right of the white dashed line in Figure 4 almost all
power will pass through . As a consequence, the maximum
transmitted power in this area approaches 13 dBm (20 mW),
cf. Table I. For the IEEE results in Figure 6, the averaging area
is larger than the array areafor all considered arrays.
In general, as the physical array size is increased the
transmitted power is distributed over a larger area which for a
small distance translates to a larger maximum transmitted
power. The FCC exposure limits results in a lower maximum
transmitted power than the ICNIRP and IEEE exposure limits.
For lower frequencies and larger arrays, the IEEE exposure
limits results in a larger maximum transmitted power
compared with the ICNIRP exposure limits. For higher
frequencies and smaller arrays the opposite is true, which is
explained by the fact that in this region the maximum
transmitted power for the IEEE limits is determined by the
spatial peak power density.
Fig. 3. Distance of maximum exposure as function of frequency and array
size for the 1 cm2 spatial averaging area employed in FCC 15-138 [19].
Fig. 4. Maximum transmitted power to be compliant with the ICNIRP RF
EMF exposure limit [7] for .
Fig. 5. Maximum transmitted power to be compliant with the FCC RF EMF
exposure limits [8, 9, 19] for .
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Fig. 6. Maximum transmitted power to be compliant with the IEEE RF EMF
exposure limit [10, 11] for .
Results on maximum transmitted power to be compliant
with the ICNIRP, FCC, and IEEE limits in Table 1 for
are provided in Figure 7 – Figure 9. For the
smallest arrays investigated, the maximum transmitted power
for the ICNIRP and FCC exposure limits is similar since far-
field conditions apply and the difference between power
density averaged over 1 cm2 and 20 cm2 is very small. In this
region, the maximum transmitted power is approximately
inversely proportional to the antenna gain, which for the
considered square-shaped arrays is independent of the
frequency. As a consequence, the largest maximum
transmitted power levels in Figures 7 and 8 display a
frequency independent behavior and are obtained for the array
with the fewest number of elements. For the IEEE limits, the
side of the averaging area at the lower frequencies is not small
compared with the distance, , which explains why the
horizontal behavior of the contour lines is not maintained as
the frequency is reduced. The vertical contour lines in Figure 9
for frequencies below 30 GHz indicate that, in this region,
most of the transmitted power will flow through the averaging
area independent of the array size.
Fig. 7. Maximum transmitted power to be compliant with the ICNIRP RF
EMF exposure limit [7] for .
Fig. 8. Maximum transmitted power to be compliant with the FCC RF EMF
exposure limits [8, 9, 19] for .
Fig. 9. Maximum transmitted power to be compliant with the IEEE RF EMF
exposure limit [10, 11] for .
For a communication system employing array antennas it
is important to relate maximum transmitted power levels to
maximum EIRP determined according to Equation (3). As an
example, the maximum EIRP to be compliant with the
ICNIRP exposure limits for is provided in
Figure 10. Since the physical array size scales with frequency,
the maximum antenna gain is frequency independent. Thus,
the variation in maximum EIRP versus frequency reflects the
frequency dependency of the maximum transmitted power, cf.
Figure 4. According to Figure 4, in a large part of the analyzed
parameter space the maximum transmitted power is constant.
The variation in maximum EIRP in this region, as observed in
Figure 10, corresponds to the variation in maximum gain as
function of array size. Plots of the maximum EIRP for the
exposure limits and distances analyzed in Figures 5-9 are
provided in Appendix A.
A summary of the results on maximum transmitted power
and maximum EIRP to comply with the different exposure
limits is provided in Table II.
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TABLE II
SUMMARY OF MAXIMUM TRANSMITTED POWER AND MAXIMUM EIRP TO COMPLY WITH THE ICNIRP [7], FCC [8, 9, 19] , AND IEEE[10, 11] RF EMF EXPOSURE
LIMITS .
(GHz)
Array area (cm2)
–
elements
Maximum transmitted power (dBm)
Maximum EIRP (dBm)
ICNIRP
FCC
IEEE
ICNIRP
FCC
IEEE
Portable1
applications
()
10
9.0 – 230
13 – 20
7 – 18
16 – 28
24 – 45
18 – 43
27 – 53
20
2.3 – 56
13 – 16
2 – 13
12 – 24
24 – 41
13 – 38
23 – 49
30
1.0 – 25
13 – 14
1 – 10
11 – 20
24 – 39
12 – 35
22 – 45
40
0.56 – 14
13
1 – 8
9 – 20
24 – 38
12 – 33
20 – 45
50
0.36 – 9.0
13
1 – 6
9 – 18
24 – 38
12 – 31
20 – 43
Mobile2
applications
()
10
9.0 – 230
20 – 26
18 – 26
27 – 31
37 – 45
37 – 43
42 – 53
20
2.3 – 56
16 – 26
14 – 26
24 – 28
37 – 41
37 – 39
39 – 49
30
1.0 – 25
16 – 26
13 – 26
21 – 27
37 – 41
37 – 38
38 – 46
40
0.56 – 14
15 – 26
13 – 26
21 – 27
37 – 40
37 – 38
38 – 46
50
0.36 – 9.0
15 – 26
13 – 26
21 – 27
37 – 40
37 – 38
38 – 46
1 FCC terminology used to denote devices intended to be used at a distance of less than 20 cm from the human body.
2 FCC terminology used to denote devices intended to be used at a distance of 20 cm or more from the human body.
Fig. 10. Maximum EIRP to be compliant with the ICNIRP RF EMF
exposure limit [7] for .
IV. DISCUSSION
The purpose with the numerical study presented above
was to provide an indication of the maximum transmitted
power levels and maximum EIRP for array antennas to be
used in future 5G mobile communication systems in order
to comply with all major RF EMF exposure standards. This
was accomplished by investigating a wide parameter space
of relevance for both UEs and low-power RBS for a
maximum exposure scenario with the arrays transmitting
directly towards the human body.
In the analysis, ground-plane backed arrays with
canonical dipole elements were considered. For comparable
arrays with other antenna elements, a similar behavior is
expected with exposure levels of the same order of
magnitude.
The results in Table II constitute a summary of the
obtained results for the considered parameter space. For a
particular application, where only a subset of the considered
frequency range and/or array size is of interest, the reader is
referred to the results in Figure 4 – Figure 10 and
Figure 13 – Figure 17.
For , the smallest arrays and highest
frequencies considered, the IEEE limits result in a
significantly lower maximum transmitted power level
compared with the ICNIRP limits. This is a consequence of
the maximum transmitted power being determined by the
spatial peak power density for the IEEE limits in this
region. As pointed out in Section II, spatial peak is not a
well-defined quantity and will depend on the assessment
method. If the same definition of spatial peak power density
had been adopted for the IEEE limits as for the FCC limits,
i.e. if a 1 cm2 averaging area had been used, the maximum
transmitted power in the lower right corner of Figure 6
would have been 13 dBm (20 mW) and consistent with the
ICNIRP limits.
The results in this paper have been obtained for array
antennas with an inter-element spacing. This
choice of inter-element spacing allows for a wide scan
range without the introduction of grating lobes. A drawback
with too dense element spacing is that the devices become
more expensive as the cost scales with the number of
transceivers. On the other hand, too wide inter-element
spacing will introduce grating lobes that will reduce the
EIRP and possibly increase the interference in the system.
An illustration of this possible trade-off is provided in
Figure 12, where the spread of maximum EIRP to be
compliant with the major exposure standards for
is given as function of inter-element distance. The
spread in maximum EIRP corresponds to the considered
ideal scan range , see Section II-C. For
no grating lobes will propagate. Here, the
obtained spread in EIRP corresponds to a reduced gain as
the arrays are scanned from broadside. As expected, the
spread gets wider with an increasing inter-element
separation distance illustrating the impact of propagating
grating lobes, which becomes larger as the scan angle
increases from broadside.
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Fig. 12. Spread of maximum EIRP as function of inter-element
separation distance for arrays of area , transmitting at 15 GHz with a
progressive phase shift , to be compliant with all major
exposure standards for For the smaller array area (
), the considered inter-element separation distance corresponds to
arrays with to elements. For the larger array area (
), the considered inter-element separation distance corresponds to
arrays with to elements.
For devices intended to be used in the immediate vicinity
of the human body, the ICNIRP and FCC exposure limits
results in a maximum transmitted power significantly below
what is specified today for existing mobile communication
technologies, e.g. 23 dBm and 24 dBm for Long Term
Evolution (LTE) [23] and WCDMA [24], respectively. This
is in-line with the findings in [16] where a single dipole
element was considered. To circumvent these problems, and
also to improve the link budget by reducing body losses, it
is conceivable that technical solutions are employed
whereby the transmitted energy is always directed away
from the human body. A reasonable EMF compliance
assessment procedure would then be based on evaluations
of the exposure behind the array antenna, at a certain
distance, , plus an evaluation of the exposure for
bystanders at a distance, , in front of the array. As
an example, Figure 11 shows the maximum transmitted
power for a array antenna with the exposure directed
away from the human body assuming and
. The maximum transmitted power is in
this case given by the bystander exposure and is found to be
in the range 26 dBm – 30 dBm for the different exposure
standards and frequencies investigated.
V. CONCLUSIONS
In this paper, a study to investigate the maximum
transmitted power and maximum EIRP to comply with all
major RF EMF exposure standards has been presented for
array antennas intended for user equipment and low-power
radio base stations in 5G mobile communication systems.
Effects of frequency, array size, distance to human body,
scan range and array topology have been considered. The
obtained results constitute valuable input to the design of
future mobile communication systems employing array
antennas with beamforming capabilities.
Fig. 11. Maximum transmitted power to be compliant with all major RF
exposure standards for a array antenna with the exposure directed
away from the human body (, ).
For devices intended to be used in the immediate vicinity
of the human body (portable devices) and exposure
scenarios where the transmitted energy is directed towards
the body, the FCC exposure limits are more restrictive than
the ICNIRP and IEEE limits. In general, the ICNIRP
exposure limits are more restrictive than the IEEE limits
with the exception of high frequencies and small arrays
where the IEEE limit on spatial peak power density will
lead to a lower maximum transmitted power. In order to
allow larger power levels, technical solutions, whereby the
transmitted energy is always directed away from the human
body, are conceivable.
For mobile devices1 very similar results are obtained for
the FCC and ICNIRP limits as the difference between the
spatially averaged power density over 1 cm2 and 20 cm2 is
small in a large part of the parameter space where far-field
conditions apply.
Depending on the applicable RF EMF exposure standard,
quite large variations in maximum transmitted power levels
and maximum EIRP may be expected for UE to be used in
future mobile communication systems. This inconsistency
will lead to different pre-requisites for different markets.
Furthermore, for UE intended to be used in close proximity
of the body, the ICNIRP and FCC exposure limits results in
a maximum transmitted power significantly below what is
specified today for existing mobile communication
technologies. If not resolved, these findings may have a
large negative impact on the performance and cost of future
mobile communication systems. A global harmonization of
the RF EMF exposure limits for frequencies above 6 GHz is
desirable with a similar margin of safety as for frequencies
below 6 GHz to protect from established adverse health
effects.
APPENDIX A
The maximum EIRP to be compliant with the FCC and
IEEE RF EMF exposure limits for are provided
in Figure 13 and Figure 14 The corresponding results for
, and the ICNIRP, FCC and IEEE RF EMF
exposure limits, are given in Figures 15-17.
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Fig. 13. Maximum EIRP to be compliant with the FCC RF EMF exposure
limits [8, 9, 19] for .
Fig. 14. Maximum EIRP to be compliant with the IEEE RF EMF
exposure limit [10, 11] for .
Fig. 15. Maximum EIRP to be compliant with the ICNIRP RF EMF
exposure limit [7] for .
Fig. 16. Maximum EIRP to be compliant with the FCC RF EMF exposure
limits [8, 9, 19] for .
Fig. 17. Maximum EIRP to be compliant with the IEEE RF EMF
exposure limit [10, 11] for .
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2169-3536 (c) 2016 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See
http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/ACCESS.2016.2601145, IEEE Access
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