Starting with commercial off-the-shelf LED light bulbs, the researcher packaged an Atheros AR9331 SoC running Linux and an ATmega328p microcontroller running the VLC communication protocols, four photodiodes and an additional power supply to support the added electronics, all within a 3D printed enclosure expanding between the bulb’s screw-base and the actual LED dome.
The low bandwidth VLC systems called EnLighting is described in a paper to be presented at the International Conference on Sensing, Communication and Networking (SECON) 2016. The VLC controller runs a software-based PHY and MAC layer and enables low-level networking between multiple light bulbs. To make use of higher level network protocols (and for testing purposes) the VLC controller is extended with a SoC running a Linux distribution for embedded wireless systems: OpenWrt which allows to run a Wi-Fi interface as an additional control channel for the experiment.
The actual VLC channel is created by modulating the bulbs’ light intensity using pulse width modulation (PWM) based on simple On-Off keying (through a cheap microcontroller), the receiving photodiodes are either directly connected to one of the MCU’s analog pins or fed to an amplifier first.
The prototypes were tested on floor lamps spaced four meters apart, sending network packets either directly (one hop in direct line of sight) or through multiple hops from one node to the next, reaching a 200bit/s data throughput with 18 bytes UDP payloads, and estimating throughputs up to 600 bit/s for a maximum UDP payload of 158 bytes.
During his experiments, Schmid also demonstrated that the modified LED bulbs could be used to localize a receiver. In localization mode, each light bulb sends a UDP packet at set intervals with their unique sender address. The receiver (either another LED bulb being added to the network or an external photodiode-enabled device) then uses the Received Signal Strength Indicator (RSSI) to estimate the distance from the receiver to the transmitting light bulbs through triangulation techniques.
This research hints at the possible development of intermediate add-on EnLighting sockets, which could embed the necessary electronics as well as drawing the necessary current from the ceiling socket.
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