Models to Evaluate EM Interference and Human Exposure for Wireless Communication Systems in...
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Models to EvaluateEM Interference and Human Exposure for Wireless Communication Systems
in Realistic Environments
COST 286 Workshop on
EMC in Diffused Communication Systems:
Current Capabilities and Future Needs
P. Bernardi and E. Piuzzi
Dept. of Electronic Engineering
“La Sapienza” University of Rome
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Outline
• Introduction– Possible EM field sources– Possible EM interference targets– Operating environment
• EM interference assessment– External and internal parameters– Numerical modeling: hybrid UTD / FDTD model
• Examples in realistic scenarios– Coupling of a microstrip line with the field radiated by a WLAN – Human exposure to a BTS antenna
• Conclusions and developments
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Possible EM field sources• Base stations (GSM and UMTS systems)
– Coverage: 100 m 10 km (macrocellular systems)– Frequency: 900 2000 MHz– Power: 25 mW 20 W– Data rate: 10 kb/s 2 Mb/s
• Wireless LANs (IEEE 802.11b, HIPERLAN)– Coverage: single room / building (microcellular systems)– Frequency: 2.4 5.2 GHz– Power: 100 mW– Data rate: 10 Mb/s
• Bluetooths– Coverage: 10 m (picocellular systems)– Frequency: 2.4 GHz– Data rate: 300 400 kb/s– Power: 1 mW
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Possible EM interference targets
• Communication systems operating in the same
frequency band– Interfering signal received by the antenna radio link performance
degradation loss of communication / data rate reduction
• Electronic equipment / Medical devices– Induced currents in a connecting cable disturbance inside the
system system malfunction– Induced disturbances inside an electronic apparatus system
malfunction / apparatus break
• Human beings– Induced power absorption inside the biological tissues
temperature elevations / specific effects adverse health effect
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Operating environment
• BTS are almost entirely installed in outdoor locations, but
the radiated field penetrates also inside buildings.
• WLANs and Bluetooths mainly operate in indoor
environment.
• The highest concentration of possible interference targets
can be expected in an indoor environment.
• Field propagation in indoor environment is dominated by
multiple reflection / diffraction processes due to the
presence of room walls and furniture.
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EM interference assessment (1/2)
• EM field levels in the environment (external parameter)
must be evaluated
• In order to assess if a dangerous interference can occur
suitable internal parameters must be estimated– Bit Error Rate (BER) for digital communication systems– Induced disturbances for electronic equipment– Specific Absorption Rate (SAR) for human beings
• For practical reasons threshold levels referred to the
external EM field are used– Maximum Carrier-to-Interference Ratio (CIR)– Immunity Level (EMC standards)– Reference Level (human exposure guidelines)
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EM interference assessment (2/2)
• Immunity levels are experimentally tested exposing the
electronic equipment to a uniform plane wave.
• Reference levels for human exposure have been
theoretically derived from threshold whole-body SAR values
(with an appropriate safety factor) assuming uniform plane
wave exposure of various body models.
• In realistic environments exposure fields are far from
being uniform plane waves.
• A direct evaluation of the relevant internal parameter
might be useful in order to establish if a source can:– Cause malfunctions to the considered electronic equipment– Pose any health risk for people
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Numerical modeling
• The problem requires characterization of– Field propagation in complex scenarios– Field interaction with electronic equipment / human body phantom
• UTD model– Efficient modeling of high-frequency field propagation– Not suitable to study interaction between the EM field and dielectric
bodies of arbitrary shape
• FDTD technique– Efficient modeling of interaction between the field and
heterogeneous bodies of arbitrary shape– Huge computational costs for complex scenarios
Hybrid UTD / FDTD model
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Tx Antenna
Equivalencesurface
FDTD Region
a
b
c
d
a) direct ray-pathb) GO ray-pathc) UTD ray-pathd) UTD ray-path
Environment Elements
UTD / FDTD model
• UTD computation of
exposure field on an
equivalence surface
2 steps
• FDTD evaluation of
induced field in the
exposed target
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Example 1: WLAN – microstrip coupling
f = 2.44 GHz; Pirr = 250 mW; Tx = λ/2 dipole reflector antenna
Tx @ (3.5; 0.0; 2.5)
The shadowed region indicates
the field computation area
Microstrip @ (1.75; 2.5; 1.0)Length = 25 cmZ0 = 50
P. Bernardi, R. Cicchetti, O. Testa, 27th URSI General Assembly, Aug. 2002
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Validation of the UTD heuristic diffraction coefficient
• Modeling of field
diffraction from
penetrable wedges and
junctions formed by thin
plates
• Evaluation of the field
inside penetrable
objects
— P. Bernardi, R. Cicchetti, O. Testa, IEEE Trans. Antennas and Propagat., Feb. 2002
◦ ◦ A. J. Booysen, C. W. I. Pistorius, IEEE Trans. Antennas and Propagat., April 1992
εr = 3 f = 30 GHz ρ = 2 λ0
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Field distributionf = 2.44 GHzPirr = 250 mWTx = λ/2 dipole reflector antennaTx @ (3.5; 0.0; 2.5)
GO Field: up to 5 reflected/transmitted contributionsUTD Field: up to 3 reflected/transmitted contributions
Empty room Furnished room
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EMI prediction – Induced currentf = 2.44 GHzPirr = 250 mWTx = λ/2 dipole reflector antennaTx @ (3.5; 0.0; 2.5)Microstrip @ (1.75; 2.5; 1.0)Z0 = 50 ; Length = 25 cm
A TL model is used to predict induced current on the microstrip line
Empty room Furnished room
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Example 2: human exposure inside a room in front of a BTS
Subject phantom:
• “Visible Human”
• 3-mm resolution
• 31 tissues
• Prad = 30 W
• G = 18 dBi
Antenna parameters:
• -3 dB hor. = 64°
• -3 dB vert. = 8°
P. Bernardi et al., IEEE Trans. Microwave Theory and Tech., Dec. 2003 (to be published)
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UTD / FDTD validation
GO UTD UTD/FDTD
rms-field maps on the central yz-sectionof the subject’s domain (GSM900 – f = 947.5 MHz)
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Exposure field distributions
GSM900 (947.5 MHz) UMTS (2140 MHz)
Field maps 1 m above the floorin the absence of the subject
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SAR distributions
GSM900 UMTS
dB W/kg
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Conclusions and developments
• A UTD / FDTD model, useful to study EM interference
problems in realistic environments, has been developed.
• The model has already been applied to study exposure of a
subject inside a room illuminated by an outdoor GSM /
UMTS base station antenna.
• The model will be applied to evaluate disturbance levels
inside realistic targets located in a complex indoor
environment where a WLAN system operates.
• The model can be used to predict possible interferences
and / or exposure hazards during the planning stage of
indoor wireless communication systems.
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Material characteristics
GSM 900 UMTS / WLAN