Terrestrial Wi-Fi to Boost Underwater Communications
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.
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