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8/3/2019 OFDMPPT_ Tapan

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Orthogonal Frequency

Division MultiplexingOFDM

Tapan Kumar Pradhan

Electronics and Tele-Communication

Regd. No- 1007214020

KONARK INSTITUTE OF SCIENCE AND TECHNOLOGY

JATANI


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Objectives

What is OFDM

How OFDM works

OFDM Advantages\Disadvantages

Applications

References


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What is OFDM ?

The single-carriermodulation system

- A single carrier system modulates information onto

one carrier using frequency, phase, or amplitudeadjustment of the carrier.

- As higher bandwidths (data rates) are used, the

duration of one bit or symbol of information becomes

smaller. The system becomes more susceptible to loss ofinformation.


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Single Carrier System

Sequential Transmissionof Waveforms

Waveforms are

Short Duration T

Waveforms Occupy

Full System Bandwidth 1/T


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FDM modulation system

- Frequency division multiplexing uses multiple sub carriers

within the same single channel.

- The total data rate to be sent in the channel is divided between

the various sub carriers.

- FDM offers an advantage in terms of narrowband frequency

interference since this interference will only affect one of the

frequency sub-bands, the other sub bands remains un-affected.


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Multi-Carrier System

Parallel Transmission

of Waveforms

Waveforms areLong Duration MT

Waveforms Occupy 1/M th

Of System Bandwidth 1/T


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Orthogonal FDM

- If the FDM system above had been able to use a set ofsubcarriers those were orthogonal to each other, a higher level of

spectral efficiency could have been achieved.

- The use of orthogonal subcarriers would allow the subcarriers

spectra to overlap, thus increasing the spectral efficiency.


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How it works ? OFDM works as the concept of frequency division

multiplexing (multiple sub carriers in a single channel).

In FDD the sub carriers are separated by guard bands

but OFDM doesnt require the guard band.

In the transmitter, binary input data is encoded,The

symbol is modulated onto sub carriers by applying the

Inverse Fast Fourier Transform (IFFT). To the output a

cyclic prefix is added to make the system robust to

multipath propagation and the signal is converted to

analog.


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The receiver has to estimate frequency offsetand symbol timing, using special training symbols in

the preamble. After removing the cyclic prefix, the

signal can be applied to a Fast Fourier Transform to

recover.


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OFDM Advantages

The OFDM transmission scheme has the following key advantages: Makes efficient use of the spectrum by allowing overlap By dividing the channel into narrowband flat fading subchannels,

OFDM is more resistant to frequency selective fading than single

carrier systems are. Eliminates ISI and ICI through use of a cyclic prefix. Using adequate channel coding and interleaving one can recover

symbols lost due to the frequency selectivity of the channel.


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OFDM Disadvantages

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 average power ratio.

It is more sensitive to carrier frequency offset and driftthan single carrier systems are due to leakage of the DFT.


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Applications

Digital Audio Broadcasting

Digital Video Broadcasting

Wireless LAN


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Books and References

The Principles of OFDM

Louis Litwin and pugel

OFDM Wireless LANs: A Theoretical and Practical Guide

Juha Heiskala and John Terry

Communications Systems Engineering.John G. Proakis and Masoud Salehi

Single and Multi-Carrier Amplitude Modulation

Lajos Hanzo, William Webb, and Thomas Keller


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THANK YOU

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