Terrestrial Wi-Fi to Boost Underwater Communications

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Orthogonal Frequency Division Multiplexing (OFDM) technology – which enables wireless computers and networks to work in homes and offices – could transform how data is gathered in oil and natural gas operations. Founded in 2007 by three faculty members from the University of Connecticut’s (UConn) School of Engineering, Aquatic Sensor Network Technology (AquaSeNT) has focused not on trying to take existing acoustic technology one step forward, but also accomplishing a quantum leap by adopting the best technology available in terrestrial radio and Wi-Fi for underwater communication and networking, CEO John Hanson told Rigzone in a recent interview.

“The founders wanted to adapt this technology not just for point to point communication, which already is being done by other companies, but to create networks, placing higher order intelligence into equipment and multiple nodes that can connect, the way a cell phone connects point to point in a seamless manner for users,” Hanson explained.  The challenges of underwater acoustic communication include echoes and reverberations and acoustic reflectors such as floating structures, water stratification, subsea equipment or the sea floor. Hanson said the company’s OFDM technology can overcome these challenges and provide the most efficient use of bandwidth, allowing for the transmission of more data than previously possible.

HOW IT BEGAN

Starting in 2005, the UConn faculty members received academic research grants for underwater wireless communication and networking from the U.S. Office of Naval Research and the National Science Foundation. After a couple of years, they realized that their combined academic work would coalesce into a body of work that has commercial value, with a focus on underwater wireless communication and networking.

UConn patented some of this work, specifically, the algorithms for implementation of underwater OFDM, which convert the digital information to be sent wirelessly into an analog signal, a process called modulation. In a classic case of university technology transfer, the company received an exclusive license from UConn in 2009 to commercialize work done in the academic labs. Small Business Innovation Research funding (SBIR) helped the company develop the technology toward commercial implementation, while ongoing academic research grants continue to create new insights on how to apply communications theory to boost the reliability and speed of underwater networks.  

Modulation can be achieved through a number of schemes, or mathematical algorithms. Two well-known examples are amplitude modulation and frequency modulation used in AM and FM radio. The goal of a modulation scheme is to place the digital data on an analog carrier signal, transmit that signal wirelessly, and then extract the original digital data from the received signal.

“In the world of communication, engineers are always looking for the best performance, to get the most information transmitted, subject to reliability,” said Hanson.

In the electromagnetic space, engineers figured out a modulation scheme that takes a range of frequencies and breaks it up into a bunch of individual subcarriers, and transmits information on all of them simultaneously. The best way to pack those subcarriers as close to each other as possible is through OFDM. Interestingly, attempting to do OFDM underwater is more difficult than in the electromagnetic world so it took time for the UConn researchers to work out all the bugs.

“But the result is that OFDM underwater brings some strong advantages, notably that it handles reflected acoustic energy much better than other approaches, while allowing more data to pass through the water. This offers commercial advantages,” said Hanson.

The Storrs, Conn.-based company started gaining traction in 2009 thanks to the SBIR grants, which allowed the concepts to be made into actual prototypes. After being shown as viable in computer simulation and in the lab, the technology was built and tested at UConn’s swimming pool, when the pool wasn’t in use. As part of UConn’s Technology Incubation Program the company maintains close contact with the university, benefiting from student interns and support from regional service providers.

The technology was next tested in regional lakes. Eventually, waterproof housings were built and the technology was tested the shallow waters of Long Island Sound, near UConn’s Marine Science campus.  For this phase of testing, a shallow water housing was used, which consists of a tube of PVC polymer with end caps.  Deepwater housings are very similar, but are typically made of stainless steel, aluminum, or titanium, to handle the extreme pressures at depth, where polymer tubes would be crushed. 

“It’s a just a trade-off between cost, weight, and durability, including rust and corrosion.”

After successful tests in Long Island Sound – an environment that has a lot of reflective acoustic energy – a number of oceanographic researchers started using AquaSeNT’s equipment; the National Oceanic and Atmospheric Administration started using the technology in the Chesapeake Bay. In oceanography applications, various sensors typically collect data on the seafloor or in the water column. Sometimes, a data logger is used to collect and store information over a long period of time, and then retrieved and the data downloaded. Other times, a buoy is placed on the surface, and a cable is run to the seafloor to bring the data to the surface. 

However, in the case of data loggers, at times the equipment fails, and the experiment is lost. And surface buoys create potential problems regarding ship navigation or vandalism, and also suffer from cable failures due to storms and general fatigue. So oceanographers often turn to wireless communication solutions, and are particularly interested in the improved robustness of the OFDM data link.

HOW ACOUSTIC UNDERWATER COMMUNICATION SYSTEMS WORK

The first speed of sound water measurements were conducted in the 1820s, and submarine signaling devices based on acoustics were used around World War I. The earliest acoustic modems first came into use in the 1960s and 1970s, which allowed for data transmission from sensors on a seabed to a surface modem. These early acoustic systems can be thought of as walkie talkies, in which a message is sent to the sensor package to wake up and send data, and then data is gathered without retrieving equipment from the seabed, Hanson said.

In acoustic transducer systems, a disc or sphere-shaped piezo ceramic crystal transducer is placed in the water and excited with an electrical signal, which causes it to expand and contract with the applied signal. The expansion and contraction converts the electronic signal into a pressure wave, which ripples throughout the water at the speed of sound like a pebble’s ripple on the water’s surface. The pressure waves eventually reach the receiver, causing it to vibrate and for the original electrical signal to be re-created. Because these pressure waves have been modulated with digital information, they carry the digital information on them to the receiver, and the receiver performs the reverse process, demodulation, and recovers the digital data from the carrier wave. 

In general, these early systems worked on a single frequency and performed okay, but often the data link would be dropped and operations would be disrupted, creating a need for more robust systems that could transmit data more reliably and with greater data rate.

“There was slow improvement in the technology, but the advent of digital electronics made a significant step forward in the ability to receive signals,” said Hanson.

Researchers in underwater acoustics saw the advances in digital technology in the telecommunications industry and wanted to apply this to underwater communication. The rate at which data can be transmitted wirelessly is related to frequency: the higher the frequency, the higher the data rate. Electromagnetic waves can transmit through the air at extremely high frequencies, but underwater only very low frequencies travel more than 32 feet (10 meters) or so, making radio communication impractical other than for very short ranges.  

“Even if you put your cell phone in a dry bag and put it underwater, it wouldn’t work; radio waves are absorbed by the water,” said Hanson.  

Some firms are developing optical based wireless data transmission, using light to transmit data through the water, which may achieve data rates exceeding that of radio. 

“But challenges with absorption and scattering are likely to limit the effective communication range.  Consequently, if you want to communicate wirelessly beyond 164 feet (50 meters) or so, you will be using acoustic technology.”  

The real breakthrough in Wi-Fi for telecommunications came in the early 1990s, when the OFDM modulation scheme came into use. The question is, can the same concepts be used underwater, in the acoustic domain, and bring the same improved robustness and higher acoustic data rates.

“Mathematically, it can work, but practically, it faces challenges of multi-path and Doppler shift,” said Hanson.

In particular, multipath – the fact that an acoustic signal moves through the water along more than one path – is a key challenge. The signal can bounce off the sea floor, off the air/water interface, or off of hard structures such as a ship’s bottom, and even reflect off thermal layers in the water. These multiple paths mean the receiver may not “hear” only the direct carrier signal, bur rather “hears” a sum of several copies of a transmitted signal, which can confuse the electronics. The Doppler shift challenge relates to the fact that acoustic signals travel with ocean currents, or perhaps reflect off waves near the surface, and this creates the appearance of a frequency shift. The electronics and algorithms in the receiver have to deal with this.

To address these issues, various filtering techniques can be used. Also, techniques that employ multiple frequencies are used, such as frequency hopping, or sweep spread spectrum, to help discern the desired signal from those that are reflections or echoes. However, multipath can still be an issue, and the computational overhead associated with addressing multipath effectively slows down data transmission. Newer acoustic systems are better with multipath, but can be much slower than they otherwise would be in an environment free of multipath and Doppler shift.  

An elegant aspect of the OFDM algorithm is that it is inherently insensitive to multipath; this is why it rapidly became the backbone of Wi-Fi in the electromagnetic realm and why AquaSeNT has developed it as a platform for underwater-networked communication, Hanson noted.

The other key advantage of broadband multi-carrier OFDM technology is the potential ability to send a lot of data through the water. By using a wide bandwidth, more subcarrier frequencies can be used, allowing more bits of data to be transmitted simultaneously. Underwater “acoustic Wi-Fi” won’t work at the same speed as Wi-Fi at an office desktop, but using the same technique does deliver more robust signals and higher data throughput,” said Hanson.

“Because the speed of sound in water is orders of magnitude slower than electromagnetic waves in the atmosphere, acoustic signals will never achieve the data throughput we enjoy with radio. But by using advanced OFDM algorithms, we can expect enough increase in data rate to enable new operational capabilities,” said Hanson.

OFFSHORE OPPORTUNITIES

Opportunities that AquaSeNT sees in the oil and gas space include mobile platforms such as autonomous underwater vehicles (AUV), which can only be communicated with wirelessly. Remotely Operated Vehicles (ROV) typically have power and fiber optic cables connecting them with an operator onboard the surface vessel, but can benefit from acoustic wireless for data collection from subsea sensors. There is discussion in the oil and gas industry of a tetherless ROV, that has the functional capabilities of traditional ROVs but can operate for a period of time without the cable; this concept can also benefit from higher data rate wireless communication to enable new missions.

Hanson said the company envisions an AUV or a remotely operated vehicle flying through an undersea field, acquiring an “underwater GPS” fix, taking photographic images, collecting data, and communicating with any number of nodes attached to subsea trees or manifolds or valves or sensor packages.

Currently, no acoustic data telemetry systems today can support video at a range that delivers practical benefit. With an OFDM modem running at high data rates, over short range, Hanson said a “freeze frame” video is possible, with an image update every few seconds. Research and development is still underway, and the company has methodologies that AquaSeNT believes holds promise of a delivering a quality image at a range that is useful to subsea engineers.

Hanson walked the show floor at the Offshore Technology Conference (OTC) in Houston in 2013, but wanted to conduct further testing before displaying the technology at OTC. At this point, the company feels confident in the reliability of its technology, and at this year’s OTC, provided information on AquaSeNT at the UConn booth at OTC’s academic showcase.

“We are an independent company, but we also maintain close connection to UConn and wanted to show we are part of a broader university initiative for this industry,” said Hanson.  

Quite a few subsea equipment manufacturers have expressed interest in learning more about “subsea acoustic Wi-Fi” and the firm is in discussions about possible trials. One of AquaSeNT’s founders has started a National Science Foundation-funded Industry/University Cooperative Research Center, or I/UCRC. The program’s objective is to encourage businesses to sponsor applied research at universities, and also to encourage university researchers to focus some effort towards research areas that directly address technical challenges that offshore businesses face.

UConn is working with the University of Washington (UW), which also has a strong portfolio of research work related to oceanography and offshore data collection, Hanson noted. The focus of UConn and UW’s I/UCRC is “smart ocean technology;” innovation to capitalize on emerging cyber technology to advance offshore activity.

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