Transmission possible

Space communication-inspired technology could improve quality of deep-sea signals

Swarms of autonomous vehicles may soon be combing the ocean floors swapping vital information on deep-sea wreckage investigations or possible naval defence missions, thanks to technology that makes underwater wireless communication faster and more reliable.

The breakthrough — that could make the transmission of clearer images, sound and videos possible — gives a glimpse into the future of a digital ocean in which integrated networks of instruments, sensors, robots and vehicles will operate in a variety of environments.

The applications not only include collaborative search missions but also ad hoc deployable sensor networks for monitoring environmental conditions such as climate change and oil spills.

The system from the Swedish Defence Research Agency FOI is claimed to be the first to reduce the effects of echoes, which limit data rate in underwater transmission.

When wireless communication is transmitted through the air by systems like Bluetooth, there are significantly fewer technical obstacles to overcome. But in seawater there is limited bandwidth available and signals are subject to signal interference. In shallow water, signals bounce from the surface and sea bed, while in deep water, acoustic waves tend to travel in the depth of water where sound moves the slowest. The scattered movement of these waves creates signal echoes, which create interference in a digital communication system.

Error correction

Erland Sangfelt, an FOI researcher behind the system, said the technology has reduced the effects of the echoes by taking advantage of turbo codes — a form of algorithmic error correction that has been used in deep space satellite communications. These codes work by adding redundant data to messages, which allows the receiver to detect and correct errors without needing to ask the sender for additional data. Turbo codes allow designers to transfer information over a limited bandwidth in the presence of data-corrupting noise.

‘We wanted to capitalise on the advancement of turbo codes with our system’s hydrophone, which is a type of receiver that transforms underwater sound signals into electrical signals,’ he said.

The turbo codes make it possible to increase data rate without increasing the power of a transmission. The main drawback of the system is it produces a relatively high time delay. ‘The delay underwater can be a few hundred milliseconds to sometimes a couple of seconds,’ said Sangfelt. ‘Normally with transmission of radio signals out of water you’re talking about microseconds. Here it’s 1,000 times more.’

Also, unlike out-of-water communication, signal transmission under seawater must be carried out at frequencies as low as 5,000hz. By comparison, mobile phones transmit pictures messages via Bluetooth at a frequency around 2.45Ghz.

Water in its pure form is an insulator, but in its natural state, it contains dissolved salts and other matter that makes it a partial conductor. The higher its conductivity, the greater the loss of radio signals which pass through it.

So, in effect, the saltier the water, the lower the frequency a radio signal needs to be to transmit without signal loss.

‘About 5,000hz would work well in the Atlantic,’ said Sangfelt. ‘In the Baltic Sea, which is not that salty, you could use frequencies twice as high.’

Sangfelt’s team is currently trialling transmitters and receivers 50m beneath the Baltic. ‘We can transfer data from a fairly long distance,’ he said. ‘We have done 60km without errors.’

He added that they were able to transmit signals at a data rate of 8,000 coded bits/sec using a 16 Quadrature amplitude modulation (QAM) modulation format.

‘The result was obtained using a single hydrophone,’ said Sangfelt. ‘This is important because we are looking for small-sized modems that can be fitted on to small platforms like autonomous underwater vehicles.’

Defence applications

Sangfelt said the AUVs could be used in a variety of defence applications. ‘Navies could use them to look for underwater mines,’ he said. ‘The information could be sent to a buoy and then transmitted through the air to a computer by radio signals or satellite.’

The major technical obstacle ahead has less to do with the communications system and more to do with the construction of the vehicles, according to Sangfelt.

‘They would have to be cost-effective,’ he said. ‘Also, most AUVs are powered by rechargeable batteries, and you would like them to have greater endurance and go several days without recharging,’ he said.

As far as the communications are concerned Sangfelt said, ‘there aren’t many challenges left.’