PAPR Reduction Method for OFDM Systems without Side Information

PAPR Reduction Method for OFDM PAPR Reduction Method for OFDM Systems without Side Information Systems without Side Information

Pornpawit Boonsrimuang, Pisit Boonsrimuang Kazuo Mori, Tawil Paungma and Hideo Kobayashi

ContentsContents

Research TopicObjective of ResearchConventional MethodsProposal of PAPR Reduction Method Performance EvaluationConclusions

Research TopicResearch Topic

Proposal of PAPR Reduction Method for OFDM Signal without Side Information

OFDM: Orthogonal Frequency Division Multiplexing

Next Generation of Wireless LANNext Generation of Wireless LANNext Generation of Wireless LANNext Generation of Wireless LAN

Indoor

GigabitGigabitOutdoor

100Mbit100Mbit

Target of Research FieldTarget of Research Field

Advantages of OFDMAdvantages of OFDM

Enable the higher data rate transmissionRobustness to multi-path fadingEfficient usage of frequency bandwidth

The OFDM has already been adopted as the standard transmission techniques in

ADSL (Asynchronous Digital Subscriber Line)Terrestrial Digital TV

WiMAX(Worldwide Interoperability for Microwave Access)

Wireless LAN(Wireless Local Area Network)

Candidate transmission technique for the next generation of wireless LAN systems

Structure of OFDM SystemStructure of OFDM SystemStructure of OFDM SystemStructure of OFDM System

System Overview

.. . Parallel to Serial

(P/S)

ModSerial to Parallel (S/P)

s(t)Add GI

Fading Channel

H()

Noise n(t)

.. . Serial to Parallel (S/P)

FFTParallel to Serial

(P/S)

r(t) Equalization

TransmitterTransmitter

ReceiverReceiver

Channel

(a) OFDM spectrum

vs.

(b) conventional FDM spectrum

IFFT

.. .Demod

.. . AMP

Channel estimation

Remove GI

Larger peak to averaged power ratio (PAPR)

Larger PAPR will cause the degradation of BER performance and expansion of spectrum

re-growth in the non-linear channel

Disadvantage of OFDMDisadvantage of OFDM

2

max 0 12

max | |, =0, , 1

[| | ]

nn N

av n

SPPAPR n N

P E S

PAPR: Peak to Averaged Power Ratio

0 100 200 300 400 5000

0.5

1

1.5

2

Sample NumberA

mp

litu

de

of O

FD

M T

ime

Dom

ain

Sig

nal

0 100 200 300 400 5000

0.5

1

1.5

2

Sample Number

Am

pli

tud

e of

OF

DM

Tim

e D

omai

n S

ign

al

Comparison of OFDM Signal with Single Carrier Signal

Comparison of OFDM Signal with Single Carrier Signal

(a) OFDM signals in time domain (b) Single carrier signal in time domain

Amplitude of OFDM time domain signal is fluctuated Amplitude of OFDM time domain signal is fluctuated much larger than that for single carrier signalmuch larger than that for single carrier signal

Operation of OFDM Signal Operation of OFDM Signal at Lower IBO in Non-Linear Channelat Lower IBO in Non-Linear Channel

Operation of OFDM Signal Operation of OFDM Signal at Lower IBO in Non-Linear Channelat Lower IBO in Non-Linear Channel

-10 -8 -6 -4 -2 0 2 4-10

-8

-6

-4

-2

0

2

4

Input Power (dB)

Out

put

Pow

er (

dB)

r=2 r=4

r=30 r=6

Linear

where F[ ] represent the AM/AM conversion characteristics of non-linear amplifier

,arg

, ,

l kj y

l k l ks F y e

-10 -8 -6 -4 -2 0 2 4-10

-8

-6

-4

-2

0

2

4

Input Power (dB)

Out

put

Pow

er (

dB)

r=2 r=4

r=30 r=6

Linear

where F[ ] represent the AM/AM conversion characteristics of non-linear amplifier

Inter-modulation Noise

Operation of OFDM Signal Operation of OFDM Signal at Higher IBO in Non-Linear Channelat Higher IBO in Non-Linear Channel

Operation of OFDM Signal Operation of OFDM Signal at Higher IBO in Non-Linear Channelat Higher IBO in Non-Linear Channel

,arg

, ,

l kj y

l k l ks F y e

-1.5 -1 -0.5 0 0.5 1 1.5-1.5

-1

-0.5

0

0.5

1

1.5

.. . Parallel to Serial

(P/S)Mod

Serial to Parallel (S/P)

s(t)Add GIIFFT

.. . AMP

-1.5 -1 -0.5 0 0.5 1 1.5-1.5

-1

-0.5

0

0.5

1

1.5

Transmitter structure of conventional OFDM

Inter-Modulation Noise Occurred Inter-Modulation Noise Occurred at Non-Linear Amplifier at Non-Linear Amplifier

Inter-Modulation Noise Occurred Inter-Modulation Noise Occurred at Non-Linear Amplifier at Non-Linear Amplifier

(a) Input of power amplifier (b) Output of power amplifier

AMP

Non-LinearNon-Linear

Required Low Input Back-Off

Inefficient Usage of Power Amplifier

Larger PAPR

N sub CarriersOFDM with N sub carriers has PAPR ratio of 10log N [dB]

Disadvantage of OFDM TechniqueDisadvantage of OFDM Technique

Several methods have been proposed to reduce the PAPR of OFDM signal

Data coding technique The main disadvantage of this method is to degrade transmission efficiencyClipping technique The main disadvantage of this method is to degrade BER performance Phase alignment technique The main disadvantage of this method is required to transmit side information which leads the degradation of transmission efficiency

Improvement of PAPRImprovement of PAPR

Conventional PAPR Reduction MethodsConventional PAPR Reduction MethodsBased on Phase Alignment Technique Based on Phase Alignment Technique The Selected Mapping (SLM) and Partial Transmission

Sequence (PTS) methods which control the phase of data sub-carrier

SLM and PTS are required to inform the phase information to the receiver as the side information (SI) for the correct demodulation

Although the conventional method could improve the PAPR performance relatively,

the transmission efficiency and system complexity would be degraded because of the necessity of

transmission of side information

Objective of ResearchObjective of Research

Improve PAPR and BER performances without

degradation of transmission efficiency

Proposal of Proposal of PAPR Reduction Method for OFDM Signal PAPR Reduction Method for OFDM Signal

Based on Phase Alignment Technique which Requires Based on Phase Alignment Technique which Requires No Side InformationNo Side Information

Proposal of PAPR Reduction MethodProposal of PAPR Reduction Methodfor OFDM Signal without Side Informationfor OFDM Signal without Side Information

Use the common weighting factor over one frame including the preamble symbols

The time-frequency domain swapping algorithm is employed in the determination of common weighting factor

The common weighting factor can be estimated together with the channel frequency response by using preamble symbols

The common weighting factor can be removed from the received data symbols by using the frequency domain equalization

Proposed method could improve the PAPR performance Proposed method could improve the PAPR performance without Side Informationwithout Side Information

Common weighting factor will be assigned for each sub-carrier over one frameCommon weighting factor will be assigned for each sub-carrier over one frame

0 1 2 N-1Number of sub-carriers

0je

1je 2je

( N 1 )je Weighting Factors for

Reducing PAPR

Structure of Proposed PAPR Reduction MethodStructure of Proposed PAPR Reduction Method

Common Common weighting factor weighting factor over one frameover one frame

1st Preamble Symbol

2nd Preamble Symbols

1st Data Symbol

Lth Data Symbol

On

e F

ram

eO

ne

Fra

me

0 10 20 30 40 50 600

0.5

1

1.5

2

2.5

3

3.5

Time domain

Am

plitu

de (

Tim

e

Dom

ain

)

Predetermined threshold level (S)

( )

,il ky

0 10 20 30 40 50 600

0.5

1

1.5

2

2.5

3

3.5

( ) inji

nE e

Am

plitu

de (

Tim

e

Dom

ain

)

Frequency domain

FFT

Basic concept of Time-Frequency Swapping Algorithm

( ) ( ), ,

( ),

( ),0

i il k l k

il k

il k

y if y Se

if y S

Determination of Weighting Factor by Using Time-Determination of Weighting Factor by Using Time-Frequency Domains Swapping AlgorithmFrequency Domains Swapping Algorithm

Flowchart of Time-Frequency Domains Swapping AlgorithmFlowchart of Time-Frequency Domains Swapping Algorithm

Calculation of Time-domain Signal

Calculation of Error Signal

FFT

0 20 40 60 80 100 1200

0.5

1

1.5

2

2.5

3

3.5

S1

2

3

0 N-1N-FFT

mi (

t)0 20 40 60 80 100 120

0

0.5

1

1.5

2

2.5

0 N-1N-FFT

1

2

ei (t)

Threshold level(S)

( )( , )

il ky

( )( , )

il ky

( i )l ,nE

( ) ( ), ,

( ),

( ),0

i il k l k

il k

il k

y if y Se

if y S

( ) ( )

, ,arg( 1) ( )

, ,

i il n l n

j X Ei il n l nX X e

( 1) ( 1) ( ), , ,arg argi i i

l n l n l nX X Determined Phase (weighting factor)Determined Phase (weighting factor)

Basic Concept of Proposed AlgorithmBasic Concept of Proposed Algorithm

0 10 20 30 40 50 60-1

0

1

2

Sampling time

Am

plit

ud

e

0 N-1

Frequency DomainN-10 1 2 3 4 5 6

IFFT

Time Domain

N-2

( ) ( ), ,

( ),

( ),0

i il k l k

il k

il k

y if y Se

if y S

S Threshold LevelThreshold Level

Basic Concept of Proposed AlgorithmBasic Concept of Proposed Algorithm

0 10 20 30 40 50 60-1

0

1

2

Sampling time

Am

plit

ud

e

0 N-1

Frequency DomainN-10 1 2 3 4 5 6

Time Domain

N-2

FFT

2 knN 1 j( i ) ( i ) Nl ,n l ,k

k 0

1E e e

N

Basic Concept of Proposed AlgorithmBasic Concept of Proposed Algorithm

0 10 20 30 40 50 60-1

0

1

2

Sampling time

Am

plit

ud

e

0 N-1

Frequency DomainN-10 1 2 3 4 5 6

Time Domain

N-2

FFT

( ) ( )

, ,arg( 1) ( )

, ,

i il n l n

j X Ei il n l nX X e

Basic Concept of Proposed AlgorithmBasic Concept of Proposed Algorithm

0 10 20 30 40 50 60-1

0

1

2

Sampling time

Am

plit

ud

e

0 N-1

Frequency DomainN-10 1 2 3 4 5 6

Time Domain

N-2

FFT

Basic Concept of Proposed AlgorithmBasic Concept of Proposed Algorithm

0 10 20 30 40 50 60-1

0

1

2

Sampling time

Am

plit

ud

e

0 N-1

Frequency DomainN-10 1 2 3 4 5 6

Time Domain

N-2

IFFT

Basic Concept of Proposed AlgorithmBasic Concept of Proposed Algorithm

0 10 20 30 40 50 60-1

0

1

2

Sampling time

Am

plit

ud

e

0 N-1

Frequency DomainN-10 1 2 3 4 5 6

Time Domain

N-2

( ) ( ), ,

( 1),

( ),0

i il k l k

il k

il k

y if y Se

if y S

S

FFT

Amplitude of OFDM signal in the time domain Amplitude of OFDM signal in the time domain

0

1

0.5

1.5

0 12080604020 100Time Sample Number

(b) (b) After T-F Swapping AlgorithmAfter T-F Swapping Algorithm

Am

pli

tud

e

0 20 40 60 80 100 1200

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0

1

3

2

0 12080604020 100Time Sample Number

(a) (a) Before T-F Swapping AlgorithmBefore T-F Swapping Algorithm

0 20 40 60 80 100 1200

0.5

1

1.5

2

2.5

3

3.5

Am

pli

tud

e

2 knN 1 j( i ) ( i ) Nl ,n l ,k

k 0

1E e e

N

( ) ( )

, ,arg( 1) ( )

, ,

i il n l n

j X Ei il n l nX X e

( 1) ( 1) ( ), , ,arg argi i i

l n l n l nX X

( 1)

,

1 1

argil n

Lj

i ln

e

L

Average for the weighting factors obtained for all symbols

Determination of Weighting FactorDetermination of Weighting Factor

Error signal in frequency Error signal in frequency domaindomain

OFDM signal in frequency domainOFDM signal in frequency domain

Determined weighting factorDetermined weighting factor

Common weighting factor over Common weighting factor over one frameone frame

(a) (a) Original OFDMOriginal OFDM Signal Signal (b) (b) Phase Rotation due Phase Rotation due to weighting factor to weighting factor

-1.5 -1 -0.5 0 0.5 1 1.5-1.5

-1

-0.5

0

0.5

1

1.5

-1

0

1

0 1-1

(a)

-1.5 -1 -0.5 0 0.5 1 1.5-1.5

-1

-0.5

0

0.5

1

1.5

0 1-1

-1

0

1

(b)

Scatter Diagram Scatter Diagram oof Preamblef Preamble and Data Symbols and Data Symbols

( 1)21

( 1) ( )

0

21( 1)

0

1

1

in

knN jji i Nk n

n

knN ji Nn

n

y X e eN

X eN

Structure of Transmitter for Proposal MethodStructure of Transmitter for Proposal Method

S/P

MOD

Xn

Data & Preamble

0

N-1

IFF

T

P/S

+GI SSPAD/A &

U/C

yk

Xi Number of

predetermined iteration or PAPR < Target

No Yes

Calculate error signal

FFTDetermination of

common weighting factor

njn n n nR X e H N

ˆ ( ) /

/

n

n

n

jn n n n n

jn n n

jn

H X e H N X

e H N X

e H

Structure of Receiver for Proposed MethodStructure of Receiver for Proposed Method

ˆ ˆ/

ˆ( ) /( )

ˆ/( )

n n

n

n n n

j jn n n n

jn n n

n

X R H

X e H N e H

X N e H

X

D/C &

A/D

Estimation

Preamble

X DEMOD DATA-GI S/P FFT P/ S1ˆ

nH

nR ˆnX

Received DataReceived Data

Estimated Channel Estimated Channel ResponseResponse

Demodulated DataDemodulated Data

Frequency Domain EqulizationFrequency Domain Equlization

n : Weighting Factor at n-th Sub-Carrier: Weighting Factor at n-th Sub-Carrier

List of Parameters List of Parameters in Performance Evaluationin Performance Evaluation

Modulation 64QAM

Demodulation Coherent

Allocated bandwidth 5MHz

Number of FFT points 256

Number of sub-carriers 64

Symbol duration 12.8 s

Guard interval 1.28 s

Non-linear amplifier SSPA

Non-linear parameter of SSPA r =2

Number of data symbols in one frame 12

Number of preamble symbols in one frame 2

Multi-path fading model

Power delay profile Exponential

Number of delay paths 16

Decay constant -1 dB

4 5 6 7 8 9 10 11 1210

-3

10-2

10-1

100

PAPR (dB)

CC

DF

(P

rob.

PA

PR

> A

bsci

ssa)

Conventional OFDMProposed method

Number of sub-carriers = 64 Frame length = 12 symbols

PAPR PerformancePAPR PerformanceModulation Method =64QAM

ConventionalConventional

ProposedProposed

PAPR Performance vs. Number of IterationsPAPR Performance vs. Number of Iterations

0 2 4 6 8 10 12 145.5

6

6.5

7

7.5

8

Number of iterations

Ave

rage

PA

PR

(d

B)

Conventional OFDMProposed method

Total sub-carriers = 64 Frame length = 12 symbols

Modulation Method =64QAM

ConventionalConventional

ProposedProposed

10 15 20 25 30 35 40 45 5010

-5

10-4

10-3

10-2

10-1

100

C/N (dB)

BE

RLinearIBO = -2dBIBO = -4dB

Multi path = 16 pathsPower Decay = -1dB Rayleight

BER PerformanceBER Performance

Modulation Method =16QAM

ConventionalConventional

ProposedProposed

IdealIdeal

10 20 30 40 50 6010

-5

10-4

10-3

10-2

10-1

100

C/N (dB)

BE

RLinearIBO = -6dBIBO = -8dB

Conventional OFDM

Proposed method

BER PerformanceBER Performance

Modulation Method =64QAM

ConventionalConventional

ProposedProposed

IdealIdeal

Proposed the PAPR reduction method for OFDM signal without side information

Use the common weighting factor over one frame including the preamble symbols

The time-frequency domain swapping algorithm is employed in the determination of common weighting factor

The common weighting factor can be removed from the received data symbols by using the frequency domain equalization

The common weighting factor can be estimated together with the channel frequency response by using preamble symbols

ConclusionsConclusions

The proposed technique can achieve the better PAPR performance and better BER performance than the conventional OFDM in the channel of non-linear amplifier and multi-path fading

ConclusionsConclusions