OFDM Signal Analysis,

Creation and

Troubleshooting

Presented by:

Riccardo Giacometti

Roberto Sacchi

Agenda

What is OFDM?

OFDM system architectures

Measuring & troubleshooting OFDM modulation quality

Case study 1: LTE

Case study 2: LTE-Advanced

Case study 3: Custom OFDM

2

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.

3

Orthogonal Subcarriers

Overlapping Carriers But

No Inter-Carrier Interference (Ideally!)

Frequency domain analog of zero inter-symbol interference (impulse response)

4

Symbol #0

Symbol #1

Symbol #2

Symbol #3

Symbol #4

Time

Freq

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

……

OFDM Symbols & Subcarriers Simplified view

5

Preamble

Symbol #1

Symbol #2

Symbol #3

Symbol #4

Time

Freq

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

……

OFDM Symbols & SubcarriersReal world view

6

OFDM vs. Single Carrier Modulation

Frequency Domain View

many carriers

BW =

#carriers x spacing

Adj Chan =

Normal Rolloff

Carrier #0

always null

OFDM

1 carrier

BW =

Sym(1+ )

Adj Chan =

Distortion

Single Carrier QAM

7

OFDM vs. Single Carrier ModulationTime Domain View

Single Carrier 64QAM

1 Sym = 1 Sample = .083 usec

Data rate = 54 Mbits/s

@ ¾ coding = 72 Mbits/s

@ 6 bits/sym = 12 Msym/s

Data rate = 54 Mbits/s

@ ¾ coding = 72 Mbits/s

@ 6 bits/sym = 12 Mbits/s

@ 48 carriers = 250 Ksym/s

1 Sym = 64 Samples = 4.0 usec

802.11a OFDM

8

Sharing the Resource: OFDMA

+

+

=

User1 (low rate): 112 subcarriers

User3 (hi-rate): 448 subcarriers

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

“Multi-Access”

9

Who Uses OFDM?W

PA

N Bluetooth 1.0

WiMedia

802.11b

802.16

Mo

bile

WL

AN

WM

AN

Bro

ad

cast

802.11a/g 802.11n

2.0 3.0

802.11ad

802.11ac

802.16d 802.16e 802.16m

Analog

1G

Analog

2G

GSM, CDMA

PDC, PHS

3G

WCDMA

CDMA2K

3.9G

FDD-LTE

TDD-LTE

4G

LTE-Adv.

ATSC

DVB

ISDB

DVB-T

ISDB-T

DVB-H

DAB

10

OFDMA Resource Map

• 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

urs

t 9

DL B

urs

t 8DL Burst 5

DL Burst 1

DL Burst 7DL Burst 4DL Burst 3

UL

-MA

P

Pre

am

ble

FC

HD

L-M

AP

Subcarr

iers

Example:

802.16e

Mobile WiMAX

11

Wireless evolution: Five competing 3.9G systems

Page 12Page 12

2GIS-136TDMA

PDCGSMIS-95Acdma

Inc

reasin

g e

ffic

ien

cy,

ba

nd

wid

th a

nd

data

rate

s

IS-95Bcdma

HSCSD iMode2.5G GPRSIS-95Bcdma

3GE-GPRSEDGE

IS-95Ccdma2000

W-CDMAFDD

W-CDMATDD

TD-SCDMALCR-TDD

3.5GHSUPAFDD & TDD

1xEV-DORelease B

1xEV-DORelease A

1xEV-DORelease 0

HSDPAFDD & TDD

3.9G3.9GLTEE-UTRA

EDGE Evolution HSPA+

802.16eMobileWiMAXTM

UMBcf 802.20

802.11g

802.11a

802.11b

802.16dFixed WiMAXTM

802.11n

802.11h

WiBRO

New OFDM Systems! New OFDM System!

Case study 1: LTE

13

14

Physical Layer Definitions: Frame Structure

Frame Structure type 1 (FDD) FDD: Uplink and downlink are transmitted separately

#0 #2 #3 #18#1 ………. #19

One subframe = 1ms

One slot = 0.5 ms

One radio frame = 10 ms

Subframe 0 Subframe 1 Subframe 9

Frame Structure type 2 (TDD)One radio frame, Tf = 307200 x Ts = 10 ms

One half-frame, 153600 x Ts = 5 ms

#0 #2 #3 #4 #5

One subframe, 30720 x Ts = 1 ms

DwPTS

Guard period

UpPTS

One slot, Tslot =15360 x Ts = 0.5 ms

#7 #8 #9

DwPTS

Guard period

UpPTS

15

Condition (DL) NRBsc NUL

symb

Normal

cyclic prefix∆f=15kHz 12 7

Extended

cyclic prefix

∆f=15kHz 12 6

∆f=7.5kHz 24 3

RB

scN

RB

scN

OFDM symbols

One slot, Tslot

:

:

x subcarriers

Resource block

x

Resource

element

(k, l)

l=0 l= – 1

subcarriers

RB

scN

DLsymbN

DLsymbN

DLsymbN

RB

scN

Condition (UL) NRBsc NUL

symb

Normal

cyclic prefix12 7

Extended

cyclic prefix12 6

Slot Structure & Physical Resource Elements

16

OFDM symbols (= 7 OFDM symbols @ Normal CP)

The Cyclic Prefix is created by prepending each

symbol with a copy of the end of the symbol

160 2048 144 2048 144 2048 144 2048 144 2048 144 2048 144 2048 (x Ts)

1 sub-frame= 2 slots

= 1 ms

1 slot= 15360 Ts

= 0.5 ms

0 1 2 3 4 5 6

etc.

CP CP CP CPCPCP

DL

symbN

Downlink Frame Structure Type 1

RS - Reference Signal (Pilot)

P-SS - Primary Synchronization Signal

S-SS - Secondary Synchronization Signal

PBCH - Physical Broadcast Channel

PCFICH – Physical Control Channel Format Indicator Channel

PHICH (Normal)– Physical Hybrid ARQ Indicator Channel

PDCCH (L=3) - Physical Downlink Control Channel

PDSCH - Physical Downlink Shared Channel

#0 #1 #8#2 #3 #4 #5 #6 #7 #9 #10 #11 #12 #19#13 #14 #15 #16 #17 #18

10 2 3 4 5 610 3 4 5 62

Time (Symbol)

LTE Air Interface: Downlink Physical Signals

BaseStation

(eNB)

UserEquipment

(UE)

P-SS - Primary Synchronization Signal

RS – Reference Signal (Pilot)

P-SS:

- Used in cell search and initial synchronization procedures

- Carries part of the cell ID (one of 3 sequences) and identifies 5 ms timing

- Transmitted on 62 out of the reserved 72 subcarriers (6 RBs) around DC at

OFDMA symbol #6 of slot #0 & #10

- Modulation sequence = One of 3 Zadoff-Chu sequences

S-SS:

- Used to identify cell-identity groups. Also identifies frame timing (10 ms)

- Carries remainder of cell ID (one of 168 binary sequences)

- Transmitted on 62 out of the reserved 72 subcarriers (6 RBs) around DC at

OFDMA symbol #5 of slot #0 & #10

- Modulation sequence = Two 31-bit binary sequences; BPSK

RS:

- Used for DL channel estimation and coherent demodulation

- Transmitted on every 6th subcarrier of OFDMA symbols #0 & #4 of every slot

- Modulation sequence = Pseudo Random Sequence (PRS). Exact sequence

derived from cell ID, (one of 3 * 168 = 504).

S-SS - Secondary Synchronization Signal

Multiple Antenna Techniques

•Multiple antenna techniques are fundamental to LTE.

•Five multi-antenna techniques have been defined for LTE to

improve eNB performance.

• Receive diversity at the mobile

• Transmit diversity at the eNB

• MIMO spatial multiplexing at the eNB

• Cyclic Delay Diversity (CDD) at eNB – used with spatial multiplexing

• Beamsteering (user specific)

MIMO TechniquesSpatial Diversity

Transmit

Spatial Diversity with

Space Time Coding

(STC)

Redundant data

improves signal quality

No increase in data

rate

combine

,switch

or

beam-

form

S0, S1, S2,

Symbols

S0, S1, S2,

r0, r1, …

r0, r1, …

S0, S1,…

h0

h1

Receive

Spatial Diversity

Reduces amplitude

variations due to fading

to improve signal quality

No increase in data

rate

Multiple Antennas Improve Signal Quality

Base Station

Antenna

Subscriber#1

#2Antenna Diversity Improves

Received Average SNR

MIMO TechniquesSpatial Multiplexing

Increases data rate and capacity

Two symbol transmitted and received simultaneous

Requires rich multipath environment so that h00, h01, h10, h11 have

low correlation

To decode it requires knowledge of the channel: h00, h01, h10, h11

S0, S1 are solved using two equations, two unknowns

MIMO only works when the channel is rich in multipath

(low channel correlation)

Vertical

Polarization

Horizontal

Polarization

Angular Spread

Antenna PolarizationAntenna Spacing Antenna Pattern

MIMO TechniquesSpatial Correlation

Spatial Correlation exists due to

• Angular Spread

• Antenna Spacing

• Antenna Pattern

• Antenna Polarization

W1910 LTE Baseband Verification LibraryCreate an algorithmic reference for own signal processing chain

LTE

TurboECODER

SRS_SF_Config=0

SRS_Enable=NO

PUCCH_PUSCH=PUSCH

NumTxAnts=Tx1

PDCCH_SymsPerSF=3;3;3;3;3;3;3;3;3;3 [[3 3 3 3 3 3 3 3 3 3]]

RB_Alloc=0;25 [[0 25]]

RB_AllocType=StartRB + NumRBs

CyclicPrefix=Normal

Bandwidth=BW 5 MHz

SpecialSF_Config=Config 4

TDD_Config=Config 0

FrameMode=FDD

NumChBits=Real Array (1x10) [[5640 5640 5640 5640 5640 5640 5640 5640 5640 5640]]

ChBit_Config=REs per subframe

NumOfLayers=1

MIMO_Mode=Spatial_Mux

MappingType=0;0;0;0;0;0;0;0;0;0 [[0 0 0 0 0 0 0 0 0 0]]

Payload=Real Array (1x10) [[2555 2555 2555 2555 2555 2555 2555 2555 2555 2555]]

Payload_Config=Transport block size

LinkDir=Downlink

L2 {LTE_TurboCoder@LTE Models}

Capture bit-true

test vectors anywhere in

the signal processing chain

Fully parameterized

algorithmic

reference modelsCompare with your implementation:

Math, C++, Fixed Pt, VHDL, Verilog

LTE Downlink Coded 2x2 MIMO example with

BER/BLER waterfalls vs noise/fading

2x2 MIMO

baseband coded source2x2 MIMO

baseband receiver

RF CHANNEL

with noise/fading

DATA

DATA

Case study 2: LTE Advanced

25

ITU-R IMT-Advanced / LTE-Advanced timeline

January 2011

SystemVue LTE-Advanced Library

26

3GPP submission to ITU-R

Sept 2009

TR36.912 v 2.2.0

R1-093731, Characteristic template

R1-093682, Compliance template

R1-093741, Link Budget template

Comparing LTE and LTE-Advanced to

IMT-Advanced

February 2011 Page 27

LTE-Advanced

LTE LTE-Advanced IMT-Advanced Peak

Data Rate

DL 300 Mbps 1 Gbps 100 Mbps

(high mobility)

1 Gbps

(low mobility)

UL 75 Mbps 500 Mbps

Peak

Spectrum

Efficiency

[bps/Hz]

DL 15 30 15

UL 3.75 15 6.75

Tx Bandwidth UL & DL Up to 20 MHz Up to 100 MHz Up to 40 MHz

MIMO

(spatial multiplexing)

DL Up to 4x4 Up to 8x8 Up to 4x4

UL N/A Up to 4x4 Up to 2x4

What’s New: LTE-Advanced at a Glance

February 2011

LTE-Advanced

Page 28

1

2

3

Carrier aggregation

• Support for up to 5 Aggregated Carriers

• Up to 100 MHz Bandwidth

Enhanced uplink multiple access

• Clustered SC-FDMA

• Simultaneous Control and Data

Higher order MIMO

• Downlink 8x8

• Uplink 4x4

Further items being studied

January 2011

SystemVue LTE-Advanced Library 29

1Coordinated

Multipoint2 Relaying

5 Self Optimizing Networks (SON)

6 Heterogeneous Networks

4Customer Premises

Equipment (CPE)CPE

LTECPE

LTE

Indoor CPE scenario Outdoor CPE scenario

CPELTE

CPELTE

Indoor CPE scenario Outdoor CPE scenario

3HeNB mobility

enhancements

SystemVue takes you from Algorithm to

Verification for LTE-Advanced

W1918 LTE-Advanced baseband

verification library

• Validate early 3GPP Rel10

architectures at the link level

• Use a trusted, independent IP

reference from Agilent

• Reduce the amount of “throw-

away” IP and scripting needed,

while accelerating design

• Quantify the effect of MIMO

capabilities early, using simulation

• Interoperate with test equipment,

even while the Standard is still

evolving

January 2011

SystemVue LTE-Advanced Library

30

SystemVue LTE-Advanced: Carrier Aggregation

Scenario number

Deployment scenarioTransmission BWs of LTE-A carriers

# of LTE-A component carriersBands for LTE-A

carriersDuplex modes

1Single-band contiguous spec. alloc. @ 3.5GHz band for FDD

UL: 40 MHz

DL: 80 MHz

UL: Contiguous 2x20 MHz CCsDL: Contiguous 4x20 MHz CCs

3.5 GHz band FDD

2Single-band contiguous spec. alloc. @ Band 40 for TDD

100 MHz Contiguous 5x20 MHz CCsBand 40 (2.3 GHz)

TDD

4

Single-band, non-contiguous spec. alloc. @ 3.5GHz band for FDD

UL: 40 MHz

DL: 80 MHz

UL: Non-contiguous 1x20 + 1x20 MHz CCsDL: Non-contiguous 2x20 + 2x20 MHz CCs

3.5 GHz band FDD

Component

Carrier 1

Component

Carrier 2

Component

Carrier 3

Component

Carrier 4

31 January 2011SystemVue LTE-Advanced Library

SystemVue LTE-Advanced: Carrier Aggregation

January 2011

SystemVue LTE-Advanced Library

32

Scenario 1 FDD DL Scenario 2 TDD DL

Scenario 4 FDD DL Scenario 4 FDD UL

SystemVue LTE-Advanced: Downlink Throughput

Throughput measurements, up to 8x8, w/active HARQ

ACK/NACK

LTE-Advanced

Channel Model

LTE-Advanced

Downlink

MIMO source

LTE-Advanced

Downlink MIMO

Receiver

33

SystemVue LTE-Advanced Library

January 2011

Case study 3: Custom OFDM

34

Channel

Encoding &

InterleavingMapping

Subcarrier

MuxIFFT

Guard

Insertion

Spectrum

ShapingIF/RF

Digital IF

and D/A

Pilots

Rapid Waveform Creation & Troubleshooting

A System and Algorithm design environment

35

OFDM Signal Troubleshooting

Examples

1. Amplitude & Phase Drift

2. Timing Errors

36

OFDM Troubleshooting

Amplitude & Phase Drift

37

Pilot Tracking Compensates (Hides) Impairments

38

CPE Display Shows the Defect Removed

~1 dB of am-

plitude droop

in 240 uSec.

EVM looks

fine with pilot

tracking ON.

39

OFDM Troubleshooting

The V-Shaped EVM Plot

Pilot phase

track is ON,

so this can’t

be phase

noise.

40

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

OFDM Troubleshooting

The V-Shaped EVM Plot

41

Analyzing Proprietary OFDM Signals

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?

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

……

Preamble

Pilot

Data

42

Analyzing Proprietary OFDM Signals

Agilent 89600 VSA

Main window

Custom OFDM

Configuration Window

43

Analyzing Proprietary OFDM Signals

Basic FFT

Parameters

Load Config File:

Pilot IQ Values

Load Config File:

Subcarrier Types

Load Config File:

Preamble IQ Values

Load Config File:

Subcarrier Modulation

44

OFDM for Simulation or Hardware Test

W1461 SystemVue

Custom OFDM source

Source

Test Waveform

VSA 89600BSimulation

Analyzer

OFDM Resource configuration info

Common

Demod / Analysis

DUT

IdlePreamble

1

Preamble

2

Data 1

Payload

Data 2

Payload

45

Additional design support for hardware verificationLeverage design environment for apps that “fall between”

FILL

HOLES

OFDM, MIMO

LTE-Advanced

WNW, Defense

Jamming, Interfere

Clutter, Targets

RF, Phase Noise

Cognitive environments

Throughput

Coded BER

DPD

MULTI-BOX

COORDINATION Digital vs. RF interfaces

Missing test coverage

Missing user hardware

NON-STD

WAVEFORMS

FADING,

IMPAIRMENTS

46

Connecting SystemVue to instrumentation

.m

C++

VHDL/Verilog

Signal

Studio(licensed)

VSAvisualization,

connectivity

TCP/IP

SCPI

FlexDCAvisualization

(free with SV)

Agilent I/O LibConnectivity

(free with SV)

PSA/PXA/MXA

Infiniuum/DCA

PXI/Modular

Simulation

Any test H/W

Customer Test Equipment

Customer Virtual Platform

Simulators & Apps

File

I/O

ESG

MXG

PSG

Arbs

Modeling Interfaces

RF impairments

Scripting

Reference IP

PNA-XX-parameters

47

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: 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/89600B48

Q & A

49

Thank you!

50