An Introduction to Free-Field Measurements of Wireless Devices in Reverberation Chambers · 2016-11-18 · An Introduction to Free-Field Measurements of Wireless Devices in Reverberation

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


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

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



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= 69ns * mean-delay=71.8ns




1 absorber

3 absorbers

7 absorbers

Jefferson North Plant, 10m



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= 67ns * mean-delay=71.2ns





1 absorber

3 absorbers

7 absorbers

flint metal, drg-drg, 10m



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


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

Documents

An Introduction to Free-Field Measurements of Wireless Devices in Reverberation Chambers · 2016-11-18 · An Introduction to Free-Field Measurements of Wireless Devices in Reverberation