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21-09-2008, 10:53 AM
Post: #1
Orthogonal Frequency Division Multiplplexing

Multi Carrier Modulation is a technique for data transmission by dividing a high ?]bit rate data stream is several parallel low bit ]rate data streams and using these low bit rate data streams to modulate several carriers. Multi Carrier Transmission has a lot of useful properties such as delay spread tolerance and spectrum efficiency that encourage their use in untethered broadband communications. OFDM is a multi carrier modulation technique with densely spaced sub carriers that has gained a lot of popularity among the broadband community in the last few years. It has found immense applications in communication systems. This report is intended to provide a tutorial level introduction to OFDM Modulation, its advantages and demerits, and some applications of OFDM.

History Of OFDM
The concept of using parallel data transmission by means of frequency division multiplexing (FDM) was published in mid 60s. Some early development can be traced back in the 50s. A U.S. patent was filled and issued in January, 1970. The idea was to use parallel data streams and FDM with overlapping sub channels to avoid the use of high speed equalization and to combat impulsive noise, and multipath distortion as well as to fully use the available bandwidth. The initial applications were in the military communications. In the telecommunications field, the terms of discrete multi tone (DMT), multichannel modulation and multicarrier modulation (MCM) are widely used and sometimes they are interchangeable with OFDM. In OFDM, each carrier is orthogonal to all other carriers. However, this condition is not always maintained in MCM. OFDM is an optimal version of multicarrier transmission schemes.

For a large number of sub channels, the arrays of sinusoidal generators and coherent demodulators required in a parallel system become unreasonably expensive and complex. The receiver needs precise phasing of the demodulating carriers and sampling times in order to keep crosstalk between sub channels acceptable. Weinstein and Ebert applied the discrete Fourier transform (DFT) to parallel data transmission system as part of the modulation and demodulation process. In addition to eliminating the banks of subcarrier oscillators and coherent demodulators required by FDM, a completely digital implementation could be built around special purpose hardware performing the fast Fourier transform (FFT). Recent advances in VLSI technology enable making of high speed chips that can perform large size FFT at affordable price.

In the 1980s, OFDM has been studied for high speed modems, digital mobile communications and high density recording. One of the systems used a pilot tone for stabilizing carrier and clock frequency control and trellis coding was implemented. Various fast modems were developed for telephone networks.
In 1990s, OFDM has been exploited for wideband data communications over mobile radio FM channels, high bit rate digital subscriber lines (HDSL, 1.6 Mb/s), asymmetric digital subscriber lines (ADSL, 1,536 Mb/s), very high speed digital subscriber lines (VHDSL, 100 Mb/s), digital audio broadcasting (DAB) and HDTV terrestrial broadcasting.


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