Measurements of Wireless Devices in Reverberation Chambers
Kate A. Remley IEEE EMC Society Distinguished Lecturer Group Leader, NIST Metrology for Wireless Systems
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OTA Test in reverberation chambers involves all except calibration services What is a Reverberation Chamber? . A shielded, highly reflective radiated-signal test chamber What you can do with one: . Create known fields (EMC/susceptibility) . Radiated emissions and power (CW and modulated-signal) . Antenna parameters (, efficiency, etc.) You can also do communication system tests . Receiver sensitivity . Throughput . EVM, BER, etc. . DUT needs realistic channel
1 BER BER .01 CBW=32.8CBW=328 kHzkHz HighLow BERBER .01 0 45 90 135 180 225 270 315 360
) Paddle angle (degrees) ) Paddle angle (degrees) m m NIST measurements of prototype 4G NISTMIMO measurements cellular telephone of wireless antennas router Fields in a Metal Box (Cavity)*
. 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)
Frozen Food
• The paddle changes internal field distributions (modes) where the high and low field values occur • Ideally, after one mode-stirring sequence, all locations in the chamber will have experienced nearly the same collection of field maxima and minima 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 or moving walls . Using multiple antennas with various locations, polarizations, etc. Reverberation Chambers Come in All Shapes and Sizes Lowest frequency of operation, uncertainties determined by chamber size, wall loss
NASA: Glenn Research Reverberation Chamber Center (Sandusky, OH) with Moving Walls Reverberation Chamber Characteristics Constructive and destructive interference for each -10 mode-stirring sample All Frequencies, one angle PCS Band, four angles -20 . Frequency domain (|S |2): 21 -30 (dB) Reflections create multipath 2 -40 |S21|
-50
Reverberation Chamber -60 Measurement 1 1.5 2 antenna Frequency (GHz)
Mode-stirring paddle
Reference . Time domain (power antenna delay profile): Decay time Device Vector Network RF under RF Analyzer of reflections depends on abs test abs P1 P2 Platform chamber reflectivity Original Applications
. Radiated immunity: . Antenna efficiency high field strength . Calibrate RF probes . components . RF/MW Spectrograph . large systems . absorption properties . Radiated emissions . Material heating . Shielding . Biological effects . cables . Conductivity and . connectors material properties . enclosures Wireless Applications . Multipath environments . Standardized over- . Rayleigh, Rician multipath the-air test methods channels: with/without . Radiated power of channel emulators mobile wireless devices . Time response: power . Receiver sensitivity delay profile, delay spread . Large-form-factor and . Biological effects of body-worn devices (with modulated-signal phantoms) exposure . Public-safety emergency equipment . Gain from multiple antenna systems . TX or RX diversity . MIMO Emulating Multipath Environments
Office Corridor Oil Refinery
Apartment Building
Automobile Plants Subterranean Tunnels NIST channel measurements: Standards development for electronic safety equipment such as firefighter beacons Channel Measurements: Denver High Rise
Receiving Transmitting antenna antenna
tripods 16062 inches cm VNA
Port 1 Port 4
Ground Plane
Fiber Optic Fiber Optic Transmitter Receiver 200 m RF Optical RF Optical Optical Fiber Replicate Environment in Reverberation Chamber • Add RF absorbing material to “tune” the decay time of the chamber • Distributed multipath (reflections) matched by chamber’s decay profile
Time response of channel replicated in chamber
0 1 absorber: rms=187 ns -5 -10 3 absorber: rms=106 ns -15
-20 7 absorber: rms=66 ns -25 ile (dB) ile f -30 -35 -40 -45
Power Delay Pro -50 -55 Reverberation chamber with -60 Large office biulding -65 absorbing material rms=59 ns -70 0 50 100 150 200 250 300 350 400 450 500 time (ns) Emulating Other Reflective Environments . Oil Refinery Emulating Other Reflective Environments
. Automobile Factories 0
-5
-10
-15
-20 o delay-spread=207ns * mean-delay=252.4ns o delay-spread=123ns * mean-delay=130.3ns -25 o delay-spread= 69ns * mean-delay=71.8ns o delay-spread=195ns * mean-delay=147.0ns -30 o delay-spread=199ns * mean-delay=161.6ns o delay-spread=261ns * mean-delay=275.3ns Power Delay Profile (dB) Profile Delay Power -35 1 absorber 3 absorbers -40 7 absorbers Jefferson North Plant, 10m -45 Jeffersonassembly North Plant, plant 50m Jefferson North Plant, 100m
-50 0 100 200 300 400 500 600 700 800 900 time (ns)
0 o delay-spread=277ns * mean-delay=290.5ns 1 absorber o delay-spread=122ns * mean-delay=130.1ns 3 absorbers o delay-spread= 67ns * mean-delay=71.2ns 7 absorbers o delay-spread=168ns * mean-delay=137.8ns -5 o delay-spread=175ns * mean-delay=142.7ns flint metal, drg-drg, 10m o delay-spread=128ns * mean-delay=105.8ns flint stampingmetal, drg-drg, 50m o delay-spread=173ns * mean-delay=176.1ns flint metal, drg-drg, 80m flint metal,plant drg-drg, 110Bm -10
-15
Power Delay Profile (dB) Profile Delay Power -20
-25
-30 0 200 400 600 800 1000 1200 1400 1600 1800 2000 time (ns) Urban Canyon Multipath Effects
-90 TX1 to RX5 -110 -100 rms = 115 ns Noise Threshold = -116 dB -110 -120 -120 -130
-130 -140
PDP (dB) PDP -140 PDP PDP (dB) -150 -150 TX1 to RX9 -160 -160 rms = 39 ns Noise Threshold = -116 dB R -170 -170 9 0 200 400 600 800 1000 1200 0 500 1000 Delay (ns) Delay (ns) R10 Line of Sight Non Line of Sight R (LOS) (NLOS) 5
T R T1 1212 •Measurements made in T3 Denver urban canyon 2009 T2 •Channel characterization: LOS and NLOS Replicating Clustered Multipath in 1 fitted simulated data Reverberation Chamber 0.9 measured Data Denver
0.8
0.7 Shortest path 0.6
0.5 PDP TX 0.4
0.3
0.2
0.1
0 0 100 200 300 400 500 600 700 800 900 1000 RX Delay [ns] • Blue: Mean of 27 NLOS Clusters of measurements exponentially • Red: RC + channel distributed signals emulator received off of • Dashed: Exponential model buildings “Channel Models” used for standardized over-the-air (OTA) tests
. Outdoor-to-indoor channel model for 700 MHz . 8 environments, hundreds of measurements . Included in proposed 3GPP reverberation- chamber-based test methods
0 700 MHz, measured Excess tap delay Relative power [dB] 4900 MHz, measured -5 700 MHz, fit [ns] 4900 MHz, fit 0 0.0 -10 40 -1.7 120 -5.2 -15 180 -7.8 210 -9.1 -20 260 -11.3 PDP(dB) -25 350 -15.2 RMS DS: 80 ns -30 Discrete version of the “NIST Model” @ 700 MHz for anechoic-chamber measurements -35 0 100 200 300 400 500 600 Delay (ns) D.W. Matolak, K.A. Remley, C.L. Holloway, and C. Gentile, “Outdoor-to-Indoor Channel Dispersion and Power-Delay Profile Reverberation chamber can easily Models for the 700 MHz and 4.9 GHz Bands,” IEEE Antennas replicate diffuse multipath and Wireless Propagat. Lett., vol. 15, 2016, pp. 441-443. 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 Reverberation Chamber: Real Channel: Loading Reduces Variations Slow Variations with Frequency -151 -20 TX1 to RX6 UNLOAD ~ 10 dB 2 7 Abs: 3.23 MHz |H(f)| (mean) = -58 dB -20 -30 4 Abs: 2.29 MHz 2 0.8 1 Abs: 1.27 MHz Mean |H(f)| -25 -40 2 Mean |N(f)| -30 ~ 5 dB 2 0.6 (dB) 2
(dB) Std Dev |H(f)| -35 -50 | 2 21
|S 0.4-40
|H(f)| -60 Correlation LOAD -45 ~ 5 dB -70 0.2-50 UnloadedLoaded Chamber = broader Loaded Chamber: 7 abs -55 coherence bandwidth -80 0 1 2 3 4 0 730 740 750 760 770 780 790 800 1931Bandwidth 1932 (MHz) 1933 1934 Frequency (MHz) Frequency (MHz) 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: ReverberationReverberation Chamber Chamber Measurement Gref =
Device Device Vector Network RFRF under under RFRF Analyzer absabs absabs testtest Base station P1 P2 PlatformPlatform emulator Total Radiated Power from Cell Phones Data from CTIA working group shows good agreement between anechoic and reverberation chambers
TRP Comparison, Free Space, Slider Open, W-CDMA Band II
Cell phone 5.00 4.50 AC RC 4.00 Anechoic Lab A Band II 3.50 Anechoic Lab B Band II 3.00 Anechoic Lab C Band II Reverb Lab A Band II 2.50 2 dB Reverb Lab B Band II 2.00 Reverb Lab C Band II Relative TRPRelative 1.50 Reverb Lab D Band II 1.00 Reverb Lab E Band II 0.50 0.00 Low Mid High Reference Channel +/-2 dB is threshold TRP Comparison, Free Space, Slider Open GSM850
5.00 4.50 AC RC 4.00 Anechoic Lab A GSM850 3.50 Anechoic Lab B GSM850 3.00 Anechoic Lab C GSM850 Reverb Lab A GSM850 • Cell phone 2.50 Reverb Lab B GSM850 2.00 testing: ca. 2010 Reverb Lab C GSM850 Relative TRP Relative 2 dB 1.50 Reverb Lab D GSM850 • But in 2017… 1.00 Reverb Lab E GSM850 0.50 0.00 Low Mid High Channel The Machine-to-Machine (M2M) Revolution . 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
* Source Cisco M2M and the Internet of Things (IoT) . Large, cellular enabled devices: . Low-cost, rough duty, solar-powered, becoming ubiquitous
Truck Boston responds, Trash can saving fuel reports when full New Orleans Illustration of name brands does not imply endorsement San Diego by NIST: other products may work as well or better Testing Large Form-Factor Devices . Integrated antennas: test of entire device required . Reverberation chamber: currently only option for single-antenna devices . Device placement not critical within chamber . Relatively low cost . OTA test issues: Large devices, loaded chamber
Bath, England Loading Provides Improved Channel Loading helps with demodulation: Test DUT response to stimulus, NOT to chamber But it introduces other “nonideal” effects
Relative mean power at Monopole12 -20 Unloaded Unloaded 1 RF 2 RF 3 RF 4 RF 5 RF 6 RF -22 RF Absorbers TwoBoxes 18 Metallic Boxes FourBoxes 18 SixBoxes -24 Theoretical StDev EightBoxes 17 17 TenBoxes -26 TwelveBoxes 16 OneRF 16 -28 Decreases spatial TwoRF ThreeRF -30 15 uniformity FourRF 15 FiveRF -32 14 SixRF 14 -34 Relative mean power (dB) (dB) power mean Relative 13 13 -36 Increases chamber loss 12 12 -38 (can calibrate out)
-40 11 11 1500Unloaded1600 17002 Bx 1800 4 Bx1900 20006 Bx 2100 8 2200Bx 230010 Bx2400 122500 Bx Frequency (MHz) increases with loading S. van de Beek et al., Characterizing large-form-factor (in percent) devices in a reverberation chamber, EMC Europe 2013. Loading and Position Stirring go Hand in Hand . Industry uses position, polarization and source stirring to reduce K factor, improve estimate of DUT performance . Non-negligible K factor a necessity: . unstirred energy . correlated samples: paddle angle, spatial, frequency
• Chamber reference for PCS channel 9262 (1.850-1.854 GHz) • VNA : 9 spatially uncorrelated positions • Combinations of all sets Measured estimate of chamber’s power transfer of data function (Gref) improves with position stirring Set-up: Absorber Placement • Standing on floor • Lying on floor • Stacked
Considerations: • Exposed absorber surface area • Exposed metal surfaces
• Absorber location 34 Comparable Loading, Different Uncertainty PCS band measurement (~1950 MHz) . Load chamber for approximately the same CBW . Chamber loss approximately the same as well . is higher when absorbers lie on floor ref . Less exposed metal surface . Higher proximity effect
Distributed on Distributed on Stacked Floor: Standing Floor: Lying CBW (MHz) 3.13 3.32 3.48 No. abs 374
(dB) -29.46 -29.69 -29.78 (dB) ref 0.15 0.30 0.35 Total Isotropy Sensitivity (TIS): Wireless Router . Absorbers stacked . Reference and DUT measurements: . 9 different locations, heights . TIS underestimated with insufficient loading 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
abs abs Agreement with anechoic chamber: . 0.2 dB, PCS band (1.850 GHz to 1.995 GHz)
. 1.95 dB, Cell band (800 MHzCorrelation,R to 900 MHz) DUTDUT
Currently, the only accepted test method for large devices involves reverberation chambers Summary: Cellular Device Testing in Reverberation Chambers OTA Tests Require Loading: . Provide realistic channel for receiver . Increase unstirred energy . Reduce spatial uniformity Samsung May 2014 Configure Chamber with Care: . Placement of absorbers, antennas affects accuracy . Validation of set-up required . RC OTA tests compare well to other methods The Future Will Bring: . OTA for multiple antenna systems . mmWave testing for 5G Watch this space for more information on over-the-air testing with reverberation chambers