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Spatial diversity processing for underwater acoustic telemetry
| Details |
Inventors: Catipovic, Josko A.; Freitag, Lee E.;
Assignee:
Primary Examiner: Pihulic; Daniel T.
Assistant Examiner:
Attorney, Agent or Firm:
An underwater acoustic telemetry system uses spatially distributed receivers with aperture sizes from 0.35 to 20 m. Output from each receiver is assigned a quality measure based on the estimated error rate, and the data, weighted by the quality measure, is combined and decoded. The quality measure is derived from a Viterbi error-correction decoder operating on each receiver. The quality estimator exploits the signal and noise differential travel times to individual sensors. The spatial coherence structure of the shallow-water acoustic channel shows relatively low signal coherence at separations as short as 0.35 m. Increasing receiver spacing beyond 5 m offers additional benefits in the presence of impulsive noise and larger scale inhomogeneities in the acoustic field. Diversity combining, even with only two receivers, can lower uncoded error rates by up to several orders of magnitude while providing immunity to transducer jamming or failure. |
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DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S) The spatial diversity system of the present invention provides a means of increasing reliability without the throughput and bandwidth cost of a very low-rate error correction scheme and provides substantial advantages during extremal channel conditions when conventional error control codes are overwhelmed and channel equalizers are unable to track the multipath fluctuations.
An acoustic telemetry link is usually only one part of what may be a very complex and expensive underwater system.
The cost of a link failure must be taken into account when such a system is designed, and anything the designer can do to reduce the possibility that the link will become inoperable must be considered.
Solutions which appear excessively complex or computationally intensive in other communication applications are frequently justified in underwater acoustic communication.
The spatial diversity system of the present invention maintains a quality check on hydrophone performance and can switch from faulty or jammed units to new ones automatically.
This allows the system operator to select, change, and redistribute receiver hydrophones without interrupting the system.
Since hydrophone failures are one of the most likely system failure modes, an automatic monitoring system is of practical value.
The present invention provides techniques for selecting and weighting the output from multiple hydrophones for optimal operation under both excellent and worst-case conditions.
Advantages of diversity reception over fluctuating channels are well documented in the literature, for example, in J.
M.
Wozencraft and I.
M.
Jacobs, Principles of Communication Engineering, New York: Wiley, 1965, and R.
Kennedy, Fading Dispersive Communication Channels, New York: Wiley, 1969.
Most conventional techniques use explicit diversity, where a number of diversity paths are deliberately excited by the transmitter, and the available power is divided among the diverse paths.
The ocean acoustic channel supports a number of independently propagating transmission paths, and it is rather difficult to avoid exciting several diversity paths with realistic acoustic communication systems, as shown in S
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Multiplexer-related 
