Communications – electrical: acoustic wave systems and devices – Underwater system – Telemetering
Reexamination Certificate
2003-09-18
2004-11-16
Lobo, Ian J. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Underwater system
Telemetering
C375S233000
Reexamination Certificate
active
06819630
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates generally to communications systems and, more particularly, to a high performance iterative and adaptive decision feedback equalizer which is especially suitable for use in underwater telemetry.
(2) Description of the Prior Art
The underwater environment provides numerous difficult obstacles for acoustic communications. The ocean acoustic channel produces large amplitude and phase fluctuations on acoustic signals transmitted therethrough causing temporal, spatial, and frequency dependent fluctuations. Multipath distortion is a significant problem. Underwater regions often experience high and/or variable sound attenuation. Ambient ocean noise influences the received signal-to-noise ratio and may require high transmission power levels to achieve suitable ratios depending on the conditions.
Presently utilized underwater coherent acoustic telemetry systems are often able to transmit M-ary Phase Shift Keying (MPSK) and M-ary Quadrature Amplitude Modulation (MQAM) signals. At the receiver end, these coherent signals may be processed by an adaptive multi-channel decision feedback equalizer (DFE). The DFE is then usually followed by a de-interleaver and an error correction decoder operating in a single pass fashion. The de-interleaver randomizes the errors and the error correction decoder tries to correct these randomly distributed errors. The error correction decoder is usually a Viterbi decoder for a convolutional code. The overall performance obtained by this type of prior art underwater telemetry system is often acceptable, but is not satisfactory in many situations. The desire for performance improvement has led to higher performance algorithms whose complexity is orders of magnitude greater than the standard decision feedback equalizer (DFE) system followed by de-interleaving and decoding. The turbo-equalization algorithm is one such algorithm that has performed much better than the normal algorithm but the cost has been extremely high complexity.
Turbo equalization and turbo coding may be applied to many detection and decoding problems. Turbo coding involves concatenation of simple component codes with an interleaver so that decoding can be performed in steps using algorithms of manageable complexity. However, the complexity of prior art turbo equalization increases exponentially with the number of channels and/or other factors, thereby making a multichannel telemetry system, as is typically utilized in underwater telemetry systems, highly complex. More particularly, the complexity of the prior art turbo-equalizer grows with channel complexity, modulation level, and spatial and/or time diversity. The complexity of a prior art turbo-equalizer is therefore orders of magnitude greater than the typical DFE structure discussed above.
The following U.S. Patents describe various prior art systems that may be related to the above and/or other telemetry systems:
U.S. Pat. No. 5,301,167, issued Apr. 5, 1994, to Proakis et al., discloses an underwater acoustic communications system that utilizes phase coherent modulation and demodulation in which high data rates are achieved through the use of rapid Doppler removal, a specialized sample timing control technique and decision feedback equalization including feedforward and feedback equalizers. The combined use of these techniques dramatically increases data rates by one and sometimes two orders of magnitude over traditional FSK systems by successfully combating fading and multipath problems associated with a rapidly changing underwater acoustic channel that produce intersymbol interference and makes timing optimization for the sampling of incoming data impossible.
U.S. Pat. No. 5,559,757, issued Sep. 24, 1996, to Catipovic et al., discloses an underwater acoustic telemetry system that 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.
U.S. Pat. No. 6,295,312 B1, issued Sep. 25, 2001, to Susan M. Jarvis, discloses a method and system for communicating in a time-varying medium. A transmitter sends transmissions of the same message data separated in time with respect to one another. A single sensor receives the transmissions. Each received transmission is buffered until all of the transmissions that were sent are received. The buffered transmissions are simultaneously processed via multichannel adaptive equalization only when all of the transmissions that were sent are received.
The above cited prior art does not disclose a system whose complexity is similar to that of the prior art decision feedback equalizer followed by a de-interleaver and an error correction decoder, but whose performance is greatly improved. The above cited prior art also does not disclose decision feedback equalizers utilizing hard and/or soft feedback from the decoder. The solutions to the above described and/or related problems have been long sought without success. Consequently, those skilled in the art will appreciate the present invention that addresses the above and other problems.
SUMMARY OF THE INVENTION
It is a general purpose of the present invention to provide an improved telemetry system.
Yet another object is to provide an augmented high performance iterative receiver algorithm for underwater acoustic telemetry.
It is another object of the present invention to provide a hard-iterative DFE structure and a soft-iterative DFE structure that is superior to the standard DFE structure.
It is yet another object of the present invention to provide a system which has linear complexity growth with the size of the symboling constellation as opposed to more complex systems such as turbo-equalization which experience exponential complexity growth.
An advantage of the present invention is that it takes advantage of the attractive features of the DFE structure such as diversity combining, modest complexity increase with channel complexity, symbol synchronization, and phase tracking while providing higher performance than a standard DFE with less complexity than the turbo-equalizer.
A feature of one embodiment of the invention combines a decision feedback adaptive equalizer (DFE) with a turbo-equalizer whereby the decision feedback equalizer or variant thereof provides a pre-processing stage for a turbo-equalizer that significantly limits the complexity of the turbo-equalizer.
An advantage of the present invention is superior performance as compared to the standard DFE structure.
Another advantage is that time or spatial signal diversity can be processed with low complexity within the DFE to provide a single stream of diversity combined symbols which can be processed with a simplified turbo-equalizer construction for use in multichannel transmissions.
Yet another advantage of the present invention is that a DFE structure may be utilized therein to take advantage of fractional spacing to help synchronize symbols.
Yet another advantage of the present invention is that a DFE structure may be utilized to reduce the extent of the channel response and therefore allow a turbo-equalizer to operate on a much shorter impulse response in order to reduce the complexity thereof.
These and other objects, features, and advantages of the present invention w
Blackmon Fletcher A.
Hagh Mohanadreza M.
Proakis John
Salehi Masoud
Kasischke James M.
Lobo Ian J.
Nasser Jean-Paul A.
Oglo Michael F.
The United States of America as represented by the Secretary of
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