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OFDM Principle
ZTE University
Objectives
After the course, you will:
Know the Concept of OFDM
Understand the OFDM Principle
Know the Benefits and Drawbacks of OFDM
Understand the Key Technologies of OFDM
Know the OFDM Application
Contents
Basic Points
Benefits and Drawbacks
Key Technologies
Application in DL
Application in UL
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Frequency selective fading Time selective fading
Radio Channel Characteristics
Multi-Path Effect
Time-Variant Channel
Frequency
Frequency
Transmitter
Receiver
Time
Power
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Radio Channel’s Problems to Solve
Time selective fading
OFDM Technology
Frequency selective fading
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What is OFDM ?
OFDM: Orthogonal Frequency Division Multiplexing, is a kind of
multi-carrier transmission.
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Why using OFDM ?
Bandwidth utilization rate is higher than FDM
Frequency selective fading is small
Time selective fading is small
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OFDM Modulation and Demodulation
Modulation
Demodulation
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OFDM Principle Diagram
Contents
Basic Points
Benefits and Drawbacks
Key Technologies
Application in DL
Application in UL
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OFDM Benefits
against frequency-selective fading
against ISI
Spectrum efficiency is maximized
Calculation is simple
increase system throughput
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OFDM Drawbacks
Sensitive to
frequency
deviation
High PAPR (Peak-
to-Average Power
Ratio)
Contents
Basic Points
Benefits and Drawbacks
Key Technologies
Cycle Prefix Technology
Channel Estimation
Synchronous technology
Adaptive Modulation and Coding
Application in DL
Application in UL
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Multi-Path Effect
GI: guarding interval
Path 2
The second symbol in Path 1 interferes with the first symbol in Path 2
Path 1
ISI (InterSymbol Interference) will occur because of multi-path effect
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If GI is Inserted
GI: Guarding Interval
Path 2
Path 1
GI
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ICI Brought by GI
subcarrier 1
Delayed subcarrier 2
GI FFT IntegrationTime = 1/subcarrier spacing
Part of subcarrier 2 causing ICI on subcarrier 1
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If CP is Inserted
Amplitude
Time GI FFT Integral Time Length OFDM Symbol
Length
CP
CP(Cycle Prefix)
Contents
Basic Points
Benefits and Drawbacks
Key Technologies
Cycle Prefix Technology
Channel Estimation
Synchronous technology
Adaptive Modulation and Coding
Application in DL
Application in UL
© ZTE Corporation. All rights reserved 19
Channel Estimation and Reference Symbols
To estimate the frequency-domain channel tap directly by
inserting reference symbols(RS).
0l
0R
0R
0R
0R
6l 0l
0R
0R
0R
0R
6l
On
e an
ten
na
po
rt
TS36.211 DL RS, normal CP,1TX
One RB
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Reference symbols Location
Normal CP Single Antenna
Normal CP Two Antenna
bearing the radio resource unit of the antenna 0
bearing the radio resource unit of the antenna 1
bearing the data resource unit Idle resource unit
Contents
Basic Points
Benefits and Drawbacks
Key Technologies
Cycle Prefix Technology
Channel Estimation
Synchronous technology
Adaptive Modulation and Coding
Application in DL
Application in UL
© ZTE Corporation. All rights reserved
Synchronization Requirements
• Carrier synchronization
• Sample value synchronization
• Symbol synchronization
OFDM synchronization
requirements
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Carrier Synchronization
• Tracking mode
• Catching mode
OFDM synchronization procedure
• Phase1： A rough estimate of the carrier
frequency, solve catching problem
• Phase2： lock and execution tracking task
OFDM synchronization phases
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Carrier and Symbol Synchronization
Contents
Basic Points
Benefits and Drawbacks
Key Technologies
Cycle Prefix Technology
Channel Estimation
Synchronous technology
Adaptive Modulation and Coding
Application in DL
Application in UL
© ZTE Corporation. All rights reserved
AMC(Adaptive Modulation and Coding)
dynamic adaption of the
modulation scheme
dynamic adaption of the
coding scheme
dynamic adaption of the
bits
Performance
enhancements
Contents
Basic Points
Benefits and Drawbacks
Key Technologies
Application in DL
Application in UL
© ZTE Corporation. All rights reserved
OFDM Time-Frequency Structure
Resource Element
Resource Block
DLsymbN OFDM symbols
One downlink slotslotT
0l 1DLsymb Nl
RB
scD
LR
BN
N
subca
rrie
rsR
Bsc
Nsu
bca
rrie
rs
RBsc
DLsymb NN
Resource blockresource elements
Resource
element ),( lk
0k
1RBsc
DLRB NNk
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Bandwidth Analysis
Nominal
Bandwidth
(MHz)
1.4 3 5 10 15 20
Number of RB 6 15 25 50 75 100
Frequency
Domain Real
Bandwidth
(MHz)
1.08 2.7 4.5 9 13.5 18
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OFDMA: Downlink multiple access
Up to 64 QAM can be used
Resistance to multi-path interference by Cyclic Prefix.
Friendly to MIMO.
Sub-carriers
Sub-frame
Frequency
Time
Time frequency resource for User 1
Time frequency resource for User 2
Time frequency resource for User 3
System Bandwidth
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OFDM vs OFDMA
Contents
Basic Points
Benefits and Drawbacks
Key Technologies
Application in DL
Application in UL
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Why select SC-FDMA?
DFT-S-OFDM (DFT-
Spread-OFDM) in the
frequency domain
OFDM
SC-FDMA
(Single Carrier
FDMA)
Not suitable for terminal
Suitable for terminal
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SC-FDMA: Uplink multiple access
0
Single Carrier Sub-frame
Frequency
Time
Time frequency resource for User 1
Time frequency resource for User 2
Time frequency resource for User 3
System Bandwidth
Up to 16 QAM can be used
Single carrier modulation achieves lower Peak to Average Ratio (PAPR)
FDMA is efficiently achieved through FFT operation
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OFDMA and SC-FDMA Comparison
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DFT-S-OFDM Process
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OFDM vs DFT-S-OFDM
• OFDM PAPR is far higher than DFT-S-OFDM • The higher modulation mode DFT-S-OFDM adopts, the higher PAPR it gets.
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Distributed DFT-S-OFDM Benefit/Drawback
• Localized DFT-S-OFDM is adopted in LTE UL.
• DFTS-OFDM signal can be spread over a
very large overall BW
Benefit
• No single sub-carrier property
• More sensitivity to frequency errors
Drawback
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Summary
What is OFDM/OFDMA?
OFDM Benefits/Drawbacks?
Related Key Technologies?
Why SC-FDMA?
Difference between DFT-S-OFDM & OFDM?
DFT-S-OFDM Types?
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