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.
