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

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

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

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

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.

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)

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

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

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

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

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

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

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

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)

UTD / FDTD validation

GO UTD UTD/FDTD

rms-field maps on the central yz-sectionof the subject’s domain (GSM900 – f = 947.5 MHz)

Exposure field distributions

GSM900 (947.5 MHz) UMTS (2140 MHz)

Field maps 1 m above the floorin the absence of the subject

SAR distributions

GSM900 UMTS

dB W/kg

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.

Material characteristics

GSM 900 UMTS / WLAN