an introduction to orthogonal frequency division multiplex ...hy439/reading/list_2010/...digital...
TRANSCRIPT
![Page 1: An Introduction to Orthogonal Frequency Division Multiplex ...hy439/reading/list_2010/...Digital Modulation Overview I and Q Components of a Signal φ A I Q Q I Q 90o I Sum Simple](https://reader035.vdocuments.us/reader035/viewer/2022071219/6056960ef163fb37e26ad137/html5/thumbnails/1.jpg)
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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 indoor environments 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
/Sec
ond/
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 signalTo travel D (Distance)Dmultipath > DdirectTX to RX < 1us
Symbol Rate = 1MSymbols/sSymbol 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 signalTo travel D (Distance)Dmultipath > DdirectTX to RX < 3.3us
Symbol Rate = 300kSymbols/sSymbol Duration = 1/300 = 3.3us
Ddirect
But the data throughput is reduced!
Maximum Symbol Delay < 3.3us
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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 * ¾ 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 frequency is 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 WaveformOFDM – 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
Q waveform
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The OFDM Radio
DigitalSection(ASIC/FPGA)
Digital I and Q Bus
D/A
D/A
Filter
Filter
90o Sum PAMixer
LocalOscillator
(LO)
IF
Modulator
DigitalSection(ASIC/FPGA)
A/D
A/D
Filter
Filter
90o PAMixer
LocalOscillator
(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 VectorMagnitude
Ratio of Measured Amplitude to Intended Amplitude (% or dB)
Magnitude Error = “IQ Error Magnitude”
MeasuredSignal
Intended Signal
Phase Error = “IQ Error Phase”
Q
I
φ
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 the quadrature, or Q-axis.
• Example: four-symbol Quadrature Phase Shift Keying (QPSK) modulator
01 = φ2 = 3π/4
00 = φ1 = π/4
10 = φ4 = –π/4
11 = φ3 = –3π/4
00
11 10
01
π
Quadrature Phase Shift Keying (QPSK)Data 01 00 11 00 01 10Φ(t)
Q
I
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Recovering the DataQPSK and 16QAM Signals
1
I+1V
00
11 10
01
-1V
-1V
Q +1V
ConstantMagnitudecircle
AssignVoltages
1100110111101111
1000100110101011
0100010101100111
0000000100100011 MagnitudeChanges
-1V
Q +1V
I+1V-1V
Noise
QPSK0.707SI(t)
-0.707-1
0.7071
SQ(t)-0.707
-101 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
φA
I
Q
Q
I
Q90o
ISum
Simple PythagorasAmplitude (Length of A) = SQRT (I2 + Q2)Phase (Angle φ) = tan-1 (Q/I)
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Representation of Signal in Complex Plane
I – axis(Real)
Q-axis(Imaginary)
φ
A
frequencysignalfc =
Asinφ
Acosφ
Independent Signal 1 Independent Signal 2
S(t) = A cos(2πfct + φ)
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Quadrature Modulator Hardware
carrier
filter
filter
Asinφ x sinθ
Acosφ x cosθ
Acosφ x cosθ - Asinφ x sinθ = Acos(φ+θ)
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Quadrature Demodulation
• The receiver can recover the two independently modulated signals, 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…
RFInput
Mixer
LocalOscillator
IQ DemodDigital IF
Error VectorMagnitude
Ratio of Measured Amplitude to Intended Amplitude (%)
Magnitude Error = “IQ Error Magnitude”
MeasuredSignal
Intended Signal
Phase Error = “IQ Error Phase”
Q
I
φ
Unit Circle
RCE (dB) = 20 log(EVM in %/100)
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I/Q Modulator ImpairmentsContributions to EVM
Basebandgenerator
GI
90°+θe
Ioffset
Qoffset
LO
I
Q
Modulator imbalance
Origin Offset
QuadratureError
RFφc
Carrier Phase Shift
Basebandgenerator
GI
90°+θe
Ioffset
Qoffset
LO
I
Q
Modulator imbalance
Origin Offset
QuadratureError
RFφc
Carrier Phase Shift
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Origin Offset Example16-QAM Constellation
I
Q
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Quadrature Error ExamplesQPSK Constellations
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Modulator Imbalance ExamplesQPSK Constellations
Excess I gain and reduced Q gain relative
to ideal constellation
Ideal Excess I Gain
Excess Q Gain
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Power Amplifier NonlinearityAnother Contributor to EVM
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ISI – Inter Symbol Interference
1100110111101111
1000100110101011
0100010101100111
00000001
Noise
ExpectedSymbol Position
Delayed ReceivedSymbol
Q +1V
0011 0010
SymbolBoundary
I+1V
-1V
-1V
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Constellation DisplayIs a Composite of all OFDM Sub-carrier Symbols
…fnIndividual Sub-carriersf5
f4f3
f2f1
QPSKConstellation
Display at T = t0
Q
I
16QAM
BPSK
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Constellation DisplayIs 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 Display
Q
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
PilotCarriers
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 or
64-QAM
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WLAN Summary
• WLAN implies Wireless LAN compatible with one the IEEE 802.11 sub standards. It is what you have in your laptop.
• WiFi is an industry consortium that defines a required subset of 802.11 to ensure better operation between different vendor’s equipment.
• EWC is an industry consortium that took the unfinished N standard, agreed upon a version, 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 channels
a
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/eOFDMA
• Worldwide Interoperability for Microwave Access • Fixed – 802.16d, point-to-point backhaul applications• Mobile – 801.16e, enhanced data services mobile
applications.
WiMAX enhancedMobile Devices
BTS
Mobile
Fixed
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RF Characteristics 802.16e Mobile WiMAX
IFFT size determines carriers Carriers
Gua
rd B
and
Gua
rd B
and
Pilot Carriers
DataCarriers
Subcarrier Spacing: 10.94 kHz
20 MHz204810 MHz10245 MHz512
1.25 MHz128
Channel Bandwidth
FFTSize
Frequency
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Symbol Transmission verses Time
OFDM Symbol Number
Pre-amble
Sub
Cha
nnel
Num
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+10k
TransitionGap
TransitionGap
DL-Map
DL 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
IQ
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Dynamic Symbol Map
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The Physical Channels are Different from the Logical Channels
Physical sub-channels are changed every symbol using a PN sequence.
-fn 0 +fn
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Link Characteristics – Summary
DistanceClose – Far away
Hig
h D
ata
Thro
ughp
ut (Q
AM
)M
ore
Sub
-cha
nnel
s us
ed
Low
Dat
a Th
roug
hput
(QP
SK
)Le
ss S
ub-c
hann
els
used
UsersMany – Few
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demo
WiMAX Measurements
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WiMAX Summary
• This is metropolitan area networking – internet to your home, office or car.
• Implies one of the 802.16 Standards
• Very similar in concept to 802.11, but the demands of multiple simultaneous users (possibly mobile) make the implementation much more complex.
• Uses scheduled transactions to ensure all paying users get access. You can get frozen out with WiFi.
• Stands for: Worldwide Interoperability for Microwave Access
• 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 MIMO configurations
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Spectrally Efficiency – SISO MIMOBits/Second/Hz
6.0
4.0
2.0
0.5WLAN802.11n
GSM WLAN802.11a/g
W-CDMAHSDPA
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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 Radio Configuration
TX1
TX2
RX1
RX2
2x2
TX1
TX2
TX3
RX1
RX2
3x2
TX1
TX2
TX3
TX4
RX1
RX2
RX3
RX4
4x4
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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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MathematicallyModel the Channel
y = Hx + n
y = Receive Vectorx = Transmit VectorH = Channel Matrixn = Noise Vector
TX1
TX2
h21h12
h11 = a+jb
RX1
RX2h22
Channel
h11 h12h21 h22
H =Header
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Correct for channel effects
RX = H * TX + n
TX1
TX2
h21h12
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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802.16e Matrix A and BRobust Symbols vs. More Symbols
11010101001010101
110001100110
TX1
TX2
RX1
RX2
TX1
TX2
RX1
RX2
11 01Matrix A – Transmit Inverse Symbols
Cov
erag
e
Matrix B – Transmit Parallel Symbols
Thro
ughp
ut
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Beam Forming
TX1
TX2
TX3
TX4
• Control the directionality and shape of the radiated pattern
Increase range, capacity and throughput
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The Beam Forming ProcessWiMAX Example - Closed Loop
Sound Channel
Feedback Channel Characteristics
Direct Beam
TX1
TX2
TX3
TX4
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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
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-aloneMIMO MasterMIMO 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 nsecForm Factor• 28/2920: 3U high, ½ rack width• MIMO Sync. Unit: 1U high, full-rack width
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Conclusions
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Speed vs. Mobility
WiMAX
HSDPAGSM
Future
OFDMAWLANSpeed
Today
Mobility
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The Long Term Evolution of Wireless
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
5MHz 20MHz 40MHz300kHz 1MHz 3MHz 10MHz
GSM
IS2000
W-CDMA
WiMAX
WLAN
Keithley instruments have 40MHz BW as standard.
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Summary
• OFDM and SISO radio configurations• OFDM and MIMO radio configurations• OFDMA• WLAN, WiMAX and the evolution to 4G
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Industry’s Leading 4x4 MIMO RF Test System
• 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-ready
instruments– Industry leading signal analysis MIMO
software