ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING

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BY www.tricksworld.net.tc

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. 3

OFDM DEFINITION OFDM also known as

Multi-Carrier or Multi-Tone Modulation

DAB-OFDMDigital Audio Broadcasting

DVD-OFDMDigital Video Broadcasting

ADSL-OFDMAsynchronous Digital

Subscriber Line Wireless Local Area Network

IEEE-802.11a, IEEE-802.11gETSI 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 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 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 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 12

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.

o 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. 14

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

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