Wimax Agilent

RF Measurements for WiMAX (802.16-2004)

IEEE Seminar on WiMAX Wireless Technology April 28, 2006

© Copyright 2006 Agilent Technologies, Inc.

A Capable and Complicated New Std. Constellation

EVM vs time/symbol

EVM vs freq/carrier

Error summary & data bits

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Why Measure? Measurements Required by the Standard (some) • These measurements tell only part of the story Measurements for Success (however you define it) • Find problems quickly and specifically • Ensure compatibility, solve compatibility or interoperability issues • Improve performance • Reduce cost • Improve manufacturability

Agenda WiMAX Overview, Specifically 802.16-2004 • Brief review here • See resources for OFDM review & tutorial Measurement & Troubleshooting Sequence • Spectrum • Frequency & time domain • Basic digital demodulation • Advanced digital demodulation Discuss Common Problems, How to Find Them Resources

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Resources, Tutorials, Demonstrations 802.11a WLAN Materials for OFDM Background • Magazine article on OFDM impairments • OFDM analysis tutorial with case studies • Measurement & setup demonstration videos (30 minutes) on CD WiMAX Resources • Recent WiMAX Ap-Notes • 89601A VSA software--free demo/tutorial license • Example captured signals (no hardware required for tutorial) See Resources Slide For Summary, Links

Contrast OFDMs--802.11a vs. 802.16 WiMAX BPSK, QPSK, 16QAM, 64QAM

4 BPSK Pilots

52 carriers, 312.5 kHz spacing

802.11a (18 MHz)

8 BPSK Pilots

200 carriers, 90 kHz spacing . . .

200 carriers, 6.7 kHz spacing

BPSK, QPSK, 16QAM, 64QAM

. . .

802.16 (20 MHz) : 10 MHz 7.0 MHz 3.5 MHz : 802.16 (1.5 MHz)

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802.16 Concepts--Uplink Sub-channelization

...

Low-rate user: 12 carriers

P B1

...

+ =

Med-rate user: 48 carriers

+

200 carrier uplink burst (192 data carriers)

Hi-rate user: 132 carriers

Terminology--Different from 802.11a Devices = Base Stations & Subscribers Downlink+Uplink RF Pair = Frame RF Burst (uplink or downlink) = Subframe Initial Sync & Training Seq = Preamble & Symbols • Short, long preamble, midamble(s) Data Block to/from Single Subscriber = Burst Instruct. to Subscribers = Frame Cntrl. Header (FCH) Relative to 802.11a: • Short training sequence Preamble symbol 1 • Channel est. sequence Preamble symbol 2 • Signal symbol FCH

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802.16 Concepts--TDD vs FDD vs H-FDD Time Division Duplexing 1 Frame (5, 10 or 20 mSec) xx MHz

P H B1

B2

B3

P B1

B4

Downlink Subframe (Base Station)

B1 P B2

P B3

...

Uplink Subframe (Subscribers)

Frequency Division Duplexing 1 Frame (20 mSec max) xx MHz

P H B1

B2

B3

B4

B5

. . . Bn

P H

...

P B1

...

Downlink Subframe (Base Station) yy MHz

P B1

B1 P B2

P B3

P B4

B1 P B5

Uplink Subframe (Subscribers)

Half-Duplex FDD xx MHz

1 Frame (20 mSec max.)

P H B1

B2

B3

B4

Downlink Subframe (Base Station)

P B1 yy MHz

B1 P B2

P B3

...

Uplink Subframe (Subscribers)

802.16 Concepts--TDD vs FDD vs H-FDD

Time Division Duplexing 1 Frame (5, 10 or 20 mSec) P H B1

B2

B3

B4 TTG P B1

Downlink Subframe (Base Station)

B1 P B2

P B3

RTG

Uplink Subframe (Subscribers)

TTG = Transmit/Receive Transition Gap RTG = Receive/Transmit Transition Gap

No transmissions are allowed during the TTG or RTG time periods

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802.16 Typical Downlink Subframe Downlink Subframe (Base Station) P H B1

B2

B3

B4

Example: B1 = QPSK B2 = QPSK B3 = 16 QAM

2 Symbols of QPSK

B4 = 64 QAM 1 Symbol of BPSK One or more symbols; all symbols within each burst have the same mod type

Consecutive bursts can use modulation different than previous bursts. Bursts are transmitted in order of decreasing robustness (QPSK is less robust than BPSK, QAM is less robust than QPSK, etc)

Connection quality

modulation types

data rates

Downlink Preamble Downlink Subframe (Base Station) P H B1

B2

B3

B4

• 2 Symbols of QPSK

Preamble 50 active 100 active QPSK carriers QPSK carriers

Symbol 1

Symbol 2

• Well defined data pattern • All BS transmit same preamble • No Pilots • 256 FFT is used, although carrier spacing is such that 64 or 128 FFTs could be used if processing was an issue • Known as a “long preamble”

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Physical Layer Meas. in the Standard Cyclic Prefix and Symbol Timing Preambles & Midambles Transmit Ranging Support Power Control Support Spectral Flatness Relative Constellation Error (RCE) Transmit Synchronization Transmitter Spectral Mask SSRTG Performance See New Application Notes (resources slide)

Well-Organized Measurement Approach An Orderly Approach Will

• Reduce setup and measurement errors (correct center frequency, span, pulse/burst parameters) • Find problems at the earliest stages of analysis • Provide more understandable and useful measurement results • Help avoid missing certain signal problems or impairments (some impairments are most clearly seen in vector measurements) • Provide the fastest path to complete troubleshooting

It is often more useful to have a clear understanding of the signal and a reliable analysis approach than to know all the technical details of the standard

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A Meas. & Troubleshooting Sequence One suggested sequence, especially for signals that are not fully understood

Frequency,

Basic

Advanced &

Frequency & Time

Digital Demod

Specific Demod

Get basics right, find major problems

Signal quality numbers, constellation, basic error vector measurements

Find specific problems & causes

Meas. & Troubleshooting Sequence

Frequency,

Basic

Advanced &

Frequency & Time

Digital Demod

Specific Demod

Get basics right, find major problems

Signal quality numbers, constellation, basic error vector measurements

Find specific problems & causes

Wideband spectrum Narrowband spectrum Frequency & Time Triggering, timing

Gated Spectrum Gated power, CCDF Time capture Spectrogram

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Frequency Meas., then Freq. & Time Frequency--Wideband Spectrum • Approximate center frequency, occupied BW, power level/range • Other signals present, spurs & interference 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

Frequency Meas., then Freq. & 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

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Simultaneous Freq. & Time Meas.

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 • 89601A occupied bandwidth marker (adjust for desired power %, but use carefully on signals with essential sidebands)

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WiMAX Parameter Summary Table

“WiMAX Frequency and Time Parameter Table”

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

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Time-Gated Measurements

Measure and Verify Time-Gated Spectrum Measurements • Spectrum vs. time, any spectrum artifacts • Power changes during burst, CCDF variations • Carrier structure, missing or extra carriers, energy at exact CF • Sidelobes (part of signal, not ACP), symmetry • Frequency accuracy, carrier spacing • Spurious, interference • Flatness, tilt/ripple • Preamble length, structure • Confirm sampling factor, guard interval

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Amplification Problems--Gain Compress. 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)

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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 (data portion of burst should not have repeated patterns

Find Problems Even if Demod. not yet Possible

• Example:

Power problem could be seen in demodulation mode, but malformed pilots prevent demodulation-measure in vector mode

Meas. & Troubleshooting Sequence Frequency,

Basic

Advanced &

Frequency & Time

Digital Demod

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

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Switch to Digital Demodulation Select Broadband Wireless Access Preset to Standard Set Nominal Bandwidth Set Guard Interval • If unknown, measure in time domain or assume 1/4 to start Analyzer Automatically Detects, Sets, Displays: • Sampling factor • Data sub-carrier modulation type • Preamble type • Result length

Set up the Measurement, Displays Use Pulse Search, Set Search Length • Minimum: 2x(on time)+1x(off time) • Reliable indicator of inconsistent burst length or burst problems Select Default Quad Display • Default is a good starting point • Select new 6-trace display if desired • Constellation, error vector time, error vector frequency, symbols/errors

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WiMAX Parameter Summary Table

“WiMAX Frequency and Time Parameter Table”

Digital Demodulation--Basic Setup

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Basic Demodulation Results

Initial Demodulation Results Constellation • Successful demodulation? • Expected modulation type(s)? • Indications of error? Symbols/Errors Table • Relative constellation error (RCE) = 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

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Multi-Format Constellation Overlaid BPSK, QPSK, 16QAM, 64QAM

Multi-Format Const. Now Color-Coded

BPSK (pilots) BPSK (FCH) QPSK 16 QAM 64QAM

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Color Combines to Show Problem Clearly

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 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

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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

Understanding IQ Errors in OFDM “Effects of physical layer impairments on OFDM systems” RF Design Magazine http://rfdesign.com/mag/radio_effects_physical_layer/

SCM

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Meas. & Troubleshooting Sequence Frequency,

Basic

Advanced &

Frequency & Time

Digital Demod

Specific Demod

Get basics right, find major problems

Signal quality numbers, constellation, basic error vector measurements

Find specific problems & causes

Demod by carrier or symbol or both Select pilot tracking types Select carrier, timing Preamble (equalization) analysis

Cross-domain & cross-measurement links Demod parameter adjustments More time capture

Advanced & Specific Digital Demod. Demod By Specific Carriers Demod by 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)

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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

New Data Type “Subframe Info”

Mod Types Detected and Listed

• Automatic measurement of individual power, symbol length, error

Preamble Type Listed, FCH Checksum Compared More, Including Detection of Many Signal Malformations

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Pilot Tracking Demodulation Is Adjusted During A Subframe Demodulation is Performed Relative to the BPSK Pilots Some Errors are “Tracked Out” as Demod Follows Pilots Tracking Types can be Enabled/Disabled Independently • Amplitude (default is off) • Phase (default is on) • Timing (default is off) Pilot Tracking Removes Close-In Phase Noise

Amplification Gain Drift (Droop) Possible Causes • Thermal • Power Supply • Gating

Would this error be “tracked out?”

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Timing Errors Caused by • Frequency error in oscillators • Wrong number of samples in guard interval Troubleshooting • Observe CPE when timing tracking is enabled

Adaptive Equalization Training Sequence (Preamble) Provided on All Bursts (downlink & uplink) Equalizer Usually Trained on 2nd Symbol of Long Preamble (100 carriers out of 200) 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

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Linear Distortion and Equalization Causes • IF Filtering • DSP Filtering • ADC Sin(x)/X compensation Troubleshooting • Reposition FFT, observe RCE • Use Data Driven EQ to improve EQ training

Complete Physical Layer Analysis 89600 Series VSA Software (89601A opt. B7S) WiMAX Signals Supported

• All channel BWs, 1.25 to 20 MHz • Uplink and downlink • Auto-detected mod. formats BPSK 64QAM • All frame lengths, guard intervals, Fs/BW values • FDD, TDD, H-FDD modes, incl. non-bursted • Subchannelization supported Broad RF Hardware Support

• 89600S VXI-based • PSA Series spectrum analyzers • 89650S bundle (PSA opt. 122) • Infiniium oscilloscopes • ESA Series spectrum analyzers • E4406 VSA transmitter tester

BW: BW: BW: BW: BW: BW:

36 MHz 10 MHz 80 MHz 0.5-13 GHz 10 MHz 8 MHz

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Physical Layer Signal Source Solutions ESG (E4438C) & PSG (E8267D) Series Vector Signal Generators With N7613A Signal Studio for WiMAX

•Uplink & downlink signals with payloads of fixed patterns, PN9 or PN15 sequences, data files •MAC messaging for rcv’r test •Full control of bursts, incl. FCH, burst, gap •Support of all modulation types •Create frames with FCH and broadcast msg. for rcv’r test •Add real-time noise for receiver sensitivity testing with AWGN option •Can be deployed as TDD, FDD, half-duplex FDD

89601A Software Demo/Training Mode • Available without contacting • • •

Agilent Does not expire Does not interfere with trial license Limited to supplied recordings

At startup if no license present

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Look Ahead to OFDMA--Spectrogram & Pwr Envelope Preamble

Zone 0 Zone 1

Look Ahead to OFDMA--Spectrogram & Zone Maps Preamble

Zone Map 0

Zone 0

Zone Map 1

Zone 1

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Resources Agilent WiMAX Portal: http://www.agilent.com/find/wimax Application Notes “Agilent WiMAX Signal Analysis” Part 1: Literature No. 5989-3037EN Part 2: Literature No. 5989-3038EN Part 3: Literature No. 5989-3039EN

“WiMAX Concepts and RF Measurements” (Literature No. 5989-2027EN)

WiMAX Frequency and Time Parameter Table (Literature No. 5989-2274EN)

RF Design Magazine article on OFDM Troubleshooting by Bob Cutler http://rfdesign.com/mag/radio_effects_physical_layer/

Trial Version 89600 VSA Software, N7613A Signal Studio for WiMAX (Download or contact Agilent)

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