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The UT 3000 MASQ Through-Water Communication System
Direct-Sequence Spread Spectrum Modulation Techniques Provide Naval and Research Submarines With Reliable Communication

AUTHORS:
Dr. Andreas Mues
Senior Vice President of Technology
L-3 ELAC Nautik GmbH
Kiel, Germany

Ross Stuart
Director of IT and Technology
L-3 Nautronix Ltd.
Fremantle, Australia

Through-water communication (TWC) has been a research subject since the early days of submarine development. Work on TWC started in Kiel, Germany, in the first decade of the 20th century when Heinrich Hecht, who later founded EMAC, tested acoustic devices for through-water signaling in the Bay of Kiel, Germany. He noted in his technical diaries: “One day we achieved a range of 100 kilometers and thought we had solved the problem just to see a range of two kilometers on the following day.” While studying the temperature and pressure dependence of sound propagation a decade later, Heinrich Barkhausen and Hugo Lichte recognized that Hecht was facing a physical problem—now known as the acoustic channel—that still affects TWC today.

In the 1940s, analog TWC systems were developed to implement through-water telephony, a technology still in use today by the majority of submarines. TWC systems remained virtually unchanged until the introduction of digital TWC in the 1990s. L-3 Nautronix was one of the pioneers in this field, adapting digital above-water communications theory for use in the undersea acoustic channel.

The development of digital TWC technology coincided with a change in the operation of naval submarines. At the time, most navies were operating diesel-electric submarines that were designed to combat warship attacks or to fulfill particular missions such as special forces insertion. For these missions, nondetection was critical, often precluding the use of active sonar transmissions, including TWC.

Recently, however, the use of submarines has been extended to include information collection, especially in a crisis situation against asymmetric threats. This new engagement is often executed in littoral waters and in cooperation with surface platforms. Quite often the new threat does not have the means to detect a dived submarine. Therefore the use of a TWC device—along with the increased detection probability—is more acceptable than it was in the past. TWC is also a necessity for operations in which the submarine is part of a naval formation.

MASQ digital spread spectrum modulation.

Development of the UT 3000
The underwater acoustic channel is generally recognized as one of the most difficult communication environments, as Hecht discovered more than a century ago. The channel is characterized by frequency-dependent path loss, multipath propagation and low propagation speed. The channel impulse response is sparse, and each channel path acts as a time-varying low-pass filter, with additional motion-inducing Doppler spreading and shifting.

To combat these problems, L-3 ELAC Nautik and L-3 Nautronix, both sister companies at L-3 Communications (New York, New York), have worked together to develop the UT 3000 MASQ Digital Underwater Communications System.

An underwater telephone, the UT 3000 is capable of voice and robust data communications. Developed by L-3 ELAC Nautik and first introduced in 2008, it represents the next generation of underwater telephones utilizing advanced digital signal processing algorithms.

L-3 Nautronix had previously developed digital TWC systems, in particular the HydroAcoustic Information Link, which is in service in the Royal Australian Navy and U.S. Navy. MASQ is the latest signaling technology to be developed and provides the foundation for the next generation of reliable TWC data systems for subsea platforms operating at speed and depth.

MASQ Spread Spectrum Signaling
The MASQ signaling is an adaption of direct-sequence spread spectrum (DSSS) modulation, which, as its name suggests, spreads the information across a wide frequency spectrum. Due to the processing gain, the attenuated signal is still detectable at the input of a receiver even if the signal power has dropped below the background noise. The DSSS modulation is particularly effective in fading channels typical of shallow-water environments.

In addition to using the frequency domain, MASQ signaling utilizes the time domain to further improve reliability by including redundant symbols in the message as part of a forward error correction scheme. This mechanism is effective against transient noise sources.

Finally, the spatial domain is exploited in the UT 3000 MASQ implementation when the transducer has multiple hydrophones. The receiver is designed to permit spatially separated hydrophones to be processed separately, combining the decoded outputs on a best symbol selection basis or best message selection basis.

While the MASQ schema has been optimized for reliability, a significant additional benefit of the MASQ signaling is its ability to operate reliably below the background noise, thereby providing a low probability of intercept or stealth capability. When configured for 100 bits per second or lower, the receiver can decode messages with a signal-to-noise ratio of -9 decibels or better.

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UT 3000 Expandability Features
When L-3 designed the UT 3000, the company anticipated that digital TWC and its applications would develop over time. The system was created so that upgrading the processor and processing were integrated features of the design.

In a basic configuration, the UT 3000 is equipped with two boards: one board for classic telegraphy and telephony and another for a noncoherent multicarrier-based digital communication schema (multifrequency shift keying). In this way, both methods can be used in parallel. This means that if a digital communication is detected, the system automatically receives and displays the received information or stores it; incoming telephony calls would be processed and made audible at the same time.

The processing power of the UT 3000 may be upgraded simply by adding processing boards. Its software is designed in a modular manner with an infrastructure common to all processing implementations and processing blocks (e.g., a fast Fourier transformation process) compiled as required by the application. This concept extends modularity from the processor to the processing. The processing software is realized in blocks in a software-defined environment, allowing third parties to add their own software applications to the system. This concept was developed in close cooperation with the Bundeswehr Technical Centre for Ships and Naval Weapons, Naval Technology and Research, WTD 71.

MASQ processing was implemented on a UT 3000 processing board hosted by one of the two extension slots. While all processors (e.g., telephony, multifrequency shift keying, MASQ) share the identical software infrastructure (e.g., electronic control, data transfer, up-down conversion) the processing elements (e.g., modulation, coding, Doppler processing) differ to achieve the desired functionality.


MASQ Software Integration
In order to fully specify the MASQ acoustic signal, several parameters need to be configured, including the signal type, code set, carrier frequency, bit rate and forward error correction. As there are several billion combinations of parameters that can make a valid signal, a mechanism to manage configurations needed to be devised. In the UT 3000 MASQ implementation, this fundamental configuration item is termed a “preset,” which defines the properties of a communication link and configures components of the transmitter and receiver. To continue this article please click here.



Dr. Andreas Mues is senior vice president of technology at L-3 ELAC Nautik. Mues attended the University of Kiel in Germany, where he studied physics and oceanology from 1985 to 1993. He leads the design of all L-3 ELAC Nautik sonar products, including mine avoidance, hydrographic survey and through-water communication.

Ross Stuart is the director of IT and technology for L-3 Nautronix. Stuart has a degree in electrical engineering from the University of Western Australia and more than 20 years of experience as a professional engineer, with expertise in systems engineering, electronic systems and underwater acoustics for both oil and gas and defense applications.




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