3 LF_BT1010_E01_1 LTE OFDM Principle 40.pdf

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

© ZTE Corporation. All rights reserved

Frequency selective fading Time selective fading

Radio Channel Characteristics

Multi-Path Effect

Time-Variant Channel

Frequency

Frequency

Transmitter

Receiver

Time

Power


Radio Channel’s Problems to Solve

Time selective fading

OFDM Technology

Frequency selective fading


What is OFDM ?

OFDM: Orthogonal Frequency Division Multiplexing, is a kind of

multi-carrier transmission.


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

Demodulation


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

frequency

deviation

High PAPR (Peak-

to-Average Power

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

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


If GI is Inserted

GI: Guarding Interval

Path 2

Path 1

GI


ICI Brought by GI

subcarrier 1

Delayed subcarrier 2

GI FFT IntegrationTime = 1/subcarrier spacing

Part of subcarrier 2 causing ICI on subcarrier 1


If CP is Inserted

Amplitude

Time GI FFT Integral Time Length OFDM Symbol

Length

CP

CP(Cycle Prefix)

Contents

Basic Points


Key Technologies


Channel Estimation



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


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


Key Technologies


Channel Estimation



Application in DL

Application in UL


Synchronization Requirements

• Carrier synchronization

• Sample value synchronization

• Symbol synchronization

OFDM synchronization

requirements


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


Carrier and Symbol Synchronization

Contents

Basic Points


Key Technologies


Channel Estimation



Application in DL

Application in UL


AMC(Adaptive Modulation and Coding)

dynamic adaption of the

modulation scheme


coding scheme


bits

Performance

enhancements

Contents

Basic Points


Key Technologies

Application in DL

Application in UL


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


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


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



System Bandwidth


OFDM vs OFDMA

Contents

Basic Points


Key Technologies

Application in DL

Application in UL


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


SC-FDMA: Uplink multiple access

0

Single Carrier Sub-frame

Frequency

Time




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


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

• 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


Summary

What is OFDM/OFDMA?

OFDM Benefits/Drawbacks?

Related Key Technologies?

Why SC-FDMA?

Difference between DFT-S-OFDM & OFDM?

DFT-S-OFDM Types?

Documents

3 LF_BT1010_E01_1 LTE OFDM Principle 40.pdf