Earth’s two-third of the area is covered by water. With the rapid advancements in technology, underwater communications have become a fast-growing field due to the wide variety of applications in commercial and military based systems such as underwater sensor networks (UWSNs), remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs).
The research on underwater wireless communication techniques has played the key role in the exploration of the oceans and other aquatic environments. However, underwater communication has big challenges to overcome, because underwater wireless networks can be seriously affected by the marine environment, noise, limited bandwidth, limited power resources, and the harsh underwater ambient conditions. Hence, the underwater communication channel often manifests severe attenuation, multipath effect, frequency dispersion, constrained bandwidth, power resources, etc. This makes the underwater communication channel one of the most complex and harsh wireless channels in nature.
About five years ago, NASA and Lincoln Laboratory came up with the Lunar Laser Communication Demonstration (LLCD) system. The project used a pulsed laser beam to transmit data from a satellite, which was orbiting the moon to earth. Although the satellite’s distance from the earth was more than 239,000 miles, the satellite transmitted the data at a whooping download speed of 622 megabits per second. This invention is being utilized for underwater communication and is breaking all the records of underwater communication.
Following are the key features of laser-based visible light for underwater communication (Lasercom) system:
- Use of water as a propagation medium
One of the major problems in underwater communications is the low data rate available due to the use of low frequencies, and the main focus is on increasing distances and bandwidth. On the other hand, scientists are attempting to reduce the energy consumption of underwater devices. The strong backscattering caused by suspended particles in water disturbs the communication at long distances. Another way to improve the connectivity is to use the system of acoustic and ultrasonic communications, but they are affected by turbid water with large particles. The utilization of radio frequency (RF) methods hardly able to reach the data rates up to 100 MB/s that works only for short distances. Undersea laser communication utilizes an underwater optical link that uses water as the propagation medium. Unlike the other conventional undersea approaches that send the beam over a wide angle with the reduction in range and data rates, the new LLCD based communication technology has the ability to achieve undersea communication that is 10,000 times more efficient than other conventional techniques.
- Acquisition scanning function
Most of the autonomous systems working above the sea surface level depend on the GPS technology for positioning and timing data; however, GPS signals do not penetrate the water medium. Submerged vehicles use costly and large navigation systems, which comprise gyroscope, accelerometer, and compass data. This position calculation is noise sensitive, which leads to a large number of errors in the hundreds of meters when the equipment is submerged for a significant period of time. The use of acoustic waves for ratification and communication does not work underwater, because acoustic signals move more slowly than radio waves. Acoustic signals take two seconds to travel back and forth across a 1.5-kilometer distance. In addition, signal transmission is affected by refraction, absorption, and scattering through the water, which leads to attenuation, a gradual loss in intensity as a signal moves through a medium, and it is way greater in liquid compared to air. There is a very high reflection of signals underwater with acoustics. The signal can bounce off the seafloor and other underwater geographic structures, which includes mediums such as the ocean’s surface and layers of water separated by differences in temperature as well as density. The shallow water and reef structures are one of the most difficult areas where communication handling is highly challenging. These position uncertainties make it very difficult for an undersea terminal to locate and establish a link with the incoming narrow optical beams. To overcome these obstacles, narrow-beam lasercom implemented three basic functions, which are pointing, acquisitioning and tracking. A transmitter terminal sends a broader beam, which will eventually scan across the estimated uncertainty region called as receiver location. The acquisition scanning function can quickly translate the incoming beam over a span of the area covering a wide milliradian-class, and as a result, the companion terminal can easily detect the beam. Beam detection helps to keep the beam centered on the acquisition and the communication detector of lasercom terminal. The acquisition and tracking are employed at both transmit and receive terminals in a bi-directional manner, making it more focused and reliable system.
- Wide bandwidth signaling features
After the two lasercom terminals get locked onto each other and are communicating, the wide bandwidth signaling features in the communication waveform determine the two vehicles precisely. The wide bandwidth signaling feature assists to calculate the relative bearing and range between vehicles with precision accuracy within a few centimeters. Recently, six members of the lasercom team tested the communication capability of this system on two underwater vehicles. In their tests, the vehicles searched and located each other within one second and transmitted hundreds of gigabytes of data in one session.
- Modulation and Information Capacity for the Photon-starved Channel
LLCD demonstrated a high rate optical communication, popularly known as “photon-starved channel.” Photon-starved channel is the system where the total signal flux relative to the data rate is very limited. Both the deep space links and undersea links are the photon-starved channels. In the case of LLCD, photon efficiency was increased by utilizing optical bandwidth and detectors sensitive to single photons. LLCD used a 5 GHz high bandwidth signaling scheme with 16-ary pulse position modulation PPM, which enabled the transmission of multiple bits of information. The PPM signaling and single photon receivers are compatible with the undersea environment, allowing the machine to compute the best achievable efficiency with PPM and single-photon sensitive receivers.
- Communication System Dynamic Range
The terminals of the lasercom system are designed in such a way that they can achieve high data rates through a long distance. The terminals would be designed for the customized low power level with variable water clarity. This robust system is able to handle distances shorter than the maximum range, which is accomplished by shortening the propagation distance by a few extinction lengths, or by clarification of water. In such a varied dynamic range, lasercom is able to deliver revolutionary Gigabyte/Sec class communication.
Many previous undersea communication systems have been designed to operate with extra powered wide-beam optical power transmitters, which fail to gain the optimal performance. This new undersea transmission technology can achieve significant performance by using actively-pointed transmitters and photon-counting receivers designed to operate with high-sensitivity waveforms and powerful error correction. In addition, the ability to transmit data from megabit to gigabit-per-second over a variety of distances will open the doors for tremendous research and development in the underwater communication area. Lasercom has the ability to upgrade the underwater defense system with its highly precise and efficient space laser technology. This high data rated, reliable communication technology can completely transform the underwater vehicle operation methods leading to the development of next-gen high-accuracy underwater vehicles.
