Final Ofdm Ppt 3

8/4/2019 Final Ofdm Ppt 3

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Implementation of an Orthogonal

Frequency Division Multiplexing(OFDM)

for high data rate over noisy channel.

Presented By

Shailesh khaparkar


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Aim of the Project

The aim of the project is to implement the OFDM

system for high data rate using MATLAB Software and

estimating the performance of it.


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Introduction

OFDMOrthogonal Frequency Division Multiplexing.

Developed in 1960s and 70s.

Definition:It is a multi carrier transmission technique, which

divides the available spectrum into many carriers, each one

being modulated by a low rate data stream.

It depends on Orthogonality

It is a special form of MCM.


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Actually, FDM systems have been common for many

decades. But, it waste too much bandwidth. This is where

OFDM sense.

Used to combat ISI due to multipath and also used in the

area of high-data-rate mobile wireless communications

such as cellular phone, satellite communication etc.,

Introduction


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Multiple Access Techniques

Multiple access schemes are used to allow many simultaneous users touse the same fixed bandwidth radio spectrum.

Sharing of the spectrum is required in order increase the user capacity

of any wireless network.

There are mainly three types of multiple access techniques

FDMA (Frequency Division Multiple Access)

TDMA (Time Division Multiple Access)

CDMA (Code Division Multiple Access)

The hybrid technique for these methods is

OFDM (Orthogonal Frequency Division Multiplexing)


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FDMA

Available bandwidth is subdivided into a number of narrower band channels.

Each user is allocated a unique frequency band in which to transmit and

receive on.

Each user is allocated a forward link channel (from the base station to themobile phone) and a reverse channel (back to the base station), each being asingle way link.


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TDMA

TDMA divides the available spectrum into multiple time slots, bygiving each user a time slot in which they can transmit or receive.

TDMA can suffer from multipath effects as the transmission rate isgenerally very high. This leads the multipath signals causing inter-symbol interference.

Fig 1.4 TDMA scheme, where each user is allocated a small time slotFig TDMA scheme, where each user is allocated a small time slot


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CDMA

Fig. Basic CDMA Generation.

CDMA is a spread spectrum technique that uses neither frequency channels ortime slots.

All users in a CDMA system use the same frequency band and transmitsimultaneously

The transmitted signal is recovered by correlating the received signal with thePN code used by the transmitter.


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Why Use OFDM?

In previous research, it has been proven that OFDM

outperforms other transmission schemes in many

aspects of multiple access transmission.

Suppression of Inter-Symbol Interference (ISI)

Allows many more users

Flexible bandwidth allocation


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OFDM System

OFDM Block Diagram


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A/D Converter (In Tx)

Sampler Quantizer Encoder

Analog Signal Discreate Time

Sequence

Quantized Signal Digital Signal

10110

Block Diagram of A/D Converter


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QAM System (In Tx)

Product

Modulator

90 phaseShifter

Product

Modulator

Message

Signals

X1(t)

X2(t)

C(t)=Acos(2 pifct)

Multiplexed

Signal s(t)

QAM SYSTEM


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QPSK Generation

Bipolarencodar

A Demultipulator AdderQPSK signal

Sin(2pifct)

Cos(2pifct)

be(t)

bo(t)

Se(t)

b(t)Binary

Data

Generation of QPSK


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Channel Characteristics

The performance of any wireless systems performance is

affected by the medium of propagation namely the

characteristics of the channel.

The different characteristics of channel are

1.Multipath

2.Attenuation

3.Doppler shift


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Multipath Effect


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Attenuation

Attenuation is the drop in the signal power when transmitting from one point to

another. It can be caused by the transmission path length,obstructions in the singlepath and multi path effects.

Shadowing of the signal can occur whenever there is an obstruction between the

transmitter and receiver.

To overcome the problem of shadowing, Txs are usually elevated as high as possible

to minimize the number of obstructions.

Delay Spread

Delay spread is the time spread between the arrival of the first and last multipathsignal seen by the receiver.

In a digital system, the delay spread can lead to inter-symbol interference.

This is due to the delayed multipath signal overlapping with the following symbols.


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Doppler Shift

When a wave source and a receiver are moving relative to

one another the frequency of the received signal will not be

the same as the source. When they are moving toward each

other the frequency of the received signal is higher then thesource, and when they are approaching each other the

frequency decreases. This is called the Doppler effect.

The amount the frequency changes due to the Dopplereffect depends on the relative motion between the source

and receiver and on the speed of propagation of the wave.


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Inter symbol interference

Combating ISI using a guard interval


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Fig. OFDM Model used for simulations

Methodology


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Flow Chart of

OFDM Systemin MATLAB


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A

B

C

D = A+B+C

A

A is still present, only with some phase shift.


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real imag

f f


Fig. An arbitrary signalA in the frequency domain

Fig. Phase shift onA is simply a rotation on the

complex axes


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Fig. SignalA and SignalBexperience 150 phase

shifts

Fig. SignalA with unknown phase and signalB

with known phase


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PLATFORMMATLAB:

Full form is MATrix LABoratory.

Developed by Math Works.

It is a high level technical computing language and interactive environment

for algorithm development, data visualization, data analysis and

numeric computation.

MATLAB contains five major sections:

1.Development Environment

2.MATLAB mathematical function library.

3.MATLAB language.

4.Graphics.

5.MATLAB application program interference.


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MATLAB features & capabilities


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Advantages of OFDM

1. Makes efficient use of the spectrum by allowing overlap.

2. By dividing the channel into narrowband flat fading sub channels,

OFDM is more resistant to frequency selective fadings than single

carrier systems.

3. Using adequate channel coding and interleaving one can recover

symbols lost due to the frequency selectivity of the channel.

4. OFDM is computationally efficient by using FFT techniques to

implement the modulation and demodulation functions

5. Provides good protection against co-channel interference and

impulsive parasitic noise.6. Preservation of orthogonality in severe multipath.

7. Used for highest speed applications.


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Disadvantages of OFDM

1.The OFDM signal has a noise like amplitude with

a very large dynamic range, therefore it requires

RF power amplifiers with a high peak to averagepower ratio.

2.It is more sensitive to carrier frequency

offset and drift than single carrier systems.

3.It is sensitive to co-channel interferences.


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Future Application of OFDM

Military HF modems

Voice band modems

ADSL & HDSL Digital Broadcasting (DAB and DVB-TV)

WLANs (IEEE 802.11 & Hiper LAN II)

Cable modems

WDM fiber optics


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Simulation&Result

Simulation is done on matlab and OFDM is compaire with qam

Bit error rate of OFDM and qam is compaired.


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QAM AND OFDM INPUT AND OUTPUT


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CONCLUSION

OFDM is a multi-carrier scheme providing high

data rates over noisy channel.

OFDM systems offers good spectral efficiency.


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References


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1-Bahai, A., and B. Saltzberg. Multicarrier Digital Communications: Theory and

Applications of OFDM. New York: Kluwer Academic/Plenum Publishers, 1999

2-Van Nee, R., and R. Prasad. OFDM Wireless Multimedia Communications. Boston:

Artech House, 2000

3-Couch II, L. W. Digital and Analog Communication Systems. New Jersey: Prentice-Hall,

1997

4-Keller, T., and L. Hanzo. Adaptive Multicarrier Modulation: A Convenient Framework

for Time-Frequency Processing in Wireless Communications. Proceedings of the IEEE88.5 (2000) 609 - 639

5-OFDM Wireless Technology, Eric Lawrey and Craig Blackburn. 2000. James Cook

University. .

6-Spread Spectrum Scene, SSS Online, Inc. 2001

7-Wireless Resource Center, PaloWireless.Com. 2001

8-OFDM Receiver for Broadband Receivers, Michael Speth. Institute for Integrated Signal

Processing Systems. 2001.


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Thanq


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Queries

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Final Ofdm Ppt 3