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Introduction

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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

An outline of OFDM modulation and demodulation

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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.

sinc01.gif

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.

Modulation

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◇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.

ofdm01.gif

◇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?

ofdm03.gif

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.

Demodulation

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◇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.

ofdm02.gif

◇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.

ofdm04.gif

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

OFDM modulation applet

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An OFDM modulation and demodulation applet
Click the “Explain” button at the bottom right of the applet for how to use it.

OFDM_00.gif

Click the image: OFDM modulation and demodulation applet

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