Visible Light Positioning

This page details a rudimentary visible light positioning system using multiple Arduino Unos based on mchr3k's Arduino Manchester Encoding library.

Lamps and photodetector not included.

Concepts

VLC

Visible light communication (VLC) works by pulsing a light source on and off to send digital data to a receiver. A high frequency makes the flicker undetectable. Researchers have pushed bandwidths of over 1Gbit/s over these communication links, but this example only uses ~1200 baud as there is no need for high data rates.

Positioning

Upon startup, each lamp (emitter) continuously transmits the same packet, which is a randomly generated 6-byte address unique to each lamp.

uint8_t data[6] = {0x6D, 0x67, 0x93, 0xDD, 0x79, 0x7A};

The receiver (detector, filter and decoder) can then be positioned under the lamps to receive the transmissions. Additional processing is done and the receiver will notify the user which lamp it's under.

Use Case

This VLP implementation can be used as an indoor positioning system (IPS) to get sub-meter positioning accuracy within a building.

Getting Started

Prerequisites

Install the required libraries.

• Manchester Encoding - Communication library
• Timer1 - Easy timer library

Usage

• There are 3 sketches, transmitter, dcfilter and receiver.
• The transmitter sketch uses the library transmission example without any delay between packets. It loops forever. This example is connected to a custom Arduino-based 10 channel light source which also accepts PWM control. You're free to use any light source down to simple LEDs. Connect the light source switch to pin TX_PIN.
• The dcfilter sketch filters the incoming analog voltage from a photodetector and converts it into a digital signal. Again, you're free to use your own, like visible light LED transceivers. I would recommend adding an amplifier circuit with automatic gain control (AGC) before the signal to the Arduino. Connect the photodetector output to SENSOR_PIN and the PIN output into the receiver. Remember to not exceed 5V on the signal.
• receiver decodes the received packets which are received from the digital signal going into RX_PIN from dcfilter's PIN and updates the LEDs depending on which lamp is detected.
• Basically, photodetector > SENSOR_PIN > filter > PIN > RX_PIN > receiver.

Receiver LED Output

	Red	Green	Yellow
No lamp	Blinking	Off	Off
Lamp 1	Off	On	Off
Lamp 2	Off	Off	On

Pictures

• Here's an idea of how everything's connected together.
• The dcfilter sketch successfully converts a noisy 80mA peak-to-peak signal into a digital signal with a slight phase shift.

Limitations

• The current version of the code only supports two lamps (not easily scalable)
• The light cones from each lamp should not overlap (there is no multiplexing scheme implemented)
• The checking algorithm is not very sophisticated yet, although there is some allowance for lost packets.
• Currently, the register level code is only only works on the Arduino Uno.

Issues

• The Manchester Encoding library sometimes (at least 4/5 times) assumes incoming transmissions are of a different baudrate. In this case, it fails to update the receiving array even though it tells the user that a message has been received. To circumvent this, I have bodged a solution that checks whether the library has fallen into this state periodically and performs a dirty soft reboot if it is stuck.
• The filter and decoder run on separate Arduinos because there is heavy use of interrupts for both sketches. The sketches should be able to be merged if a lower baudrate is used.
• The Serial.print function cannot be used on the receiver as it seems to crash the Arduino Uno. LED outputs are used instead.