Post on 06-Apr-2018
transcript
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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