Download - 3 LF_BT1010_E01_1 LTE OFDM Principle 40.pdf
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OFDM Principle
ZTE University
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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
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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
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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)
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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)
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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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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
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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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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
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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
![Page 26: 3 LF_BT1010_E01_1 LTE OFDM Principle 40.pdf](https://reader033.vdocument.in/reader033/viewer/2022050802/5695d06c1a28ab9b029269e0/html5/thumbnails/26.jpg)
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AMC(Adaptive Modulation and Coding)
dynamic adaption of the
modulation scheme
dynamic adaption of the
coding scheme
dynamic adaption of the
bits
Performance
enhancements
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Contents
Basic Points
Benefits and Drawbacks
Key Technologies
Application in DL
Application in UL
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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
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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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