LTE FDD OFDM Technology

ZTE University

Contents

Basic Principle Benefits and Drawbacks Key Technologies Application in DL Application in UL

Radio Channel Characteristics

Multi-Path Effect Time Dispersion causes

ISI (InterSymbol Interference

When Transmitter sends a Pulse signal, the receiver receives:

Time-Variant Channel Doppler Shift is one

form Frequency

Dispersion causes ICI (InterChannel Interference)

Frequency

Frequency

Transmitter

Receiver

Power Power

Time

What is OFDM ?

OFDM: Orthogonal Frequency Division Multiplexing, is a kind of multi-carrier transmission;

In frequency domain, OFDM divides channel into some sub-channels overlapped between adjacent sub-channels. These sub-channels are orthogonal.

Implement of CP can undermine ISI caused by Delay Spread

OFDM Modulation and Demodulation

Modulation

Demodulation

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

GI is inserted in order to eliminate ISI When GI is longer than max delay spread of channel, the multi-path component of

a symbol doesn’t interfere with next symbol

ICI Brought by GI

subcarrier 1

Delayed subcarrier 2

GI FFT IntegrationTime = 1/subcarrier spacing

Part of subcarrier 2 causing ICI on subcarrier 1

CP: Cyclic Prefix

FFT integral time length can’t contain the entire waves because GIs are included. The orthogonality between subcarriers is detroyed.

If CP is Inserted Amplitude

Time

GI FFT Integral Time Length

OFDM Symbol Length

CP refers to the prefixing of a symbol with a repetition of the end, in order that the orthogonality between subcarriers is not undermined due to GI.

FFT integral time length can contain integer number of waveforms of subcarriers in multi-path, as long as time delay of every path is less than GI.

CP: Cyclic Prefix

CP

OFDM Principle Diagram

OFDM Frequency-Time Structure

Each column corresponds to one OFDM symbol Each row corresponds to one OFDM subcarrier

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

OFDM vs OFDMA

Contents

Basic Principle Benefits and Drawbacks Key Technologies Application in DL Application in UL

OFDM Benefits Dividing large bandwidth into small subcarriers, can be effectively

against frequency-selective fading

It can be effectively against ISI, so it is suitable for high-speed data transmission in multi-path environment.

Spectrum efficiency is maximized. Because of the orthogonality between subcarriers, adjancent subchannels overlap.

Modulation/demodulation can be achived by IFFT/FFT. Calculation is efficient and simple.

It can achieve the different UL and DL transmission data rate by using different number of subchannels.

It can take full advantage of high SNR subchannels to increase system throughput by dynamic subchannel allocation.

OFDM Drawbacks

Sensitive to frequency deviation: Frequency shift occurred in transmission process(such as Doppler Shift),

or the frequency difference between a receiver local oscillator and the carrier frequency of transmitter, would undermine the orthogonality between subcarriers of OFDM system, leading to inter-channel signal interference.

High PAPR (Peak-to-Average Power Ratio): The output of OFDM modulation is the superposition of multiple sub-

channel. If the signal phases in multiple sub-channels are the same at a certain time, superimposed signal instantaneous power will be far greater than the signal average power, resulting in a high PAPR. High PAPR not only brings higher requirements on transmitter Power Amplifier (PA) linearity, but also reduces the amplifier efficiency.

Contents

Basic Principle Benefits and Drawbacks Key Technologies Application in DL Application in UL

18

Channel Estimation and Reference Symbols

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

Using knowledge about the RS, the receiver can estimate the frequency-domain channel around the location of the RS.

The reference symbols should have a sufficiently high density in both the time and the frequency domain to estimate entire time/frequency grid.

0l0R

0R

0R

0R

6l 0l0R

0R

0R

0R

6l

One

ant

enna

por

t

TS36.211 DL RS, normal CP,1TX

One RB

19

Frequency Interleaving

The consequence bits are spread entire the frequency band.

AMC

AMC: Adaptive Modulation and Coding OFDM can achieve dynamic adaption of the modulation

sheme, coding scheme and bit amount on every subcarrier according to the fading situation of frequency-selective channel.

Thus great performance enhancements can be achived in OFDM system.

Contents

Basic Principle Benefits and Drawbacks Key Technologies Application in DL Application in UL

OFDM Time-Frequency Structure

Resource Element The smallest resource unit

Resource Block Contains 12 subcarriers in

frequency domain Contains 7 OFDM symbols in

time domain(6 OFDM symbols when Extended CP is used)

DLsymbN OFDM symbols

One downlink slotslotT

0l 1DLsymb Nl

RB

scD

LR

BN

N

subc

arri

ers

RB

scN su

bcar

rier

s

RBsc

DLsymb NN Resource block

resource elements

Resource element ),( lk

0k

1RBsc

DLRB NNk

1 RB : • 180kHz (i.e. 12 x 15KHz )in

frequency domain• 0.5 ms (i.e. one slot)in time

domain

Bandwidth Analysis

Bandwidth = Sub-frame x No. of sub-frame in each RB x No. of RB Bandwidth of one sub-frame = 15KHz No. of sub-frame each RB = 12

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

Contents

Basic Principle Benefits and Drawbacks Key Technologies Application in DL Application in UL

Why SC-FDMA?

OFDM: High PAPR Lower PA (Power Amplifier) efficiency

Lower battery efficiency Not suitable for terminal

SC-FDMA (Single Carrier FDMA): Lower PAPR Higher PA efficiency Longer battery

life Suitable for terminal DFT-S-OFDM (DFT-Spread-OFDM) is the

implementation method of SC-FDMA in the frequency domain, which performs DFT-based precoding before IFFT modulation in the OFDM modulation process.

SC-FDMA: Uplink multiple access

Up to 16 QAM can be used

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

FDMA is efficiently achieved through FFT operation

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

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.

Zero Fill

The output of the DFT to equidistant inputs of the IDFT with zeros inserted in between.

The output of the DFT is mapped to consecutive inputs of the IFFT.

Localized DFTS-OFDMLocalized DFTS-OFDM

SD-FDMA Multiple Access

Distributed DFT-S-OFDM Benefit/Drawback

Benefit of distributed DFT-S-OFDM Possibility for additional frequency diversity as even a

low-rate distributed DFTS-OFDM signal (small DFT size M) can be spread over a potentially very large overall BW.

Drawback of distributed DFT-S-OFDM No single sub-carrier property More sensitivity to frequency errors and has higher

requirements on power control

• Localized DFT-S-OFDM is adopted in LTE UL.

Summary

What is OFDM/OFDMA? OFDM Benefits/Drawbacks? Related Key Technologies? Why SC-FDMA? Difference between DFT-S-OFDM & OFDM? DFT-S-OFDM Types?