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*Resources**Radio technology**Orthogonal Frequency Division Multiplexing(OFDM)*

This Java applet allows you to experience the basics of OFDM modulation and
demodulation visually.

It’s interesting to look at the spectrogram and try changing the various
parameters. You can understand the background to the characteristics of the
signal.

To run the applet, click the picture.

*Java Runtime Environment version 5.0 or higher is required to run the applet.

Download:http://www.java.com/en/download/index.jsp

OFDM (Orthogonal Frequency Division Multiplexing) is a modulation method used
for earth digital broadcasting and wireless LAN.

OFDM modulation is classified as multicarrier modulation, but it’s characterised
by the mutual intersection of the respective subcarriers. The subcarrier
frequency bands overlap, but since the subcarriers mutually intersect, the
information signal is demodulated on the receiving side without any problem.
This intersection also makes efficient use of bandwidth, enabling limited radio
wave resources to be used effectively. In other words, because they intersect,
the spectra of each subcarrier don’t interfere with each other.

The applet below shows the OFDM spectrum. The spectrum takes the shape of a sinc
function. The spectrum null point of each carrier wave is arranged in such a way
as to coincide with the peak of the next subcarrier so that the signal can be
distinguished.

Click the image: OFDM spectrum applet

OFDM uses several subcarriers and since the bit stream of the information signal
is distributed over each subcarrier, the symbol rate is slower by the number of
subcarriers compared with 1-bit 1-symbol modulation. For this reason, it’s
resistant to multipath interference. Also, by adding a guard interval, it’s
possible to reduce symbol interval interference. These characteristics make OFDM
a modulation method that’s resistant to interference.

However, in an actual radio communication environment, multipath and propagation
loss can cause fading, which results in demodulation errors. For this reason, it
must be combined with error correction technology.

Types of subcarrier modulation include BPSK, QPSK and QAM. The number of bits
assigned to subcarriers is 1 bit for BPSK, 2 bits for QPSK, and 6 bits for
64QAM.

The most familiar OFDM technology is terrestrial digital TV broadcasting. This
has 5,617 subcarriers and uses QPSK and 64QAM for subcarrier modulation.

This applet has 16 subcarriers and the modulation method is QPSK.

◇The principles of modulation

The figure
below is a block diagram of the analog circuit of the OFDM modulator. The
subcarrier modulation method is QPSK, and there are 16 subcarriers (channels).

The 2 bits of the digital information signal are assigned to each subcarrier by
a serial/parallel converter. QPSK is 2-bit 4-value modulation, so I and Q are
assigned respectively to 1 channel and the subcarrier is modulated by a mixer.

In this way, the I and Q signals of each modulated channel are added in the time
base direction by their respective adders. The added I and Q signals undergo IQ
modulation to an intermediate frequency and finally, an OFDM waveform is
obtained in the intermediate frequency by adding I and Q.

◇Digital OFDM modulation

Using an analog
circuit for OFDM modulation would make the hardware enormously big, and since it
is also inaccurate, synchronous demodulation on the receiving end is not
possible. For this reason, OFDM modulation is actually performed digitally as in
the diagram below. The principles of OFDM modulation have long been understood,
but recent advances in digital signal processing have enabled it be used in
commercial applications for the first time.

I and Q are mapped in the information signal, and an inverse discrete Fourier
transform (IDFT) is applied to the IQ signal of each channel to produce a QPSK
modulated wave with 1 symbol interval. After D/A conversion, this signal is
converted to an intermediate frequency using a mixer, and finally, an OFDM
waveform is obtained by adding I and Q signals. IDFT requires a lot of
processing, but actually, the hardware performs discrete inverse fast Fourier
transform (IFFT). Although it is surprising that OFDM modulation can be achieved
by arithmetic, but this is the fruit of our ancestors’ intelligence and effort.
Incidentally, Fourier who established the foundations of the Fourier transform
operation was born in 1768, and it comes as a surprise to learn that back then
he was researching theory that is still hard for us to understand today. Is it
possible that he anticipated the world we live in today?

Although we’ll leave the details to specialist textbooks, providing a guard
interval with OFDM is a method of diminishing the impact of multipath fading.

A guard interval is the second half of the OFDM symbol copied in front of the
symbol. The guard interval time must be longer than the time taken to arrive by
waves delayed by multiple paths. Even with a guard interval added, the
individual channels are consecutive sine waves so that orthogonality is
maintained, and even if FFT is applied on the receiving side, modulation is
performed correctly.

◇The principles of demodulation

The figure
below is a block diagram of OFDM demodulation using an analog circuit. The
intermediate frequency OFDM waves are down-converted using a mixer, and the
required signal is obtained with an LPF. Using a mixer to add the subcarrier
frequency to the signal obtains the I and Q information signals.

◇Digital OFDM demodulation

Demodulation with
an analog circuit is impossible due to the problem of synchronization. For this
reason, demodulation is actually digital. Demodulation uses discrete Fourier
transform (DFT), but actually, the hardware performs discrete fast Fourier
transform (FFT). Performing FFT enables sampling of the OFDM waveform spectrum,
and that itself represents the information signal.

With OFDM demodulation, carrier wave synchronization and symbol synchronization are problems. Solutions include using a guard interval and embedding a pilot signal.

An OFDM modulation and demodulation applet

Click the “Explain” button at the bottom right of the applet for how to use it.