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An Introduction to

Orthogonal Frequency Division Multiplex

Technology

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Agenda

Part One OFDM and SISO radio configurations SISO Single Input Single Output Radio Topology

Why use OFDM? Digital Modulation Overview

Multi-path Issues

OFDM and WLAN

OFDMA and WiMAX

Test Equipment Requirements Part Two OFDM and MIMO radio configurations

MIMO Multiple Input Multiple Output Radio Topology

MIMO and WLAN

MIMO and WiMAX

Beam Forming Test Equipment Requirements

Conclusion Technology Overview and Test Equipment Summary

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What is SISO?

Single-Input Single-Output

Traditional SISO Architecture

RadioMAC Radio MAC

One radio, only one antenna used at a time (e.g., 1 x 1 ) Antennas constantly switched for best signal path

Only one data stream and a single data channel

DataData

Single Data Channel

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System Standards using OFDM

Wireless

IEEE 802.11a, g, j, n (WiFi) Wireless LANs

IEEE 802.15.3a Ultra Wideband (UWB) Wireless PAN IEEE 802.16d, e (WiMAX), WiBro, and HiperMAN Wireless MANs

IEEE 802.20 Mobile Broadband Wireless Access (MBWA)

DVB (Digital Video Broadcast) terrestrial TV systems: DVB-T, DVB-H, T-DMB

and ISDB-T DAB (Digital Audio Broadcast) systems: EUREKA 147, Digital Radio Mondiale,

HD Radio, T-DMB and ISDB-TSB

Flash-OFDM cellular systems

3GPP UMTS & 3GPP@ LTE (Long-Term Evolution), and 4GWireline

ADSL and VDSL broadband access via POTS copper wiring

MoCA (Multi-media over Coax Alliance) home networking

PLC (Power Line Communication)

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Why Orthogonal Frequency Division Multiplex?

High spectral efficiency provides more data services.

Resiliency to RF interference good performance in

unregulated and regulated frequency bands

Lower multi-path distortion works in complex indoorenvironments as well as at speed in vehicles.

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Spectrally Efficiency OFDM

4G/LTE(OFDM)

2G(GMSK)

3G(CDMA)

4.0

0.5

2.0

Bits/Second

/Hz

WiMAXGSM WLAN802.11a/g

W-CDMAHSDPA

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Why OFDM?

Resiliency to RF interference.

The ISM Band (Industrial Scientific and Medical) is a set of

frequency ranges that are unregulated.

Most popular consumer bands

915MHz Band (BW 26MHz)

2.45GHz Band (BW 100MHz)

5.8GHz Band (BW 100MHz)

Typical RF transmitters in the ISM band include

Analog Cordless Phones (900MHz)

Microwave Ovens (2.45 GHz)

Bluetooth Devices (2.45GHz)

Digital Cordless Phones (2.45GHz or 5.8GHz)

Wireless LAN (2.45GHz or 5.8GHz).

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The Multi-Path ProblemExample: Bluetooth Transmitter & Receiver

TX1 RX1

Ceiling

Floor

Maximum time for signal

To travel D (Distance)

Dmultipath > Ddirect

TX to RX < 1us

Symbol Rate = 1MSymbols/s

Symbol Duration = 1/1E6 = 1us

Ddirect

Maximum Symbol Delay < 1us

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Single Carrier Single Symbol

Bluetooth, GSM, CDMA and other communications standards

use a single carrier to transmit a single symbol at a time.

Data throughput is achieved by using a very fast symbol

rate.

W-CDMA - 3.14 Msymbols/secBluetooth 1 Msymbols/sec

A primary disadvantage is that fast symbol rates are more

susceptible to Multi-path distortion.

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Slow the symbol rateReduce the previous examples symbol rate by a third

TX1 RX1

Ceiling

Floor

Maximum time for signal

To travel D (Distance)

Dmultipath > Ddirect

TX to RX < 3.3us

Symbol Rate = 300kSymbols/s

Symbol Duration = 1/300 = 3.3us

Ddirect

But the data throughput is reduced!

Maximum Symbol Delay < 3.3us

k ithl

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Improve the throughput -

use more than one carrier!

802.11a-g WLAN example

Q

I

Q

I

Q

312.5kHz

I

312.5kHz

I

Q

312.5kHz

I

Q

312.5kHz

Low symbol rate per carrier * multiple carriers = high data rate

250 kbps symbol rate * 48 sub-carriers * 6 coded bits /sub-carrier * coding rate = 54 Mbps

( for 64QAM )

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Sub Carrier Spacing

The sub-carriers are spaced at

regular intervals called the

sub-carrier frequency spacing

(F). The sub-carrier frequency

relative to the center frequencyis k F where k is the sub-carrier number.

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Symbol to WaveformTraditional Serial Symbol Transmissions

I

Q

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Symbol to Waveform

OFDM Parallel Symbol Transmissions

f1

f2

f3

f4

fn

OFDM Symbol Period

Multiple carriers will transmit multiple symbols in parallel.

Carriers may have different modulations BPSK, QPSK 64QAM.

I waveform

Qwaveform

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The OFDM Radio

Digital

Section

(ASIC/

FPGA)

Digital I and Q

Bus

D/A

D/A

Filter

Filter

90o Sum PA

Mixer

Local

Oscillator

(LO)

IF

Modulator

Digital

Section

(ASIC/

FPGA)

A/D

A/D

Filter

Filter

90o PA

Mixer

Local

Oscillator

(LO)

IF

Demodulator

IFFT

FFT

TX RF/uW

RX RF/uW

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Key Measurements: Constellation and EVM

Pilot Symbols

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Vector Signal Analyzer MeasurementsModulation Quality Analysis

Input Signal Analyzer Display Measurement Analysis

I

Q

I

Q

Q

I

QPSK

8PSK

16QAM

etc

Error Vector

MagnitudeRatio of Measured Amplitude

to Intended Amplitude (% or dB)

Magnitude Error = IQ Error Magnitude

Measured

Signal

Intended Signal

Phase Error = IQ Error Phase

Q

I

Unit Circle

RCE (dB) = 20 log(EVM in %/100)

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EVM Metrics

P = RMS Power

Source: Wikipedia

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Introduction to Constellation Diagrams

A constellation diagram is a representation of a digital modulation

scheme in the complex plane.

The real and imaginary axes are often called the in phase, or I-axis and thequadrature, or Q-axis.

Example: four-symbol Quadrature Phase Shift Keying (QPSK)

modulator

01 = 2 = 3/4

00 = 1 = /4

10 = 4 = /4

11 = 3 = 3/4

00

11 10

01

Quadrature Phase Shift Keying (QPSK)

Data 01 00 11 00 01 10(t)

Q

I

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Recovering the DataQPSK and 16QAM Signals

1

I

+1V

00

11 10

01

-1V

-1V

Q +1V

Constant

Magnitudecircle

Assign

Voltages

1100110111101111

1000100110101011

0100010101100111

0000000100100011 Magnitude

Changes

-1V

Q +1V

I

+1V-1VNoise

QPSK0.707SI(t)

-0.707

-1

0.707

1

SQ(t)-0.707

-1

01 00 11 00 01 10

0.5

-0.5-1

1

0.5

-1

1

0100 1100 0110

16QAMSI(t)

SQ(t)

-0.5

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Digital Modulation Overview

I and Q Components of a Signal

AI

Q

Q

I

Q

90o

ISum

Simple PythagorasAmplitude (Length of A) = SQRT (I2 + Q2)

Phase (Angle ) = tan-1 (Q/I)

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Representation of Signal in Complex Plane

I axis

(Real)

Q-axis

(Imaginary)

A

frequencysignalfc =

Asin

Acos

Independent Signal 1 Independent Signal 2

S(t) = A cos(2fct + )

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Quadrature Modulator Hardware

carrier

filter

filter

Asin x sin

Acos x cos

Acos x cos - Asin x sin = Acos(+)

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Quadrature Demodulation

The receiver can recover the two independently modulatedsignals, even though they share the same carrier frequency.

Q

I

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Vector Signal Analyzer MeasurementsModulation Quality Analysis

Input Signal Analyzer Display Measurement Analysis

I

Q

I

Q

Q

I

QPSK

8PSK

16QAM

etc

RF

Input

Mixer

Local

Oscillator

IQ DemodDigital IF

Error VectorMagnitude

Ratio of Measured Amplitude

to Intended Amplitude (%)

Magnitude Error = IQ Error Magnitude

Measured

Signal

Intended Signal

Phase Error = IQ Error Phase

Q

I

Unit Circle

RCE (dB) = 20 log(EVM in %/100)

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I/Q Modulator Impairments

Contributions to EVM

Baseband

generator

GI

90+e

Ioffset

Qoffset

LO

I

Q

Modulator

imbalance

Origin

Offset

Quadrature

Error

RFc

Carrier

Phase Shift

Baseband

generator

GI

90+e

Ioffset

Qoffset

LO

I

Q

Modulator

imbalance

Origin

Offset

Quadrature

Error

RFc

Carrier

Phase Shift

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Origin Offset Example

16-QAM Constellation

I

Q

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Quadrature Error Examples

QPSK Constellations

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Modulator Imbalance Examples

QPSK Constellations

Excess I gain andreduced Q gain relative

to ideal constellation

Ideal Excess I Gain

Excess Q Gain

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Power Amplifier Nonlinearity

Another Contributor to EVM

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ISI Inter Symbol Interference

1100110111101111

1000100110101011

0100010101100111

00000001

Noise

Expected

Symbol Position

Delayed Received

Symbol

Q +1V

0011 0010

Symbol

Boundary

I+1V

-1V

-1V

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Constellation Display

Is a Composite of all OFDM Sub-carrier Symbols

fnIndividual Sub-carriers

f5f4

f3f2

f1

QPSK

Constellation

Display at T = t0Q

I

16QAM

BPSK

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Constellation Display

Is a Composite of all OFDM Sub-carrier Symbols and time

t2

t0

Q

I

Q

I

Q

I

Q

I

Q

I

Q

I

Q

I

Q

I

Q

I

Q

I

Q

I

Q

I

Symbols vs. Time

tn

t4t3

t2t1

Constellation

DisplayQ

I

t1

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Data and Pilot Carriers

Used as reference for phase and amplitude to demodulate

the data in the other sub-carriers.

Carriers -1 to -n Carriers 1 to n

Sub-carrier -1

Pilot

Carriers

Sub-carrier 1

Frequency

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Example: WLAN (802.11a/g)

Modulation Technique OFDM

Bandwidth 16.25MHz

Number of sub-carriers 52

Sub-carrier numbering -26 to + 26

Sub-carrier spacing 312.5kHz

Maximum sub-carrier symbol rate 250 kHz (64QAM)

Pilot sub-carriers -21, -7, +7 and +21 (BPSK)

Packet Structure Preamble Header Data Block

SUB Carrier Modulation Types - BPSK, QPSK, 16-QAM or64-QAM

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WLAN Summary

WLAN implies Wireless LAN

compatible with one the IEEE

802.11 sub standards. It iswhat you have in your laptop.

WiFi is an industry consortium

that defines a required subset

of 802.11 to ensure better

operation between different

vendors equipment.

EWC is an industry consortium

that took the unfinished N

standard, agreed upon aversion, and is attempting to

field solutions prior to 802.11n

ratification.

Not a finished standard yet.

Like g, but up to 600Mbps

OFDM

MIMO

20 & 40 MHz channels

n

Japanese version of g that

uses half the sample rate.j

What you can easily buy now

same as a, but at 2.4 GHz

g

11 Mbps CCK, 2.4 GHz

(Legacy, not OFDM)b

54 Mbps OFDM, 5.9 GHz

Band, 20 MHz channelsa

Means802.11

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demo

WLAN OFDM - Test Equipment Requirements

Frequency Coverage up to 5.8GHz

Modulation Bandwidth up to 16.25MHz

802.11a/g Signal Creation and Analysis Capability

Keithley instruments 2820 and 2920 VSA and VSG have a frequency

range of 6 GHz and 40 MHz bandwidth as standard.

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OFDM to OFDMA

OFDM as a modulation technique is not multi user* all

sub-carriers in a channel are used to facilitate a single link.

OFDMA assigning different number of sub-carriers to

different users in a similar fashion as in CDMA.

* 802.11 WLAN supports multiple users with FDMA (frequency-division multiple access)

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WiMAX 802 16d/

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WiMAX 802.16d/e

OFDMA

Worldwide Interoperability forMicrowave Access

Fixed 802.16d, point-to-point backhaul applications

Mobile 801.16e, enhanced data services mobile

applications.

WiMAX enhanced

Mobile Devices

BTS

Mobil

e

Fixed

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RF Characteristics 802.16e Mobile WiMAX

IFFT size determines carriers Carriers

GuardBa

nd

GuardBand

Pilot

Carriers

DataCarriers

Subcarrier Spacing: 10.94 kHz

20 MHz204810 MHz1024

5 MHz512

1.25 MHz128

Channel

Bandwidth

FFT

Size

Frequency

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Symbol Transmission verses Time

OFDM Symbol Number

Pre-

amble

SubC

hannelNum

ber

0

.

.

.

.

.

.

.

n

Each OFDM symbol

consists of n sub-carrier

symbols

k+1 k+2 k+3 k+4 k+5 k+6 k+7 k+8 k+9 k+10 k

Transition

Gap

Transition

GapDL-

MapDL Burst UL Burst

802.16e can use time-division or frequency-

division multiplexing between the up and down-link

bursts.

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Transmitting Multiple Symbols Simultaneously

I

Q

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Dynamic Symbol Map

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Dynamic Symbol Map

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Link Characteristics Summary

DistanceClose Far away

HighDataThroug

hput(QAM

)

MoreS

ub-channe

lsused

Low

Da

taThroughp

ut(QPSK)

LessSu

b-channels

used

UsersMany Few

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demo

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demo

WiMAX Measurements

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WiMAX Summary

This is metropolitan areanetworking internet to yourhome, office or car.

Implies one of the 802.16Standards

Very similar in concept to 802.11,but the demands of multiplesimultaneous users (possibly

mobile) make the implementationmuch more complex.

Uses scheduled transactions toensure all paying users getaccess. You can get frozen out

with WiFi. Stands for: Worldwide

Interoperability forMicrowaveAccess

SOFDMA (Scalable OFDM Multiple

Access)

SOFDMA interoperates with OFDMA,but requires new equipment.

Adds MIMO

The current version of the standard,

upgraded to include mobile wireless.802.16e-2005

OFDMA (OFDM multiple access)

2-11 GHz (no regulatory approval

above 5.9 GHz)

Practical rate: 10 Mbps over 2 km

Fielded system for fixed-point access

(to the home or office)802.16-2004

(aka 802.16d)

Means802.16

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O / O

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OFDM/A to MIMO

MIMO uses multiple transmitters and receivers that are

modulated with OFDM/A.

Both WLAN (802.11n) and WiMAX (802.16e) have MIMOconfigurations

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Spectrally Efficiency SISO MIMO

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Spectrally Efficiency SISO MIMOBits/Second/Hz

6.0

4.0

2.0

0.5

WLAN

802.11n

GSM WLAN

802.11a/g

W-CDMA

HSDPA

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Wh i MIMO diff t f t d d OFDM

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Why is MIMO different from standard OFDM

40MHz 40MHz 40MHz 40MHz

~ 4 x Information, but with 4 x the BW

40MHz

~ 3.5 x Information, but with 1 x the BW

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MIMO R di C fi ti

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MIMO Radio Configuration

TX1

TX2

RX1

RX22x2

TX1TX2

TX3

RX1

RX2

3x2

TX1

TX2

TX3

TX4

RX1

RX2

RX3

RX4

4x4

www.keithley.com

MIMO requires lots of paths!

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MIMO requires lots of paths!

If you have two unknown

transmitted signals and two

measurements at the

receivers. If the two

measurements are

sufficiently independent, you

can solve for the transmitted

symbols!

www.keithley.com

Mathematically

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yModel the Channel

y = Hx + n

y = Receive Vector

x = Transmit Vector

H = Channel Matrix

n = Noise Vector

TX1

TX2

h21h12

h11 = a+jb

RX1

RX2

h22

Channel

h11 h12h21 h22

H =

Header

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Correct for channel effects

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Correct for channel effects

RX = H * TX + n

TX1

TX2

h21

h12

h11 = a+jb

RX1

RX2

RX1RX2

TX1TX2

= - nh22

Channel

h11 h12h21 h22

DataData

Header

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demo

2x2 Measurement Example

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2x2 Measurement Example

www.keithley.com

802.16e Matrix A and B

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Robust Symbols vs. More Symbols

11010101001010101

110001100110

TX1

TX2

RX1

RX2

TX1

TX2

RX1

RX2

11 01

Matrix A Transmit Inverse Symbols

Coverage

Matrix B Transmit Parallel Symbols

Through

put

www.keithley.com

Beam Forming

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Beam Forming

TX1

TX2

TX3TX4

Control the directionality and shape of the radiated pattern

Increase range, capacity and throughput

www.keithley.com

The Beam Forming Process

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WiMAX Example - Closed Loop

Sound Channel

Feedback Channel Characteristics

Direct Beam

TX1

TX2

TX3

TX4

www.keithley.com

Typical Types of Beam Forming

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Typical Types of Beam Forming

Statistical Eigen Beam Forming (EBF)

Advantage Quickly builds a channel model to form a beam,

making it ideal for mobile applications. Disadvantage Not as efficient as Maximum Ratio Transmission.

Maximum Ratio Transmission (MRT)

Advantage builds a very accurate channel model, thus improving

throughput and coverage.

Disadvantage Slow processing times limit to static transmissions

www.keithley.com

MIMO Instrument Requirements

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MIMO Instrument Requirements

2820 VSA2820 VSA(Master)

MIMO Synchronization Unit

2920 VSG2920 VSG

(Master)

MIMO Synchronization Unit

Hardware

All 28/2920 VSAs are identical standard units

Flexibility to use 2820 VSAs as stand-alone generators User-definable instrument configuration setting

Stand-alone

MIMO Master

MIMO Slave

System

Common LO and clock signals for all analyzers

Master provides LO, 100 MHz digital clock, and Trigger

Sync to MIMO Synchronization Unit

MIMO Sync. Unit distributes a common LO, common

100 MHz clock, and synchronized Trigger to all units

Signal sampling alignment within +1 nsec

Form Factor

28/2920: 3U high, rack width

MIMO Sync. Unit: 1U high, full-rack width

www.keithley.com

Conclusions

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www.keithley.com

Speed vs. Mobility

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p y

WiMAX

HSDPA

GSM

Future

OFDMAWLANSpeed

Today

Mobility

www.keithley.com

The Long Term Evolution of Wireless

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g

GSM / W-CDMA HSDPA/UPA DVB-H

LTEGMSK, QPSK CDMA Long Term EvolutionOFDMA

SISO MIMO

802.11a-b-g-j 802.16e 802.11n, 802.16e Wave 2

www.keithley.com

Instrument Bandwidth Requirements

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q

5MHz 20MHz 40MHz300kHz 1MHz 3MHz 10MHz

GSM

IS2000

W-CDMA

WiMAX

WLAN

Keithley instruments have 40MHz BW as standard.

www.keithley.com

Summary

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OFDM and SISO radio configurations

OFDM and MIMO radio configurations

OFDMA

WLAN, WiMAX and the evolution to 4G

www.keithley.com

Industrys Leading 4x4 MIMO RF Test System

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Industry-Leading Performance

Flexible 2, 3, or 4-channel

configurations 40MHz signal bandwidth

1 nsec signal sampler synchronization

1 nsec peak-to-peak signal sampler

jitter 1 peak-to-peak RF-carrier phase jitter

High-performance: -40dB EVM (Error

Vector Magnitude)

Uses standard MIMO-readyinstruments

Industry leading signal analysis MIMO

software