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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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Inter symbol interference
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Inter symbol interference
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
Inter symbol interference
Fig. An arbitrary signalA in the frequency domain
Fig. Phase shift onA is simply a rotation on the
complex axes
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Inter symbol interference
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