Tadashi Ebihara

Underwater Acoustic Communication With an Orthogonal Signal Division Multiplexing Scheme in Doubly Spread Channels

Underwater acoustic (UWA) communication is an ongoing challenge because of the heavy time spread by multipath and Doppler spreads. In this paper, we propose a UWA communication system using orthogonal signal-division multiplexing (OSDM), a scheme that measures the multipath profile without an adaptation or interpolation process, to achieve stable communication in doubly spread channels. We previously evaluated the performance of an OSDM scheme in a UWA communication system in both an experiment and simulations. In the present study, we experimentally compared the performance of OSDM and existing communication schemes - single-carrier with decision feedback equalizer (DFE) and orthogonal frequency-division multiplexing (OFDM)-in a test tank with respect to communication quality, data rate, frame length, and calculation complexity. We found that OSDM with a multichannel receiver is attractive in terms of communication quality; it achieved a far better bit error rate (BER) performance compared to the other schemes in both static and dynamic channels with various input signal-to-noise ratios, although the complexity is less than that achieved with single-carrier DFE. Based on these findings, we suggest that OSDM can provide a highly reliable communication environment for UWA communication with multipath and Doppler spread, such as in shallow water.

Visualization of Mode Conversion of Lamb-Type Waves in Glass Plates Using Pulsed Light Source

Presently, a number of visualization methods of sound waves are investigated in order to understand their behaviors in various mediums. In this study, we employ the visualization method using photoelasticity. In this method, the stress in elastic plates is visualized using the birefringence effect arising from ultrasound stress, through two orthogonal polarization plates. As visualization objects, we observed Lamb and Lamb-type waves under some conditions. Among them, the condition that the layer changes during the wave propagation is very interesting. Under this condition, not only Lamb and Lamb-type waves but also a propagating wave along the boundary appears at the same time. In this study, we can observe the mode conversion of elastic waves by changing the boundary condition during the wave propagation.

Experimental Study of Doppler Effect for Underwater Acoustic Communication Using Orthogonal Signal Division Multiplexing

This paper is about the underwater acoustic (UWA) communication using orthogonal signal division multiplexing (OSDM) in shallow water, whose environment is time spread and frequency spread. In this paper, the Doppler effect – Doppler shift and spread – for UWA communication using OSDM is mainly considered. The effects of Doppler shift and Doppler spread are evaluated in a test tank with a moving platform on a stable water surface and with a stable platform with a moving water surface, respectively. Doppler shift correction, which has been considered in simulation-based studies, is found to work effectively. In relation to the effect of Doppler spread, the experimental result well agrees with the simulation result. Through this study, it is confirmed that a smaller frame length is preferable because it enables the measurement of the UWA channel frequently so that it can keep up with channel changes.

Study of Doppler Shift Correction for Underwater Acoustic Communication Using Orthogonal Signal Division Multiplexing

In this study, we apply Doppler shift correction schemes for underwater acoustic (UWA) communication with orthogonal signal division multiplexing (OSDM) to achieve stable communication in underwater acoustic channels. Three Doppler correction schemes, which exploit the guard interval, are applied to UWA communication with OSDM and evaluated in simulations. Through a simulation in which only the Doppler effect is considered, we confirmed that by adapting schemes to UWA communication with OSDM, we can correct large Doppler shifts, which addresses the usual speed of vehicles and ships. Moreover, by considering both the Doppler effect and channel reverberation, we propose the best possible combination of Doppler correction schemes for UWA communication with OSDM. The results suggest that UWA communication with OSDM may lead to high-quality communication by considering channel reverberation and large Doppler shifts.

Basic Study of Orthogonal Signal Division Multiplexing for Underwater Acoustic Communication: A Comparative Study

Underwater acoustic (UWA) communication in shallow water has been challenging owing to multipath-induced signal distortion and motion-induced Doppler distortion. In this study, we investigate the performance of a UWA communication system using orthogonal signal division multiplexing (OSDM) on intersymbol interference (ISI) and Doppler shift in a simulation, by comparing it with an existing communication system that uses coherent modulation with an adaptive filter, to clarify whether OSDM is suitable for UWA communication in shallow water. The obtained results clarify that OSDM is robust to heavy ISI, and it is expected to be suitable for a rapidly time-varying channel. For example, OSDM can improve the output signal-to-noise ratio (SNR) of 9.4 dB in a shallow water channel with a multipath delay time dispersion of 20 symbols, compared with the UWA communication system with decision feedback equalizer (DFE). However, it is also clarified that OSDM is more sensitive to the Doppler effect compared with the existing communication system, and there is the necessity of applying frequency-offset compensation, because the permissible Doppler shift, when the output SNR falls to -3 dB, for OSDM is less than half that for the UWA communication system with DFE.

Direction of Arrival Estimation Based on Delayed-Sum Method in Reverberation Environment

Microphone arrays have been used for estimating the direction of arrival (DOA). Owing to intercorrelation among direct and reflected sounds, there are difficulties in estimating DOA using microphone array in highly reverberant environments. The purpose of this research is to estimate the correct DOA in such environments using a small number of microphones with the aid of signal processing. The proposed signal processing uses the relationship that direct signals always arrive earlier than reflected signals. By comparing the delayed-sum signals corresponding to candidate direct signals, the DOA of direct sounds could be determined correctly. We have confirmed the usefulness of the proposed method by conducting several experiments. This proposed method is different from conventional methods because it has high tolerance to the effect of reverberation; it not only enables estimation of DOA but can also be applied to the measurement of the delay of time of flight in reverberant environments.

Defect visualization in billets by time reversal of scattered waves from defects

There are several defect inspection methods for billets using ultrasound. One of the methods is pulse transmission with computerized tomography, and this method enables the visualizion of defects in billets. However, this method requires many paths to visualize. Therefore, we propose a method of time reversal of scattered waves from defects, which enables the visualization of defects with fewer paths. In this paper, we show the flow of our method and describe our experiment for evaluating the method. As a result, the image of defects visualized by our method is clearer than those of defects visualized by simple back-propagation methods. In addition, our method enables visualization of defects with fewer paths.

Improvement of Power Efficiency for Underwater Acoustic Communication Using Orthogonal Signal Division Multiplexing over Multiple Transducers

In underwater acoustic (UWA) communication, power efficiency is one of the important characteristics. This paper is about multistream transmission using orthogonal signal division multiplexing (OSDM) as a technique to increase power efficiency. In this work, the performance of multistream transmission using OSDM is evaluated both experimentally in a test tank and by numerical simulation. Through this study, it is confirmed that the multistream transmission scheme is effective in enhancing the power efficiency compared with the single-stream transmission using higher order modulation. Moreover, the performance of multistream transmission using OSDM is compared with the existing scheme, multistream transmission using orthogonal frequency division multiplexing (OFDM). The obtained results suggest that multistream transmission using OSDM is attractive because it can achieve the same bit-error rate (BER) and the same data rate with less power of the signal, compared with the reference. Although the calculation cost of OSDM in the receiver remains as an issue, multistream transmission using OSDM may contribute to high-speed UWA communication because of its excellent power efficiency.

Chip-Interleaved Multiple Access for Underwater Acoustic Communication and Its Performance Evaluation in Propagation Simulation

In this paper, we review a chip-interleaved multiple-access (CIMA) method for underwater acoustic communication. CIMA is commonly used for wireless radio communication, but it is rarely applied to underwater communication. We evaluated whether this CIMA method is applicable to underwater acoustic communication. We also evaluated the performance of the communication method in which a multipath profile is measured in real time by applying the CIMA method, and evaluated its performance in a simulation. The obtained results suggest that CIMA can contribute actively to underwater communication, and that the proposed communication method can reconstruct transmitted data precisely even in a multipath environment using a multipath profile obtained from a pilot signal.

Doppler-Resilient Orthogonal Signal-Division Multiplexing for Underwater Acoustic Communication

Underwater acoustic (UWA) channels are characterized by a severe spread in time and frequency, and are usually labeled as “doubly spread channels.” In this paper, we propose Doppler-resilient orthogonal signal-division multiplexing (D-OSDM), to provide a highly reliable communication environment in doubly spread channels for UWA communication. D-OSDM multiplexes several data vectors in addition to a pilot vector, and preserves orthogonality among them even after propagation through doubly spread channels, under the assumption that the channel can be modeled by a basis expansion model (BEM). We describe the signal processing steps at the transmitter and the receiver for D-OSDM, and evaluate its performance by both simulations and experiments. To generate a doubly spread channel, a test tank with a wave generator is employed. The obtained results suggest that D-OSDM can provide low-power and high-quality UWA communications in channels with large delay and Doppler spreads; for example, D-OSDM succeeds to achieve a block error rate (BLER) of 10 -3 while BEM-based orthogonal frequency-division multiplexing (OFDM) has a BLER floor of 10 -2 in the experiments. Equivalently, D-OSDM can reduce the signal power required for communications to achieve the same BER significantly. Overall, it was found that D-OSDM can become a powerful communication tool for underwater operations.