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High-Speed Acoustic Communication For AUVs Through MIMO Techniques
MIMO Acoustic Communication Prototype on a Gavia AUV Achieves High Data Rates in Shallow Waters of the Delaware Bay

Dr. Aijun Song
Physical Ocean Science and Engineering Program

Dr. Mohsen Badiey
Professor and Director
Physical Ocean Science and Engineering Program

Dr. Arthur C. Trembanis
Coastal Sediments, Hydrodynamics and Engineering
College of Earth, Ocean and Environment
University of Delaware
Newark, Delaware

The payload module, flooded section and nosecone for the MIMO test bed.

One of the prevailing challenges to oceanographic research has been a lack of adequate sampling of the marine realm. Historically, ship-based and mooring-based data formed the bulk of observational understanding. In recent years, Lagrangian platforms such as AUVs have come to play an important role as mobile sampling platforms. AUVs provide compelling benefits for both the places they can access (e.g., hostile environments) and the types of missions they can conduct (e.g., repeated precise patterns).

Underwater wireless technology is critical to the AUV missions. Being untethered has the benefit of decoupling the AUV from the surface and allows for more stable maneuvers through the water column. Underwater wireless technology can also make possible real-time and adaptive AUV operations. This could be revolutionary for ocean exploration and management. For example, AUVs enhanced by high-data-rate communication capabilities could share optical or sonar survey images with surface platforms in real time. Real-time monitoring and disaster response in the ocean could then be possible.

Challenges in Underwater Communication
Acoustic waves are widely used as information carriers in the ocean since both electromagnetic and optical waves are strongly attenuated in seawater. This leads to acoustic communication as the main means for data telemetry in the ocean. Present acoustic communication technologies using a single transmitter have limited data rates due to the narrow bandwidth available in the underwater environment. As the communication channel, the ocean has extensive multipath, better known as echoes, and rapidly changing characteristics (e.g., sharp density stratification and breaking waves).

In a typical coastal communication channel, the available bandwidth is only a few tens of kilohertz, compared with a few hundreds of megahertz of bandwidth in radio frequency wireless communication. The multipath typically ranges up to tens of milliseconds, which translates into severe intersymbol interference (ISI), a blurring together of subsequent communication transmissions. As a major hurdle to high-speed communication, the ISI in the ocean is much more severe than those encountered in radio frequency wireless communication. Due to these challenges, commercial acoustic modems often have data rates of less than one kilobit per second in the coastal environment, a speed that can only support short text messages.

MIMO Communication
Recent studies show that significant data rate increases can be achieved by simultaneously transmitting multiple data streams from a bank of transmitters in wireless communication. Taking advantage of spatial differences of the signals from different transmitters, multiple data streams can be recovered at multiple receivers at the same time and at the same frequency. The transmission of multiple data streams provides increased data rates, similar to communicating through multiple, independent links between the sender and recipient. This is known as multiple-input multiple-output (MIMO) communication. As a major technological driver, the MIMO technique is responsible for the multifold data rate increase in radio frequency wireless communication.

The use of multiple transducers for communication in the underwater environment is still in the early stages of research. In addition to the multipath effects, cross-talk among different transducers results from the usage of multiple transmitters in MIMO communication. The challenge is to treat both multipath propagation and cross-talk in the dynamic ocean. Most of the studies in the literature used transducers with enough element separation to minimize the cross-talk.

Significant data rate increases have recently been demonstrated in at-sea experiments by different groups, including the research team at the University of Delaware (UD).

MIMO communication was recently tested by the UD team on a small-size AUV. This compact test bed is capable of simultaneous transmission of multiple digital data streams using multiple transducers. Due to the space constraint, physical separation among the source elements is limited'only a couple of inches for the 25-kilohertz transducers. Field tests were conducted in 2010 to examine the acoustic transmissions as well as the AUV navigation with the communication test bed in the Delaware Bay. The acquired acoustic communication data were processed by advanced signal-processing techniques, which addressed both the cross-talk and multipath effects. MIMO communication was demonstrated from the two transducers on the AUV to a multielement receiving array. To continue this article please click here.

Dr. Aijun Song is a faculty member of the physical ocean science and engineering program in the College of Earth, Ocean and Environment at the University of Delaware. His research interests include underwater acoustic wave propagation, digital communication theory and advanced signal processing in mobile radio frequency and underwater acoustic environments. He received his Ph.D. in electrical engineering from the University of Delaware in 2005.

Dr. Mohsen Badiey is a professor and director of the physical ocean science and engineering program in the College of Earth, Ocean and Environment at the University of Delaware. His research interests are physics of sound and vibration, experimental and theoretical aspects of acoustical oceanography, and underwater acoustic communication. He is a fellow of the Acoustical Society of America. He received his Ph.D. from the University of Miami in 1988.

Dr. Arthur Trembanis is the director of the Coastal Sediments, Hydrodynamics and Engineering Laboratory (CSHEL) in the College of Earth, Ocean, and Environment at the University of Delaware. The work of CSHEL involves the development and utilization of advanced oceanographic instrumentation, particularly autonomous underwater vehicles. He received a bachelor's degree in geology from Duke University in 1998 and a Ph.D. in marine sciences from the Virginia Institute of Marine Sciences in 2004.

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