Coherent Underwater Acoustic Communications Research Articles

Lake experimentation of in-band full-duplex underwater acoustic communications

In-band full-duplex (IBFD) communications is a promising technique to achieve spectral-efficient communication in the underwater environment. Characteristics of self-interference and methods to suppress the self-interference are crucial to attaining IBFD acoustic communications. In this effort, we analyze the characteristic of the self-interference for the underwater acoustic channel through experimental measurements. Self-interference reduction using physical separation and digital cancellation is investigated under different transmitter-receiver geometries. An iterative IBFD receiver combining joint channel estimation, SI cancellation, and multichannel decision feedback equalizer is developed. The feasibility of coherent IBFD underwater acoustic communications was tested in a lake environment. The self-interference suppression methods and the receiver performance were evaluated using the experimental measurements. IBFD communications was demonstrated at 80 m. With 10 dB local self-interference source level reduction, IBFD communications was extended to the 170 m range. [Work supported by the National Science Foundation.]

Adaptive estimation of sparse channel based on modified RLS for coherent underwater acoustics communications

A new channel estimation approach used in coherent underwater acoustic communication system is presented in this paper. The proposed method consists of two steps: (1) obtain the initial channel estimation based on least square (LS) and the sparse characteristics of underwater acoustic channels; (2) track the channels using the modified recursive least squares (RLS). In the first step, the threshold value to set all coefficients below it to zero is deduced. In the second step, a modified RLS based on the idea of RLS and Kalman filter terminology is proposed. Experimental results show that the proposed method can track underwater acoustic channels well. The lower symbol error rate (SER) and higher output SNR can be achieved.

Time reversal MFSK acoustic communication in underwater channel with large multipath spread

It has been recognized that, compared with coherent acoustic communication, Multiple Frequency Shift Keying (MFSK) underwater acoustic communication offers the advantages of low complexity, easy-implementation and channel tolerance, but it is subject to significant performance degradation caused by inter-symbol interference (ISI) when the multipath spread is larger than symbol duration. The time reversal is capable of effectively suppressing the channel multipath by the means of temporal-spatial focusing, which has been widely examined and applied in coherent underwater acoustic communication systems. However, there is a lack of investigations to incorporate the time reversal with MFSK communication. In this paper, we report a multi-channel time reversal MFSK receiver, in which the channel estimate is initially obtained and periodically updated by matched filtering of the sync preamble, meanwhile, down-conversion is adopted to reduce computational complexity. The performance of the proposed receiver is evaluated in a shallow water channel with severe multipath spread, in terms of bit error rate (BER) and robustness upon time variations.

Measurements and predictions of surface Doppler and arrival time spread in a variety of wind conditions

Measurements from a bistatic surface scattering experiment are presented. Acoustic waveforms in the 200 kHz—400 kHz (VHF) frequency regime were transmitted and received in a variable speed wind wave channel at the Scripps Institution of Oceanography. Physics based analytic predictions for Doppler shift and arrival time spread are compared to experimental measurements. The effect of source/receiver depth and surface wave shape on acoustic properties of the surface multipath are quantified as a function of wind speed. The importance of transmit waveform Doppler sensitivity of common underwater communications signals is quantified for various surface scattering regimes. This work has direct application to the improved performance of phase coherent underwater acoustic communications systems and the remote sensing of gravity-capillary waves.

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.

Underwater Acoustic Communication in a Highly Refractive Environment Using SC–FDE

Single-carrier frequency-domain equalization (SC-FDE) is a promising technique for coherent underwater acoustic communications. The method has a lower peak-to-average power ratio than orthogonal frequency-division multiplexing (OFDM) and is more resistant to frequency offset. In this work, SC-FDE performance is evaluated using data collected during the 2009 Cooperative Array Performance Experiment (CAPEx09) that took place near Seattle, WA, USA. CAPEx09 had two densely populated vertical receiving arrays and a highly refractive environment, features that make it suitable for examining issues related to sensor positioning in range and depth. Two-channel equalization results show only a weak dependence on the spacing between the two sensors. Communications performance is interpreted in terms of relevant propagation physics, including acoustic ducting and rough surface scattering. The implications for undersea acoustic networks are discussed.

Time Reversal in Underwater Acoustic Communications

How to achieve high data rate coherent underwater acoustic (UWA) communications is a challenging topic due to several unique properties of UWA environments, especially in shallow water acoustic environments. Time reversal, or phase conjugation in the frequency domain, is a process engages in achieving high data rate and reliable high frequency coherent communications in time-varying UWA environments, while reducing the system complexity significantly compared with traditional methods, such as using large arrays of sensors, or redundantly transmitting at different time. Therefore, timereversal technique has been approved to be a promising technique in future UWA communications and networks. In his paper, we will present an overview of the application of the time-reversal process to acoustic communications. The content includes basic concept and mechanism of timereversal in UWA communications, passive and active timereversal UWA communications, application challenges of the time-reversal technique in UWA communications and networks.

Signal processing in underwater acoustic communication system for manned deep submersible “Jiaolong”

In this report, signal processing in underwater acoustic communication system for manned deep submersible “Jiaolong” is introduced. 1. Four communication methods are integrated to meet different needs: (1) coherent underwater acoustic communication, with a variable transmission rate from 5kbps to 15kbps, to transmit images. (2) Non-coherent underwater acoustic communication, with a transmission rate 300bps, to transmit texts, instructions, and sensor data. (3) Spread spectrum underwater acoustic communication, with a transmission rate 16bps, to transmit instructions. (4) Underwater voice communication, using analogy modulation method to transmit human voice. 2. Signal processing method in coherent communication mainly consists of concatenation of decision feedback equalizer and Turbo decoder, and wavelet based image compression with fixed length coding. In the equalizer, Doppler compensation, multichannel combining and equalizer coefficients updating are all using fast self-optimized adaptive algorithm. 3. A linear hydrophone array is lowered from the mother ship to certain depth, and spatial diversity combining technology is adopted. From July to August 2011, diving trial of “Jiaolong” was carried out in the Pacific Ocean. The communication distance can cover nearly all ocean depth. The covering conical area is wider than 100 degree. An optical/acoustic image could be transmitted in 7 or 14 seconds.

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.

A fast algorithm for computing doppler introduced by sea surface gravity waves.

The technical advances in phase coherent underwater acoustic communications and the increasing availability of relatively inexpensive commercial modems have created a renewed interest in utilizing sound channel models for predicting modem performance. While accurate channel models already exist, their usefulness for modeling extensive “what‐if” scenarios is limited by the computational resources required. Acoustic field models based on raytracing methods are a fast and efficient approach to sound channel modeling. They readily lend themselves to estimating the impulse response or time arrival structure of a hypothetical sound channel. The computed impulse response can then be convolved with the waveform transmitted by the modem to obtain an estimate of the timeseries observed at a hypothetical receiver. The existing models for addressing the Doppler effects introduced by sea surface gravity waves into the receiver timeseries are accurate, but relatively resource hungry. A fast algorithm for modeling Doppler based on simple postprocessing of the eigenrays computed by the raytracing based field model will be presented.

Physics‐based performance prediction model for a coherent communications algorithm.

A demodulation algorithm for coherent underwater acoustic communications may start by applying a finite impulse response (FIR) filter matched to the response of the channel. Relevant design parameters are the length of the FIR filter and the rate at which it must be updated. The performance of the algorithm as a function of these design parameters can then be quantified in terms of the mean‐squared error (MSE) in the soft demodulation output. In terms of propagation physics, the length of the FIR filter is related to the number of acoustic paths retained as usable signal, and the required update rate is related to the time variation of the channel. In the present study, scattering by the sea surface is the assumed source of time variation. A model is developed for the MSE as a function of the design parameters. The Kirchhoff approximation is used to model acoustic paths that may have undergone multiple reflections by the sea surface. Required model inputs include the grazing angle, the significant wave height, and the dominant period for the surface waves. Model predictions are compared to published experimental results. [Work supported by ONR.]

Impact of source depth on coherent underwater acoustic communications

A recent paper [Song et al., J. Acoust. Soc. Am. 123, 856-865 (2008)] investigated ocean variability impact on coherent underwater acoustic communications (8-16 kHz) for a single near-seafloor transmitter in shallow water during an extended period (27 h). This letter extends that investigation to various source depths and receiver subarrays. Specifically, the middle water column source, which is either in or out of the thermocline, experiences performance variability of 6-7 dB in terms of output signal-to-noise ratio. Further, the source below the thermocline consistently outperforms the source above the thermocline when the receiver subarray is located below the thermocline.

Effect of reflected and refracted signals on coherent underwater acoustic communication: Results from the Kauai experiment (KauaiEx 2003)

The performance of a communications equalizer is quantified in terms of the number of acoustic paths that are treated as usable signal. The analysis uses acoustical and oceanographic data collected off the Hawaiian Island of Kauai. Communication signals were measured on an eight-element vertical array at two different ranges, 1 and 2 km, and processed using an equalizer based on passive time-reversal signal processing. By estimating the Rayleigh parameter, it is shown that all paths reflected by the sea surface at both ranges undergo incoherent scattering. It is demonstrated that some of these incoherently scattered paths are still useful for coherent communications. At range of 1 km, optimal communications performance is achieved when six acoustic paths are retained and all paths with more than one reflection off the sea surface are rejected. Consistent with a model that ignores loss from near-surface bubbles, the performance improves by approximately 1.8 dB when increasing the number of retained paths from four to six. The four-path results though are more stable and require less frequent channel estimation. At range of 2 km, ray refraction is observed and communications performance is optimal when some paths with two sea-surface reflections are retained.

Effects of internal waves on acoustic coherent communications during SW06

During the SW06 experiment conducted east off the New Jersey continental shelf from July to September of 2006, internal wave surface expression and water temperature profiles were recorded when phase shift keying signals (PSK) were transmitted to study internal wave effects on coherent underwater acoustic communications at low to mid frequency bands. The collected communications data are processed using time reversal multichannel combining followed by a decision feedback equalizer. Due to the presence of internal waves, the acoustic channels show a time-varying property. Continuous channel updates prior to time reversal combining are required to combat fast channel fluctuations. The communications performance in terms of output signal-to-noise ratio exhibits significant change due to the internal waves. Further, the internal wave effects on the performance of coherent acoustic communications exhibit a frequency- and depth- dependency. The choices of receiver parameters also can be affected by the presence of internal waves.

High data rate coherent underwater acoustic communications during KauaiEx and MakaiEx

During two acoustic communications experiments conducted around Kauai Island, Hawaii (KauaiEx, 2003 and MakaiEx, 2005), various coherent communications data with concurrent environmental measurements were collected under different experimental settings. The collected communications data are processed by a newly proposed receiver, which consists of time‐reversal multichannel combining followed by a single channel DFE. Continuous channel updates along with Doppler tracking are used prior to time reversal combining to combat fast channel variations. The receiver can successfully demodulate different types of coherent communications signals, including phase shift keying (PSK) and quadrature amplitude modulation (QAM) signals, at different symbols rates for different source/receiver settings, such as fixed source/receiver, drifting source/receiver, and towed source. Selected high data rate communication results will be presented to show the effectiveness of the receiver. The receiver performance in relation to the environmental variability also will be shown.

Coherent time reversal communications in a shallow multipath environment

Underwater acoustic channel is one of the less reliable communication channels due to its reverberant properties produced by the surface and the bottom of the sea and Doppler spreading caused by the transmitter and the receiver movements. Strong intersymbol interference (ISI) caused by time‐varying multipath environments and relatively fast channel variations are two of the major challenges for practical implementation of coherent underwater acoustic communications. In this paper a phase conjugation is considered as a method for mitigating intersymbol interference in coherent communication and this technique reducing the complexity of underwater receivers. Phase conjugation uses time reversal to remove intersymbol interferences. This method performs the time reversal operation in the computer at the receiver instead of time reversed propagation through the sea. This paper presents numerical simulations results of coherent communications using this technique. Phase conjugation processing in acoustic communications was demonstrated with the transmission of BPSK and QPSK modulation schemes. Different messages were sent simultaneously to different depths at different ranges in 100 m deep shallow water. Simulation results suggest that the phase conjugation technique may be used as a potential application to undersea communications, especially in an environment with significant multipath.

Erratum: “Impact of ocean variability on coherent underwater acoustic communications during the Kaual experiment (KauaiEx)” [J. Acoust. Soc. Am.123 (2), 856–865 (2008)

Erratum: “Impact of ocean variability on coherent underwater acoustic communications during the Kaual experiment (KauaiEx)” [J. Acoust. Soc. Am.123 (2), 856–865 (2008)

Impact of ocean variability on coherent underwater acoustic communications during the Kauai experiment (KauaiEx)

During the July 2003 acoustic communications experiment conducted in 100 m deep water off the western side of Kauai, Hawaii, a 10 s binary phase shift keying signal with a symbol rate of 4 kilosymbol/s was transmitted every 30 min for 27 h from a bottom moored source at 12 kHz center frequency to a 16 element vertical array spanning the water column at about 3 km range. The communications signals are demodulated by time reversal multichannel combining followed by a single channel decision feedback equalizer using two subsets of array elements whose channel characteristics appear distinct: (1) top 10 and (2) bottom 4 elements. Due to rapid channel variations, continuous channel updates along with Doppler tracking are required prior to time reversal combining. This is especially true for the top 10 elements where the received acoustic field involves significant interaction with the dynamic ocean surface. The resulting communications performance in terms of output signal-to-noise ratio exhibits significant change over the 27 h transmission duration. This is particularly evident as the water column changes from well-mixed to a downward refracting environment.

Underwater Acoustic Communications Using Time Reversal

This paper contains theoretical and experimental results on the application of the time-reversal process to acoustic communications in order to improve data telemetry in the ocean. A coherent underwater acoustic communication system must deal with the inter-symbol interference caused by the time-varying, dispersive, shallow-water ocean environment. An approach is demonstrated that takes advantage of the focal properties of time reversal. The spatial and temporal compression available at the time-reversal focus mitigates channel fading, reduces the dispersion caused by the channel, and increases the signal strength. Thus, a time-reversal communication system does not require spatial diversity at the receiver, i.e., an array of receiving sensors, but takes advantage of spatial diversity at the transmitter. The time-reversal communications system concept is demonstrated using experimental data collected in shallow water. Data telemetry bit rates of 500 bps (BPSK) and 1000 bps (QPSK) with bit error rates of 0 out of 4976 bits and 254 out of 9953 bits, respectively, were obtained when transmitting to a receiver at a distance of 10 km, with a carrier frequency of 3500 Hz, and a 500 Hz bandwidth. In a shallow-water upslope region, bit error rates of 15 out of 4976 bits and 14 out of 4976 bits were achieved over the same distance. In neither case was complex processing at the receiver used (i.e., channel equalization, error correction coding). Time-reversal transmissions are intercompared with single source and broadside transmissions and shown to have superior results in both range independent and dependent bathymetries. The time-reversal performance appears limited by self-generated inter-symbol interference. In addition, an initial look at the application of a single channel adaptive channel equalizer to received time-reversal communication sequences is presented. The same properties that are beneficial to a single channel receiver are also beneficial to adaptive channel equalization. A single channel RLS DFE equalizer is cascaded with the received time-reversal sequences and shown to further reduce scatter in the I/Q plane. The bit error rate decreased in all but one of the cases

Surface wave focusing and acoustic communications in the surf zone

The forward scattering of acoustic signals off of shoaling surface gravity waves in the surf zone results in a time-varying channel impulse response that is characterized by intense, rapidly fluctuating arrivals. In some cases, the acoustic focusing by the curvature of the wave crest results in the formation of caustics at or near a receiver location. This focusing and the resulting caustics present challenges to the reliable operation of phase coherent underwater acoustic communications systems that must implicitly or explicitly track the fluctuations in the impulse response. The propagation physics leading to focusing are studied with both experimental data and a propagation model using surface wave profiles measured during the collection of the experimental data. The deterministic experimental and modeled data show good agreement and demonstrate the stages of the focusing event and the impact of the high intensity arrivals and rapid fluctuations on the ability of an algorithm to accurately estimate the impulse response. The statistical characterization of experimental data shows that the focusing by surface gravity waves results in focused surface reflected arrivals whose intensity often exceeds that of the direct arrival and the focusing and caustic formation adversely impacts the performance of an impulse response estimation algorithm.