Successful Modulation Analysis
in 3 Steps
Ben Zarlingo
Application Specialist
Agilent Technologies Inc.
January 22, 2014
•© Agilent Technologies, Inc. 2014
This Presentation
Focus on Design, Validation, Troubleshooting
Techniques Apply to Any Source of Sampled Signal Data
• RF signal analyzers (modular or bench-top), oscilloscopes, digitizers
• Math and simulation tools (VSA as simulation element), custom software
• Logic analyzers, sampled data from FPGAs
Efficient Path to Success as You Define it
• Design complete—performance, compatibility, interoperability
• Minimize risk, surprises, rework, delays
• Optimize cost, power consumption, manufacturability
Leverage Your Knowledge, Experience, Insights
• Augment your skill, not a substitute for it
• Displays that reveal unexpected problems
Agenda
Measurement & Troubleshooting Sequence
Structured Around Three Steps
• Spectrum, frequency & time domain, vector
• Basic digital demodulation
• Advanced digital demodulation
Measurement Examples to Illustrate Sequence
Example Errors to Illustrate Displays, Techniques to Find
Problems
Additional Resources
Understanding,
Measuring in
3 Domains
Precision Engineering
in a Time-Varying world
• Time
• Frequency
• Modulation
Design, Optimize,
Troubleshoot to Meet
Specs in All Domains
at Once
3 Step Measurement & Troubleshooting Sequence
One suggested sequence, especially for
signals that are not fully understood
Get basics right,find major problems
Signal qualitynumbers, constellation,
basic error vector measurements
Find specificproblems & causes
Frequency,
Frequency & Time
Basic
Digital Demod
Advanced &
Specific Demod
Time & Frequency Analysis
Time & Frequency Domain
Amplitude & Phase or I/Q
Log, Linear
Time-Selective, Time-Gated
CCDF, PSD
•Spectrum
•IF Time
Spectrum
RF Envelope
IF Time
See Problem Clearly in Vector, Not in Demodulation
Defect: Short Training Field is Too Short
Possible Problems with Synchronization, Demodulation
See Problem Clearly in Demodulation, Not in Vector
Defect:
Scaling Error
on 64QAM Only
BPSK (pilots)
BPSK (FCH)
QPSK
16 QAM
64QAM
Meas. & Troubleshooting Sequence
Wideband spectrumNarrowband spectrumFrequency & TimeTriggering, timing
Gated SpectrumGated power, CCDFTime captureSpectrogram
Get basics right,find major problems
Quantitative errors,constellations, basicerror vector meas.
Find specificproblems & causes
Frequency,
Frequency & Time
Basic
Digital Demod
Advanced &
Specific Demod
Real-Time Spectral Meas. & Triggering
Frequency Measurement, then Frequency & Time
Frequency--Wideband Spectrum
• Approximate center frequency, occupied BW, power level/range
• Other signals present, spurs & interference
• Wideband also for non-demod measures such as ACPR
Frequency--Narrowband Spectrum ~1.1x(nominal BW)
• More accurate center frequency
• Transition to frequency & time
• Spectrum alone (even with averaging) is inadequate for pulsed signals
with AM
• Accurate spectrum requires triggering
Persistence, Cumulative History, Density
Understand Spectral Occupancy, Interference, Hopping
Cumulative history
Persistence
Trigger on real-time
spectrum, with logic
Density or Histogram
Frequency Measurement, then Frequency & Time
Simultaneous Freq. & Time Measurements
• Set time to log magnitude (burst envelope)
• Select IF triggering, pre-trigger delay, adjust trigger level, add holdoff
(holdoff is often essential for pulsed signals with AM)
• Stabilize acquisition to make all other measurements reliable
• Adjust time record length to see entire burst(s)
– Use very large number of frequency/time points
Simultaneous Freq. & Time Meas.
Center Frequency & Bandwidth Measurements
Measure and Verify
Frequency & Time Measurements
• Center frequency, occupied bandwidth
• Amplitude--average, and variations during burst (transients, drift)
• Turn-on & turn-off behavior, on/off ratio
• Burst length, duty cycle, unanticipated frequency/time variations
• Band power measurements
• 89600B occupied bandwidth marker & centroid–use carefully on signals
with essential sidebands or asymmetry
Time-Gated Spectrum Measurements
Time-Gating Setup (example: measuring preamble)
• Set main time length to approx. 5 symbol times
• Enable gating, set gate length for desired signal segment and RBW, then
set gate length equal to the “OFDM symbol time” to see preamble sym.
• Set initial gate delay (beginning of time gate) to match pre-trigger delay
Select Appropriate Gate Windows (RBW Shape)
• Flat Top for amplitude accuracy, Uniform for frequency resolution
Time-Gated CCDF
• Preamble vs. data
Time-Gated Measurements
Measure Data Portion of Frame Only, See 5 dB Tilt, Ripple
Amplification Problems—Gain Compression
Before Amplification After Amplification
Use Time-Gated CCDF to Investigate Different Modulation Types
Amplification Effects
Gain Drift
• ADC reference changes with thermal effects
• Amplifier gain changes with temperature
• Power supply effects (sag/surge with loads)
Transients (usually occur at beginning of bursts)
• Fast thermal
• Short term power supply instability
• Oscillator instability (power supply/other couplings)
Measured Here and With Advanced Demodulation Operations
In-band Impairments: IQ Errors, Spectrogram View
Use Time Capture & Spectrogram to See Signal Without Gaps,
With Adjustable Overlap
I-Q Errors Produce Energy at Symmetric Frequencies
Explore Resource Allocation, Transmitter Power
View Any Signal From Trace Butter
Does actual resource allocation match the plan?
Spectrogram Shows Structure, Resource Allocation
Other Meas. Before Digital Demodulation
Time Capture
• Reduce uncertainty by analyzing known signal (useful during transition to
digital demodulation)
• Provides for “real-time” & overlapped analysis
• Identify patterns not otherwise seen
• Capture 2-10 bursts (generally avoid very large captures)
Spectrogram
• See entire burst in frequency and time on one display
• Find subtle patterns, errors (For example, data portion of burst should not
have repeated patterns)
Meas. & Troubleshooting Sequence
Set up demod & displaysConstellationError SummaryError vector spectrumError vector time
Cross-domain &cross-measurement links
Parameter adjustmentMore time capture
Get basics right,find major problems
Signal qualitynumbers, constellation,
basic error vector measurements
Find specificproblems & causes
Frequency,
Frequency & Time
Basic
Digital Demod
Advanced &
Specific Demod
Basic Demodulation Results
EVM vs time/symbol
Constellation
EVM vs freq/carrier
Error summary
Initial Demodulation Results
Constellation
• Successful demodulation?
• Expected modulation type(s)?
• Indications of error?
Symbols/Errors Table
• EVM, MER, RCE = Typically EVM of data and pilot carriers
• Pilot & common pilot errors (CPE)
• I/Q errors including gain imbalance, quadrature error, delay mismatch
• Carrier frequency error, symbol clock error
Understanding IQ Errors in OFDM
“Effects of physical layer impairments on OFDM systems” RF Design
Magazine
http://defenseelectronicsmag.com/site-files/defenseelectronicsmag.com/files/archive/rfdesign.com/images/archive/0502Cutler36.pdf
SCM
64QAM Scaling Error Defect
BPSK (pilots)
BPSK (FCH)
QPSK
16 QAM
64QAM
Error Displays in Time & Frequency
Initial Demodulation Results (cont.)
Error Vector Spectrum
• All symbols shown on Y-axis for each carrier on X-axis
• All-symbol average for each carrier is shown
• Examine for patterns/trends by carrier, differences between carriers & pilots
• Spurs, interference will affect individual or few carriers, for all symbols
Error Vector Time
• All carriers shown on Y-axis for each symbol on X-axis
• All-carrier average for each symbol is shown
• Examine for patterns or changes according to symbol (time)
• Impulsive errors (DSP, interference, clocks, power) will affect all carriers for
an individual symbol or group of symbols
Time and Frequency Error Displays are
Complementary
Error vs. Time
or SymbolError vs. Frequency
or Subcarrier
Spurious Signal is Obscure in One Domain
and Clear in Another
Initial Demodulation Results (cont.)
Coupled Markers
• Identify a symbol by time or frequency or error magnitude
• Link a symbol across time and frequency domains, and between different
display types
• Link error peaks to constellation points, amplitude values, specific carriers,
time points in a burst, as a way to pinpoint error mechanism
• Identify specific time instant or frequency to examine with advanced &
specific demodulation techniques (next)
Change Measurement & Display Parameters Without Taking
New Data
Use Time Capture to Provide Consistent Signal & Error
Behavior
Meas. & Troubleshooting Sequence
Demod by carrieror symbol or both
Select pilot tracking typesSelect carrier, timingPreamble (equalization) analysis
Cross-domain &cross-measurement links
Demod parameter adjustmentsMore time capture
Get basics right,find major problems
Signal qualitynumbers, constellation,
basic error vector measurements
Find specificproblems & causes
Frequency,
Frequency & Time
Basic
Digital Demod
Advanced &
Specific Demod
Advanced & Specific Digital Demod.
Demod Results for Specific Carriers
Demod Results for Specific Symbols
Enable/Disable Pilot Tracking of Amplitude, Phase, Timing
Data Sub-Carrier Manual Select
Symbol Timing Adjust
Equalizer Training Select (preamble only, preamble + data)
Preamble Error Measurements
X & Y-Axis Scaling (display zoom; actual demod results are
not changed)
Time and Frequency-Specific Demod.
Demodulate a Specific Carrier
• Find frequency-specific problems on a single carrier or at band edge
• Demodulate pilots only, and compare to data carriers
Demodulate A Specific Time Interval
• Modulation type changes with symbol time, and error may change along
with it
• Identify impulsive, intermittent, or periodic error sources
• Turn on/off, power supply, settling, or thermal effects
Simultaneous Frequency & Time-Specific Demod.
• Find subtle defects such as DSP errors or impulsive interference that only
affect a specific carrier/frequency at a specific time or over a specific time
interval
Adaptive Equalization
Corrects Linear Errors Only
Training Sequence (Part of Preamble) Provided on All Bursts
(downlink & uplink)
Equalizer Usually Trained on Two or More Symbols of
Preamble
Equalizer can be Trained on Preamble Only (typical) or on
Entire Burst
Midambles may also be Provided
Results of Equalization can be Viewed, Measured, used to
Find Problems
Automatic Detection and Customized Data Tables
• Modulation Types Detected and Listed
• Automatic Measurement of Individual Power, Symbol Length, Error
• Signal Elements Listed, Signal Structure Summarized
Pilot Tracking
Demodulation Is Adjusted Symbol-by-Symbol
Demodulation is Performed Relative to the Pilots
Some Errors are “Tracked Out” as Demodulation Follows Pilots
Tracking Types can be Enabled/Disabled Independently
• Amplitude
• Phase
• Timing
Pilot Tracking Removes Close-In Phase Noise
Pilot Tracking Disabled to Show Errors
Pilot Tracking Compensates for (Hides) Errors
Selectively Enable/Disable Tracking Types
Common Pilot Error (CPE) Quantifies Defect(s)
CPE Trace
Shows
Amplitude
Droop
Additional Resources
89600 VSA software free demo (non-expiring) and trial licenses. Go to
www.agilent.com/find/89600vsa to download and explore in demo mode (all
functions on pre-recorded signals only) or click on “Trials & Licenses” to use a full-
featured version for a limited time.
“Effects of physical layer impairments on OFDM systems” by Robert Cutler, RF
Design Magazine http://defenseelectronicsmag.com/site-
files/defenseelectronicsmag.com/files/archive/rfdesign.com/images/archive/0502Cutler36.pdf
“Bringing New Power and Precision to Gated Spectrum Measurements” by Tom
Wright, Joe Gorin, Ben Zarlingo, from High Frequency Electronics magazine,
August 2007
http://www.highfrequencyelectronics.com/Archives/Aug07/HFE0807_Zarlingo.pdf
“Optimize OFDM Via Phase-Noise Injection” by Ben Zarlingo, from Microwaves & RF
magazine, 10/2012
http://mwrf.com/systems/optimize-ofdm-phase-noise-injection
“Measuring Agile Signals and Dynamic Signal Environments” Agilent application note,
literature number 5991-2119EN, May 2013