16-06-2014, 02:09 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 centers
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 processing
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 purpose,
we shall use, as an example, one of the proposed OFDM signals of the Digital
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 transmission
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 generates
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 correcting
codes together with time and frequency interleaving yields even more robustness
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.
There are two main drawbacks with OFDM, the large dynamic range of the
signal (also referred as peak-to average [PAR] ratio) and its sensitivity to frequency
errors. These in turn are the main research topics of OFDM in many research centers
around the world, including the SARL.
FFT Implementation
The first task to consider is that the OFDM spectrum is centered on c f ; i.e.,
subcarrier 1 is 7.61
2 MHz to the left of the carrier and subcarrier 1,705 is 7.61
2 MHz to
the right. One simple way to achieve the centering is to use a 2N-IFFT [2] and T/2
as the elementary period. As we can see in Table 1, the OFDM symbol duration,
TU, is specified considering a 2,048-IFFT (N=2,048); therefore, we shall use a
5
4,096-IFFT. A block diagram of the generation of one OFDM symbol is shown in
Figure 2.1 where we have indicated the variables used in the Matlab code. The
next task to consider is the appropriate simulation period. T is defined as the elementary
period for a baseband signal,
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 research
and innovations are done in the receiver. For example, the frequency sensitivity
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 process
is shown in Figure 3.1.
OFDM is very sensitive to timing and frequency offsets [2]. Even in this ideal
simulation environment, we have to consider the delay produced by the filtering
operation. For our simulation, the delay produced by the reconstruction and demodulation
filters is about td=64/Rs. This delay is enough to impede the reception,
and it is the cause of the slight differences we can see between the transmitted
and received signals
Conclusion
We can find many advantages in OFDM, but there are still many complex problems
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 simulation
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.