Freitag Lee E.

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Lee E.

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Now showing 1 - 8 of 8
  • Article
    Multicarrier communication over underwater acoustic channels with nonuniform Doppler shifts
    (IEEE, 2008-04) Li, Baosheng ; Zhou, Shengli ; Stojanovic, Milica ; Freitag, Lee E. ; Willett, Peter
    Underwater acoustic (UWA) channels are wideband in nature due to the small ratio of the carrier frequency to the signal bandwidth, which introduces frequency-dependent Doppler shifts. In this paper, we treat the channel as having a common Doppler scaling factor on all propagation paths, and propose a two-step approach to mitigating the Doppler effect: 1) nonuniform Doppler compensation via resampling that converts a "wideband" problem into a "narrowband" problem and 2) high-resolution uniform compensation of the residual Doppler. We focus on zero-padded orthogonal frequency-division multiplexing (OFDM) to minimize the transmission power. Null subcarriers are used to facilitate Doppler compensation, and pilot subcarriers are used for channel estimation. The receiver is based on block-by-block processing, and does not rely on channel dependence across OFDM blocks; thus, it is suitable for fast-varying UWA channels. The data from two shallow-water experiments near Woods Hole, MA, are used to demonstrate the receiver performance. Excellent performance results are obtained even when the transmitter and the receiver are moving at a relative speed of up to 10 kn, at which the Doppler shifts are greater than the OFDM subcarrier spacing. These results suggest that OFDM is a viable option for high-rate communications over wideband UWA channels with nonuniform Doppler shifts.
  • Technical Report
    Ocean acoustical ray-tracing : Software Ray
    (Woods Hole Oceanographic Institution, 1992-10) Bowlin, James B. ; Spiesberger, John L. ; Duda, Timothy F. ; Freitag, Lee E.
    A new computer program for accurate calculation of acoustic ray paths through a range-varing ocean sound channel has been written. It is based on creating a model of the speed of sound in the ocean, consistent with input data, that produces the smoothest possible wavefronts. This scheme eliminates "false caustics" from the wavefront. It may be useful in calculating an approximate solution to the full wave equation at megameter ranges.
  • Article
    Multipurpose acoustic networks in the integrated Arctic Ocean observing system
    (Arctic Institute of North America, 2015) Mikhalevsky, Peter N. ; Sagen, Hanne ; Worcester, Peter F. ; Baggeroer, Arthur B. ; Orcutt, John A. ; Moore, Sue E. ; Lee, Craig M. ; Vigness-Raposa, Kathleen J. ; Freitag, Lee E. ; Arrott, Matthew ; Atakan, Kuvvet ; Beszczynska-Möller, Agnieszka ; Duda, Timothy F. ; Dushaw, Brian D. ; Gascard, Jean-Claude ; Gavrilov, Alexander N. ; Keers, Henk ; Morozov, Andrey K. ; Munk, Walter H. ; Rixen, Michel ; Sandven, Stein ; Skarsoulis, Emmanuel ; Stafford, Kathleen M. ; Vernon, Frank L. ; Yuen, Mo Yan
    The dramatic reduction of sea ice in the Arctic Ocean will increase human activities in the coming years. This activity will be driven by increased demand for energy and the marine resources of an Arctic Ocean accessible to ships. Oil and gas exploration, fisheries, mineral extraction, marine transportation, research and development, tourism, and search and rescue will increase the pressure on the vulnerable Arctic environment. Technologies that allow synoptic in situ observations year-round are needed to monitor and forecast changes in the Arctic atmosphere-ice-ocean system at daily, seasonal, annual, and decadal scales. These data can inform and enable both sustainable development and enforcement of international Arctic agreements and treaties, while protecting this critical environment. In this paper, we discuss multipurpose acoustic networks, including subsea cable components, in the Arctic. These networks provide communication, power, underwater and under-ice navigation, passive monitoring of ambient sound (ice, seismic, biologic, and anthropogenic), and acoustic remote sensing (tomography and thermometry), supporting and complementing data collection from platforms, moorings, and vehicles. We support the development and implementation of regional to basin-wide acoustic networks as an integral component of a multidisciplinary in situ Arctic Ocean observatory.
  • Article
    Synthetic baseline navigation using phase-coherent acoustic communication signals
    (Acoustical Society of America, 2019-12-31) Gallimore, Eric ; Anderson, Mark ; Freitag, Lee E. ; Terrill, Eric
    The development of a synthetic baseline navigation technique that self-localizes an autonomous underwater vehicle (AUV) using intermittent acoustic communications signals received by a single transducer is described, along with field results from in-ocean tests. The method uses the phase measurement at the output of a second-order phase-locked loop to create fine-scale pseudorange estimates in addition to, or in the absence of, a one-way travel time measurement based on the arrival time of the acoustic data packet. These range measurements are incorporated by an adaptive particle filter. This technique allows the vehicle navigation system to take advantage of multiple phase-derived range measurements made over the duration of a communication packet. These measurements, when incorporated with an appropriate filter and vehicle kinematic model, improve vehicle navigation at no additional cost in navigation-specific acoustic transmissions. This approach was demonstrated and evaluated with data collected at-sea using a REMUS 100 AUV (Hydroid, Inc., Pocasset, MA).
  • Technical Report
    Design study for a moored surface-scanning sonar
    (Woods Hole Oceanographic Institution, 1993-06) Freitag, Lee E. ; Plueddemann, Albert J. ; Merriam, Steve
    This report contains the results of a design study for a surface scanning sonar instrument capable of long-term deployment on ocean moorings. The instrument is intended to sample the bubble field just below the ocean's surface and compute the backscattered intensity and Doppler velocity in small unit volumes. The principal motivation for the development of such an instrument is to enhance the study of upper ocean processes by utilizing the ability of the sonar to detect surface waves and Langmuir circulation. Important design parameters for the instrument are investigated and a detailed design proposed. Key technical issues such as the trade-offs among spatial resolution, temporal resolution, velocity precision, total range, and power are discussed. The azimuthal motion of the instrument on a mooring is considered as a potential problem, and possible solutions are discussed. Matlab functions used for the investigations are included in an appendix.
  • Article
    Multichannel detection for wideband underwater acoustic CDMA communications
    (IEEE, 2006-07) Stojanovic, Milica ; Freitag, Lee E.
    Direct-sequence (DS) code-division multiple access (CDMA) is considered for future wideband mobile underwater acoustic networks, where a typical configuration may include several autonomous underwater vehicles (AUVs) operating within a few kilometers of a central receiver. Two receivers that utilize multichannel (array) processing of asynchronous multiuser signals are proposed: the symbol decision feedback (SDF) receiver and the chip hypothesis feedback (CHF) receiver. Both receivers use a chip-resolution adaptive front end consisting of a many-to-few combiner and a bank of fractionally-spaced feedforward equalizers. In the SDF receiver, feedback equalization is implemented at symbol resolution, and receiver filters, including a decision-directed phase-locked loop, are adapted at the symbol rate. This limits its applicability to the channels whose time variation is slow compared to the symbol rate. In a wideband acoustic system, which transmits at maximal chip rate, the symbol rate is down-scaled by the spreading factor, and an inverse effect may occur by which increasing the spreading factor results in performance degradation. To eliminate this effect, feedback equalization, which is necessary for the majority of acoustic channels, is performed in the CHF receiver at chip resolution and receiver parameters are adjusted at the chip rate. At the price of increased computational complexity (there are as many adaptive filters as there are symbol values), this receiver provides improved performance for systems where time variation cannot be neglected with respect to the symbol rate [e.g., low probability of detection (LPD) acoustic systems]. Performance of the two receivers was demonstrated in a four-user scenario, using experimental data obtained over a 2-km shallow-water channel. At the chip rate of 19.2 kilochips per second (kc/s) with quaternary phase-shift keying (QPSK) modulation, excellent results were achieved at an aggregate data rate of up to 10 kb/s
  • Technical Report
    WHOI acoustic telemetry project interim report 12/1/88 - 6/1/89
    (Woods Hole Oceanographic Institution, 1989-07) Catipovic, Josko A. ; Freitag, Lee E.
    This interim report covers the progress of the acoustic telemetry project during the period 12/1/88 to 5/15/89. In general, the work followed the format specified in WHOI proposal No. 5674.1. The major exception was the deletion of the transmitter array development task and a corresponding funding decrease from $242,242 to $170,000. In addition, the period for the funding was extended to June 30, partly due to a two month delay in project startup. The telemetry project was centered around the construction, programming and testing of a digital receiver prototype capable of supporting future signal processing algorithms in real-time over ocean acoustic channels. The baseline receiver consists of a two-channel analog quadrature demodulator, and interface to a multiprocessor receiver for digital signal processing. The software developed includes routines for command and control of the analog demodulator, data handling and formatting, and minimal software to digitally implement an incoherent MFSK demodulator, synchronizer and data decoder. Data storage and display programs were also completed to facilitate the performance analysis of the unit during testing. The system was tested in Woods Hole harbor at data rates up to 4800 bits/sec. The acoustic channel was time-dispersive Rayleigh fading, and performance close to theoretical expectations was achieved. We are confident that the system error behavior is arising from channel-caused effects and known deficiencies in system performance, such as excessive synchronizer steady-state jitter.
  • Article
    Inversion of surficial sediment thickness from under-ice acoustic transmission measurement
    (Acoustical Society of America, 2021-01-15) Chotiros, Nicholas P. ; Hope, Gaute ; Storheim, Espen ; Hobaek, Halvor ; Freitag, Lee E. ; Sagen, Hanne
    The under-ice acoustic transmission experiment of 2013, conducted under ice cover in the Fram Strait, was analyzed for bottom interactions for the purpose of developing a model of the seabed. Using the acoustic signals, as well as data from other sources, including cores, gravimetric, refraction, and seismic surveys, it was deduced that the seabed may be modeled as a thin surficial layer overlaid on a deeper sediment. The modeling was based on the Biot–Stoll model for acoustic propagation in porous sediments, aided by more recent developments that improve parameter estimation and depth dependence due to consolidation. At every stage, elastic and fluid approximations were explored to simplify the model and improve computational efficiency. It was found the surficial layer could be approximated as a fluid, but the deeper sediment required an elastic model. The full Biot–Stoll model, while instrumental in guiding the model construction, was not needed for the final computation. The model could be made to agree with the measurements by adjusting the surficial layer thickness.