Communications – electrical: acoustic wave systems and devices – Underwater system – Telemetering
Reexamination Certificate
2002-05-14
2004-02-03
Lobo, Ian J. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Underwater system
Telemetering
C367S131000, C367S119000, C367S904000
Reexamination Certificate
active
06687188
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method and device for phase coherent underwater acoustic communications. More particularly, the invention relates to a system for long range underwater acoustic communications using a combination of arrays and sub-arrays.
BACKGROUND ART
Phase Coherent Underwater Acoustic Communications (ACOMMS)
The ocean presents an acoustic communication channel, which is band-limited and temporally variable. Propagation in the horizontal can be severely influenced by macro and micro multipath variability. Vertical propagation is often less severely impacted by the multipath.
Incoherent communication schemes, using for example frequency shift keying algorithms, are used for line of sight propagation conditions in which multipath has minimal impact on the signals of interest. At long ranges, symbol rates for incoherent communications are limited by the multipath symbol interference. Additional processing (such as error encoding) is often required to remove the bit errors (due to symbol interference). The available frequency band is limited by frequency fading.
Coherent communication schemes use the available bandwidth more efficiently and provide higher data rates than the incoherent schemes for horizontal transmission of signals in a multipath environment. The state of the art systems use a (recursive) minimum least-mean-square (MLMS) approach for equalizing and updating the channel. The MLMS approach requires a certain minimum signal-to-noise ratio (SNR) at the receivers, of typically 10-15 dB. Maximum data rate and minimum bit error rate depend critically on the temporal properties of the channel impulse response function. The recursive least square (RLS) algorithm is computationally intensive and only a limited number (typically <4) of channels can be supported by prototype systems for real time communications.
An algorithm for phase coherent acoustic communications is described in U.S. Pat. Nos. 5,301,167 and 5,844,951, incorporated herein by reference. The latter patent extends the algorithm from a single receiver to multiple receivers; it uses jointly a phase locking loop and channel equalizer to adaptively correct for the channel temporal variation to minimize bit errors. The communication signals are transmitted by grouping symbols into packets. Each packet begins with a short pulse (e.g., a Barker code of 13 symbols of binary phases) used for symbol synchronization and an initial estimate of the multipath arrival structure. It is followed by a data packet beginning with a training data set with known symbols to estimate the carrier frequency (shift) and train the equalizer. The equalizer is updated by estimating the symbol errors using either the known symbol as in the training data or a decision on the received symbol. The number of tap coefficients is estimated from the impulse response deduced from the probe/trigger pulse. Carrier frequency is estimated from the training data. The data are fractionally sampled, typically 2 samples per symbol, and the most popular schemes for signal modulation are binary phase shift keying (BPSK) and/or quadrature phase shift keying (QPSK) signals. Channel impulse response and equalizer update requires a minimal input signal-to-noise (SNR) ratio for minimal bit errors. Multiple receivers using spatial diversity are often required for successful communications.
Underwater Acoustic Communication Channel
The underwater acoustic communication channel is different from the RF channel in three respects: (1) the long multipath delay due to sound refraction and long duration of reverberation from the ocean boundary; (2) the severe signal fading due to time-variable transmission loss; and (3) the high Doppler spread/shift, i.e., the variability and offset of receiver frequency and phase relative to the transmitter resulting from the media and/or platform motion. The Doppler spread determines the signal coherence time assuming that the equalizer is able to update itself within the given coherence time. Because of these differences, the various techniques for radio frequencies (RF) communications cannot be applied directly to underwater acoustic communications.
Wireless radio communications are by line of sight with some multi-paths by reflection from nearby building and structure. Multi-path interference can usually be removed by antenna beamforming using an antenna on a horizontal plane. The array configuration can be designed with element spacing and configurations based on a plane wave model: the array aperture determines the width of the beam and element spacing determines the level of the side-lobes. Multi-paths in the oceans arrive with different vertical depletion/elevation angle. Array beamforming and diversity combining techniques can be used to mitigate the signal spreading by multi-path propagation. These two techniques are based on fundamentally different principles. To combat multi-paths, a vertical array or a planner array having some vertical aperture will be required. An array must have wide spacing between elements and hence large (vertical) aperture to combat signal fading by diversity combining. An array must have close spacing between elements to achieve the array gain by coherent beamforming. How to achieve both depends on the spatial coherence of the signal, which is normally not an issue in RF communications.
Multipath delays in underwater acoustic channels can last tens to hundreds of milliseconds, causing inter-symbol interference to extend over tens to hundreds of symbols depending on the carrier frequency and symbol rate. Inter-symbol interference in RF channels is orders of magnitude less and thus easier to deal with. Doppler shift of carrier frequency in underwater acoustic channels is several orders of magnitude larger than that of the RF channel since the sound speed is many orders lower than the speed of light. Hence, carrier frequency identification and symbol synchronization are critical for underwater systems. In addition, Doppler spread is non-negligible in the underwater communication channel as sound propagates through a random ocean medium and scatters from moving surfaces.
In a random medium, signal phase and amplitude fluctuations resulting from propagation through random environments are range, source, and receiver depth- and frequency-dependent. The temporal scale of fluctuations dictates the rate of adaptation for a coherent processor. The magnitude of the fluctuations determines how well the adaptation will work. Since successful communications require a sufficient input signal-to-noise ratio (SNR), appropriate placement of the source and receiver are necessary to avoid the “shadow” zones (areas where transmission loss is high). Random media increase the probability of signal fading; signal fading occurs when multipath arrivals interfere destructively.
The effects of random media on acoustic communications can be grouped into five areas: (a) signal amplitude fluctuations, which affect the ability of the modem to trigger on the probing signal (e.g., Barker code) and to decode the symbols properly; (b) signal phase fluctuations, which affect the performance of the phase locked loop; (c) temporal coherence of impulse response functions, which affects the performance of the channel equalizer; (d) Doppler spread and frequency coherence bandwidth, which limit the maximum data rate of underwater acoustic communications in an ocean channel; and (e) spatial coherence of the multipath signals, which determines the optimal use of multiple receivers. The effects of the ocean acoustic environments on the performance of phase coherence communications requires an environmental adaptive approach to estimate the signal propagation and noise characteristics in a particular ocean environment to improve the communication algorithm performance.
Multi-channel data have been processed using (1) conventional/adaptive beamforming followed by channel equalization or (2) adaptive multi-channel combining with spatial diversity. The purpose of (1) conventional or adaptive beamforming can be to i
Karasek John J.
Legg L. George
Lobo Ian J.
The United States of America as represented by the Secretary of
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