Plugin-Introduction Orthogonal Frequency Division Multiplex
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A G R E A T E R M E A S U R E O F C O N F I D E N C E
www.keithley.com
Copyright 2004 Keithley Instruments, Inc.1
An Introduction to
Orthogonal Frequency Division Multiplex
Technology
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Copyright 2007-2008 Keithley Instruments, Inc.2
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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y
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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
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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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
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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!
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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
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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
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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
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The Beam Forming Process
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WiMAX Example - Closed Loop
Sound Channel
Feedback Channel Characteristics
Direct Beam
TX1
TX2
TX3
TX4
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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
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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
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Conclusions
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Speed vs. Mobility
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p y
WiMAX
HSDPA
GSM
Future
OFDMAWLANSpeed
Today
Mobility
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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
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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.
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Summary
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OFDM and SISO radio configurations
OFDM and MIMO radio configurations
OFDMA
WLAN, WiMAX and the evolution to 4G
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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