30-04-2014, 03:53 PM
OFDM Simulation Using Matlab
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Abstract
Orthogonal frequency division multiplexing (OFDM) is becoming the chosen
modulation technique for wireless communications. OFDM can provide large data
rates with sufficient robustness to radio channel impairments. Many research cen-
ters in the world have specialized teams working in the optimization of OFDM for
countless applications. Here, at the Georgia Institute of Technology, one of such
teams is in Dr. M. A. Ingram's Smart Antenna Research Laboratory (SARL), a part
of the Georgia Center for Advanced Telecommunications Technology (GCATT).
The purpose of this report is to provide Matlab code to simulate the basic proc-
essing involved in the generation and reception of an OFDM signal in a physical
channel and to provide a description of each of the steps involved. For this pur-
pose, we shall use, as an example, one of the proposed OFDM signals of the Digi-
tal Video Broadcasting (DVB) standard for the European terrestrial digital television
(DTV) service.
Introduction
In an OFDM scheme, a large number of orthogonal, overlapping, narrow band
sub-channels or subcarriers, transmitted in parallel, divide the available transmis-
sion bandwidth. The separation of the subcarriers is theoretically minimal such that
there is a very compact spectral utilization. The attraction of OFDM is mainly due to
how the system handles the multipath interference at the receiver. Multipath gen-
erates two effects: frequency selective fading and intersymbol interference (ISI).
The "flatness" perceived by a narrow-band channel overcomes the former, and
modulating at a very low symbol rate, which makes the symbols much longer than
the channel impulse response, diminishes the latter. Using powerful error correct-
ing codes together with time and frequency interleaving yields even more robust-
ness against frequency selective fading, and the insertion of an extra guard interval
between consecutive OFDM symbols can reduce the effects of ISI even more.
Thus, an equalizer in the receiver is not necessary.
OFDM Reception
As we mentioned before, the design of an OFDM receiver is open; i.e., there
are only transmission standards. With an open receiver design, most of the re-
search and innovations are done in the receiver. For example, the frequency sensi-
tivity drawback is mainly a transmission channel prediction issue, something that is
done at the receiver; therefore, we shall only present a basic receiver structure in
this report. A basic receiver that just follows the inverse of the transmission proc-
ess is shown in Figure 3.1.
Conclusion
We can find many advantages in OFDM, but there are still many complex prob-
lems to solve, and the people of the research team at the SARL are working in
some of these problems. It is the purpose of this project to provide a basic simula-
tion tool for them to use as a starting point in their projects. We hope that by using
the specifications of a working system, the DBV-T, as an example, we are able to
provide a much better explanation of the fundamentals of OFDM.