Engineers have been working on a seemingly straightforward project at Woods Hole Oceanographic Institution in Falmouth, Massachusetts, on a dock that has seen more oceanographic equipment deployed and recovered than most people will see in a lifetime: retrieving information in real time from the deep ocean. The problem is that the majority of people’s go-to resources for wireless data transmission—cellular frequencies, radio signals, and the electromagnetic spectrum in all of its practical forms—stop functioning as soon as they come into contact with saltwater. nor gradually, nor with a decline in performance. Within meters of the surface, they perish. For practical wireless communication, the deep ocean could be compared to a Faraday cage with an enormous water bill.
Physics is the constraint, and there isn’t a traditional fix for it. Underwater RF communication is not feasible for anything deeper than the shallowest applications due to the rates at which water absorbs radio frequency energy. Because of this, submarines use incredibly low frequency signals to communicate with the surface. These signals require massive ground-based antenna arrays and convey data at rates estimated in bits per minute, which is sufficient for brief coded communications but insufficient for anything approaching contemporary data interchange.
Historically, the communication options for scientific instruments, autonomous underwater vehicles, and monitoring sensors placed at depth have been either acoustic (transmitting information as sound waves, which travel far but slowly and at low bandwidth) or physical (transporting the data to the surface on a tether or by recovery of the instrument).
The majority of the current infrastructure is located in the acoustic option. Since sound moves through water at a speed of about 1,500 meters per second, acoustic modems have been the mainstay of underwater communication for many years. They operate at great distances. They do deep work. Bandwidth is the issue. Compared to surface wireless technologies, acoustic modems typically operate at data rates measured in kilobits per second, which are orders of magnitude slower. A few kilobits per second presents a bottleneck that restricts what is feasible in terms of real-time monitoring in an era where a single oceanographic sensor array might be producing continuous streams of temperature, salinity, chemical, and auditory data simultaneously.
American maritime engineers have been working on a hybrid acoustic-optical strategy that views the issue as two distinct problems with different solutions. When optical techniques are unable to create a communication link over hundreds or thousands of meters of water, acoustics takes care of the range and connectivity layer. The data rate layer is handled via optical communication, which uses blue-green laser wavelengths that are more effective at penetrating seawater than other frequencies. When instruments are close enough for optical communication to be feasible, high-bandwidth bursts are transmitted over short distances. By combining the two, neither technique needs to be used exclusively. When circumstances permit, the optical modem handles the heavy lifting while the acoustic modem keeps the connection intact.
The OFDM modulation technology used in underwater acoustic channels is customized for the unique distortion characteristics of acoustic transmission over water. It is derived from terrestrial wireless communications and is the same fundamental method used in WiFi and 4G networks. The sea isn’t a pristine passage. The physical boundaries of the seafloor and surface, temperature gradients, and salinity layers all reflect and scatter acoustic signals, resulting in what engineers refer to as multipath interference—versions of the same signal arriving at different times, smearing together in ways that distort the data being transmitted. By dividing the signal over numerous narrowband subcarriers that are each more resilient to this distortion, OFDM reduces interference that would taint a single-carrier signal and permits more precise information to pass through.
The component that has the most potential to alter the monitoring landscape is the development of tiny sensor tags. For the majority of research initiatives, deploying numerous conventional deep-sea instruments at once is impractical due to their size, power consumption, and high cost. It is possible to deploy small, pressure-tolerant acoustic tags at scale—attached to marine organisms for tracking, dispersed throughout a seafloor area for environmental monitoring, or embedded in infrastructure for continuous status reporting. These tags are made of soft encapsulation materials that compress without failing under extreme pressure. Their low power consumption eliminates the requirement for tethered electricity, which restricts traditional deep-sea instruments, and allows them to operate on battery power for extended periods of time.
