| Orthogonal
Frequency Division Multiplplexing |
Definition
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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