an introduction to free-field measurements of wireless ... · an introduction to free-field...

An Introduction to Free-Field Measurements of Wireless Devices in Reverberation Chambers

Kate A. Remley, Group LeaderMetrology 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 (Γ, 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

m)

32.8 kHz Low BER0 45 90 135 180 225 270 315 360

.01

1

Paddle angle (degrees)

BE

R

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

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 in various locations

Reverberation Chambers Come in All Shapes and Sizes

Lowest frequency of operation, uncertainties determined by chamber size, wall loss

NASA: Glenn Research Center (Sandusky, OH)Reverberation Chamber

with Moving Walls

Reverberation Chamber CharacteristicsConstructive 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)

|S21

|2 (dB

)

All Frequencies, one anglePCS 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

Channel Emulation Rayleigh, Rician

multipath channels Time response: power

delay profile, RMS delay spread

BER, EVM, other metrics

Biological effects of RF modulated-signal exposure Gain from multiple

antenna systems TX or RX diversity MIMO

Emulating Multipath Environments

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

354

67

8109

1112131415

1621

20

19

18

17

VNA measurement test locations are in pink

Ground Plane

Transmittingantenna

tripods62 inches

VNA

Port 1 Port 4

200 mOpticalFiber

Receivingantenna

Fiber OpticReceiver

OpticalRF

Fiber OpticTransmitter

OpticalRF

Replicate Environment in Reverberation Chamber

Reverberation chamber with absorbing material

0 50 100 150 200 250 300 350 400 450 500time (ns)

-70

-65

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

Pow

er D

elay

Pro

file

(dB

)

1 absorber: τrms=187 ns



Large office biulding τrms=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)

Pow

er D

elay

Pro

file

(dB

)

o delay-spread=207ns * mean-delay=252.4ns


o delay-spread= 69ns * mean-delay=71.8ns




1 absorber3 absorbers7 absorbersJefferson North Plant, 10mJefferson North Plant, 50mJefferson North Plant, 100m

0 200 400 600 800 1000 1200 1400 1600 1800 2000-30

-25

-20

-15

-10

-5

0

time (ns)

Pow

er D

elay

Pro

file

(dB

)

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 absorber3 absorbers7 absorbersflint metal, drg-drg, 10mflint metal, drg-drg, 50mflint metal, drg-drg, 80mflint metal, drg-drg, 110Bm

Automobile Factories

assembly plant

stamping plant

Channel Measurements of “Clustered” Multipath

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 (d

B)

TX1 to RX5τrms = 115 nsNoise Threshold = -116 dB

0 500 1000-170

-160

-150

-140

-130

-120

-110

Delay (ns)

PD

P (d

B)

TX1 to RX9τrms = 39 nsNoise 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 datameasured Data Denver

RX

TX

Shortest path

Welton Street

17 t

h S

tree

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

NIEHS Study of Cell-Phone Radiation Long-term study on health effects

related to wireless exposure

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 degraded 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

σ ~ 5 dB

0 1 2 3 4-55

-50

-45

-40

-35

-30

-25

-20

-15

Bandwidth (MHz)

|S21

|2 (dB

)

Unloaded ChamberLoaded Chamber: 7 abs

σLOAD~ 5 dB

σUNLOAD ~ 10 dB

T

T

R12T1

T2

T3

R12

R9

R5

R10

T

T

R12T1

T2

T3

R12

R9

R5

R10

Frequency (MHz)

1931 1932 1933 1934

Cor

rela

tion

0

0.2

0.4

0.6

0.8

17 Abs: 3.23 MHz

4 Abs: 2.29 MHz

1 Abs: 1.27 MHz

Loaded = broader CBW

Cellular Device Testing: How is it done? Reference measurement provides:

Chamber loss: Transfer function of chamber Spatial uniformity of averaged fields in chamber Rotating platform:

Gref = <Gref,p>

DUT measurement: Same set-up as Ref Assume GDUT = Gref


Reference antenna

Measurement antenna


RF abs

Device under test

RF abs

Platform

Vector Network Analyzer

P1 P2

Gref


Base station emulator

Reference antenna

Measurement antenna


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

Rel

ativ

e TR

P

Anechoic Lab A Band IIAnechoic Lab B Band IIAnechoic Lab C Band IIReverb Lab A Band IIReverb Lab B Band IIReverb Lab C Band IIReverb Lab D Band IIReverb Lab E Band II

TRP Comparison, Free Space, Slider Open GSM850

0.000.501.001.502.002.503.003.504.004.505.00

Low Mid High

Channel

Rel

ativ

e TR

P

Anechoic Lab A GSM850Anechoic Lab B GSM850Anechoic Lab C GSM850Reverb Lab A GSM850Reverb Lab B GSM850Reverb Lab C GSM850Reverb Lab D GSM850Reverb Lab E GSM850

Total Radiated Power from Cell PhonesData from CTIA working group shows good agreementbetween 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

1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500-40

-38

-36

-34

-32

-30

-28

-26

-24

-22

-20

Frequency (MHz)

Rel

ativ

e m

ean

pow

er (d

B)

Relative mean power at Monopole12

UnloadedTwoBoxesFourBoxesSixBoxesEightBoxesTenBoxesTwelveBoxesOneRFTwoRFThreeRFFourRFFiveRFSixRF

Increases chamber loss (calibrate out)

Loading Decreases Spatial UniformityLoading 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.

σ increases with loading (in percent)

0 1 2 3 4 5 6 7 8-120

-115

-110

-105

-100

-95

-90

-85

-80

-75

-70

Time (µs)

Rel

ativ

e Po

wer

(dB)

PDP for different loading at Monopole2

UnloadedTwoBoxesFourBoxesSixBoxesEightBoxesTenBoxesTwelveBoxesOneRFTwoRFThreeRFFourRFFiveRFSixRF

Decreases decay time (replicates real world)

Unloaded 2 Bx 4 Bx 6 Bx 8 Bx 10 Bx 12 Bx11

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 AbsorbersMetallic BoxesTheoretical StDev

Decreases spatial uniformity

Loading and Position Stirring go Hand in Hand Non-negligible K factor a necessity:

unstirred energy correlated samples: paddle angle, spatial, frequency

Industry uses position, polarization, source stirring to reduce average K factor, improve estimate of DUT

• Chamber reference for PCS channel 9262 (1.850-1.854 GHz)

• VNA : 9 spatially uncorrelated positions

• Combinations of all sets of data

26

How to place the absorbers?

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 𝜎𝜎𝐺𝐺ref is higher when absorbers lie on floor

Less exposed metal surface Higher proximity effect

Various metrics: PCS band measurement

Distributed on Floor: Standing Stacked Distributed on

Floor: LyingCBW (MHz) 3.13 3.32 3.48No. abs 3 7 4𝐺𝐺ref (dB) -29.46 -29.69 -29.78𝜎𝜎𝐺𝐺ref (dB) 0.15 0.30 0.35

K factor (dB) -5.06 -4.36 -5.26

Stirring Sequence is Important Stirring mechanisms influence results differently Each chamber will have a different “mix” of optimal stirring

M = antenna positionsN = paddle angles

Cart 2x

y

x

y

Cart 1

Cart 3 (dashed) on ground

7steps

13steps

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

Antenna Placement is ImportantUnstirred energy: increased K factor, reduced spatial uniformity

Antenna placement guidelines: Orient away from each other Cross polarize Aim toward stirrersDUT: Unknown pattern?

1 2 3 4-5

-4

-3

-2

-1

0

1

2

3

Antenna Configuration

K Fa

ctor

(dB

)

Cell BandPCS Band

Dir/Dir

Omni/Dir

Dir/Omni

Omni/Omni

TX/RX pairs

K factor can be higher with directional transmit antennas

Relationship between antennas and absorber

is also important

30

30 cm

11

Rotating paddle

RF abs blocks

Measurement antenna

Reverberation Chamber (Top)

50 cm

100 cm120 cm150 cm

PCS Band

Proximity Effect Test

A systematic error: lower received power

Measure Gref successively closer to absorber

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 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 Nom BadAC1 -100.50 -99.00 -94.20AC2 -102.80 -100.00 -94.20RC1 -103.58 -100.29 -93.40RC2 -101.46 -98.22 -92.12RC3 -101.93 -98.66 -90.91Spread+/- 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 Proximity effect: No effect at 1 λ (at fc) Coherence bandwidth: Verified >3.84 MHz (4.42 MHz) Reference: Nine locations, DUT one location

DUT

abs

fc+ f (MHz)

1920 1925 1930 1935 1940 1945

Cor

rela

tion,

R

0

0.2

0.4

0.6

0.8

1

CBW (from 100 MHz BW) = 4.42 MHz

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)

Total Isotropy Sensitivity: Wireless Router Absorbers stacked Reference and DUT measurements:

Various heights Various locations

How does TIS change as a function of loading?

Total Isotropic Sensitivity

Number of absorbers

1 2 3 4 5 6 7

PTI

S (d

Bm

)

-105.5

-105

-104.5

-104

-103.5

-103

-102.5TIS values averaged over paddle positions

Millimeter-Wave Wireless for “5G”

Goal is uncertainty ≈ 1 % oru = 1

𝑁𝑁+ 𝐾𝐾2 ≤ 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: Reverberation chambers represent reliable and repeatable test

facilities that have the capability of simulating key characteristics of many multipath environments for the testing of wireless devices

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

Watch this space for more information on over-the-air testing

with reverberation chambers

an introduction to free-field measurements of wireless ... · an introduction to free-field...

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