Models to Evaluate EM 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 b, 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 Equivalence surface FDTD Region a b c d a) direct ray-path b) GO ray-path c) UTD ray-path d) 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; P irr = 250 mW; Tx = λ/2 dipole reflector antenna (3.5; 0.0; 2.5) The shadowed region indicates the field computation area (1.75; 2.5; 1.0) Length = 25 cm Z 0 = 50  P. Bernardi, R. Cicchetti, O. Testa, 27 th 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 ◦ ◦ A. J. Booysen, C. W. I. Pistorius, IEEE Trans. Antennas and Propagat., April 1992 ε r = 3 f = 30 GHz ρ = 2 λ 0

Field distribution f = 2.44 GHz P irr = 250 mW Tx = λ/2 dipole reflector antenna (3.5; 0.0; 2.5) GO Field: up to 5 reflected/transmitted contributions UTD Field: up to 3 reflected/transmitted contributions Empty roomFurnished room

EMI prediction – Induced current f = 2.44 GHz P irr = 250 mW Tx = λ/2 dipole reflector antenna (3.5; 0.0; 2.5) (1.75; 2.5; 1.0) Z 0 = 50  ; Length = 25 cm A TL model is used to predict induced current on the microstrip line Empty roomFurnished room

Example 2: human exposure inside a room in front of a BTS Subject phantom: “Visible Human” 3-mm resolution 31 tissues P rad = 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 (to be published)

UTD / FDTD validation GOUTDUTD/FDTD rms-field maps on the central yz-section of the subject’s domain (GSM900 – f = MHz)

Exposure field distributions GSM900 (947.5 MHz) UMTS (2140 MHz) Field maps 1 m above the floor in 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