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