Underwater Free space optical communication promising alternative for Short range links. Considered to be point to point. New optical front end proposed the concept of smart transmitters and receivers. Smart Receivers is capable of detecting angle of arrival of signals. Smart transmitters electronically steers output beam towards particular direction. Estimates water quality from back scattered light.
Advantages are non mechanical pointing and tracking on a moving underwater vehicle. Providing sensory information to underwater vehicles. Duplex multi user system with spatial diversity allows for simultaneous reception from two non co-located transmitters. It monitors optical backscattering while transmitter is active. Beam attenuation coefficient: atio of energy absorbed or scattered from an incident power per unit distance. Single Scattering albedo ratio of scattering coefficient to beam attenuation coefficient.
Goal is to develop a quasi omnidirectional system that reduces pointing and tracking requirements. Characteristics are increased FOV and angle of arrival estimation. 3-D spherical array of lenses all focusing to a 2 D planar array of photodiodes. A prototype constructed using seven lenses and seven photodiodes.
Lens at the receiver - Research in the domain of indoor optical wireless in use of spherical photodiode arrays for increasing FOV. Existing optical front-end arrays use - Photodiode arrays with no lenses, Single lens with multiple photodiodes and Multiple lenses focusing on separate photodiodes. Intensity of light received can be used to estimate the angle of arrival of light.
Photodiode output combining - Connect the array of photodiodes in parallel. An ideal combining technique to maintain bandwidth, minimize noise and maximize SNR. Linear diversity combing techniques are Equal Gain Combining(EGC) and Maximum Selection Combining(SEL).
Characteristics of Smart Transmitters are increased directionality and electronic switched beamsteering. Design consists of a truncated hexagonal pyramid with seven LEDs. Each LED is coupled with its own lens that converges the wide FOV of the LED to a narrower beam in a particular direction.
Characterization of the Receiver Lens photodiode Array - Experiments were conducted for the receiver pointed in all directions and intensities were observed at all photodiode outputs stored as a function of the spherical co-ordinates. A pan and rotate system. constructed using digital servos.Seven amplified photodiode outputs digitized using 8 channel digitizer.
Results show that design also capable of acting as smart system. Backscatter estimation experiment demonstrates linear relationship between return beam intensity and channel attenuation coefficient. Smart receivers increased field of view ability to estimate angle of arrival. Smart transmitters allows electronic switched beam steering.