Visible Light Communication in the Dark
Zhao Tian, Andrew T. Campbell, Xia Zhou
Dartmouth College
Problem: What If the Light Is Off?
Why Visible Light? Operating on visible light band, Visible Light
Communication (VLC) holds a great potential to solve the spectrum
crunch, complementing RF technologies, such as Wi-Fi.
Problem: The VLC link ceases when the lights are off
VLC in the dark: A new VLC primitive
•  Always-on. Extend the application scenario of VLC to 24/7: the
VLC link sustains even if the light source appears dark
•  Imperceptible. VLC up-link for mobile devices. The minimum
luminance shift incurred by the LED is 0.09 lx
•  Energy-efficient. The ultra-low energy consumption makes it
appealing to Internet of Things (IoT) devices, given the tight
energy budget. 46.8 µW power consumption at the LED
How to reduce the duty cycle?
Results: Illuminance, Power, and Data Rate
•  Narrow the ON time.
ü  Set ON time to its minimum, the physical constraint
q  Turn-on time of the LED
q  Rise- and fall-time limits of the switching circuitry
q  Bandwidth of the photodiodes
•  Widen the period time (ON time + OFF time).
? Trade-off
q  Small: bright light
q  Large: low data rate and flickering effect
The illuminance gradually drops to the ambient light level with the
increase of the period time. The minimum illuminance shift incurred
by the LED is only 0.09 lx. Imperceptible to human eyes in the day.
屏幕快照 2013-­‐03-­‐06 12.05.21 AM The power is proportional to the duty cycle. In our testbed, the
power of the LED of VLC in the dark can reach as low as 46.8 µW.
Implementation: Make the Switching Fast
Design: Decrease the Duty Cycle
We do not completely switch off the light; instead, we reduce its
luminance to “negligible” but still maintain its ability of
communication.
We make the LED light appear off by decreasing its flashing duty
cycle.
Reducing the peak intensity will curtail the communication distance
according to the inverse-square law.
With the On-Off-Keying (OOK) modulation scheme (1 bit per
period), the data rate we achieve is 190.7 bps.
•  Transmitter
q  Fast LED.
q  Dedicated MOSFET gate driver. High current to drive the
electrical switch for high speed
q  Short wires. Avoid reflection of signal in high speed circuit.
q  FPGA as micro-controller. Clock-cycle control granularity
•  Receiver
q High bandwidth photodiode. Trade off between the bandwidth
and sensing distance.
q High speed analog-to-digital (ADC). The sampling rate of
USRP N200 is 100 MS/s
q Coaxial cable.
q Impedance matching. Voltage follower between the photodiode
and the USRP
Discussion: Throughput and Network
•  Boosting the link rate. Pulse Position Modulation (PPM) can improve
the data rate by one order.
•  Supporting longer distance. Try more sensitive photodiodes.
•  Building network of VLC in the dark. Design the MAC protocol that
is suitable for the existence of multiple links.
•  Ambient light aware adaptation. Adapt the data rate and illuminance
according to the ambient light.
•  Potential application. Near field communication and IoT devices.
References
•  T. Borogovac, M. B. Rahaim, M. Tuganbayeva, and T. D. Little. “Lights-off” visible
light communications. In Proceedings of 2nd IEEE Globecom 2011 Workshop on
Optical Wireless Communications, pages 797–801. IEEE, 2011.