Orthogonal frequency division muLtiplexing

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Orthogonal frequency division muLtiplexing

• BY
• www.tricksworld.net.tc

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DISADVANTAGES OF RADIO PROPAGATION

• path loss
• fading
• Doppler shift
• multipath delay spread

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OFDM INTRODUCTION

• Orthogonal
  frequency-division multiplexing (OFDM) is a
  frequency-division multiplexing (FDM) scheme
  utilized as a digital multi-carrier modulation
  method. A large number of closely-spaced
  orthogonal sub-carriers are used to carry data.
  The data is divided into several parallel data
  streams or channels, one for each sub-carrier.
  Each sub-carrier is modulated with a conventional
  modulation scheme (such as quadrature amplitude
  modulation or phase shift keying) at a low symbol
  rate, maintaining total data rates similar to
  conventional single-carrier modulation schemes in
  the same bandwidth.

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OFDM DEFINITION

• OFDM also known as
• Multi-Carrier or Multi-Tone Modulation
• DAB-OFDM
• Digital Audio Broadcasting
• DVD-OFDM
• Digital Video Broadcasting
• ADSL-OFDM
• Asynchronous Digital Subscriber Line
• Wireless Local Area Network
• IEEE-802.11a, IEEE-802.11g
• ETSI BRAN (Hyperlan/2)

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HISTORY OF OFDM

• The origins of OFDM development started in the
  late 1950s with the introduction of Frequency
  Division Multiplexing (FDM) for data
  communications.
• In 1966 Chang patented the structure of OFDM and
  published the concept of using orthogonal
  overlapping multi-tone signals for data
  communications.
• In 1971 Weinstein introduced the idea of using a
  Discrete Fourier Transform (DFT) for
  implementation of the generation and reception of
  OFDM signals, eliminating the requirement for
  banks of analog subcarrier oscillators. suggested
  that the easiest implementation of

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CONTD.

• This presented an opportunity for an easy
  implementation of OFDM, especially with the use
  of Fast Fourier Transforms (FFT), which are an
  efficient implementation of the DFT.
• Until the late 1980s that work began on the
  development of OFDM for commercial use, with the
  introduction of the Digital Audio Broadcasting
  (DAB) system.

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TRANSMITTER
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OFDM Modulator
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OFDM Demodulator
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RECIEVER
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CHARACTERISTIS AND PRINCIPLE OF OPERATION

• ORTHOGONALITY
• In OFDM, the sub-carrier frequencies are
  chosen so that the sub-carriers are orthogonal to
  each other, meaning that cross-talk between the
  sub-channels is eliminated and inter-carrier
  guard bands are not required. This greatly
  simplifies the design of both the transmitter and
  the receiver unlike conventional FDM, a separate
  filter for each sub-channel is not required.
• The orthogonality requires that the
  sub-carrier spacing is ?f k/(TU) Hertz, where
  TU seconds is the useful symbol duration (the
  receiver side window size), and k is a positive
  integer, typically equal to 1.

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• CHANNEL CODING AND INTERLEAVING
• OFDM is invariably used in conjunction with
  channel coding (forward error correction), and
  almost always uses frequency and/or time
  interleaving.
• The reason why interleaving is used on OFDM is to
  attempt to spread the errors out in the
  bit-stream that is presented to the error
  correction decoder, because when such decoders
  are presented with a high concentration of errors
  the decoder is unable to correct all the bit
  errors, and a burst of uncorrected errors occurs.
• The information is typically FEC encoded and
  interleaved prior to modulation

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Versions of ofdm

• MIMO OFDM
• Multiple Input, Multiple Output Orthogonal
  Frequency Division Multiplexing is a technology
  developed by Iospan Wireless that uses multiple
  antennas to transmit and receive radio signal
• According to Iospan,
• "In this environment, radio signals bounce off
  buildings, trees and other objects as they travel
  between the two antennas. This bouncing effect
  produces multiple "echoes" or "images" of the
  signal. As a result, the original signal and the
  individual echoes each arrive at the receiver
  antenna at slightly different times causing the
  echoes to interfere with one another thus
  degrading signal quality.

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• The MIMO system uses multiple antennas to
  simultaneously transmit data, in small pieces to
  the receiver, which can process the data flows
  and put them back together.
• VOFDM (VECTOR OFDM)
• VOFDM (Vector OFDM) uses the concept of MIMO
  technology and is also being developed by Cisco
  Systems.
• WOFDM (WIDEBAND OFDM)
• WOFDM - Wideband OFDM, developed by Wi-Lan,
  develops spacing between channels large enough so
  that any frequency errors between transmitter and
  receiver have no effect on performance.

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• FLASH-OFDM
• Flash-OFDM (Fast Low-latency Access with
  Seamless Handoff Orthogonal Frequency Division
  Multiplexing), which is also referred to as
  F-OFDM, is a system that is based on OFDM and
  specifies also higher protocol layers. It has
  been developed and is marketed by Flarion.
  Flash-OFDM has generated interest as a
  packet-switched cellular bearer, where it would
  compete with GSM and 3G networks.

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applications

• WIRED APPLICATION
• ADSL and VDSL broadband access via POTS copper
  wiring.
• Power line communication (PLC).
• Multimedia over Coax Alliance (MoCA) home
  networking.
• ITU-T G.hn, a standard which provides high-speed
  local area networking over existing home wiring
  (power lines, phone lines and coaxial cables).

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Advantages

• Can easily adapt to severe channel conditions
  without complex equalization
• Robust against narrow-band co-channel
  interference
• Robust against Intersymbol interference (ISI) and
  fading caused by multipath propagation
• High spectral efficiency
• Efficient implementation using FFT
• Low sensitivity to time synchronization errors
• Tuned sub-channel receiver filters are not
  required (unlike conventional FDM)

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Disadvantages

• Sensitive to Doppler shift.
• Sensitive to frequency synchronization problems.
• High peak-to-average-power ratio (PAPR),
  requiring linear transmitter circuitry, which
  suffers from poor power efficiency.
• Loss of efficiency caused by Cyclic prefix/Guard
  interval

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CONCLUSION

• Performs better than a single modulated carrier
  in multipath fading
• With a properly implemented guard interval
• Time waveform appears periodic
• orthogonality of subcarriers is ensured
• ISI and ICI are eliminated

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REFERENCES

• http// Orthogonal frequency-division
  multiplexing - Wikipedia, the free
  encyclopedia.htm
• http//loadrunner.uits.iu.edu/upgrade/
• http//www.springerlink.com
• http//www.wpi.edu/Admin/IT/Ofdm/history.
• http//Ofdm.stanford.edu
• http//news.Ofdm.edu/prAreaSelect.cfm