3 Lf_bt1010_e01_1 Lte Ofdm Principle 40

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

Radio Channel Characteristics 

Multi-Path Effect



Time-Variant Channel

Transmitte r

Power

Receiver

Frequency

Frequency

Time

Frequency selective fading

© ZTE Corporation. All rights reserved

Time selective fading

Radio Channel’s Problems to Solve

Time selective fading Frequency selective fading

OFDM Technology © ZTE Corporation. All rights reserved

What is OFDM ?

OFDM: Orthogonal Frequency Division Multiplexing, is a kind of multi-carrier transmission. © ZTE Corporation. All rights reserved

Why using OFDM ? 

Bandwidth utilization rate is higher than FDM



Frequency selective fading is small



Time selective fading is small


OFDM Modulation and Demodulation

Modulation

Demodulatio n


OFDM Principle Diagram


Contents 

Basic Points






Key Technologies



Application in DL



Application in UL

OFDM Benefits against frequency-selective fading against ISI Spectrum efficiency is maximized Calculation is simple increase system throughput


OFDM Drawbacks


Sensitive to

High PAPR (Peak-

frequency

to-Average Power

deviation

Ratio)

Contents 

Basic Points






Key Technologies 

Cycle Prefix Technology



Channel Estimation



Synchronous technology



Adaptive Modulation and Coding



Application in DL



Application in UL

Multi-Path Effect Path 1

Path 2

GI: guarding interval

The second symbol in Path 1 interferes with the first symbol in Path 2

ISI (InterSymbol Interference) will occur because of multi-path effect


If GI is Inserted

Path 1

Path 2

GI GI: Guarding Interval


ICI Brought by GI

Part of subcarrier 2 causing ICI on subcarrier 1

subcarrier 1

Delayed subcarrier 2

GI


FFT IntegrationTime = 1/subcarrier spacing

If CP is Inserted

Amplitud e

CP

GI

FFT Integral Time Length OFDM Symbol Length

CP(Cycle Prefix) © ZTE Corporation. All rights reserved

Time

Contents 

Basic Points










Channel Estimation









Application in DL



Application in UL

Channel Estimation and Reference Symbols 

To estimate the frequency-domain channel tap directly by inserting reference symbols(RS).

One RB One antenna port

R0

R0

R0

R0

R0

R0

l0

R0

R0

l6 l0

TS36.211 DL RS, normal CP,1TX © ZTE Corporation. All rights reserved 19

l6

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










Channel Estimation









Application in DL



Application in UL

Synchronization Requirements OFDM synchronization

requirements • Carrier synchronization • Sample value synchronization

• Symbol synchronization


Carrier Synchronization

OFDM synchronization procedure • Tracking mode • Catching mode

OFDM synchronization phases • Phase1： A rough estimate of the carrier frequency, solve catching problem

• Phase2： lock and execution tracking task


Carrier and Symbol Synchronization


Contents 

Basic Points










Channel Estimation









Application in DL



Application in UL

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






Key Technologies



Application in DL



Application in UL

OFDM Time-Frequency Structure One downlink slot Tslot

DL N symb OFDM

symbols DL RB k  N RB N sc  1

Resource block



Resource Element



Resource Block

Resource element (k , l )

N scRB

DL NRB  NscRB

subcarriers subcarriers

DL RBresource elements N symb  N sc

k 0 l0


l

DL N symb

1

Bandwidth Analysis Nominal Bandwidth

1.4

3

5

10

15

20

6

15

25

50

75

100

1.08

2.7

4.5

9

13.5

18

(MHz)

Number of RB

Frequency Domain Real Bandwidth (MHz)


OFDMA: Downlink multiple access System Bandwidth Sub-carriers

Sub-frame

Frequency

Time frequency resource for User 1 Time frequency resource for User 2

Time

Time frequency resource for User 3



Up to 64 QAM can be used



Resistance to multi-path interference by Cyclic Prefix.



Friendly to MIMO.


OFDM vs OFDMA


Contents 

Basic Points






Key Technologies



Application in DL



Application in UL

Why select SC-FDMA?

OFDM

SC-FDMA (Single Carrier FDMA)

Not suitable for terminal

Suitable for terminal 

DFT-S-OFDM (DFTSpread-OFDM) in the frequency domain


SC-FDMA: Uplink multiple access System Bandwidth Single Carrier Sub-frame

Frequency

Time frequency resource for User 1 Time frequency resource for User 2

Time

0


Time frequency resource for User 3



Up to 16 QAM can be used



Single carrier modulation achieves lower Peak to Average Ratio (PAPR)



FDMA is efficiently through FFT operation

achieved

OFDMA and SC-FDMA Comparison


DFT-S-OFDM Process


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.


Distributed DFT-S-OFDM Benefit/Drawback Benefit

• DFTS-OFDM signal can be spread over a very large overall BW Drawback • No single sub-carrier property • More sensitivity to frequency errors • Localized DFT-S-OFDM is adopted in LTE UL. © ZTE Corporation. All rights reserved

Summary 

What is OFDM/OFDMA?



OFDM Benefits/Drawbacks?



Related Key Technologies?



Why SC-FDMA?



Difference between DFT-S-OFDM & OFDM?



DFT-S-OFDM Types?


3 Lf_bt1010_e01_1 Lte Ofdm Principle 40

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