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Flexible OFDM Signal Generation,Analysis and Troubleshooting

Copyright © 2011 Agilent Technologies

Communications Designer’s Ideal

Flexible, Precise Signal Generation– Create ideal, error-free signals– Modify signal characteristics, structure– Create impaired signals– Create test signals– Inject anywhere in block diagram (BB, IF, RF, analog, digital)– Simulated & physical signals


– Simulated & physical signals

Flexible, Sensitive Signal Analysis– Spectrum, vector, measurements– Flexible, accurate, configurable modulation analysis– Highly specific results for troubleshooting– Analyze anywhere in block diagram (BB, IF, RF, analog, digital)– Same analysis, algorithms, UI for simulated and physical signals

2

AgendaAgenda

What is OFDM? OFDM system architecturesRapid waveform development techniquesMeasuring & troubleshooting OFDM modulation qualityAnalyzing proprietary OFDM signals


Analyzing proprietary OFDM signalsHow can Agilent help you?

3

What is OFDM?Orthogonal Frequency Domain Multiplexing

OFDM is a modulation format that achieves:– high data throughput by transmitting on hundreds or

thousands of carriers simultaneously.


– high spectral efficiency by spacing the carriers very closely.

– high data integrity by transmitting at a relatively slow symbol rate.

4

Orthogonal SubcarriersOverlapping Carriers But No Inter-Carrier Interference (Ideally!)


5

Symbol #2

Symbol #3

Symbol #4

OFDM Symbols & Subcarriers Simplified view


Symbol #0

Symbol #1

Time

Freq

-5 -4 -3 -2 -1 0 +1 +2 +3 +5+4Subcarrier Number

……

6

Symbol #2

Symbol #3

Symbol #4

OFDM Symbols & SubcarriersReal world view


Preamble

Symbol #1

Time

Freq


……

7

OFDM vs. Single Carrier ModulationFrequency Domain View

many carriers

BW = #carriers x spacing

OFDM

1 carrierBW =

Sym(1+α)

Single Carrier QAM


#carriers x spacing

Carrier #0 always null

Sym(1+α)

8

OFDM vs. Single Carrier ModulationTime Domain View

Single Carrier 64QAM802.11a OFDM


1 Sym = 1 Sample = .083 usec

Data rate = 54 Mbits/sData rate = 54 Mbits/s

1 Sym = 64 Samples = 4.0 usec

9

Sharing the Resource: OFD MA

+

=

User1 (low rate): 112 subcarriers

“Multi-Access”


+

=

User3 (hi-rate): 448 subcarriers

User2 (med-rate): 280 subcarriers 840 subcarrier signal

10

OFDMA Resource Map

DL B

urst 9

DL B

urst 8

DL Burst 5DL Burst 1

DL Burst 7DL Burst 4DL Burst 3

UL-M

AP

Pream

bleF

CH

DL-M

AP

Sub

carr

iers

Example:802.16e Mobile WiMAX


• Shows allocation of subcarriers by time and frequency.• Subcarriers are usually grouped into logical channels.• Each channel can have different modulation, power level, coding, etc.

Symbol #

DL Burst 2DL Burst 6

DL B

urst 9

DL B

urst 8

11

Summary: How OFDM Achieves its Goals1. High throughput:

• an 800-subcarrier system with 64QAM mapped to each subcarrier can transmit 800 x 8 = 6400 bits per symbol.

2. Bandwidth efficiency: • with DSP techniques (FFT and IFFT), subcarrier spacing can be reduced to

theoretical minimum, i.e. mathematically orthogonal (don’t expect to see individual subcarriers!)

3. Data integrity: multi-subcarrier symbol structur e has advantages: • symbol is long relative to most impulse noise.


• symbol is long relative to most impulse noise.• single-freq interferer only disturbs 1-2 subcarrier s, not entire signal.• built-in amplitude and phase references (pilots) al low signal to be re-

synchronized and/or equalized for each symbol.• symbol can be cyclically extended for multipath immunity:

12

Who Uses OFDM?

WPA

N Bluetooth 1.0WiMedia

802.11b

802.16

WLA

NW

MA

N

802.11a/g 802.11n

2.0 3.0

802.11ad802.11ac

802.16d 802.16e 802.16m


Mob

ileB

road

cast

Analog

1GAnalog

2GGSM, CDMAPDC, PHS

3GWCDMACDMA2K

3.9GFDD-LTETDD-LTE

4GLTE-Adv.

ATSC

DVBISDB

DVB-TISDB-T

DVB-HDAB

13

LTE FeaturesFeature Capability

Access modes FDD & TDD – with same frame structure

Frame structure also aligned with UMTS 1.28 Mcps TDD

Variable channel BW 1.4, 3, 5, 10, 15, 20 MHz

Baseline UE capability 20 MHz UL/DL, 2 Rx, one Tx antenna

User Data rates DL 172.8 Mbps / UL 86.4 Mbps @ 20 MHz BW


14

(2x2 DL SU-MIMO & non-MIMO 64QAM on UL)

Downlink transmission OFDM using QPSK, 16QAM, 64QAM

Uplink transmission SC-FDMA using QPSK,16QAM, 64QAM

DL Spatial diversity Open loop TX diversity

Single-User MIMO up to 4x4 supportable

UL Spatial diversity Optional open loop TX diversity, 2x2 MU-MIMO,

Optional 2x2 SU-MIMO


15

AgendaAgenda




16

OFDM Waveform DesignA System Architect’s view

What do we want to accomplish:• High throughput • Reliable – independent of channel or user• Resistive to other interference• Low PAPR• Low Baseband/DSP resource usage• Compliance (or unique)

What are our design options?• Number of sub-carriers (spacing & bandwidth)• Encoding and Interleaving• Modulation order (mapping types)• Length of CP• Preamble structure• Pilot structure/scheme• MANY more…


Channel Encoding & Interleaving

Mapping Subcarrier Mux

IFFTGuard

InsertionSpectrumShaping

IF/RFDigital IF and D/A

Pilots

17

General OFDM Frame Structure

Idle Preamble 1 Preamble 2 Data 1 Data 2

OFDM symbol

OFDM symbol

OFDM symbol

Block Block Block


symbol symbol symbol

OFDM symbol

OFDM symbol

OFDM symbol

Block Block Block

Idle

Preamble

Data

18

Preamble Structure

preN

postN…Prefix

CPPostfix

CPBlock Block

L

postN


××

=RBlockSize

RDFTSizeL

, if Preamble sequence is defined in frequency domain

, if Preamble sequence is defined in time domain

where R is defined as the repeat times of preamble sequence.

GiLN post ×= GiLN pre ×=,

19

Pilot StructureNo pilot Continuous pilot Scattered pilot

Continuous pilotand scattered pilot


Data subcarrierData subcarrier

pilot subcarrier

Data subcarrier

pilot subcarrier

Data subcarrier

Scattered pilot subcarrier

Pilot Subcarrier

20

Symbol Windowing

•Rectangular pulse shaping can introduce phase discontinuity

•Reduction in multi-path fading immunity

No Window


With Window

21

Payload OFDM Symbol Structure

preN

PrefixCP DFT size

DFTSize

GuardIntervalType=CycleShift


preN

zeros DFT size

DFTSize

GuardIntervalType=Zeros

GiDFTSizeN pre ×= , where Gi is defined as the guard interval in parameter GuardInterval.

22

OFDM Waveform DesignEvaluating the System Architecture

Did we make all the correct choices?• Number of sub-carriers (spacing & bandwidth)• Encoding and Interleaving• Modulation order (mapping types)• Length of CP• Preamble structure• Pilot structure/scheme• MANY more…

Too much out of band

Synch issues

“How does the system perform?”

“What to fix?”




IFFTGuard



Pilots

Easy to disrupt

ICI issues

of bandissues

PAPR is too high for this

amp

Not efficient enough

Channel issue

23

OFDM Waveform DesignEvaluating the System Architecture

Did we make all the correct choices?• Number of sub-carriers (spacing & bandwidth)• Encoding and Interleaving• Modulation order (mapping types)• Length of CP• Preamble structure• Pilot structure/scheme• MANY more…

Better windowing

Preamble adjustment

“How does the system perform?”

“What to fix?”




IFFTGuard



Pilots

Better encoding

Longer CP

windowingadjustment

Employ Clipping

technique

Higher order

Better pilot

structure

24

AgendaAgenda




25

Agilent SystemVue for physical layer designA new environment for system-level modeling and verification

C++C++

General Purpose Embedded

Open algorithmic modeling interface: math/MATLAB, C++, HDL SystemVue: Cross-domain framework for model-based d esign

Fixed-point VHDL / Verilog

Fixed-point VHDL / Verilog

ANSI-C or C++ANSI-C or C++

FPGA/ASIC Embedded

DSPEmbedded

Detailed Specs & Test Benches

Detailed Specs & Test Benches

RF Systems PHY IP


Target FlowTarget FlowRTOS

SynthesisSynthesisTarget ProcessorTarget ProcessorIDE/Tools

ImplementImplement(S/W app)

ImplementImplement(custom H/W)

ImplementImplement(targeted S/W)

Electrical (circuit) Electrical (circuit) Design

Physical Design Physical Design Assembly / Fab

PHY system integration and verification

Mainstream EDA Flows RF EDA Flows

Complete a working PHY using combinations of Software, RF/BB Hardware, Simulation, and Measurements

Test

26

Rapid Waveform Creation & TroubleshootingA System and Algorithm design environment




IFFTGuard



Pilots

27





IFFTGuard



Pilots

28





IFFTGuard



Pilots

29

Rapid Creation Example (continued)


• All OFDM system parameters of source are set in this GUI. The switch of Preamble 1 and Preamble 2, Pilot 1 and Pilot 2 are also set in it

• The number of OFDM symbols of Data 1 and Data 2(if available) payload are set in this GUI

• Symbol windowing function between OFDM symbols is also can be selected or not

30



•The Preamble construction interface shows a multi-format structure

•Specialized indexing

•Multi-format is generally desired (ex. 802.11a)

31


•This GUI shows all parameters of the Data 1 and Data 2 payload (if available).

•Specify Mod type


32



•User interface for Pilot configuration •Most of OFDM communications (IEEE 802 series) have only one kind of pilot (continuous pilot or scattered pilot)

•Power line communication standard (G3-PLC) is an example with no-pilot format

•Wireless video formats (DVB-T, DVBT2 and etc) generally have two kinds of pilot (continuous pilot and scattered pilot)

33

Example: Peak to Average Power RatioDVB-T2 Algorithm

PAPR reduction algorithm

•Reserved carriers or Peak reduction tones

•Virtually distortionless


34

AgendaAgenda




35

Communications Designer’s Ideal

Flexible, Precise Signal Generation– Create ideal, error-free signals– Modify signal characteristics, structure– Create impaired signals– Create test signals– Inject anywhere in block diagram (BB, IF, RF, analog, digital)– Simulated & physical signals


– Simulated & physical signals

Flexible, Sensitive Signal Analysis– Spectrum, vector, measurements– Flexible, accurate, configurable modulation analysis– Highly specific results for troubleshooting– Analyze anywhere in block diagram (BB, IF, RF, analog, digital)– Same analysis, algorithms, UI for simulated and physical signals

36

OFDM Demodulation

1. Isolate waveform for 1 symbol;synchronize in Freq, Time, Phase

2. Perform FFT.3. Map subcarrier I-Q values

back to QAM constellations

4. Convert constellation

....

FFT


4. Convert constellation states to data bits.

.23 + j.71 -.71 + j.23 .71 + j.71

1011 0110 100137

QErrorVector

OFDM Signal Analysis

1. Isolate waveform for 1 symbol; synchronize in Freq, Time, Phase

2. Perform FFT.3. Map subcarrier I-

Q values back to QAM constellations

....

FFT


I

Vector MagnitudeIdeal

Measured

constellations4. Compute

standardconstellation metrics (EVM, SNR, etc.) for each subcarrier in each symbol .23 + j.71 -.71 + j.23 .71 + j.71

How to display? How to display?

38

How to Display OFDM SignalsRMS Avg. vs. Time

Time Domain

One dot per subcarrier.

Symbol number (time)

Meas. Result(e.g. EVM)

RMS Avg. vs. Freq


39

Freq Domain

One dot per symbol.

Subcarrier number (freq)

Meas. Result(e.g. EVM)

RMS Avg. vs. Freq

39

Measuring Modulation Quality


40

89600 Vector Signal Analysis software

Tests the spectrum, time, modulation characteristic s of wireless signals

– Hi resolution FFT-based spectrum measurements

– High quality time tools: PVT, CCDF, pulse, signal capture/play

– Advanced modulation analysis of >70 signal formats

– Works with >30 platforms – signal analyzers, scopes, logic analyzers, simulation software

Target application : Wireless R&D – Evaluation & T-shooting


shootingHelps designers understand signal details so they can fix their toughest PHY layer problems.

Target market: Wireless signaling Cell Comm. Military Comm.

Wireless Networking Satellite Comm.Public Safety Radio Radar Wireless Connection Electronic warfare

41

OFDM Signal Troubleshooting

Case Studies1. Amplitude & Phase Drift2. Timing Errors3. Spurious Interference4. Clipping


4. Clipping5. I-Q Errors

42

OFDM Troubleshooting Case Study #1: Amplitude & Phase Drift


43

Pilot Tracking Compensates (Hides) Impairments


44

CPE Display Shows the Defect Removed

~1 dB of am-plitude droopin 240 uSec.


EVM looksfine with pilottracking ON.

45

OFDM Troubleshooting Case Study #2: The V-Shaped EVM Plot

Pilot phase track is ON,

so this can’t


so this can’t be phase

noise.

46

OFDM Troubleshooting Case Study #2: The V-Shaped EVM Plot


Remember: dots correspond to a point in time and

frequency

Error increases linearly as a function of frequency offset

Several potential causes:• I-Q time offset • Symbol clock error

47

OFDM Troubleshooting Case Study #3: Single-Frequency Interference

Only carrier -24 has high EVM


Impact on RMS EVM isn’t very high

48

OFDM Troubleshooting Case Study #3: Single-Frequency Interference

Set to analyzecarrier –24 only.


EVM constellation is square, rotated and off-center.

49

Interpreting the Error Vector Constellation

• Square shape indicates that entire • Origin Offset indicates the “real” error,

• Subcarrier -24 only• EVM vs. Time• Polar (I-Q) axes• Zoomed scale


• Square shape indicates that entire signal constellation is multiplied by a constant

• Rotation indicates that it is a complex constant

• Conclusion: the equalizer is setting the wrong value for this bin. This is an effect -- not a cause

• Origin Offset indicates the “real” error, i.e. an extra signal in this bin

• Stable display = interferer is coherent with the OFDM signal (spur?)

• Conclusion: an extra signal on-bin at carrier –24; it’s confusing the equalizer at this bin

50

OFDM Troubleshooting Case Study #4: Power Amp Clipping

Minor clippingNo clipping


51

OFDM Troubleshooting Case Study #5: I-Q Errors


Clue: EVM of pilots is high, but with little symbol-to-symbol variation.

52

Clue! BPSK pilots have split into two vertical dots� I-Q Quadrature Skew

OFDM Troubleshooting Case #5: Impact of 5 Degrees of I-Q Quadrature

Skew


Looks like BPSK with 90degree phase rotation.

Concept: I-Q errors create images of the original signal.

53

I-Q Quadrature ErrorsCreate a scaled version of the original signal at image frequency.

Sinewave case:

Perfect I-Q balance

-F +F00o

f

OFDM case (BPSK pilots):

+N–N Fc Normal signal at carrier +N

Carrier -N’s error signal is at carrier +N


I-Q balance

Imperfect I-Q balance

-F +F0

Phase depends on type of error.

90of ΣΣΣ

QResulting signal at

carrier +NI

Q

I

Q

I

54

Resource Allocation, I-Q Accuracy

I-Q Errors Produce Energy at Symmetric FrequenciesExplore Resource Allocation, Transmitter PowerCreate Test Signals for Troubleshooting


55

OFDMA--Spectrogram & Pwr Envelope

24 pt level 1– 20 pt level 2

• 18 pt level 3

Understanding resource allocationand power, without demodulation

Preamble

Zone 0

Copyright © 2011 Agilent TechnologiesZ

one 1

OFDMA--Spectrogram & Zone MapsPreamble

Zone 0

Zone M

ap 0Does actual resource allocationmatch the plan?


Zone 1

Zone M

ap 0Z

one Map 1

AgendaAgenda




58

Analyzing Proprietary OFDM Signals

-5 -4 -3 -2 -1 0 +1 +2 +3 +5+4……

Preamble

Pilot

Data


Demodulator needs to know:• basic time, freq and FFT parameters.• which subcarriers are pilots?• which subcarriers are preambles?• what are the expected I-Q values for each preamble and pilot subcarrier?• what is the expected modulation format for each data subcarrier?


……

59


Agilent 89600 VSA Main window


Custom OFDMConfiguration Window

60


Basic FFT

Load Config File:Pilot IQ Values

Load Config File:Subcarrier Types

Load Config File:Preamble IQ Values

Load Config File:Subcarrier Modulation


Basic FFTParameters

61

Configuration Files

Example:ResourceMap.txt Describes function of each subcarrier, every symbol.

Symbol #0Symbol #1

Symbol #2


Symbol #2Symbol #3

Symbol #4

Symbol #5

Subcarrier type: 0 = data 1 = pilot 3 = preamble 4 = null repeat

62

Configuration Files

Configuration Files– Resource Modulation.txt – Describes modulation format for each

subcarrier.– Preamble I-Q.txt – expected IQ value for each preamble subcarrier.– Pilot I-Q.txt – expected IQ value for each pilot.


Features to simplify configuration: – Auto-detect pilot I-Q – can eliminate Pilot I-Q file– Auto-detect data subcarrier modulation format – simplify Resource

Mod file– Loop continuously through last N symbols – shorter config files– Modulation format table – modify all data subcarrier modulation formats

simultaneously, by changing one value in table.63

AgendaAgenda




64

OFDM for Simulation or Hardware Test

W1461 SystemVue

Custom OFDM source

Source

Test Waveform

Analyzer

DUT


VSA 89600BSimulation

OFDM Resource configuration info

CommonDemod / Analysis

IdlePreamble

1Preamble

2Data 1

PayloadData 2

Payload

65

Additional design support for hardware verification

Leverage design environment for apps that “fall between”

OFDM, MIMOLTE-Advanced

WNW, Defense

Jamming, InterfereClutter, TargetsRF, Phase NoiseCognitive environments

NON-STDWAVEFORMS

FADING,IMPAIRMENTS


FILL HOLESThroughput

Coded BERDPD

MULTI-BOXCOORDINATION Digital vs. RF interfaces

Missing test coverageMissing user hardware

66

Connecting SystemVue to instrumentation

TCP/IPSCPI

File I/O

Modeling InterfacesRF impairmentsScriptingReference IP


.m C++

VHDL/Verilog

Signal Studio

(licensed)

VSAvisualization,connectivity

FlexDCAvisualization

(free with SV)

Agilent I/O LibConnectivity

(free with SV)

PSA/PXA/MXAInfiniuum/DCAPXI/ModularSimulation

PSA/PXA/MXAInfiniuum/DCAPXI/ModularSimulation

Any test H/WCustomer Test EquipmentCustomer Virtual Platform

Simulators & Apps

Any test H/WCustomer Test EquipmentCustomer Virtual Platform

Simulators & Apps

ESGMXGPSGArbs

ESGMXGPSGArbs

PNA-XX-parameters

67

Summary

For more information about flexible OFDM– App note: http://cp.literature.agilent.com/litweb/pdf/5990-6998EN.pdf

For more information about Agilent SystemVue– OFDM demonstration:


– OFDM demonstration: http://www.youtube.com/watch?v=IFtCuKKi8Jw

– SystemVue for OFDM: http://www.agilent.com/find/eesof-systemvue-ofdm

For more information about Agilent VSA– http://www.agilent.com/find/89600B

68

Q & A Q & A


69

Thank you!


Thank you!

70

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