An Introduction to Free-Field
Measurements of Wireless Devices
in Reverberation Chambers
Kate A. Remley, Group Leader
Metrology for Wireless Systems
NIST measurements of wireless router
What is a Reverberation Chamber? A shielded, highly reflective free-field test chamber
What you can do with one: Create known fields (EMC/susceptibility)
Radiated emissions and power (CW and modulated-signal)
Antenna parameters (G, efficiency, etc.)
NIST measurements of prototype 4G
MIMO cellular telephone antennas
You can also do communication
system tests Receiver sensitivity
Throughput
EVM, BER, etc.
DUT needs realistic channel
0 45 90 135 180 225 270 315 360.01
1
Paddle angle (degrees)
BE
R
0 45 90 135 180 225 270 315 360-100
-80
-60
-40
-65 dBm-55 dBm-45 dBm-35 dBm-25 dBm
Paddle position (degrees)
Re
ce
ive
d p
ow
er
(dB
m)
32.8 kHz Low BER
0 45 90 135 180 225 270 315 360
.01
1
Paddle angle (degrees)
BE
R
0 45 90 135 180 225 270 315 360-100
-80
-60
-40
-55 dBm-45 dBm-35 dBm-25 dBm
Paddle position (degrees)
Re
ce
ive
d p
ow
er
(dB
m)
328 kHz High BER
Fields in a Metal Box (A Shielded Room)*
In a metal box, the fields have well defined modal
distributions.
Some locations have very high field values
Some locations have very low field values
* With thanks to Chris Holloway
Fields in a Metal Box with Large, Rotating
Scatterer (Paddle)
• The paddle changes locations where the high and
low field values occur
• After one mode-stirring sequence, all locations in the
chamber will have experienced nearly the same
collection of field maxima and minima
Frozen Food
A “Statistical” Test Chamber Quantities measured in the reverberation chamber
are averaged over a mode-stirring sequence
Randomize fields with
Mode-stirring paddles
Changing physical position
Using multiple antennas: various locations, polarizations
Reverberation Chambers Come
in All Shapes and SizesLowest frequency of operation,
uncertainties determined by
chamber size, wall loss
NASA: Glenn Research
Center (Sandusky, OH)Reverberation Chamber
with Moving Walls
Chamber Electrical Characteristics
Constructive and destructive interference for each
mode-stirring sample
Time domain (power
delay profile): Decay time
of reflections depends on
chamber reflectivity
1 1.5 2-60
-50
-40
-30
-20
-10
Frequency (GHz)
|S2
1|2
(dB
)
All Frequencies, one angle
PCS Band, four angles
• Frequency domain (|S21|2):
Reflections create multipath
Reverberation Chamber
Reference
antenna
Measurement
antenna
Mode-stirring paddle
RF
abs
Device under test
RF
abs
Platform
Vector Network
Analyzer
P1 P2
Original Applications
Radiated Immunity
components
large systems
Radiated Emissions
Shielding
cables
connectors
enclosures
Antenna efficiency
Calibrate RF probes
RF/MW Spectrograph
absorption properties
Material heating
Biological effects
Conductivity and
material properties
Wireless Applications
Standardized over-
the-air test methods
Radiated power of
mobile wireless devices
Receiver sensitivity
Large-form-factor and
body-worn devices (with
phantoms)
Public-safety emergency
equipment
Multipath environments
Rayleigh, Rician multipath
channels: with/without
channel emulators
Time response: power
delay profile, delay spread
Channel models
Biological effects of
modulated-signal
exposure
Gain from multiple
antenna systems
TX or RX diversity
MIMO
Cellular Wireless:
Over-the-Air Tests Required Network providers assess performance of every
wireless device model on their network:
Total Radiated Power (TRP)
Total Isotropic Sensitivity (TIS)
OTA testing traditionally done in anechoic chambers
Reverberation chambers can be used as well!
Modulated Signals in RCs:
What is different from EMC Testing? Receiver needs a realistic “frequency flat” channel
Loading required: Add RF absorber to chamber
Coherence bandwidth should match DUT design
Spatial uniformity decreases
Position stirring required
Real Channel:
Slow Variations with Frequency
Reverberation Chamber:
Loading Slows Variations
730 740 750 760 770 780 790 800
-80
-70
-60
-50
-40
-30
-20
Frequency (MHz)
|H(f
)|2 (
dB
)
Mean |H(f)|2
Mean |N(f)|2
Std Dev |H(f)|2
TX1 to RX6
|H(f)|2 (mean) = -58 dB
s ~ 5 dB
0 1 2 3 4-55
-50
-45
-40
-35
-30
-25
-20
-15
Bandwidth (MHz)
|S21|2
(dB
)
Unloaded Chamber
Loaded Chamber: 7 abs
sLOAD
~ 5 dB
sUNLOAD ~ 10 dB
T
T
R12
T1
T2
T3
R12
R9
R5
R10
T
T
R12
T1
T2
T3
R12
R9
R5
R10
Loaded =
broader CBW
Cellular Device Testing: How is it done?
Reference measurement provides:
Chamber loss: Transfer function Gref of chamber
Spatial uniformity of averaged fields in chamber
Rotating platform:
Gref = <Gref,p>
DUT measurement:
Same set-up as Ref
Assume GDUT = Gref
Reverberation Chamber
Reference
antenna
Measurement
antenna
Mode-stirring paddle
RF
abs
Device under test
RF
abs
Platform
Vector Network
Analyzer
P1 P2
Gref
Reverberation Chamber
Base station
emulator
Reference
antenna
Measurement
antenna
Mode-stirring paddle
Calibrated
cable
RF
abs
Device under test
RF
abs
Platform
GDUT
TRP Comparison, Free Space, Slider Open, W-CDMA Band II
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
Low Mid High
Reference Channel
Rela
tive T
RP
Anechoic Lab A Band II
Anechoic Lab B Band II
Anechoic Lab C Band II
Reverb Lab A Band II
Reverb Lab B Band II
Reverb Lab C Band II
Reverb Lab D Band II
Reverb Lab E Band II
TRP Comparison, Free Space, Slider Open GSM850
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
Low Mid High
Channel
Re
lati
ve
TR
P
Anechoic Lab A GSM850
Anechoic Lab B GSM850
Anechoic Lab C GSM850
Reverb Lab A GSM850
Reverb Lab B GSM850
Reverb Lab C GSM850
Reverb Lab D GSM850
Reverb Lab E GSM850
Total Radiated Power from Cell Phones
Data from CTIA working group shows good agreement
between anechoic and reverberation chambers
Cell phone
AC RC
AC RC
+/-2 dB is threshold
2 dB
2 dB
• Cell phone
testing: ca. 2010
• But in 2016…
By 2019: 11.5 billion mobile devices (world population 7.6 B)*
M2M/IoT growing faster than smart phones
2014: 495 million
2019: 3 billion
The Machine-to-Machine Revolution
* Source Cisco
Testing Large Form-Factor Devices Integrated antennas: test of entire device required
Reverberation chamber: now only option for SISO tests
Device placement not critical within chamber
Relatively low cost
OTA test issues: Large, lossy DUTs
Loading Decreases Spatial Uniformity
Loading helps with demodulation
but introduces other “nonideal” effects
S. van de Beek, et al., Characterizing large-form-factor
devices in a reverberation chamber, EMC Europe 2013.
1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500-40
-38
-36
-34
-32
-30
-28
-26
-24
-22
-20
Frequency (MHz)
Rela
tive m
ean p
ow
er
(dB
)
Relative mean power at Monopole12
Unloaded
TwoBoxes
FourBoxes
SixBoxes
EightBoxes
TenBoxes
TwelveBoxes
OneRF
TwoRF
ThreeRF
FourRF
FiveRF
SixRF
Increases chamber loss
(can calibrate out)
0 1 2 3 4 5 6 7 8-120
-115
-110
-105
-100
-95
-90
-85
-80
-75
-70
Time (s)
Rela
tive P
ow
er
(dB
)
PDP for different loading at Monopole2
Unloaded
TwoBoxes
FourBoxes
SixBoxes
EightBoxes
TenBoxes
TwelveBoxes
OneRF
TwoRF
ThreeRF
FourRF
FiveRF
SixRF
Decreases decay time
(replicates real world)Unloaded 2 Bx 4 Bx 6 Bx 8 Bx 10 Bx 12 Bx
11
12
13
14
15
16
17
18
Unloaded 1 RF 2 RF 3 RF 4 RF 5 RF 6 RF
11
12
13
14
15
16
17
18
RF Absorbers
Metallic Boxes
Theoretical StDev
Decreases spatial
uniformity
s increases with loading
(in percent)
Loading and Position Stirring go Hand in Hand
Spatial lack of uniformity a necessity:
unstirred energy
correlated samples: paddle position, location, frequency
Industry uses position, polarization, source stirring to
improve estimate of DUT performance
• Chamber reference for
PCS channel 9262
(1.850-1.854 GHz)
• VNA : 9 spatially
uncorrelated positions
• Combinations of all sets
of data
18
Set-up: Absorber Placement
Considerations:
• Exposed absorber
surface area
• Exposed metal surfaces
• Proximity to antennas
• Standing on floor
• Lying on floor
• Stacked
Comparable Loading, Different Uncertainty
Load chamber for approximately the same CBW
Chamber loss approximately the same as well
𝜎𝐺refis higher when absorbers lie on floor
Less exposed metal surface
Higher proximity effect
PCS band measurement (~1950 MHz)
Distributed on Floor: Standing
StackedDistributed on
Floor: Lying
CBW (MHz) 3.13 3.32 3.48
No. abs 3 7 4
𝐺ref (dB) -29.46 -29.69 -29.78
𝜎𝐺ref
(dB) 0.15 0.30 0.35
Set-up: Stirring Sequence is Important Stirring mechanisms influence results differently
Each chamber will have a different “mix” of optimal stirring
M = antenna positions
N = paddle angles
Cart 2x
y
x
y
Cart
1Cart 3 (dashed)
on ground
7
steps
13
steps
Measured and modeled uncertainty
at the three locations
K.A Remley, R.J Pirkl, H.A Shah, and C.-M. Wang, "Uncertainty fromchoice of mode-stirring technique in reverberation-chambermeasurements," IEEE Trans. Electromagnetic Compat., vol. 55, no.6,pp. 1022-1030, Dec. 2013.
Measure effects of paddle angle
and antenna position at three
locations in a loaded chamber
Set-up: Antenna Placement is ImportantUnstirred energy: increased K factor, reduced spatial uniformity
Antenna placement guidelines:
Orient away from each other
Cross polarize
Aim toward stirrers
DUT: Unknown pattern?
1 2 3 4-5
-4
-3
-2
-1
0
1
2
3
Antenna Configuration
K F
acto
r (d
B)
Cell Band
PCS Band
Dir/Dir
Omni/Dir
Dir/Omni
Omni/Omni
TX/RX pairs
Relationship between
antennas and absorber
is also important
Good Set-up = Good Results Must account for
placement and amount of RF absorber
number, type, and correlation of mode-stirring samples
antenna type and placement
Good comparison between chambers:
throughput vs. input power
Results show good repeatability and
comparison with anechoic methods
From MOSG140604Two devices tested in three different reverberation chambers
lab1, R1lab1, R2lab1, R3lab2, R1lab2, R2lab2, R3lab3, R1lab3, R2lab3, R3
lab1, R1lab1, R2lab1, R3lab2, R1lab2, R2lab2, R3lab3, R1lab3, R2lab3, R3
Lab Good Nominal Bad
AC1 -100.50 -99.00 -94.20
AC2 -102.80 -100.00 -94.20
RC1 -103.58 -100.29 -93.40
RC2 -101.46 -98.22 -92.12
RC3 -101.93 -98.66 -90.91
Spread+/- 1.54 1.04 1.65
From MOSG131207
TRP for Large M2M Device
Wireless, solar-powered trash compactor
TRP measured for W-CDMA signal (BW = 3.84 MHz)
Loading: stacked absorbers
Coherence bandwidth: Verified >3.84 MHz (4.42 MHz)
Antenna proximity effect: No effect at 1 λ (at fc)
Reference: Nine locations, DUT one location
DUT
abs
DUT
abs
Agreement with anechoic chamber:
0.2 dB, PCS band (1.850 GHz to 1.995 GHz)
1.95 dB, Cell band (800 MHz to 900 MHz)
OTA Tests to Model Multipath Environment
NIST channel measurements: Standards development for
electronic safety equipment such as firefighter beacons
Apartment Building
Oil RefineryOffice Corridor
Subterranean TunnelsAutomobile Plants
Channel Measurements: Denver High Rise
North
South
East
West1
2
35
4
67
8109
11121314
1516
21
20
19
18
17
VNA measurement test locations are in pink
Ground Plane
Transmitting
antenna
tripods
62 inches
VNA
Port 1 Port 4
200 m
Optical
Fiber
Receiving
antenna
Fiber Optic
Receiver
OpticalRF
Fiber Optic
Transmitter
OpticalRF
Replicate Environment in Reverberation
Chamber
Reverberation chamber with
absorbing material0 50 100 150 200 250 300 350 400 450 500
time (ns)
-70
-65
-60
-55
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
Po
we
r D
ela
y P
rofile
(d
B)
1 absorber: rms=187 ns
3 absorber: rms=106 ns
7 absorber: rms=66 ns
Large office biulding
rms=59 ns
Time response of channel
replicated in chamber
• Add RF absorbing material to “tune”
the decay time of the chamber
• Distributed multipath (reflections)
matched by chamber’s decay profile
Emulating Other Reflective Environments Oil Refinery
Emulating Other Reflective Environments
0 100 200 300 400 500 600 700 800 900-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
time (ns)
Po
we
r D
ela
y P
rofile
(d
B)
o delay-spread=207ns * mean-delay=252.4ns
o delay-spread=123ns * mean-delay=130.3ns
o delay-spread= 69ns * mean-delay=71.8ns
o delay-spread=195ns * mean-delay=147.0ns
o delay-spread=199ns * mean-delay=161.6ns
o delay-spread=261ns * mean-delay=275.3ns
1 absorber
3 absorbers
7 absorbers
Jefferson North Plant, 10m
Jefferson North Plant, 50m
Jefferson North Plant, 100m
0 200 400 600 800 1000 1200 1400 1600 1800 2000-30
-25
-20
-15
-10
-5
0
time (ns)
Po
we
r D
ela
y P
rofile
(d
B)
o delay-spread=277ns * mean-delay=290.5ns
o delay-spread=122ns * mean-delay=130.1ns
o delay-spread= 67ns * mean-delay=71.2ns
o delay-spread=168ns * mean-delay=137.8ns
o delay-spread=175ns * mean-delay=142.7ns
o delay-spread=128ns * mean-delay=105.8ns
o delay-spread=173ns * mean-delay=176.1ns
1 absorber
3 absorbers
7 absorbers
flint metal, drg-drg, 10m
flint metal, drg-drg, 50m
flint metal, drg-drg, 80m
flint metal, drg-drg, 110Bm
Automobile Factories
assembly
plant
stamping plant
Urban Canyon Multipath Effects
T
T
R12
T1
T2
T3
R12
R9
R5
R10
•Measurements made in
Denver urban canyon 2009
•Channel characterization:
LOS and NLOS
0 200 400 600 800 1000 1200
-170
-160
-150
-140
-130
-120
-110
-100
-90
Delay (ns)
PD
P (
dB
)
TX1 to RX5rms
= 115 ns
Noise Threshold = -116 dB
0 500 1000
-170
-160
-150
-140
-130
-120
-110
Delay (ns)
PD
P (
dB
)
TX1 to RX9rms
= 39 ns
Noise Threshold = -116 dB
Line of Sight Non Line of Sight
Replicating Clustered Multipath in
Reverberation Chamber
0 100 200 300 400 500 600 700 800 900 10000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Delay [ns]
PD
P
fitted simulated data
measured Data Denver
RX
TX
Shortest path
Welton Street
17
th
Str
ee
t
Glenarm Pl
TX
RX
1
RX
2
RX
3
Transmitter Site
Parking lot
• Blue: Mean of 27 NLOS
measurements
• Red: RC + channel
emulator
• Dashed: Exponential
model
Clusters of
exponentially
distributed signals
received off of
buildings
Channel Models Used for
Standardized OTA tests
Outdoor-to-indoor channel model for 700 MHz
8 environments, hundreds of measurements
“NIST Model” included in 3GPP reverberation-
chamber-based test methods
0 100 200 300 400 500 600-35
-30
-25
-20
-15
-10
-5
0
Delay (ns)
PD
P (
dB
)
700 MHz, measured
4900 MHz, measured
700 MHz, fit
4900 MHz, fit
Excess tap delay
[ns]
Relative power [dB]
0 0.0
40 -1.7
120 -5.2
180 -7.8
210 -9.1
260 -11.3
350 -15.2
D.W. Matolak, K.A. Remley, C.L. Holloway, and C. Gentile,
“Outdoor-to-Indoor Channel Dispersion and Power-Delay Profile
Models for the 700 MHz and 4.9 GHz Bands,” IEEE Antennas
and Wireless Propagat. Lett., vol. 15, 2016, pp. 441-443.
Discrete version of the “NIST Model”
for anechoic-chamber measurements
RMS DS: 80 ns
@ 700 MHz
Reverberation chamber can easily
replicate diffuse multipath
Millimeter-Wave
Wireless for “5G”
Goal is uncertainty ≈ 1 % or
u =1
𝑁+ 𝐾2 ≤ 0.0001
Very low K factor required
Left: 10,000 paddle positions at 1 antenna position
Right: 100 paddle positions at 100 antenna positions
5G Wireless Concepts:
Massive MIMO
Tiered spectrum
(licensed and
unlicensed)
Millimeter-wave
frequencies
Low Uncertainty
Required:
High carrier frequencies
High modulation
bandwidthsStay tuned …
Some issues: Angle-of-arrival information lacking
Advanced transmission, multiple antenna systems
Test methods (CTIA, 3GPP groups)
Instantaneous channel can be problematic for receiver (even if
mean characteristics are OK)
Field non-uniformity increases with loading and loading is often
required for receiver tests
Testing devices with repeaters is difficult
absorber
wireless device antenna
mode stirrers
horn antenna
Reverberation Chambers for Wireless Test
Some benefits: Capable of simulating key characteristics of many multipath
environments for the testing of wireless devices
For OTA test reverberation chambers are: Accurate – uncertainties on par with anechoic methods
Able to provide realistic distributed power delay profile
Suitable for testing diversity and MIMO gain (due to multipath)
Cost effective
Space efficient
Reverberation Chambers for Wireless Test
0 100 200 300 400 500 600-35
-30
-25
-20
-15
-10
-5
0
Delay (ns)
PD
P (
dB
)
700 MHz, measured
4900 MHz, measured
700 MHz, fit
4900 MHz, fit
Excess tap delay
[ns]
Relative power [dB]
0 0.0
40 -1.7
120 -5.2
180 -7.8
210 -9.1
260 -11.3
350 -15.2
D.W. Matolak, K.A. Remley, C.L. Holloway, and C. Gentile,
“Outdoor-to-Indoor Channel Dispersion and Power-Delay Profile
Models for the 700 MHz and 4.9 GHz Bands,” IEEE Antennas
and Wireless Propagat. Lett., vol. 15, 2016, pp. 441-443.
The “NIST Model” for building
penetration: 8 environments
RMS DS: 80 ns @ 700 MHz
Watch this space for more
information on over-the-air testing
with reverberation chambers