Aijun Song

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

During the July 2003 acoustic communications experiment conducted in 100m deep water off the western side of Kauai, Hawaii, a 10s binary phase shift keying signal with a symbol rate of 4kilosymbol∕s was transmitted every 30min for 27h from a bottom moored source at 12kHz center frequency to a 16 element vertical array spanning the water column at about 3km 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 o...

Passive Time Reversal Acoustic Communications Through Shallow-Water Internal Waves

During a 12-h period in the 2006 Shallow Water Experiment (SW06), binary phase shift keying (BPSK) signals at the carrier frequencies of 813 and 1627 Hz were propagated over a 19.8-km source-receiver range when a packet of strong internal waves passed through the acoustic track. The communication data are analyzed by time reversal processing followed by a single-channel decision feedback equalizer. Two types of internal wave effects are investigated in the context of acoustic communications. One is the rapid channel fluctuation within 90-s data packets. It can be characterized as decreased channel coherence, which was the result of fast sound-speed perturbations during the internal wave passage. We show its effect on the time reversal receiver performance and apply channel tracking in the receiver to counteract such fluctuation. The other one is the long-term (in the scale of hours) performance degradation in the depressed waveguide when the internal waves passed through the acoustic track. Even with channel tracking, the time reversal receiver experiences average 3-4-dB decrease in the output signal-to-noise ratio (SNR). Such long-term performance degradation is explained by the ray approximation in the depressed waveguide.

Time Reversal Receivers for High Data Rate Acoustic Multiple-Input–Multiple-Output Communication

A low-complexity receiver is proposed for high-frequency underwater acoustic multiple-input-multiple-output (MIMO) channels. The receiver uses time reversal combining followed by a single-channel decision feedback equalizer (DFE) to deal with the intersymbol interference. Periodical MIMO channel estimation is employed to track fast channel fluctuations. Both serial and parallel interference cancellation techniques are integrated with time reversal DFE to address the cochannel interference (CoI) in underwater MIMO systems. Two channel estimation algorithms are also implemented. It was demonstrated through the experiment conducted at Kauai, HI in 2005 that the proposed receiver can deal with the fast-fluctuating, dispersive MIMO channel at the carrier frequency of 37.5 kHz. Parallel interference cancellation combined with matching pursuit channel estimation was shown to provide significant performance improvements, indicating the receiver algorithm can effectively suppress the CoI. Four streams of binary phase-shift keying (BPSK) sequences at an aggregate rate of 16 kb/s and quadrature phase-shift keying (QPSK) sequences at a rate of 32 kb/s were demodulated at low bit error rates. These data rates corresponded to bandwidth efficiencies of 2.29 b/s/Hz or 4.57 b/s/Hz in a dynamic underwater environment, where the source and the receiver were drifting at a 2-km range.

Experimental Demonstration of Underwater Acoustic Communication by Vector Sensors

Acoustic communication often relies on a large size array with multiple spatially separated hydrophones to deal with the challenging underwater channel. This poses limitation to its application in compact size underwater platforms. In this paper, acoustic communication by vector sensors is demonstrated by the data collected during a high frequency acoustic experiment, where a vector sensor array was drifting in the ocean. It is shown that the multichannel receiver using a single vector sensor can offer significant size reduction for coherent acoustic communication at the carrier frequency of 12 kHz, compared with a pressure sensor line array. Further, the performance difference between vector sensors and pressure sensors varies at communication ranges. At close ranges (up to 160 m), both a single vector sensor and a vector sensor array can offer significant performance gain compared with the pressure sensor array. At longer ranges (up to 1080 m), the single vector sensor has the same performance with the pressure sensor array, on average. The vector sensor array consistently provides gain at all ranges over the pressure sensor array since additional information of the acoustic field is utilized by vector sensors.

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.

Generalized equalization for underwater acoustic communications

Severe time-dispersion of the channel is one of the limiting factors for high data rate acoustic communications in shallow water. To overcome the large time spread, the research community has devoted enormous effort in the design of high efficient, low complexity equalizers. The goal of these equalizers is to convert the time-dispersive channel into a unit-tap impulse function. In another research direction, code division multiple access (CDMA) and orthogonal frequency division multiplexing (OFDM) systems are designed to combat the multi-path effect. However, using OFDM or CDMA alone to combat the multi-path effect often forces system designers to reduce the data rate in such severe time-dispersive channels. In this paper, we propose to adopt a class of generalized equalizers to generate an effective channel with a desired length. These equalizers are referred to as channel-shortening equalizers. Then, other advanced modulation schemes, such as OFDM and CDMA, can be employed to overcome the remaining time-dispersion of the channel. Compared to the unit-tap equalization, channel-shortening equalizers can provide quality performance at low computation cost. Examples of current channel-shortening equalizers are based on minimum mean-squared error (MMSE) and maximum shortening signal-to-noise ratio (MSSNR) criteria. In this paper, these algorithms are compared in the context of underwater acoustic communications. Their performance, complexity, and implementation in acoustic communications are investigated. In the digital subscriber line (DSL) channel, where channel-shortening equalizers are often applied to, MSSNR algorithms perform better than MMSE algorithms. However, we show that the MMSE algorithm is more suitable for the underwater acoustic channels with lower complexity and robustness to noise. With the MMSE method, a channel-shortening equalizer with 20 taps can convert a typical 120-tap acoustic channel into a 33-tap shortened channel. Using data obtained from the Kauai experiment, June-July 2003, it shows that the mean of the effective signal-to-interference ratio (SIR) achieves 15.7 dB. For the same channel, the optimum unit-tap equalization needs more than 200 taps to achieve the same effective SIR

Multichannel combining and equalization for underwater acoustic MIMO channels

In order to achieve high data rate digital communications, multiple-input/multiple-output (MIMO) techniques have attracted growing interests in the underwater acoustic communication studies. In this paper, multichannel combining and decision feedback equalization (MCC/DFE) has been proposed for underwater acoustic MIMO channels. In order to overcome the difficulties introduced by the fast fluctuating channel, Doppler tracking and frequent channel estimation are performed. Then time reversal combining followed by a single channel DFE is used to demodulate individual symbol sequences transmitted by the multiple element source. To improve the performance, successive interference cancellation is also incorporated into the receiver structure. Using data from the Makai experiment conducted around Kauai Island, HI, 2005, we have shown that the achievable data rate can be increased up to 4 times using the same bandwidth as single source systems. For example, 32 kilobits/s could have been achieved by simultaneous transmission of four 4 kilosymbols/s 4-phase shift keying (QPSK) symbol sequences when both the source and the receiver were drifting at a 2 km range in the ocean.

Time reversal receivers for underwater acoustic communication using vector sensors

Acoustic communication often relies on a large size array with multiple spatially separated hydrophones to deal with the challenging underwater channel. This poses serious limitation to its application at compact size underwater platforms, for example, autonomous underwater vehicles. In this paper, we propose to use vector sensors to achieve reliable acoustic communication. Using experimental data, we show the usefulness of particle velocity channels for acoustic communication. Further, to deal with the dynamic ocean environment, a time reversal multichannel receiver is proposed to utilize particle velocity channels. Our results show that the receiver using vector sensors can offer significant size reduction, compared to the receiver based on the pressure sensors, while providing comparable communication performance.

Parabolic equation modeling of high frequency acoustic transmission with an evolving sea surface

The present paper examines the temporal evolution of acoustic fields by modeling forward propagation subject to sea surface dynamics with time scales of less than a second to tens of seconds. A time-evolving rough sea surface model is combined with a rough surface formulation of a parabolic equation model for predicting time-varying acoustic fields. Surface waves are generated from surface wave spectra, and stepped in time using a Runge-Kutta integration technique applied to linear evolution equations. This evolving, range-dependent surface information is combined with other environmental parameters and input to the acoustic model, giving an approximation of the time-varying acoustic field. The wide-angle parabolic equation model manages the rough sea surfaces by molding them into the boundary conditions for calculations of the near-surface acoustic field. This merged acoustic model is validated using concurrently-collected acoustic and environmental information, including surface wave spectra. Data to model comparisons demonstrate that the model is able to approximate the ensemble-averaged acoustic intensity at ranges of about a kilometer for acoustic signals of around 15 kHz. Furthermore, the model is shown to capture variations due to surface fluctuations occurring over time scales of less than a second to tens of seconds.

Time reversal multiple-input/multiple-output acoustic communication enhanced by parallel interference cancellation

Multiple-input/multiple-output (MIMO) techniques can lead to significant improvements of underwater acoustic communication capabilities. In this paper, receivers based on time reversal processing are developed for high frequency underwater MIMO channels. Time reversal followed by a single channel decision feedback equalizer, aided by frequent channel updates, is used to compensate for the time-varying inter-symbol interference. A parallel interference cancellation method is incorporated to suppress the co-channel interference in the MIMO system. The receiver performance is demonstrated by a 2008 shallow water experiment in Kauai, Hawaii. In the experiment, high frequency MIMO signals centered at 16 kHz were transmitted every hour during a 35 h period from an 8-element source array to a wide aperture 16-element vertical receiving array at 4 km range. The interference cancellation method is shown to generate significant performance enhancement, on average 2-4 dB in the output signal-to-noise ratio per data stream, throughout the 35 h MIMO transmissions. Further, communication performance and achieved data rates exhibit significant changes over the 35 h period as a result of stratification of the water column.