Acoustic OFDM Technology and System

Acoustic OFDM Technology and System

Hosei Matsuoka (NTT DOCOMO, Japan)
DOI: 10.4018/978-1-4666-2217-3.ch005
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Abstract

This chapter presents a method of aerial acoustic communication in which data is modulated using OFDM (Orthogonal Frequency Division Multiplexing) and embedded in regular audio material without significantly degrading the quality of the original sound. It can provide data transmission of several hundred bps, which is much higher than is possible with other audio data hiding techniques. The proposed method replaces the high frequency band of the audio signal with OFDM carriers, each of which is power-controlled according to the spectrum envelope of the original audio signal. The implemented system enables the transmission of short text messages from loudspeakers to mobile handheld devices at a distance of around 3m. This chapter also provides the subjective assessment results of audio clips embedded with OFDM signals.
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Introduction

Because of the increasing popularity of mobile handheld devices that offer high-level audio functions, there is a strong possibility of using sound for data transmission from ordinary loudspeakers to the handheld devices. Given wireless short-range communications such as Bluetooth, etc., mobile handheld devices can easily share some data with laptop computers without wires. While the data rates for sound are relatively low compared to media such as radio, sound links can establish an attractive interface for service access. That is, a commercial on the TV or radio can be used to load the URL of the company selling the item into the user's mobile device. Sound is attractive for such applications since no additional hardware infrastructure is needed.

For transmitting a simple URL or e-mail address in a reasonable time (a few seconds), data rates more than hundreds of bps are necessary. Audio data hiding techniques enable the data transmission over aerial links in parallel with regular audio materials, but they can offer only very low data rates less than a hundred bps. Most of them are designed for watermarks and so the data rates for aerial acoustic communications are even lower, because when a signal is broadcast into the air and re-sampled with a microphone, the signal will be subjected to possibly unknown nonlinear modification resulting in phase change, echo, frequency drift, etc. Different approaches for aerial acoustic communications have been proposed. They use the same modulation techniques as radio communications such as ASK and FSK. They send the modulated signal en clair. While these methods can provide higher data rates, from hundreds of bps to more than 1kbps, their sound can be annoying and they cannot be transmitted in parallel with pre-existing sounds such as voice or music. Ultrasound acoustic communication can achieve even higher data rates and is imperceptible to human ears, but ordinary low cost loudspeakers and microphones do not support ultrasound, which prevents the use of ordinary audio devices. Given these considerations, the goals of Acoustic OFDM system are:

  • 1.

    To transmit at data rates around hundreds of bps.

  • 2.

    To use the audible audio band so that ordinary low cost loudspeakers and microphones can be used.

  • 3.

    To embed information in pre-existing sound almost in a perceptually transparent fashion.

Acoustic OFDM system is designed to satisfy the above requirements. The main feature of the Acoustic OFDM is replacing the high frequency band of the audio signal with OFDM modulated data signal where each carrier is power-controlled according to the spectrum envelope of the original audio signal. It can suppress the original audio quality degradation and transmit high bit-rate data.

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Background

Several audio data hiding techniques have been proposed in recent years. Most of them are designed for watermarks and so offer robustness to D/A-A/D conversion. When an audio source is transmitted as an analog signal across a clean analog line and re-sampled, the absolute signal magnitude and sample quantization change. However, acoustic communication over aerial links is more challenging, because when a signal is broadcast into the air and re-sampled with a microphone, the signal will be subjected to possibly unknown nonlinear modification resulting in phase change, amplitude change, echo, frequency drift, etc. Therefore, most existing data hiding techniques cannot be applied to aerial acoustic communications. The typical data hiding techniques for audio and their problems are described below.

Echo Hiding

Motivated by the fact that the HAS (Human Auditory System) cannot distinguish an echo from the original when delay and amplitude of the echo are appropriately controlled, this method employs two different delay times to carry binary information (Gruhl, 1996). Unfortunately, the signal experiences damping oscillation in the loudspeaker and environmental reflection both of which result in a variety of echoes, which lowers the data rate; its actual data rate becomes insufficient for practical applications.

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