Communicating underwater has been a challenge for decades. Ever since technologies like radio and lasers came on the scene, people have been looking for a way to operate some sort of similar communication system under the sea.
However, water doesn't play well with radio waves nor handle light very well.
Despite these setbacks, engineers found one potential medium to communicate underwater — acoustic signals. However, it came with its own challenges, such as the strength needed to transmit signals long distances and the disruption to marine ecosystems when that strength was ramped up. Despite these setbacks, there is still a need for viable communication methodology for underwater infrastructure, and research teams worldwide are continuing to look at the problem.
Sound waves
The most used form of underwater communication protocol is sound. Sounds are widely used by underwater animals like whales and dolphins to communicate. Some studies estimate that the songs of humpback whales can travel over 10,000 km underwater. However, plenty of variables affect that.
Frequency is one of the most important determinants of an acoustic communications system. Whales use extremely low-frequency sounds to communicate over long distances. Their lower range, the sounds that can travel the farthest, can reach down to around 20 Hz. While that might be great for distance, it's not particularly useful for sending large amounts of data quickly. Higher-frequency sounds don't travel nearly as well, though they can relay information faster.
Water conditions can also dramatically affect sounds' ability to carry information underwater. Salinity, temperature and pressure affect how well sound can travel. There's even a layer of the ocean known as the SOund Fixing and Ranging (SOFAR) channel that allows sound to travel far distances with minimal energy loss.
However, sending signals like that can disrupt the natural communication systems of the cetaceans they sought to emulate. High power systems can even damage the hearing of nearby whales and other species biologically attuned to sound waves. So, while this remains the most popular form of underwater communication, researchers have had to develop workarounds to avoid causing irreparable damage to underwater ecosystems.
Many of these workarounds are based on signal processing. Technologies like frequency shift keying (FSK) and multiplexing are becoming more common underwater. Some research groups have focused exclusively on error-correcting algorithms that attempt to make up for data bits lost in the ocean waves. However, hardware implementation is also an important aspect. There is active research on "acoustic modems," a device that translates acoustic signals into digital ones and vice versa. They are like old telephone modems used for electrical signals over phone lines, except they use water as their medium and attempt to translate sound waves.
Underwater optical wireless communications
There's already a ton of optical communication under the water. Most of the internet is tied together using optical fiber cables that run along the sea floor. However, light can also be used as a communication medium in water, albeit over short distances.
Underwater optical wireless communications (UOWC) can support higher data rates at lower latency levels than traditional forms like acoustic signals. However, they are limited in their range, especially by certain water conditions like turbulence or increasing depth.
One key workaround for that limitation is to set up relays similar to nodes used in modern mesh systems. If an optical signal is strong enough to get from one repeater to another, it can then be broadcast on again to the next. In some cases, a self-organizing mesh network could form around optical communication nodes using lasers to talk to each other and hopefully back to a base station that can network in with above-water infrastructure.
To this end, there has been an effort to contribute "floats" spread throughout the world's oceans that could act as potential relay stations, with almost 4,000 operational as of a few years ago. Standardizing their communication protocol, error handling and networking infrastructures could enable an optical communications network to form around them. However, there is still the challenge of getting optical signals into the depths, where most light from the sun is already absorbed.
Several research teams are competing to showcase high-bandwidth, long-distance UOWC systems. Papers with titles like "100 m / 500 Mbps underwater optical wireless communication using an NRZ-OOK modulated 520 nm laser diode" and "34.5 m underwater optical wireless communication with 2.70 Gbps data based on a green laser diode with NRZ-OOK modulation" have flourished in the last few years.
Both of those paper titles refer to non-return-to-zero (NRZ) on-off-keying (OOK). These are two separate modulation techniques commonly combined in short-range optical communication systems. OOK is what can be considered a typical "0" and "1" transmission. When combined with NRZ, it changes a little bit. In NRZ, when transmitting a "1," the signal is held at a certain level for the entire time of the bit transmissions, whereas transferring a "0" would allow the signal to vary but doesn't necessarily have to be sitting at "0" the whole time. This system makes transmitting in noisy environments, like underwater, much more accessible and could hold the key to opening a wider world of UOWC.
Other systems
Radio communication in salt water has proven difficult, though some teams are still attempting to improve its range and reliability. Other researchers focus on more novel techniques, like magnetic induction.
It is particularly effective for short-range transmission in turbid water, which negatively impacts almost all other forms of underwater communication. It's also much less power-intensive than alternative forms. This work, which is being pioneered by a lab at MIT, has a long way to go before it can be commercially adopted, though.
Magnetic induction isn't the only technique catching the eye of MIT engineers, though — a press release from 2023 talks about a hybrid acoustic system that uses a phenomenon called underwater backscatter to enable kilometer-range high bandwidth signals using relatively low power. Other hybrid systems, like a combined optical-acoustic system developed by Shimadzu Corporation in Japan, hope to accentuate the advantages of each technique while downplaying their shortcomings.
Watery future
Underwater sensors and vehicles are becoming more and more common as we continue to expand our knowledge of the world's oceans. Setting up an "Internet of Underwater Things" might seem like a pipe dream of oceanic and communications enthusiasts hoping to bring the wider, wet world into the realm of modern-day communications. But as companies, researchers, labs and even countries compete to set new records for the distance, speed and reliability of their underwater communications channels, it's only a matter of time before the World Wide Web starts to include more and more waterlogged equipment.
About the author
Andy Tomaswick is an engineer and freelance writer who is passionate about education, space exploration and improving the world through technology. When he's not engineering or writing, he spends time with his family or runs in circles to stay in shape.
