ofdm for optical communications

OFDM for optical communications

ByB.Bindu Bhavani

M.Tech II Sem10N81D7001

2

Motivation

• High bit-rate wireless applications in a multipath radio environment.

• OFDM can enable such applications without a high complexity receiver with optical communication.

• OFDM is part of WLAN, DVB, and DAB standards and is a strong candidate for some of the 4G wireless

technologies.

OutlineOutlineIntroductionOFDM basicsOFDM Block diagramAdvantages and disadvantagesApplicationsOFDM in optical communicationConclusion

3

OFDMOFDMOFDM is a FDM scheme used as

digital multicarrier modulation method.

In this method a large closely spaced orthogonal subcarriers are used to carry the data.

4

Single Carrier SystemSingle Carrier SystemSequential

Transmission of

Waveforms Waveforms

are Short Duration T

Waveforms Occupy

Full System Bandwidth 1/T

Multi-Carrier SystemMulti-Carrier System Parallel

Transmission of

Waveforms Waveforms

are Long Duration

MT Waveforms

Occupy 1/M th Of System

Bandwidth 1/T

FDM FDM OFDM OFDM

Frequency Division Multiplexing

OFDM frequency dividing

EARN IN SPECTRAL EFFICIENCY

OFDM Signal

Ch.1

Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10

Ch.3 Ch.5 Ch.7 Ch.9Ch.2 Ch.4 Ch.6 Ch.8 Ch.10

Ch.1

Conventional multicarrier techniques

Orthogonal multicarrier techniques

50% bandwidth saving

Frequency

Frequency

ORTHOGONALITY Digital communication systems

In time domain In frequency domain

OFDM Two conditions must be considered for the

orthogonality between the subcarriers. 1. Each subcarrier has exactly an integer number

of cycles in the FFT interval. 2. The number of cycles between adjacent

subcarriers differs by exactly one.

* 1 ,

0 , i j

i jx t x t dt

i j

* 1 ,

0 , i j

i jX f X f df

i j

1 12 2 2

0 0

e es s ss s

s s

k n n kN Nj t t j t t j t tt T t TT T T

n n kt tn n

d dt d e dt d T

ORTHOGONALITY

Time domain

Frequency domain

Example of four subcarriers within one OFDM symbol

Spectra of individual subcarriers

FFT-based OFDM System

Serial-to-Parallel

Converter

SignalMapper

IFFTParallel-to-Serial

Converter

GuardIntervalInsertion

Serial Data Input

x bits0d

1d

1nd

0s

1s

1ns

D/A &Low pass

Filter

Up-Converter

Down-Converter

A/D Guard

IntervalRemoval

Serial-to-Parallel

ConverterFFT

One-tapEqualizer

SignalDemapper

Parallel-to-Serial

Converter

Serial Data

Output

0dx bits

1d

1ˆ

nd

0s1s

1ˆ ns

Channel

)(ts

Time

Frequency

Subchannels

Fast Fourier Transform

Guard Intervals

Symbols

FFT-based OFDM System OFDM Transmitter

Signal Mapper(QPSK)

IFFTParallel-to-Serial Converter

Guard IntervalInsertion

Serial-to-Parallel

Converter

2d

1nd

Serial

Data

Input

1s

2s

1ns

x bits

D/A&

LowpassFilter

1x 1d

2x

1nx

x=[0,0,0,1,1,0,1,1,….]

x1=[0,0]

x2=[0,1]

x3=[1,0]

x4=[1,1]

Q

.I

.

.

. 00

01

11

10

.I

.

.

. 00

01

11

10

Q

.

.

.

. 00

01

11

10

I.

.

.

. 00

01

11

10

d1=1

d2=i

d3=-1

d4=-i

…..

FFT-based OFDM System OFDM Transmitter

DATA CP

CP

CP

CP

CP

1

1

1

i

i

d

é ùê úê úê úê ú= ê úê úê úê ú-ê úë û

M

0 10 20 30 40 50 60 70-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

[ ]-0.09, -0.003-0.096i, , 0.01+ 0.247i, -0.035-0.0472is= L

0 10 20 30 40 50 60 70 80-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

Signal Mapper(QPSK)

IFFTParallel-to-Serial Converter

Guard IntervalInsertion

Serial-to-Parallel

Converter

2d

1nd

Serial

Data

Input

1s

2s

1ns

x bits

D/A&

LowpassFilter

1x 1d

2x

1nx

0 10 20 30 40 50 60 70 80-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

SERIES AND PARALLEL CONCEPTS

In OFDM system design, the series and parallel converter is considered to realize the concept of parallel data transmission.

Serial-to-Parallel

Converter

Serial data

Parallel data

s bT NTbT 2 bT 00 t t

The process of mapping the information bits onto the signal constellation plays a fundamental role in determining the properties of the modulation.

An OFDM signal consists of a sum of sub-carriers, each of which contains M-ary phase shift keyed (PSK) or quadrature amplitude modulated (QAM) signals.

Modulation types over OFDM systems Phase shift keying (PSK) Quadrature amplitude modulation (QAM)

MODULATION/MAPPING

MAPPING - PHASE SHIFT KEYING

M-ary phase shift keying Consider M-ary phase-shift keying (M-PSK) for which

the signal set is

where is the signal energy per symbol, is the symbol duration, and is the carrier frequency.

This phase of the carrier takes on one of the M possible values, namely ,

where

2 12cos 2 0 , 1, 2,...,s

i c ss

iEs t f t t T i M

T M

2 1i i M 1,2,...,i M

sEsTcf

Wireless Access Tech. Lab.

CCU Wireless Access Tech. Lab.

CCU Wireless Access Tech. Lab.

Mapping – Quadrature Amplitude

Modulation

The transmitted M-ary QAM signal for symbol i can be expressed as

where E is the energy of the signal with the lowest amplitude, and , and are amplitudes taking on the values, and,

where M is assumed to be a power of 4. The parameter a can be related to the average signal

energy ( ) by

2 2cos 2 sin 2 , 0 ,n n c n c

E Es t a f t b f t t T

T T

2, , 3 , , log 1 ,n na b a a M a

3

2 1sEa

M

na nb

sE

2E a

18

Multiple Data Rates/Modes

IFFT AND FFT

Inverse DFT and DFT are critical in the implementation of an OFDM system.

IFFT and FFT algorithms are the fast implementation for the IDFT and DFT.

In the IEEE 802.11a, the size of IFFT and FFT is N=64.

21

0

1 [ ] [ ]

N j knN

k

IDFT x n X k eN

21

0

[ ] [ ]N j kn

N

n

DFT X k x n e

OFDM - Spectrum

The FFT (and its inverse, the IFFT) are used to create a multitude of orthogonal subarriers using just a single TX.

Subcarrier orthogonality must be preserved in the presence of timing jitter frequency offset fading.

Frequency

Magnitude

T0

GUARD INTERVAL AND CYCLIC EXTENSION

OFDM symbol OFDM symbol duration .

Guard Interval DATA

gT T

Guard interval

FFT integration duration

OFDM symbol duration

total gT T T


Two different sources of interference can be identified in the OFDM system. Intersymbol interference (ISI) is defined as the

crosstalk between signals within the same sub-channel of consecutive FFT frames, which are separated in time by the signaling interval T.

Inter-carrier interference (ICI) is the crosstalk between adjacent subchannels or frequency bands of the same FFT frame.


For the purpose to eliminate the effect of ISI, the guard interval could consist of no signals at all.

Guard interval (or cyclic extension) is used in OFDM systems to combat against multipath fading.

:guard interval :multi path delay spread

In that case, however, the problem of intercarrier interference (ICI) would arise.

The reason is that there is no integer number of cycles difference between subcarriers within the FFT interval.

gT

delay spreadT

g delay spreadT T

Guard Interval Inserted Between Adjacent Symbols to Suppress ASI


To eliminate ICI, the OFDM symbol is cyclically extended in the guard interval.

This ensures that delayed replicas of the OFDM symbol always have an integer number of cycles within the FFT interval, as long as the delay is smaller than the guard interval.

Guard Interval(Cyclic Extension)

Cyclic Prefix Inserted in Guard Interval to Suppress Adjacent Channel Interference (ACI)

Cyclic Prefix: Comparision

CP

TTc

copy

CP functions: •It acomodates the decaying transient of the previous symbol

•It avoids the initial transient reachs the current symbol

CP

Pas

sin

g t

he

chan

nel

h(n

)

h(n)=(1)–n/n n=0,…,23

Including the Cyclic Prefix

Symbol: 8 periods of fi


Initial transientremains within

the CP

Final transientremains within

the CP

The inclusion of a CPmaintains the orthogonality

Pas

sin

g t

he

chan

nel

h(n

)

Initial transient Decaying transient

Channel:



Without the Cyclic Prefix

Loss of orthogonality

To combat the time dispersion: including ‘special’ time guards in the symbol transitions

ADVANTAGES AND DISADVANTAGES

Advantages Immunity to delay spread

Symbol duration >> channel impulse response Guard interval

Resistance to frequency selective fading Each subchannel is almost flat fading

Simple equalization Each subchannel is almost flat fading, so it only needs a

one-tap equalizer to overcome channel effect. Efficient bandwidth usage

The subchannel is kept orthogonality with overlap.


Disadvantages The problem of synchronization

Symbol synchronization Timing errors Carrier phase noise

Frequency synchronization Sampling frequency synchronization Carrier frequency synchronization

Need FFT units at transmitter, receiver The complexity of computations


Sensitive to carrier frequency offset The effect of ICI

The problem of high peak to average power ratio (PAPR) Problem 1. It increased complexity of the analog-to-digital

and digital-to-analog converters. Problem2. It reduced efficiency of the RF power amplifier. The solutions

1.Signal distortion techniques,which reduce the peak amplitudes simply by nonlinearly distorting the OFDM signal at or around the peaks.

2.Coding techniques using a special forward-error-correction code

3. It is based on scrambling each OFDM symbol with different scrambling sequences and then the sequence that gives the smallest PAP ratio is selected.

Applications

Digital Audio Broadcasting (DAB)Digital Video Broadcasting (DVB)Asymmetric Digital Subscriber Line (ADSL)Wireless LAN IEEE 802.11a

Wireless networking, device connectivityProposed for 802.16 standard

Connection between subscriber's transceiver station and a base transceiver station

OFDM Systems

System FFT Size

NumberCarriers

ChannelSpacing

kHz

BandwidthMHz

SampleRateMHz

SymbolDurationsec

Data

Rate

Mbits/s

HyperLAN/2 64 524

312.5 16.25 20 3.20.8

6-54

802.11a 64 524

312.5 16.56 20 3.2 0.8

6-54

DVB-T 20481024

1712842

4.464 7.643 9.174 224 0.68-14.92

DAB 20488192

1536 1.00 1.536 2.048 24/48/96msec

3.072

ADSL 256 (down)64 (up)

36-1277-28

4.3125 1.104 1.104 231.9 0.64-8.192

OPTICAL OFDM SYSTEM

TYPES OF OPTICAL OFDM

CO-OFDM DDO-OFDM FSO- OFDM

DDO- OFDM

LM-DDO-OFDM

NM-DDO-OFDM

FSO-OFDM TRANSMISSION SYSTEM

ReferencesReferences [1] Richard van Nee, Ramjee Prasad, OFDM wireless multimedia communication, Artech House

Boston London, 2000.

[2] Ahmad R. S. Bahai and Burton R. Saltzberg, Multi-carrier digital communications - Theory and applications of OFDM, Kluwer Academic / Plenum Publishers New York, Boston, Dordrecht, London, Moscow, 1999.

[3] Ramjee Prasad, “OFDM based wireless broadband multimedia communication,” Letter Notes on ISCOM’99, Kaohsiung, Taiwan, Nov. 7-10, 1999.

[4] L. Hanzo, W. Webb and T. Keller, Single- and multi-carrier quadrature amplitude modulation – Principles and applications for personal communications, WLANs and broadcasting, John Wiley & Sons, Ltd, 2000.

[5] Mark Engels, Wireless Ofdm Systems: How to Make Them Work? Kluwer Academic Publishers.

[6] Lajos Hanzo, William Webb, Thomas Keller, Single and Multicarrier Modulation: Principles and Applications, 2nd edition, IEEE Computer Society.

[7] Zou, W.Y.; Yiyan Wu, “ COFDM: An overview ” Broadcasting, IEEE Transactions on, Vol. 41, Issue 1, pp. 1 –8, Mar. 1995.

[8] Emmanuel C. Ifeachor & Barrie W. Jervis, Digital signal processing – A practical approach, Addision-Wesley, 1993.

[9] Blahut, R. E., Fast Algorithms for digital processing. Reading, Ma: Addison-Wesley, 1985.

[10] Simon Haykin, Communication Systems, John Wiley & Sons, Inc., 3rd edition, 1994.

[11] Roger L. Peterson, Rodger E. Ziemer, David E. Borth, Introduction to spread spectrum communications, Prentice Hall International Editions, 1995.

Conclusion

Thank You!!

Questions??

OFDM 802.11a

ofdm for optical communications

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