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Introduction to OFDM. Fire Tom Wada Professor, Information Engineering, Univ. of the Ryukyus Chief Scientist at Magna Design Net, Inc wada@ie.u-ryukyu.ac.jp http://www.ie.u-ryukyu.ac.jp/~wada/. What is OFDM?. OFDM =Orthogonal Frequency Division Multiplexing

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introduction to ofdm

Introduction to OFDM

Fire Tom Wada

Professor, Information Engineering, Univ. of the Ryukyus

Chief Scientist at Magna Design Net, Inc

wada@ie.u-ryukyu.ac.jp

http://www.ie.u-ryukyu.ac.jp/~wada/

System Arch 2007 (Fire Tom Wada)

what is ofdm
What is OFDM?
  • OFDM=Orthogonal Frequency Division Multiplexing
  • Many orthogonal sub-carriers are multiplexed in one symbol
    • What is the orthogonal?
    • How multiplexed?
    • What is the merit of OFDM?
    • What kinds of application?

System Arch 2007 (Fire Tom Wada)

outline
Outline
  • Background, history, application
  • Review of digital modulation
  • FDMA vs. Multi-carrier modulation
  • Theory of OFDM
  • Multi-path
  • Summary

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why ofdm is getting popular
Why OFDM is getting popular?
  • State-of-the-art high bandwidth digital communication start using OFDM
    • Terrestrial Video Broadcasting in Japan and Europe
    • ADSL High Speed Modem
    • WLAN such as IEEE 802.11a/g/n
    • WiMAX as IEEE 802.16d/e
  • Economical OFDM implementation become possible because of advancement in the LSI technology

System Arch 2007 (Fire Tom Wada)

japan terrestrial video broadcasting service
Japan Terrestrial Video Broadcasting service
  • ISDB-T (Integrated Services Digital Broadcasting for Terrestrial Television Broadcasting)
  • Service starts on 2003/December at three major cities (Tokyo, Nagoya, Osaka)
  • Full service area coverage on 2006
  • 5.6MHz BW is divided into 13 segments (~430KHz BW)
  • HDTV: 12 segments
  • Mobile TV : 1 segment
  • SDTV: 4 segment
  • Analog Service will end 2011

System Arch 2007 (Fire Tom Wada)

brief history of ofdm
Brief history of OFDM
  • First proposal in 1950’s
  • Theory completed in 1960’s
  • DFT implementation proposed in 1970’s
  • Europe adopted OFDM for digital radio broadcasting in 1987
  • OFDM for Terrestrial Video broadcasting in Europe and Japan
  • ADSL, WLAN(802.11a)

System Arch 2007 (Fire Tom Wada)

digital modulation basics
Digital modulation basics
  • Digital modulation modulates three parameters of sinusoidal signal.
  • A, θk fc,
  • Three type digital modulation:
    • ASK : Amplitude Shift Keying
    • PSK : Phase Shift Keying
    • FSK : Frequency Shift Keying

OFDM uses combination of ASK and PSK such as QAM, PSK

System Arch 2007 (Fire Tom Wada)

symbol waveform
Symbol Waveform

1

0

1

0

0

Digital Information

carrier

ASK

PSK

FSK

System Arch 2007 (Fire Tom Wada)

Symbol length

multi bit modulation
Multi bit modulation

carrier

1

0

1

0

0

BPSK

1bit per symbol

11

01

00

01

10

QPSK

2bit per symbol

Symbol length

System Arch 2007 (Fire Tom Wada)

mathematical expression of digital modulation
Transmission signal can be expressed as follows

s(t) can be expressed by complex base-band signal

Mathematical expression of digital modulation

Indicates carrier sinusoidal

Digital modulation

Digital modulation can be expressed by the complex number

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constellation map
Constellation map
  • (ak + jbk) is plotted on I(real)-Q(imaginary) plane

QPSK

Q

I

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quadrature amplitude modulation qam
Quadrature Amplitude Modulation (QAM)

16QAM

64QAM

Q

Q

I

I

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summary of digital modulation

Q

I

Summary of digital modulation
  • Type of modulation: ASK,PSK,FSK,QAM
  • OFDM uses ASK,PSK,QAM
  • Digital modulation is mathematically characterized by the coefficient of complex base-band signal
  • Plot of the coefficients gives the constellation map

System Arch 2007 (Fire Tom Wada)

frequency division multiple access fdma
Frequency Division Multiple Access (FDMA)
  • Old conventional method (Analog TV, Radio etc.)
  • Use separate carrier frequency for individual transmission

Channel separation

Occupied BW

fc1

fc2

fc3

fcN

Radio frequency

Carrier frequency

Guard band

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japan vhf channel assignment
Japan VHF channel assignment
  • Channel Separation = 6MHz

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multi carrier modulation
Multi-carrier modulation
  • Use multiple channel (carrier frequency) for one data transmission

LPF

LPF

DEMULTIPLEX

MULTIPLEX

data

data

LPF

System Arch 2007 (Fire Tom Wada)

spectrum comparison for same data rate transmission
Spectrum comparison for same data rate transmission

Multi carrier

frequency

Single carrier

frequency

OFDM

frequency

System Arch 2007 (Fire Tom Wada)

ofdm vs multi carrier
OFDM vs. Multi carrier
  • OFDM is multi carrier modulation
  • OFDM sub-carrier spectrum is overlapping
  • In FDMA, band-pass filter separates each transmission
  • In OFDM, each sub-carrier is separated by DFT because carriers are orthogonal
    • Condition of the orthogonality will be explained later
  • Each sub-carrier is modulated by PSK, QAM

Thousands of PSK/QAM symbol can be

simultaneously transmitted in one OFDM symbol

System Arch 2007 (Fire Tom Wada)

ofdm carriers
OFDM carriers
  • OFDM carrier frequency is n・1/T

Symbol period T

System Arch 2007 (Fire Tom Wada)

sinusoidal orthogonality

Orthogonal

Orthogonal

Sinusoidal Orthogonality
  • m,n: integer, T=1/f0

Orthogonal

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a sub carrier of f nf 0
Amplitude and Phase will be digitally modulatedA sub-carrier of f=nf0

n cycles

Time

t=0

t=T

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base band ofdm signal
Base-band OFDM signal

n=0

n=1

n=2

n=3

n=4

n=5

n=6

sB(t)

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how a n b n are caluculated from s b t demodulation procedure
How an,bn are caluculated from sB(t)- Demodulation Procedure -
  • According to the sinusoidal orthogonality, an,bn can be extracted.
  • In actual implementation, DFT(FFT) is used
  • N is roughly 64 for WLAN, thoudand for Terrestrial Video Broadcasting

System Arch 2007 (Fire Tom Wada)

pass band ofdm signal
Pass-band OFDM signal
  • SB(t) is upcoverted to pass-band signal S(t)
  • fc frequency shift

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actual ofdm spectrum
Actual OFDM spectrum

fc+kf0

fc+(k+1)f0

fc+(k-1)f0

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ofdm power spectrum
OFDM power spectrum
  • Total Power spectrum is almost square shape

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ofdm signal generation
OFDM signal generation
  • Direct method needs

N digital modulators

N carrier frequency generator

 Not practical

  • In 1971, method using DFT is proposed to OFDM siganal generation

System Arch 2007 (Fire Tom Wada)

ofdm signal generation in digital domain
Define complex base-band signal u(t) as follows

Perform N times sampling in period T

OFDM signal generation in digital domain

u(k) = IFFT (dn) = IFFT(an + jbn)

System Arch 2007 (Fire Tom Wada)

ofdm modulator
OFDM modulator

MAP

S

/

P

I-DFT

P

/

S

Real

Bit stream

Imag

BPF

AIR

generated0~dN-1

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ofdm demodulation
OFDM demodulation

dn = FFT(u(k))

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ofdm demodulator too simple
OFDM demodulator (Too simple)

Tuner

LPF

S

/

P

DFT

P

/

S

A

/

D

Channel

π/2

LPF

DEMAP

Bit Stream

System Arch 2007 (Fire Tom Wada)

summary of ofdm signal
Summary of OFDM signal
  • Each symbol carries information
  • Each symbol wave is sum of many sinusoidal
  • Each sinusoidal wave can be PSK, QAM modulated
  • Using IDFT and DFT, OFDM implementation became practical

Time

Symbol periodT=1/f0

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multi path
Multi-path
  • Delayed wave causes interference

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multi pass effect
Multi-pass effect

Symbol k-1

Symbol k

Symbol k+1

  • Inter symbol interference (ISI) happens in Multi-path condition

No multi-path

T=1/f0

Sampling Period

Direct

Multi-path

Delayed

Sampling Period

System Arch 2007 (Fire Tom Wada)

guard interval t g
Guard Interval Tg

OFDM symbol(1/f0)

Tg

  • By adding the Gurard Interval Period, ISI can be avoided

Tg

Copy signal

Tg

OFDM symbol (1/f0)

Direct

Delayed

Sampling Period

System Arch 2007 (Fire Tom Wada)

multi path36
Multi-path
  • By adding GI, orthogonality can be maintained
  • However, multi-path causes Amplitude and Phase distortion for each sub-carrier
  • The distortion has to be compensated by Equalizer

System Arch 2007 (Fire Tom Wada)

multiple frequency network
Multiple Frequency Network

f3

  • Frequency utilization is low

f1

f1

Area 3

Area 4

Area 1

f2

Area 2

System Arch 2007 (Fire Tom Wada)

single frequency network
Single Frequency Network

f1

  • If multi-path problem is solved,SFN is possible

f1

f1

Area 3

Area 4

Area 1

f1

Area 2

System Arch 2007 (Fire Tom Wada)

that s all for introduction
That’s all for introduction
  • Feature of OFDM
    • High Frequency utilization by the square spectrum shape
    • Multi-path problem is solved by GI
    • Multiple services in one OFDM by sharing sub-carriers (3 services in ISDB-T)
    • SFN
    • Implementation was complicated but NOW possible because of LSI technology progress

System Arch 2007 (Fire Tom Wada)