Passive Time Reversal Communications Research Articles

Investigation of a channel tracking based time reversal processing for underwater acoustic communication

This paper presents an investigation of a channel tracking based passive time reversal (PTR) communication method for mobile single-input/multiple-output communications in rapidly time-varying underwater acoustic channel. To investigate its performance in the non-stationary channels, the method is applied to a synthetic dataset generated by the underwater acoustic moving source model using normal mode method. The transmitted signal is modulated by the single-carrier modulation scheme using the carrier frequency of 20 kHz and bandwidth of 4, 8, and 12 kHz. From the comparisons with conventional PTR approach, the following advantages were found: (1) the quality of the equalization result hardly degrades even if the sound source moving speed increased, (2) suboptimal diversity combining is performed in the sense of the energy utilization of the multipath, (3) the dependence on the bandwidth is relatively small, and a significant performance improvement is seen especially for narrowband signals.

The relationship of time-reversal and multi-channel decision feedback equalization in underwater acoustic communication

The relationship between the performance of the passive time-reversal (PTR) communication technique and multi-channel decision feedback equalization for moving-source single-input/multiple-output (SIMO) communication problems in the multipath-rich shallow sea environment is investigated. Both methods are applied to a synthetic dataset of an underwater acoustic moving-source model, generated by the normal mode method, to evaluate utilization of the wavefield energy as an adaptive diversity-combining method. The results show that PTR followed by decision feedback equalization (PTR-DFE) is the more robust technique in a moving-source environment with moderate source speed. In particular, PTR-DFE can utilize most of the energy in moderate-speed conditions. It is also discussed how the performance of each method can be limited in the moving-source SIMO problem. The results indicate that the performance of PTR-DFE can be limited by the convergence property and filter tap length limitations of the DFE in problems with rapidly moving sources.

Analysis of passive time-reversal communication performance in shallow water with underwater sound channel

Analysis of passive time-reversal communication performance in shallow water with underwater sound channel

Estimate of Passive Time Reversal Communication Performance in Shallow Water

Time reversal processes have been used to improve communication performance in the severe underwater communication environment characterized by significant multipath channels by reducing inter-symbol interference and increasing signal-to-noise ratio. In general, the performance of the time reversal is strongly related to the behavior of the q -function, which is estimated by a sum of the autocorrelation of the channel impulse response for each channel in the receiver array. The q -function depends on the complexity of the communication channel, the number of channel elements and their spacing. A q -function with a high side-lobe level and a main-lobe width wider than the symbol duration creates a residual ISI (inter-symbol interference), which makes communication difficult even after time reversal is applied. In this paper, we propose a new parameter, E q , to describe the performance of time reversal communication. E q is an estimate of how much of the q -function lies within one symbol duration. The values of E q were estimated using communication data acquired at two different sites: one in which the sound speed ratio of sediment to water was less than unity and one where the ratio was higher than unity. Finally, the parameter E q was compared to the bit error rate and the output signal-to-noise ratio obtained after the time reversal operation. The results show that these parameters are strongly correlated to the parameter E q .

Performance analysis of passive time reversal communication technique for multipath interference in shallow sea acoustic channel

In this study, the performance of passive time reversal (PTR) communication techniques in multipath rich underwater acoustic environments is investigated. It is recognized empirically and qualitatively that a large number of multipath arrivals could generally raise the demodulation result of PTR. However, the relationship between multipath and the demodulation result is hardly evaluated quantitatively. In this study, the efficiency of the PTR acoustic communication techniques for multipath interference cancelation was investigated quantitatively by applying a PTR-DFE (decision feed-back filter) scheme to a synthetic dataset of a horizontal underwater acoustic channel. Mainly, in this study, we focused on the relationship between the signal-to-interference ratio (SIR) of datasets and the output signal-to-noise ratio (OSNR) of demodulation results by a parametric study approach. As a result, a proportional relation between SIR and OSNR is confirmed in low-SNR datasets. It was also found that PTR has a performance limitation, that is OSNR converges to a typical value depending on the number of receivers. In conclusion, results indicate that PTR could utilize the multipath efficiently and also withstand the negative effects of multipath interference at a given limitation.

Performance analysis of passive time reversal communication technique for multi-path interference with synthetic data

The interference by time-varying and intense underwater acoustic multi-path is one of the most obstructive phenomena in underwater acoustic communication. Time-Reversal (TR) communication techniques utilize the reciprocity of acoustic channels to converge the spread signals to the original one, and are considered one of the most effective methods to overcome multi-path interference as shown in many past studies. It is recognized empirically that the large number of multi-path arrival could generally raise the demodulation result of TR. However, its relationship is rarely evaluated quantitatively. In this paper, we investigate the efficiency of the TR acoustic communication techniques for the multi-path interference cancelation by applying our passive TR method to the synthetic datasets of horizontal acoustic communication in shallow water environment. Mainly, we focused on the relation between the Signal to Interference Noise Ratio (SINR) and the output SNR of demodulation result. The result shows: 1) SINR and output SNR has a nearly linear relation up to a limitation, and 2) above the limitation, output SNR reaches its ceiling or slightly decreases.

Self-synchronization and spatial diversity of passive time reversal communication.

Time reversal (TR) achieves spatial and temporal focusing in complex environments. In the context of communications, passive (uplink) TR is equivalent to active (downlink) TR with the communications link being in opposite directions but with the same theoretical performance. The benefit of passive TR, however, is its natural self-synchronization, making TR communications robust and readily extended to cases involving temporal diversity and synthetic aperture communications. Self-synchronization is examined analytically and demonstrated with shallow water experimental data. In addition, the impact of spatial diversity on communications performance is investigated in terms of the aperture and element spacing of the 64-element vertical receive array.

Passive time reversal communication with cyclic shift keying over underwater acoustic channels

Abstract Many advanced channel estimation and equalization algorithms have been proposed for high-rate coherent underwater acoustic communications. However, these algorithms are usually computationally demanding or ineffective under low signal-to-noise ratio (SNR). Energy-efficient or reliable communications over complex underwater multipath channels are required in many applications. In this paper, passive time reversal with cyclic shift keying (CSK) spread spectrum using hyperbolic frequency modulated (HFM) waveform is proposed for reliable point-to-point underwater acoustic communication. At the transmitter, multiple bits information is represented by the cyclically shifted version of the basic HFM waveform. Therefore, the CSK scheme provides a higher data rate than conventional direct sequence spread spectrum. At the receiver, the peak position of the cyclic correlator output is estimated to recovery information. A modified passive time reversal receiver for the proposed HFM based CSK is designed to: (1) focus the multipath signal without estimating the channel impulse response to increase the received SNR; (2) and alleviate the requirement for accurate symbol synchronization. Simulation and lake experimental results verify the proposed method and show its potential for low cost and reliable underwater acoustic communication systems.

해수면에 의한 신호 응답 강도의 시변동성 특성이 적용된 벨홉 기반의 수중음향 통신 채널 모델링 및 수동 시역전 통신 응용

본 논문은 시변 해수면을 생성하고 KA(Kirchhoff Approximation) 기반으로 산란계수를 시뮬레이션하여 결정론적 모델인 벨홉 임펄스 응답에 적용함으로써 시변동성 채널을 생성한다. 1D Pierson-Moskowitz 해수면 스펙트럼과 가우시안 상관 함수를 이용하여 일정한 속도로 변화하는 시 변동성 해수면을 사용하였다. 산란계수는 벨홉의 채널 임펄스 응답의 신호 응답 강도에 적용한다. 실제 실측 데이터에서 해수면 반사 성분을 분리하여 시 변동성 특성에 대한 도플러 파워 스펙트럼을 구하고, 해수면 산란계수 시뮬레이션의 결과와 비교하여 해수면에 사용된 가우시안 상관 함수의 상관 시간을 추정하였다. 최종적으로 생성된 시변동성 채널에 수동 시역전 통신 시나리오를 가정하고 기법을 적용하여 비트에러율 및 채널응답 상관계수 시뮬레이션을 수행하였다. This paper represents generation of time-varying underwater acoustic channels by performing scattering simulation with time-varying ocean surface and Kirchhoff approximation. In order to estimate the time-varying ocean surface, 1D Pierson-Moskowitz ocean power spectrum and Gaussian correlation function were used. The computed scattering coefficients are applied to the amplitudes of each impulse of BELLHOP simulation result. The scattering coefficients are then compared with measured doppler spectral density of signal components which were scattered from ocean surface and the correlation time used in the Gaussian correlation function was estimated by the comparison. Finally, bit-error-rate and channel correlation simulations were performed with the generated time-varying channel based on passive time-reversal communication scenario.

Recent experiment results of long-range time-reversal communication in deep ocean

In the Japan Agency for Marine-Earth Sicence and Technololgy (JAMSTEC), a project to develop new autonomous underwater vehicle (AUV) is being planned, which will have the capability to cruise long distances over several hundred kilometers. Achieving acoustic communication with such a long-range AUV, even at a low data transmission rate, will be important. Time reversal is an attractive solution for such a long-range communication, by converging multipath signals and decreasing intersymbol interference (ISI). Thus, we have researched on time-reversal communication in the deep ocean, have proposed a method of combining time reversal and adaptive equalization, and have executed various at-sea experiments in the deep ocean. In the first experiments, both active and passive time-reversal communication were performed at the range of 10 km and it was shown that time reversal could enable communication under many multipath interferences. The subsequent experiments were carried out in various ranges for passive time-reversal communication. In our latest trial, communication at the range up to 1,000 km was demonstrated at the data rate of 100 bps at the frequency of 500 Hz. In this paper, the results of these experiments are described.

Joint passive-phase conjugation with adaptive multichannel combining for coherent underwater acoustic communications

This paper presents a receiver structure which exploits spatial diversity by adaptive multichannel combining, which improves the performance of passive time reversal communications realized by passive-phase conjugation (PPC). PPC processing achieves pulse compression for the time delayed arrivals at the receiver, and this property is used for coherent communications to reduce the computational load. The presented structure takes advantage of pulse compression and performs adaptive multichannel combining, where the number of taps for adaptive multichannel processing is significantly reduced in order to decrease the computational load. With a previous output mean square error (MSE), the adaptive combining minimizes current output MSE, where spatial diversity is exploited by the adaptive combining. This structure improves performance of the passive time reversal approach, even though the taps for combining span one symbol interval. The performance improvement is demonstrated by a set of real data collected in a recent sea experiment, which was conducted in a range dependent acoustic channel over a range of 4 km.

Experiment Results of Time-Reversal Communication at the Range of 300 km

We have researched on long horizontal time-reversal communication in the deep ocean, to be used for communication with autonomous underwater vehicle (AUV) in future, and proposed the method of combining time reversal with adaptive equalization. The pervious at-experiments were carried out at the ranges from 10 to 100 km with a virtual time-reversal array. In our latest sea trial, the experiment for passive time-reversal communication was executed with a real receiver array at the range of 300 km. This was the first implementation of time-reversal communication at such a range as an experiment intended for communication including the source movement. In this experiment, the source was suspended from the research vessel, thus drifted due to the ship motion. The proposed method could achieve error-free demodulation in spite of such drifting motion. The M-sequence signal and chirp pulse were used as the probe signal for passive time reversal. The results in case of M-sequence were better than those in case of chirp pulse.

Time-Reversal Communication in Deep Ocean: Result of Second At-Sea Experiment

We have researched long horizontal time-reversal (phase-conjugate) communication through simulations and have proposed a method of combining the effect of time reversal and the processing of an adaptive equalizer for realizing communication with a sparse time-reversal array and communication with a moving target. In the first at-sea experiment, only passive time-reversal experiments were performed. In this second experiment, both trials of active and passive time-reversal communications were executed. As results, it is confirmed that it is possible to achieve bi-directional communication by active and passive time reversals, and that time reversal is superior to conventional methods. It is also proved that at least ten source-receivers for the time-reversal array can achieve time-reversal communication and that the performance of time-reversal communication is hardly affected by the depth of the time-reversal array.

Performance prediction of passive time reversal communications

A recent paper [M. Stojanovic J. Acoust. Soc. Am. 117, 1173-1185 (2005)] presented theoretical performance bounds of time reversal communications. In this letter, the performance of time reversal communications is evaluated using at-sea experimental data and compared to the theory for two different approaches; (1) time reversal alone and (2) time reversal combined with channel equalization. It is found that approach (1) shows a good agreement between theory and data. On the other hand, approach (2) indicates that the data is about 3-5 dB below the theory which assumes perfect knowledge of the channel, an infinite number of taps to remove the intersymbol interference, and no need for phase tracking. Taking these into consideration, the theoretical performance can provide a useful upper bound for predicting performance of time reversal communications.

First Experiment Result of Time-Reversal Communication in Deep Ocean

We have researched time-reversal (phase-conjugate) acoustic technology for long horizontal communication by simulations, and have proposed a method of combining time reversal and the use of an adaptive equalizer to realize communication even with a sparse time-reversal array and the communication with a moving underwater vehicle. An experiment in a real sea was executed in Suruga-bay at a frequency of 500 Hz at a distance of 10 km, and its results are described in this paper. In the trial, the projector oonlyn the array side malfunctioned. Thus only a passive time-reversal communication experiment was performed. It was confirmed that communication could be achieved using only 10 elements of the array by the proposed method at a water depth of 1,100 m.

Multiuser Communications Using Passive Time Reversal

A recent paper (Song , IEEE Journal of Oceanic Engineering, vol. 31, no. 2, pp. 170-178, 2006) demonstrated multiple-input-multiple-output (MIMO) communications in shallow water using active time reversal where the time reversal array (i.e., base station) sent different messages to multiple users simultaneously over a common bandwidth channel. Passive time reversal essentially is equivalent to active time reversal with the communications link being in the opposite direction. This paper describes passive time reversal communications which enables multiple users to send information simultaneously to the time reversal array. Experimental results at 3.5 kHz with a 1-kHz bandwidth demonstrate that as many as six users can transmit information over a 4-km range in a 120-m-deep water using quaternary phase-shift keying (QPSK) modulation, achieving an aggregate data rate of 6 kb/s. Moreover, the same data rate has been achieved at 20-km range by three users using 16 quadrature amplitude modulation (16-QAM).

Multiuser communications using passive time reversal

A recent paper [Song et al., IEEE J. Ocean. Eng. 31, 165–173 (2006)] demonstrated multipleinput/multiple-output (MIMO) multi user communications in shallow water using active time reversal where the time reversal array (i.e., base station) sent different messages to multiple users simultaneously over a common bandwidth channel. Passive time reversal essentially is equivalent to active time reversal with the communications link being in the opposite direction. This paper describes MIMO passive time reversal communications, which enables multiple users to send information simultaneously to the time reversal array. Experimental results at 3.5 kHz with a 1-kHz bandwidth demonstrate that as many as six users can transmit information over a 4-km range in 120-m-deep shallow water using QPSK modulation, achieving an aggregate data rate of 6 kbits/s. Moreover, the same data rate has been achieved at 20 km range by three users using 16-QAM modulation.

Spatial diversity in passive time reversal communications

A time reversal mirror exploits spatial diversity to achieve spatial and temporal focusing, a useful property for communications in an environment with significant multipath. Taking advantage of spatial diversity involves using a number of receivers distributed in space. This paper presents the impact of spatial diversity in passive time reversal communications between a probe source (PS) and a vertical receive array using at-sea experimental data, while the PS is either fixed or moving at about 4knots. The performance of two different approaches is compared in terms of output signal-to-noise ratio versus the number of receiver elements: (1) time reversal alone and (2) time reversal combined with adaptive channel equalization. The time-varying channel response due to source motion requires an adaptive channel equalizer such that approach (2) outperforms approach (1) by up to 13dB as compared to 5dB for a fixed source case. Experimental results around 3kHz with a 1kHz bandwidth illustrate that as few as two or three receivers (i.e., 2 or 4m array aperture) can provide reasonable performance at ranges of 4.2 and 10km in 118m deep water.

Spatial diversity in passive time reversal communications

A time reversal mirror exploits spatial diversity to achieve spatial and temporal focusing, a useful property for communications in an environment with significant multipath. This paper presents the impact of spatial diversity on passive time reversal communications between a single probe source and a vertical receive array using at-sea experimental data, while the probe source is either fixed or moving at about 4 knots. The performances of two different approaches are compared: (1) time reversal alone and (2) time reversal combined with adaptive channel equalization. In the presence of source motion, the motion-induced Doppler shift is coarsely estimated using a decision-feedback phase-locked loop with a training sequence and then the received time series is resampled prior to the demodulation process. The time-varying channel responses due to source motion require an adaptive channel equalizer such that time reversal combined with the equalizer outperforms time reversal alone by up to 13 dB as compared to 5 dB for a fixed source case. The experimental results around 3 kHz with a 1-kHz bandwidth illustrate that even two or three receivers (i.e., 2- or 4-m aperture) can provide resonable performance at 4.2- and 10-km ranges in a 118-m deep water.