Paul Hursky

Mitigation of Intercarrier Interference for OFDM Over Time-Varying Underwater Acoustic Channels

Orthogonal frequency division multiplexing (OFDM) has emerged as a promising modulation scheme for underwater acoustic (UWA) communications, thanks to its robustness to channels with severe time dispersion. Compared to conventional single-carrier systems, for which complicated equalization schemes are usually required, OFDM systems are in general much simpler to implement as detection can be carried out symbol-by-symbol over time-dispersive channels. In this paper, we focus on cyclic-prefixed OFDM over time-varying UWA channels. To cope with the intercarrier interference (ICI) that arises at the receiver side because of the time variations in the channel, we consider two ICI-mitigation techniques. In the first scheme, the ICI coefficients are explicitly estimated, and minimum mean square error linear equalization based on such estimates is performed. In the second approach, no explicit ICI estimation is performed, and detection is based on an adaptive decision-feedback equalizer applied in the frequency domain across adjacent subcarriers. To cope with the phase variations of the ICI coefficients, phase-tracking loops are introduced in both ICI-mitigation schemes. The effectiveness of the presented schemes is demonstrated through simulation results, as well as real data collected in a recent experiment conducted in shallow water off the western coast of Kauai, HI, in June 2008.

Effects of ocean thermocline variability on noncoherent underwater acoustic communications.

The performance of acoustic modems in the ocean is strongly affected by the ocean environment. A storm can drive up the ambient noise levels, eliminate a thermocline by wind mixing, and whip up violent waves and thereby break up the acoustic mirror formed by the ocean surface. The combined effects of these and other processes on modem performance are not well understood. The authors have been conducting experiments to study these environmental effects on various modulation schemes. Here the focus is on the role of the thermocline on a widely used modulation scheme (frequency-shift keying). Using data from a recent experiment conducted in 100-m-deep water off the coast of Kauai, HI, frequency-shift-key modulation performance is shown to be strongly affected by diurnal cycles in the thermocline. There is dramatic variation in performance (measured by bit error rates) between receivers in the surface duct and receivers in the thermocline. To interpret the performance variations in a quantitative way, a precise metric is introduced based on a signal-to-interference-noise ratio that encompasses both the ambient noise and intersymbol interference. Further, it will be shown that differences in the fading statistics for receivers in and out of the thermocline explain the differences in modem performance.

Adaptive passive fathometer processing

Recently, a technique has been developed to image seabed layers using the ocean ambient noise field as the sound source. This so called passive fathometer technique exploits the naturally occurring acoustic sounds generated on the sea-surface, primarily from breaking waves. The method is based on the cross-correlation of noise from the ocean surface with its echo from the seabed, which recovers travel times to significant seabed reflectors. To limit averaging time and make this practical, beamforming is used with a vertical array of hydrophones to reduce interference from horizontally propagating noise. The initial development used conventional beamforming, but significant improvements have been realized using adaptive techniques. In this paper, adaptive methods for this process are described and applied to several data sets to demonstrate improvements possible as compared to conventional processing.

Adjoint modeling for acoustic inversion

The use of adjoint modeling for acoustic inversion is investigated. An adjoint model is derived from a linearized forward propagation model to propagate data-model misfit at the observation points back through the medium to the medium perturbations not being accounted for in the model. This adjoint model can be used to aid in inverting for these unaccounted medium perturbations. Adjoint methods are being applied to a variety of inversion problems, but have not drawn much attention from the underwater acoustic community. This paper presents an application of adjoint methods to acoustic inversion. Inversions are demonstrated in simulation for both range-independent and range-dependent sound speed profiles using the adjoint of a parabolic equation model. Sensitivity and error analyses are discussed showing how the adjoint model enables calculations to be performed in the space of observations, rather than the often much larger space of model parameters. Using an adjoint model enables directions of steepest descent in the model parameters (what we invert for) to be calculated using far fewer modeling runs than if a forward model only were used.

Mitigation of intercarrier interference in OFDM systems over underwater acoustic channels

Transmission schemes based on orthogonal frequency division multiplexing (OFDM) are promising for underwater acoustic (UWA) communications, thanks to their robustness to large multipath spreads. On the other hand, due to the significant time variations in the UWA channels, the effectiveness of the standard OFDM receivers is impaired by intercarrier interference (ICI). In this paper, we propose two novel ICImitigation techniques. In the first scheme, the ICI coefficients are explicitly estimated, and minimum mean square error (MMSE) equalization based on such estimates is performed. In the second approach, no explicit ICI estimation is performed, and detection is based on an adaptive decision-feedback equalizer (DFE) applied in the frequency domain across different subcarriers. We report simulation results showing the effectiveness of the proposed schemes, compared to the performance of a standard OFDM receiver neglecting ICI, and results obtained by processing real data recorded in the recent KAM08 experiment.

Point-to-point underwater acoustic communications using spread-spectrum passive phase conjugation

The ocean is often a complex multipath channel and progress has been made in developing equalization algorithms to overcome this. Unfortunately, many of these algorithms are computationally demanding and not as power-efficient as one would like; in many applications it may be better to trade bit rate for longer operational life. In 2000 the U.S. Navy was developing an underwater wireless acoustic network called Seaweb, for which a number of modulation schemes were being tested in a series of SignalEx experiments. This paper discusses two modulation schemes and associated receiver algorithms that were developed and tested for Seaweb applications. These receiver designs take advantage of time reversal (phase conjugation) and properties of spread spectrum sequences known as Gold sequences. Furthermore, they are much less complex than receivers using adaptive equalizers. This paper will present results of testing these signaling and receiver concepts during two experiments at sea.

Multiinput multioutput OFDM for shallow‐water UWA communications

We investigate performance of turbo coded multiple‐input multiple‐output (MIMO)‐OFDM systems with layered space time (LST) architectures for underwater acoustic (UWA) channels by using simulations and results from the AUVfest experiment performed in June 2007. MIMO systems have been shown to be promising in the sense that they increase the reliable transmission rates significantly without consuming additional bandwidth and power. Robustness of OFDM systems with cyclic prefix or zero padding to ISI channels are also well known; so the combination of MIMO techniques and OFDM can be regarded as a promising technology for shallow water UWA communications which is characterized by severe bandwidth limitations and long intersymbol interference (ISI) spans. The paper reviews necessary components of a MIMO‐OFDM communication system, including, time and frequency synchronization, channel estimation, and tracking of the varying channel parameters. Modifications necessary to make the system suitable for UWA channels are summarized. Results of the AUVFest 2007 experiment are very promising; for instance, 2×2 MIMO‐OFDM can reach up to 60 Kbps transmission for a bandwidth of 16 KHz with simple receiver structures for a range of 2000 m. In addition to the coherent system, differential and unitary space‐time coded MIMO‐OFDM scenarios are also considered.

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.

Matched field processing with data-derived modes

The authors demonstrate MFP using data-derived modes and the sound speed profile, using no a priori bottom information. Mode shapes can be estimated directly from vertical line array data, without a priori knowledge of the environment and without using numerical wave field models. However, it is difficult to make much headway with data-derived modes alone, without wave numbers, since only a few modes at a few frequencies may be captured, and only at depths sampled by the array. Using a measured sound speed profile, the authors derive self-consistent, complete sets of modes, wave numbers, and bottom parameters from data-derived modes. Bottom parameters enable modes to be calculated at all frequencies, not just those at which modes were derived from data. This process is demonstrated on SWellEx-96 experiment data. Modes, wave numbers, and bottom parameters are derived from one track and MFP based on this information is demonstrated on another track.

High-frequency (8–16 kHz) model-based source localization

Matched-field or model-based processing has now been widely demonstrated for improving source localization and detection in ocean waveguides. Most of the processing approaches become increasingly sensitive to fluctuations or uncertainties as the frequency increases. As a result, there has been very limited work above 1 kHz and there is a perception that above several kilohertz the technique cannot be applied. We have conducted acoustic communications experiments in a variety of shallow water sites around coastal areas of the United States. These experiments show that a clear multipath structure is readily observed even in the 8–16 kHz band. Furthermore, it is shown that model-based processing can then be exploited to localize sources at these high frequencies out to ranges of several kilometers.