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Acoustic communication is critical for underwater application such as deep ocean scientific explorations, off-shore industrial controls, and ocean environment monitoring. This is because other techniques using electromagnetic waves such as RF communications are difficult for underwater applications due to the strong absorption of water in such a frequency. Optical communication, on another hand, suffers from the light scattering from micro-particles or marine life making long range underwater optical communication very challenging. Therefore, using acoustic waves to transmit information is currently the dominate technique for underwater applications including data collection and remote control of off-shore benthic stations. However, the low frequency bandwidth available for acoustic communication limits the data transmission rate and information capacity or content. We propose and experimentally demonstrate here a new approach using the orbital angular momentum (OAM) of acoustic vortex beams which provides a new and independent channel that enhances the data transmission rate by eight fold. The OAM multiplexing method demonstrated here will impact significantly on future underwater communications.

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In this paper, a visible light communication (VLC) based indoor positioning system (IPS) employing a dedicated algorithm and using the dual-tone multi-frequency technique is proposed. Unlike the time-division multiplexing based VLC-IPS, the proposed positioning system doesn't require synchronization between the transmitter and receiver, which makes it a simple, robust and cost effective. To test the accuracy of the proposed method, an experimental test-bed using 3 white light-emitting diodes and a photomultiplier tube is developed. The results show that the proposed VLC-IPS is highly accurate with an average positioning error of about 1.6cm, which far lower than many existing indoor position systems.

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