Technical Session of ITU-T SG5 Meeting in Geneva
Evaluation of Human Exposure Levels
around Radio Base Stations in Korea
April 16, 2012
Byung Chan Kim, Ph.D.
bckima@etri.re.kr
::: ETRI, The Future Wave :::
Contents
Background
Motivation
Research Issues in Korea
Spatial Averaging
Time Averaging
Predicting Formula
Evaluating Method
Uncertainty
2
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Background: Study on EMF & Health Effect
Computational Technique
EMF Detection Technique
Computation
Near
field
EMF
SAR
Far
field
guideline
Induced
current
Visual
model
Evaluation
Procedure
Application
Protection
EMF: ElectroMagnetic Field
3
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Background: Frequency Range
Cell
phones
TV
Power
Line
AM
radio
FM
radio
Microwave
oven
Heating
Lamp
Tanning
booth
Medical
X-rays
Frequency, Hz
102
104
106
108
1010
1012
Radio
1014
1016
1018
X-ray
Infrared
Microwave
1020
Ultraviolet
Non-ionizing
Ionizing
Low induced
currents
High induced
currents
Electronic
excitation
Broken bonds
No proven effect
Heating
Photochemical effects
DNA damage
4
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Background: Basis for Limiting Exposure
Limiting Basis
induced current
thermal effect,
excitation
thermal effect
10 MHz
300 GHz
100 kHz
Dosimetric Quantity
current density
100 kHz
SAR
power density
10 MHz
10 GHz
300 GHz
Dosimetry: Measurement or determination by calculation of internal electric field strength or induced current density or SAR in humans or animals
exposed to EMFs
5
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Background: Safety Limits
Limits-1
-1
Reference Level
Basic Restriction
• The
fundamental
quantities
• Alternative means for determining
that
determine the physiological response of
compliance: Electric field strength (E),
the human body to EMFs: Current density
Magnetic field strength (H), Equivalent
(J),
plane power density (S)
Specific
absorption
ratio
(SAR),
Power density(S)
• It applies to a situation where the
electromagnetic field is not influenced by
• It applies to a situation with the body
the presence of a body
present in the field
6
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Background: Safety Limits
Limits-2
-2
<Reference levels for GP>
Frequency range
E-field (V/m)*
H-field (A/m)*
- 1 Hz
-
3.2×104
1 Hz ～ 8 Hz
10,000
3.2×104/f2
8 Hz ～ 25 Hz
10,000
4,000/f
0.025 kHz ～ 0.8 KHz
250/f
4/f
0.8 kHz ～ 3 kHz
250/f
5
3 kHz ～ 150 kHz
87
5
0.15 MHz ～ 1 MHz
87
0.73/f
1 MHz ～ 10 MHz
87/f1/2
0.73/f
10 MHz ～ 400 MHz
28
0.073
400 MHz ～ 2000 MHz
1.375f1/2
0.0037f1/2
2 GHz ～ 300 GHz
61
0.16
Note 1. Field value is rms and unit of frequency is same with leftmost column
2. The field strength are to averaged over any 6-min period
ICNIRP: International Committee on Non-Ionizing Radiation Protection
7
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Background: Field Region
Reacttive
near-field
Radiating
near-field
Radiating
far-field
Distance from the source
0
E⊥H
Z = E/H
Measured
quantity
2D2/λ
λ/4
No
≠ Z0
Locally
≒ Z0
SAR
E or H
Yes
≒ Z0
E or H
Source: ITU-T Recommendation K.61, 2008
8
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Background: Determine the Compliance
Compliance-1
-1
Dosimetry in near field
< Setups >
σEi
SAR =
ρ
2
Ei : rms [V/m],
σ: conductivity [S/m],
9
:ρ
density [kg/m3]
::: ETRI, The Future Wave :::
Background: Determine the Compliance
Compliance-2
-2
Evaluation of maximum permissible exposure in far field
Spectrum [subcenter]
Spectrum [subcenter]
1
120
0.1
Electri field [V/m]
Electricfield [dBuV/m]
100
80
60
40
0.01
0.001
20
0
850
860
870
880
890
900
0.0001
1830
910
Freq. [MHz]
1835
1840
1845
1850
1855
1860
Freq.[MHz]
10
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Motivation
How can we evaluate reasonably the impact of EMF on the human body
in far field ?
What method should be used to measure the exposure practically ?
11
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Research Issues in Korea
Spatial averaging: Number of points
Time averaging: 6 min or others at each point
Prediction: Possible maximum field level location
?
Uncertainty: Probability function
direct
reflection
12
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Spatial Averaging: Previous Status
Multi-path reflections & Non uniform field distributions
Natural and Man-made structures
No global standard regarding number of points
Australia
France
Portugal
Belgium
Italy
EN50383
Reference height[m]
1.5
0.75
1.1,1.5,1.7
-
1.1,1.5,1.9
-
No. of points
5
9
3
6
3
-
dimension
[height*width, m]
1.0
1.0*0.5
-
1.2*0.4
-
0.7*0.4
Obtaining value
Max.
Ave.
Ave.
Ave.
Ave.
Ave.
nj
ni
∑E ∑E
i
i =1
Deviation =
E template − E global
E global
=
−
ni
j
j =1
nj
nj
∑E
j
j =1
nj
13
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Spatial Averaging: Previous Study
By small scale fading: N points have to be independent
The correlation distance
• Theoretically 0.4 λ and empirically 0.8 λ
• Usually 0.5 λ
Error estimation
Rayleigh model
− N N −1
p (γ ) = e
γ
∑
j=0
j
1  N 

 − e − N γ
j!  γ 
N −1
∑
j=0
1
(N γ
j!
)j
2
2
2
Y, b
Rician model ( a = µ X + µ :standard
deviation)

p (γ ) = Q N 


Na 2
,
b2
N
γ
(2 +
a2
b2


)  − QN 




Na 2
,
b2
N γ (2 +
a2
b2

)


Source: E. Larcheveque et al., “Analysis of electric field averaging for in-situ radiofrequency exposure assessment,” IEEET Trans. on VTC, vol.54, no.4, pp.1245-1250, 2005.
14
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Spatial Averaging: Rationale
Assessed quantity
• Electric and magnetic field strength or equivalent plane wave power density
At a given location
• Diffraction and reflections
• Different phases and amplitudes
Spatial variations known as small-scale fading
• It does not allow a repeatable exposure level
• We should use an averaging scheme.
15
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Spatial Averaging: Approach for New Method
To determine the number of measurement points
• Imitate the space occupied by human
• Analysis the spatial variations of electric field strength
To consider the effect of environments
• Urban
• Suburbs
• Rural
Isotropic
Electric Probe
GPS Receiver Computer
노트북
1.7 m
Tripode
1.5 m
1.7 m
1.5 m
1.1 m
RS232
1.5 m
Calibrated
Cable
1.1 m
0.2 m
Spectrum Analyzer
16
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Spatial Averaging: Analysis
Maximum mean value occur when 6 or 9 points are used in averaging process.
Unit : V/m
No. of points
27
9
6
BS1
BS2
BS3
BS4
BS5
BS6
Ave.
0.2866
0.2713
0.2308
0.3514
0.0299
0.0457
Ave.(-)
0.2888
0.2714
0.2312
0.3616
0.0274
0.0477
Ave.( 0 )
0.2835
0.2712
0.2313
0.3511
0.0331
0.0424
Ave.(+)
0.2874
0.2712
0.2312
0.3616
0.0274
0.0477
Ave.(-)
0.2867
0.2716
0.2301
0.3457
0.0300
0.0461
Ave.( 0 )
0.2826
0.2709
0.2312
0.3590
0.0315
0.0440
Ave.(+)
0.2881
0.2711
0.2309
0.3835
0.0275
0.0473
9
6
9
6
9
9
Number of Max.
average point
The more number of base stations we select, the better statistical meanings we can get.
However, the purpose of these measurements does not determine the statistical
significance (p-value estimation) between “an exposure to EMF from base station antenna”
and “that’s impact on the human health” but check the levels of exposure.
17
::: ETRI, The Future Wave :::
Spatial Averaging: Korea
Korea’s
’s Method
Location of the main organs of human and theoretical correlation distance
Reliable and repeatable exposure assessment
N
∑
1.7 m
1.5 m
( Eˆ i ) 2
i =1
E=
N
or
N
∑
1.1 m
i =1
H =
0.2 m
( Hˆ i ) 2
N
or
N
∑ Sˆ
S =
i
i =1
N
N : number of points
Eˆ : root mean square
i
Source: Byung Chan Kim et al., “Methods of Evaluating Human Exposure to Electromagnetic Fields Radiated from Operating Base Stations in Korea,”
Bioelectromagnetics, vol. 29, no. 7, pp. 579-582, Oct. 2008.
18
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Time Averaging: Previous Status
Number of points for spatial averaging: 3, 6, 9, 20 points (ITU-T K.61 and IEC62232)
Measurement time
• ICNIRP guidelines: Recommend averaged over any 6 min period
• Frequencies between 100 kHz and 10 GHz.
Source: ITU-T Recommendation K.61, 2008
19
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Time Averaging: Previous Study
Established biological and health effects
• Temperature (eyeball) rise of more than 1 degree
Tissue
Mass
density
(kg/m3)
Thermal
conductivity
(W/m K)
Specific heat
capacity
(J/Kg K)
Volumetric
perfusion
(m3/m3s)
Equilibration
time
(min)
Conductivity
(K kg/W)
Skeletal muscle
1050
0.50
3465
0.9
17
300
Kidney
1050
0.54
3700
61
0.27
4.4
Liver
1060
0.52
3600
15
1.1
18
Adipose tissue
Brain
Blood
950
1040
1060
0.27
0.54
0.51
3100
3640
3720
0.5
7.3
-
25
2.2
-
480
36
-
Heat transfer model
∂T (x, t)
∂ 2T ( x, t)
−α
+ β (T ( x, t) − Ta ) = q(x, t)
∂t
∂x 2
Max{∆T (t )} = σ LSAR[1− exp(−t / τ )]
LSAR = (10 − 8r), r: exposed mass/total mass
Source: “Gunnar Brix, Martin Seebass, Gesine Hellwig, Jurgen Griebel, “Estimation of Heat Transfer and Temperature Rise in Partial-Body Regions
during MR Procedures: An analytical approach with respect to safety considerations,” Magnetic Resonance Imaging, vol. 20, pp. 65-76, 2002.
20
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Time Averaging: Rationale of Time Reduction
The averaging time
• means the appropriate time over which exposure is averaged
• is 6 min at each point
• increases as the number of points increases
• increasing means an increase in cost
• is related to the time constant for partial body heating: present of a human body
• therefore, is not always necessary in all far field assessments
Reference levels at any point
• are evaluated in the absence of a human body
• are intended to be spatially averaged values body of the exposed individual
21
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Time Averaging: Approach and Analysis
Analyzed the eighteen periods of time from 180 to 10 sec: 10 sec interval
Unit : dBuV/m
Time (sec)
Base station
1
360
180
60
40
10
109.14
109.14
109.16
109.14
109.08
2
109.80
109.79
109.79
109.79
109.80
3
107.07
107.07
107.09
107.07
107.11
4
111.59
111.55
111.60
111.63
111.81
5
92.56
92.36
92.34
92.33
92.56
6
90.41
90.28
90.76
90.84
90.74
7
97.38
97.26
97.39
97.40
97.67
The estimated standard uncertainties: 0.09 ~ 0.40 dB (t-distribution; t.025=2.05, DoF=29)
Base station
1
2
3
4
5
6
7
8
9
10
Arithmetic average(dBuV/m)
110
117
106
109
114
100
93
107
110
99
0.25 0.62 0.98
0.46
0.39 1.07 0.59 0.40 0.58 0.51
Tr/2(confidence level: 95%, DoF: 29) 2.05 2.05 2.05
2.05
2.05 2.05 2.05 2.05 2.05 2.05
0.17
0.15 0.40 0.22 0.15 0.22 0.19
Standard deviation (SD)
Standard uncertainty
0.09 0.23 0.36
DoF: Degree of Freedom
22
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Time Averaging: Korea
Korea’s
’s Method
Compare the SD with uncertainty of measurement drift
• Between the 360 sec average value and different time periods
• No overlapping time period: 1~60, 61~120, 121~180, 181~240, 241~300, 301~360 sec
• Maximum value of SD: 0.58 dB at 10 sec
• Uncertainty for repeated measurement: 0.4 dB
0.6
BS 1
BS 2
BS 3
BS 4
BS 5
BS 6
BS 7
n
• Average:
∑
1
q=
qj
n j=1

n
• Standard
s(qj ) = 1
(qj −q)2
deviations: 
n−1 j=1
∑


n

s(qj )
1 1
(q −q)2
= tr / 2
s(q) = tr / 2
n
n n−1 j=1

Standard deviation [dB]
uncertainty for measurement drift
0.4
0.2
∑
0
0
20
40
60
80
100
120
140
160
180
Time period [sec]
Source: Byung Chan Kim et al., “Reduction of averaging time for evaluation of human exposure to radiofrequency electromagnetic fields from cellular
base stations,” IEICE Trans. on Communications, vol. E93-B, no. 7, pp. 1862-1864, July 2010.
23
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Predicting Formula: Korea
Korea’s
’s Method
Need to predict the possible maximum exposure position
G = 4π
Transmitter
U (θ , φ )
, Pin G = 4π U (θ , φ )
Pin
d-wave
rd
r-wave
U (θ , φ ) = r 2W rad
ht
α
rr1
θi
rr 2
r 2
E
r
r
1
= r 2 Re[ E × H * ] = r 2
2
2η
r2
2
U (θ , φ ) =
Eθ ( r , θ , φ )
2η
Receiver
hr
Eθ
2
max
=η
2
rg
rr1
U max (θ , φ ) =
ht
I02
4πr 2
r2
× Eθ
2η
η I02
= Pin G , I 0 =
2π
r
r
r
r
Et = E d + E r = ( 1+ Γ ) E d
Γ=
βl
βl 

 cos( 2 sin α ) − cos( 2 ) 


cos α




η
η Pin G
2π
βl
βl 

 cos( 2 sin α ) − cos( 2 ) 


cos α




− (ε r′ − j 60 λσ ) sin θ i + (ε r′ − j 60 λσ ) − cos 2 θ i
(ε r′ − j 60 λσ ) sin θ i + (ε r′ − j 60 λσ ) − cos 2 θ i

 ht + h r
 ht − hr
θ i = sin −1  sin α ∗ 

24
ηI 0 2
8π 2
2π Pin G
r
je − jβ r
E t = aˆ θ ( 1 + Γ )
r
Image transmitter
rt
I 0 e − jβ r
E = aˆθ ( 1 + Γ ) jη
2π r
max
=




::: ETRI, The Future Wave :::
Evaluating Methods: Protocols and Status
Depending on the operation state,
• Put on the market: EN50383 (certification measurement)
• Put into service (EEM): RRL Notice 2010-64, EN50400
• Offer a service (In situ): TTAS.KO-06.0125, EN50492, IEC62232
1.7 m
1.5 m
1.1 m
0.2 m
EEM: Electromagnetic Environment Measurement
25
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Evaluating Methods: Korea
Korea’’ method for In Situ
Focused on human beings: human-centric
Korean standard was established in December, 2006.
Assume spatially averaged value / Take total exposure ratio (TER) of same frequency
Only 1 point in any positions within space occupied by human body
0.2 m
0.2 m
1.7 m
1.7 m
1.5 m
1.5 m
1.1 m
1.1 m
: Point
: Point
: Position
: Position
Basic
Precision
26
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Evaluating Methods: Korea
Korea’’ method for In Situ
Evaluation at place of concern
• Where the general public have voiced concerns about EMF
• Reference for the WHO in developing its anticipated Environmental Health Criteria (EHC)
on radiofrequency
50
1
Total
Hospital
Commercial area
Residencial area
School
Apatment complex
0.1
40
Number of positions
Fractin of exposure limit
0.01
0.001
0.0001
1E-005
30
20
1E-006
10
1E-007
0
1E-008
s
Ho
al
pit
x
a
al
ol
ple Tot
ho
are
Sc
om
ial
ial
c
c
c
r
t
n
e
n
e
si d
me
mm
art
Re
Co
Ap
a
are
1E-008
1E-007
1E-006
1E-005
0.0001
0.001
0.01
0.1
1
Fraction of exposure limit
Source: Byung Chan Kim et al., “Methods of Evaluating Human Exposure to Electromagnetic Fields Radiated from Operating Base Stations in Korea,”
Bioelectromagnetics, vol. 29, no. 7, pp. 579-582, Oct. 2008.
27
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Uncertainty: Previous study
Several kinds of uncertainty sources exist
The rationale of probability function is insufficient
Divisor of U-shape probability function:
Budget
,2Rectangular:
Uncertainty Source
3
, Triangular:
6
, Normal:
2.0
Probability Function
Measurement equipment
Calibration
Normal
Isotropy
Normal
Linearity
Rectangular
Measurement device
Normal
Noise
Normal
Power chain
Normal
Physical parameters
Drifts in output power of the EUT, probe, temperature and humanity
Rectangular
Perturbation of the environment
Rectangular
Influence of the body
Post processing
Spatial averaging
Rectangular
Source: EN50492, 2008
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Uncertainty: Rationale
A measured quantity is not measured directly, but is determined from other quantities
through a functional relationship
Q = f ( R1 , R2 , LL, RN ) : measured quantity
q = f (r1, r2 , LL, rN ) : estimate
Uncertainty: The variance of the mean
s(qj )
1
n
∑
1
s(q) =
=
(q −q)2 ,
n
n n−1 j=1
n
∑
1
q=
qj
n j=1
Exposure limit
Assessment result
95%
2σ
29
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Uncertainty: Approach & Analysis
Type A: Be obtained by repeated measurement
Type B: Be estimated by the data on the calibration or specification sheet
Contribution components
Physical parameters
A
Mechanical constraints
Post-processing
10
U ncertainty for P ow er D rift (dB )
Standard deviation
Standard uncertainty
1
0.1
0.01
Power drift (measurement drift), Body influence, noise
Tripod positioning, Mismatch between receiving antenna
(or probe) and base station’s antenna
Spatial averaging
Measurement equipment
Deviation between Different Distances (dB)
B
Uncertainty sources
Calibration, Power receiver, Isotropy /linearity of probe,
Cable, Power chain
8
10
| 1m-2m |
| 1m-3m |
| 2m-3m |
1
0.1
0.01
Daily Input Power Variations (dB)
Type
Urban
Suburban
Rural
7
6
5
4
3
2
1
0.001
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 2526272829 30
Base Stations
Power drift
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
0 1 2 3 4 5 6 7 8 9 10 1112131415 1617 1819 2021 222324 2526 2728 2930
Number of Days
Base Stations
Body influence
30
Mismatches
::: ETRI, The Future Wave :::
Uncertainty: Krea
Krea’s
’s Methods
The estimated uncertainty satisfy the recommendation by international standard (under 6
dB )
Evaluation
type
Contribution
components
Uncertainty
sources
Probability
function
Uncertainty
value [dB]
Divisor
DoF
Rationale of distribution
Standard
uncertainty [dB]
Power drift
Student-t
0.67
2.05
29
Small sample, unknown
population mean & variance
0.33
Influence of body
Student-t
0.22
2.05
29
Small sample, unknown
population mean & variance
0.11
Mechanical constraint
Mismatches
Student-t
1.29
2.05
29
Small sample, unknown
population mean & variance
0.63
Post-processing
Spatial averaging
Student-t
0.99
2.26
9
Small sample, unknown
population mean & variance
0.44
Calibration
Normal
3.00
2.00
∞
Data sheet
1.50
Spectrum analyzer
Normal
0.34
2.00
∞
Calibration sheet
0.17
Isotropy
Normal
1.50
2.00
∞
Calibration sheet
0.75
Linearity
Normal
0.89
2.00
∞
Calibration sheet
0.45
Cable
Normal
0.40
2.00
∞
Calibration sheet
0.20
Physical parameter
A
B
Measurement
equipment
Root Sum Square of all standard uncertainty (√∑u2)
Combined uncertainty
Expanded uncertainty
Student-t
1.95
1.96
1,447
DoF and t-distribution table
1.95
3.82
Source: Byung Chan Kim et al., “Uncertainty Estimation for Evaluating of Human Exposure Levels to RF Electromagnetic Fields from Cellular Base
Stations,” Accepted in IEEE Trans. on EMC and will publish soon.
31
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Thank you and Questions ?
32
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