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 Frequency, Frequency & Time Basic Digital Demod Advanced & Specific Demod Get basics right, find major problems Signal quality numbers, constellation, basic error vector measurements Find specific problems & causes
Time & Frequency Analysis Time & Frequency Domain Amplitude & Phase or I/Q Log, Linear Time-Selective, Time-Gated CCDF, PSD Spectrum Spectrum RF Envelope IF Time 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 BPSK (pilots) BPSK (FCH) QPSK 16 QAM 64QAM Defect: Scaling Error on 64QAM Only
Meas. & Troubleshooting Sequence Frequency, Frequency & Time Basic Digital Demod Advanced & Specific Demod Get basics right, find major problems Quantitative errors, constellations, basic error vector meas. Find specific problems & causes Wideband spectrum Narrowband spectrum Frequency & Time Triggering, timing Gated Spectrum Gated power, CCDF Time capture Spectrogram 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 Density or Histogram Persistence Cumulative history Trigger on real-time spectrum, with logic
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 Measured Here and With Advanced Demodulation Operations 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)
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
Spectrogram Shows Structure, Resource Allocation Does actual resource allocation match the plan?
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 Frequency, Frequency & Time Basic Digital Demod Advanced & Specific Demod Get basics right, find major problems Signal quality numbers, constellation, basic error vector measurements Find specific problems & causes Set up demod & displays Constellation Error Summary Error vector spectrum Error vector time Cross-domain & cross-measurement links Parameter adjustment More time capture
Basic Demodulation Results EVM vs freq/carrier Constellation Error summary EVM vs time/symbol
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 Symbol Error 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 Frequency, Frequency & Time Basic Digital Demod Advanced & Specific Demod Get basics right, find major problems Signal quality numbers, constellation, basic error vector measurements Demod by carrier or symbol or both Select pilot tracking types Select carrier, timing Preamble (equalization) analysis Find specific problems & causes Cross-domain & cross-measurement links Demod parameter adjustments More time capture
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 fullfeatured version for a limited time. Effects of physical layer impairments on OFDM systems by Robert Cutler, RF Design Magazine http://defenseelectronicsmag.com/sitefiles/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