Transcript Document 7421654

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)
HISTORY OF OFDM
The origins of OFDM development started in the
late 1950’s 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 1980’s 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
OFDM DEMODULATOR
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 subchannels 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 Δf = 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 bitstream 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 WiLan, develops spacing between channels large
enough so that any frequency errors between
transmitter and receiver have no effect on
performance.
o
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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 FOFDM, is a system that is based on OFDM and
specifies also higher protocol layers. It has been
developed and is marketed by Flarion. FlashOFDM has generated interest as a packetswitched 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

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