ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING(OFDM)

byB.KesavaRao, Asst.Prof.

MGIT

UNIT-8

Index

• Introduction

• Basic Principles of Orthogonality

• Single Vs Multi channel systems

• OFDM block diagram

• OFDM signal Mathematical representation

Objective of OFDM

Traditional vs. OFDM Communication

Effect of ISI

OFDM- Introduction

• The major requirement of modern wireless communication systems are high capacity and variable bit rate transmission with high BW efficiency.

• But, the wireless environment signal usually impaired by fading and multipath delay spreading .

• The traditional single carrier mobile communication systems do not perform well because it suffers extreme fading of the signal and Inter Symbol Interference(ISI).

• This leads to a high probability of errors and the system’s overall performance becomes very poor.

• Because of its high-speed data transmission and effectiveness in combating the frequency selective fading channel, OFDM technique is widely used in wireless communication nowadays.

• Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique, which divides the available spectrum into many subcarriers, each one being modulated by a low data rate stream.

Introduction to ofdm

Orthogonal Frequency Division Multiplexing (OFDM) is a frequency division multiplexing (FDM) scheme used as a digital multi carrier modulation (MCM) method.

OFDM is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies.

The main idea behind the OFDM is that since low-rate modulations are less sensitive to multipath, the better way is to send a number of low rate streams in parallel than sending one high rate waveform.

A large number of closely spaced orthogonal subcarriers are used to carry data.

OFDM is a promising technique for achieving high data rate and combating multipath fading in Wireless Communications.

OFDM DEFINITION

OFDM = Orthogonal FDM Carrier centers are put on orthogonal frequencies ORTHOGONALITY - The peak of each signal coincides with

other signals Subcarriers are spaced by 1/Ts

Ofdm time and frequency response

0 2 4 6 8 10 12-0.4

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0

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FREQUENCY DOMAIN: OFDM Subcarriers 2 through 10

Frequency (Normalized by 1/Tofdm)

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1.5TIME DOMAIN: 2 OFDM subcarriers (BPSK)

Time (Normalized by Tofdm)

Ofdm versus fdm

OFDM-Introduction Contd..• OFDM can be viewed as either a modulation technique or a

multiplex technique.– Modulation technique

• Viewed by the relation between input and output signals– Multiplex technique

• Viewed by the output signal which is the linear sum of the modulated signals

Input Signal

OFDM System

Output Signal

S/P

frequency frequency

freq

uenc

y re

spon

se

freq

uenc

y re

spon

se

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OFDM is a considerable option when the channel introduces ISI

Applications: ADSL, DAB, DVB, Hiperlan/2, ...

Why OFDM?

OFDM – Orthogonal Frequency Division Multiplexing

• Uses the entire bandwidth • Splits bandwidth into subchannels

• Short symbol times

• This causes ISI

• Sends information in parallel

• OFDM: orthogonal subcarriers

Single Carrier Multicarrier

Modulation techniques: monocarrier vs. multicarrier

To improve the spectral efficiency:

To use orthogonal carriers (allowing overlapping)Eliminate band guards between carriers

– Selective Fading

– Very short pulses

– ISI is compartively long

– EQs are then very long

– Poor spectral efficiency because of band guards

Drawbacks

– It is easy to exploitFrequency diversity

– Flat Fading per carrier

– N long pulses

– ISI is comparatively short

– N short EQs needed

– Poor spectral efficiency because of band guards

Advantages

Furthermore

– It allows to deploy2D coding techniques

– Dynamic signalling

N carriers

B

Pulse length ~ N/B

Similar toFDM technique

– Data are shared among several carriers and simultaneously transmitted

B

Pulse length ~1/B

– Data are transmited over only one carrier

Channel

Guard bands

Channelization

OFDM and Multicarrier Transmission

• Single and multicarrier transmissionSingle carrier transmission

1cos(2 )f t

ib ( )s tS/P

1cos(2 )f t

2cos(2 )f t

cos(2 )Nf t

ib ( )s t

Multicarrier carrier transmission

2/8

Orthogonality in OFDM

• Orthogonality Time domain frequency domain

Band pass signal

Where is the equivalent low pass signal of

if , n is an non-integer i.e. Then

*1 2( ) ( ) 0x t x t dt

*1 2( ) ( ) 0X f X f df

2 ( ) 2( ) cos(2 ( ) ) Re Re ( )c m ci f f t i f tm c m lmx t f f t e x t e

2( ) mi f tlmx t e ( )mx t

1 2 1 22 2 2 ( )*12

0 0

sin( )( )

T Ti f t i f t i f f t i fTfTe e dt e dt e

f

f T n nfT

12 0

OFDM and Multicarrier Transmission

(A)

(E)

(D)

(C)

(B)

1cf f 2cf f 3cf f 4cf f 5cf f

1

T

1

T

Orthogonal

Non-orthogonal

Orthogonal, n=3

Orthogonal, n=2

Orthogonal, n=1(OFDM)

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OFDM and Multicarrier Transmission

1subcarrier f

2subcarrier f

3subcarrier f

4subcarrier f

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Time domain Frequency domain

Orthogonal Frequency Division Modulation

Data coded in frequency domain

N carriers

B

Transformation to time domain:each frequency is a sine wavein time, all added up.

f

Transmit

Symbol: 8 periods of f0

Symbol: 4 periods of f0

Symbol: 2 periods of f0

+

Receivetime

B

Decode each frequencybin separately

Channel frequencyresponse

f

f

Time-domain signal Frequency-domain signal

Principles OFDM Some processing is done on the source data, such as coding for correcting errors, interleaving and mapping of bits onto symbols. An example of mapping used is QAM. The symbols are modulated onto orthogonal sub-carriers. This is done by using IFFT. Orthogonality is maintained during channel transmission. This is achieved byadding a cyclic prefix to the OFDM frame to be sent. The cyclic prefix consists ofthe L last samples of the frame, which are copied and placed in the beginning ofthe frame. It must be longer than the channel impulse response. Synchronization: the introduced cyclic prefix can be used to detect the start of each frame. This is done by using the fact that the L first and last samples are the same and therefore correlated. This works under the assumption that one OFDM frame can be considered to be stationary. Demodulation of the received signal by using FFT Channel equalization: the channel can be estimated either by using a trainingsequence or sending known so-called pilot symbols at predefined sub-carriers. Decoding and de-interleaving

Advantages of OFDM

• High spectral efficiency• Simple implementation by FFT• Low receiver complexity• Robust ability for high-data rate transmission over multipath

fading channel.• High link in terms of link adaption.• OFDM eliminates Inter Symbol Interference (ISI) through the

use of a cyclic prefix.• OFDM is less sensitive to sample timing offsets than the

single carrier systems.• OFDM eliminates the need for equalizers

Disadvantages of OFDM

Synchronization

Need FFT units at transmitter, receiver

Sensitive to carrier frequency offset

High peak to average power ratio

Application of OFDM

•  Worldwide Interoperability for Microwave Access (WiMAX).

• Terrestrial Digital Audio Broadcasting (DVB-T).

• Wireless Metropolitan Area Network (WMAN).(IEEE

802.16d).

• Wireless Local Area Network (WLAN). ETSI HiperLAN.

• Digital Audio Broadcasting (DAB).

• Digital Video Broadcasting (DVB).

• High Definition Television (HDTV).

• Broadband Internet Access