DECT and
GSM
RF
Exposure Measurements
Dina SimuniC and Milan &vkoviC*
University of Zagreb, Faculty of Electrical Engineering and Computing, Dept.
for
Radiocommunications,
Unska 3,10000 Zagreb, Croatia
*Ericsson-Nikola Tesla, Krapinska 45,10000 Zagreb, Croatia
1.
Introduction
Measurements of
DECT
(Digital Enhanced
Cordless Telecommunications system) and
GSM
(Global System for Mobile Communications)
signals have been performed in the working
environment in order to find out whether
RF
exposure limits are exceeded in frequency bands
occupied by
two
mentioned systems. In the
building, DECT system replaces classic wired
telephony by installed DECT base stations,
directly connected to PSTN (Public Switched
Telephone Network). Most of base stations are in
corridors, but some are very close to working
tables. Coverage
of
a base station in the building
is between
50
and 100 m; outside is
up
to
300
m.
As reference levels
for
RF
exposure limits IEEE
values are used.
10 ms frame having 24 timeslots, with a common
scheme for transmitting and receiving data packets
of various length at 1152 kpbs within one
or
more
of those timeslots. Since the radio signals are
transmitted in bursts, there is a difference between
the peak power and the average power value.
When a DECT system transmits a burst, it has a
peak power up
to
250
mW. If it transmits during
one timeslot out of 24, the average power is
approximately 10 mW. The usual average
output
power of a base station can change from
10
mW
to
125
mW, depending on a number
of
calls,
which base station controlls.
If
there are two or
three calls, then the average power is
approximately 20-30 mW.
GSM is a paneuropean cellular system, which is
more and more used in Croatia. The system uses
two frequency bands: one
for
uplink: 890-915
MHz and for downlink: 935-960 MHz, with
channel spacing of 200
H z ,
which gives 124
channel pairs for usage. GSM is also a TDMA
system, with a frame of 4.615 ms, divided into 8
timeslots
of
577
ps
duration. This gives 217
timeslots pe seconds, while transmitting.
3.
Measurements
Measurements are performed according to Fig.
1.
2.
DECT and GSM Systems
DECT has a total spectrum allocation of 20 MHz,
from 1880.928-1898.208 MHz. That allocation is
divided into ten carriers, each separated by 1.728
MHz. Speech and data can be transfened with a
speed
of
32 kbps. Since the used access is TDMA
(Time Division Multiple Access),
all
DECT
equipment is capable of working on any DECT
frequency, which ensures that the cordless
equipment can always choose the best possible
channel. Multicarrier TDMA scheme is based on a
0-7803-6369-8/00/$10.00
02000
IEEE
1096
base station, close to a working table d
=
4.5 m
from the base station at heights h
=
1
.I-1.7 m and
on the working table (d
=
5.5 m, h
=
1
m). The
care has been taken that the calibrated dipole
antenna is as far as possible from metallic
surfaces, for the sake of measurement accuracy.
All three orthogonal polarizations have been
measured, i.e. polarization parallel with x-axis,
marked as a-polarization (horizontal); one parallel
with y-axis, marked as b-polarization (vertical)
and the one parallel with z-axis, marked as c-
polarization.
Fig. 1
Measurement scheme
In the scheme spectrum analyzer Anritsu MS710C
(with a frequency range of
10
lcHz
-
23 GHz) is
installed on an electromagnetic transparent tripod.
Calibrated half-wave dipoles are used as antennas
in two different frequency ranges (DECT and
GSM).
For
DECT measurements dipole is
0.07
m
long and for GSM 0.15 m. As it can be seen from
Fig.
1,
DECT base station is positioned
on
2.5 m
height. First measurements have been performed
at a distance d
=
1.5 m from the base station in the
corridor, and then also in the working area, behind
the wall (distance d
=
3.6 m). In order to perform
measurements, it was necessary to find the
nearlfar field distance, according to the well-
known formula:
r
=
2D2A
(1)
where
r
is the far field distance, D is dipole length,
and
h
is a wavelength. At GSM frequencies
r
=
14
m, at DECT frequencies
r
=
0.06
m.
I'herefore, we can conclude that we can measure
electric field at distances d
>
1.5
m in the far field
and then calculate the power density from the
relation:
3.1
.I
Measurements of DECT signal in corridor
Frequency spectrum of a-polarization measured in
the corridor
(d
=
1.5
m) is shown in Fig.
2.
For the two other polarizations (b- and c-)
measured output spectrum had the same shape, but
other maximum values.
As
recommended in
IEEE
SCC
28.
C9j.3-1991 Revision: Draji
Recommended Practice for Measurements and
Computations with Respect to Human Exposure
to
Radio Frequency Electromagnetic Fields
[4]
the
height and width were changed, in order to deduce
about the spatial averaged value. The values
dependent on height for b-polarization are shown
on Fig.
3.
On the spectrum analyzer resolution bandwidth
(RBW)
was chosen as 3 MHz, because of the
DECT channel bandwidth (1.728 MHz).
REF
-1
0
-20
I
I
I
1
I
'.
..
m
E
-30
-40
0
-50
d
-60
-70
-80
1880 928
1898 208
S
=
E21377(W/m2)
(2)
f(M
H z )
Fig.
2
3.1
DECT signal measurements
As
mentioned,
DECT
measurements have been
performed at several locations:
d
=
1.5 m from the
DECT fiequency spectrum
on
spectrum
analyzer
1097
-
1
12
- 2 0 "
-22
5
-25
'
height
h(m)
13
14
15
'
" " '
16
17
'
Polarization
Measured
Power
a
-50 dBm
b
-55 dBm
C
-61dBm
'
I
I
,
-37
5
-
-40'
'
'
I
'
Fig3. Measuredpower at
d=1.5
mfrom
DECT
base station, depending on heights
h=l.l-l.7m
for b-polarization
Results for all three polarizations show that the
measured power values take the values between
-26 dBm and -42 dBm. In order to compare these
values with the
RF
exposure standards, it is
necessary
to
calculate power density. Power
density
(W)
and power (P) are interrelated with
effective area,
so
it may be written:
W=PIA,
where effective area
of
a half-wave dipole is:
Polarization
Measured
uower
a
-65 dBm
b
-40 dBm
C
-64dBm
Table
2.
Measured power at
d=4.5
m,
h=1.6
m
from
DECT base station
The third measurement refers to a position on a
working table (d=5.5 m, h =1 m), shown in Table
3.
Polarization
a
Measured
power
-65 dBm
b
-66 dBm
C
-62dBm
(3)
In this area, the maximum of -40 dBm is measured
at d=4.5 m, h=1.6 m.
This
value corresponds to
0.03
mWlm2.Minimum is -66 dBm, which gives
76 nW/m2.
Here it is important to comment on the fact that
the values,
as
well
as
spatial distribution
of
electromagnetic waves, show that standing waves
occur. Standing waves are formed due to
summation of direct and reflected electromagnetic
waves.
3.2
GSMsignal measurements
GSM signal measurements are performed on the
same positions as DECT measurements. The same
spectrum analyzer was used with another, GSM
dipole antenna with
1
=
0.15 m. Frequency range
of measured signal was
890
-
960 MHz.
A,
=
0.13
k2
(4)
Relations (3) and (4) give for the maximum value
of -26 dBm power density
of
0.76 mW/mz and for
the minimum of -42 dBm power density
of
19.2 pW/m2. Since we are looking
for
the worst
case exposure, the maximum
will
be compared to
RF
exposure limits.
3.1.2 Measurements of DECT signal in the
working area
The next measurements are performed in the
working environment, which is separated from the
base station by a wall (Fig. 1).
The first measurement is performed very close to
the wall (d
=
3.6 m; h
=
1.9 m). The measured
values are given in Table 1
:
1098
Resolution bandwidth (RBW) was for this
measurement 300
kHz,
because the channel
spacing is 200
kHz.
3.2.1 GSM measurements in corridor
Results for a-, b- and c-polarization give peak
value of -47 dBm at 890 MHz (when the mobile
unit was switched on). The maximum level was
-52 dBm (without anybody talking on the mobile
phone).
This gives the maximum power density of
1.44 pW/m2, measured in
GSM
frequency band.
3.2.2 GSM measurements in working area
Results for all three polarizations in working area
at distance d
=
3.6
m
and height of 1.9
m
show
that the maximum is -52.5 dBm, whereas at d
=
4.5
m,
h
=
1.6
m the maximum is -58 dBm. At
d=5.5
m,
h=l
m
for a-, b- and c-polarizations
maximum is -57.3 dBm (0.13 pW/m2).
The maximum measured value in the working
area is found when the mobile unit is switched on.
Therefore, this value will be compared to
RF
exposure limits.
4.
Comparison
of
results with
RF
exposure
maximum permissible level of power density is
12.6
W/m2.
According to
[4],
the measured values for borh
systems were spatially averaged over an area of
0.35
m
width and 1.25
m
height, perpendicular to
earth.
Comparison for
GSM
gives the following result:
the maximum occurs with the switched on mobile
unit very close to measuring equipment and it is
1.44 pW/m2. The ratio between the measured
maximum and permissible level (6.17 W/m2) is
0.23*
Comparison for DECT gives ratio between the
measured maximum (0.76 mW/m2) and
permissible level (12.6
W/m2)
of
0.06*103.
5.
Conclusion
limits
Possible adverse effects of electromagnetic fields
to human health have caused concem in working
population. In order to estimate eventual
overexposure, results of measurements will be
compared to values of IEEE Standard
[ 5 ] .
IEEE
Standard has two levels: controlled and
uncontrolled environment. Since in this case
persons who are workers are not aware of the
potential for exposure as a concomitant of
employment, the levels applicable for uncontrolled
environment are taken for comparison purposes.
In this frequency range (300-3000 MHz),
permissible power density is f/1500 (mW/cm2),
with averaging time of 30 minutes. This would
[lean that at the central GSM frequenc
Of
225 MHz permissible level is
6.17
W/m
.
At
entral DECT frequency (1890 MHz) the
Far-field measurements with spectrum analyzer
and
two
different dipole antennas have been
performed. The dipoles were tuned for
GSM
and
DECT frequency range. The results show that
measured values in working area and corridor are
significantly lower (five orders of magnitude) than
maximum permissible exposure levels in
corresponding frequency ranges, given in
IEEE
Standard.
References
[ I ]
J.A. Phillips,
G.
Mac Namee: Personal Wireless
Communication with DECT and PWT, Artech House
Mobile Communications Library, 1998 Artech House,
Inc.
[2] The Mobile Communications Handbook, edited by J.D.
Gibson, CRC Press
&
IEEE Press, 1996
[3] C.A. Balanis: Antenna Theory, Analysis and Design,
John
Wiley
&
Sons,
Inc., 1997, New York, Chichester,
Brisbane, Toronto, Singapore
[4] IEEE SCC 28. C95.3-1991
Revision. Draft
Recommended Practice for Measurements and
Computations with Respect to Human Exposure to Radio
Frequency Electromagnetic Fields, 3
kHz
to
300
GHz,
1999
[5] IEEE C95.1-1991 Standard
for
Safety Levels with
Respect to Human Exposure
to
Radio
Frequency
Electromagnetic Fields, 3
kHz
to
300
GHz
,
Standards
Coordinating Committees. 1992
Y
1099
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