Device to Device Underwater Optical Wireless Communication

Communication in free space by means of radio frequency (RF) carrier has been in use since old age. However, it has certain drawbacks and optical wireless communication has come to the forefront. Optical free space communication (FSO) has many benefits like huge licence free bandwidth, ultra-high data rates because of the ultra-high speed of sound, inherent security and ease of installation.

In this case, optical sources like LED, LASER are used as transmitters while photodetector is used as receiver. A RF modulated signal is used to modulate the intensity of optical sources and the optical signal is released into space where information is transmitted in the form of photons. The information transmitted will be directly proportional to the intensity of the optical field. At the receiver, direct detection will be performed, where according to the intensity of the optical field, electrical signal will be generated and data can be recovered. Now in the latest scenario, this FSO can be extended to underwater applications also.

Underwater optical wireless communication (UOWC) can be used in military applications, offshore explorations, disaster management etc. There are many wireless sensor nodes embedded on the sea or ocean beds which collect data from the aquatic environment. These data need to be relayed to the ships, submarines or other underwater vehicles. There are onshore data processing centre above the water surface which exchanges data with the ships and satellites with RF or FSO links. There are many relaying vehicles like autonomous underwater vehicles (AUV) or remotely operated underwater vehicles (ROV) which helps in exchange of data among the sensor nodes and ships or submarines. These vehicles interchange data among themselves through optical wireless links.

This aquatic environment poses a lot of challenges in designing the channel model because of its turbid environment. The devices need to be energy efficient also due to lack of power source in aquatic environment. The signal will suffer from scattering and absorption effects due to presence of organic matter, detritus, chlorophyll and other suspended particles in water. Ocean currents also induce sudden changes in temperature and pressure thereby leading to fluctuations in refractive index of water. Hence propagation of light is affected. The link range in UOWC will be moderate, as a result of which cooperative UOWC communication will be needed (with the help of relays). Current research is going on to design the accurate channel model which takes all the propagation effects in seawater into account. This will be beneficial for device-to-device (D2D) communication underwater.

Anirban Bhowal Research: Wireless Communication Communication Lab-1 Email: abhowal@iitg.ac.in