In this video
we demonstrate a lightweight VLC system
and it's application in indoor positioning
More details can be found in our Mobisys 2015 paper
We used three demo scenarios
to explain our design motivation and innovation
Existing schemes on establishing communication
between smart devices and lighting lamps
mostly relying on modulating light intensity
Those schemes
requires increasing the pulse rate
above 1000Hz to avoid flickering
The high pulse rate
however
adds high processing overhead
for resource constrained devices
Instead of modulating the light intensity
we come out the idea to control the polarization of light
which human eyes can not perceive
We borrow the liquid crystal and polarizing film from the LCD technologies
with those technologies we can control the polarization of light by applying different voltages
In LCD, the back light is first polarized by a polarizing film
When the voltage of the liquid crystal is low
the structure of liquid crystal can twist the polarization of the polarized light by 90 degree
so that the light will pass the second polarizing film and LCD illustrates bright
When the voltage of the Liquid Crystal is high
the liquid crystal will not change the lights' polarization properties
so that the light will be blocked by the second polarizing film
and LCD illustrates black at this time
Our basic idea is very similar to the LCD
Since human eyes can not perceive polarization changes
we move the second polarizing film to receiver side
In this way the modulation can only be detected by equipped receivers
In the prototype system
the VLC transmitter consists of a polarizing film
a liquid crystal layer, a disperser and a control board
The two electrodes are connected to the control board
which is continually changing the voltage of the liquid crystal layer
For convenience we used LED lamps in this demonstration
Note that any other light such as florescent light
solar light can also be used as light source
The transmitter works by simply attaching to the lamp
and facing to the light source
As we can see
the modulated light has no difference from normal light
However if we view through a polarizing film
it can be see that colors change significantly because of the modulated light
Our VLC design
can benefit a vast range of smart devices
including those with limited computational resources
we have verified our system in wearables
such as Google glass
but for better illustration
we use the Samsung Galaxy s2 smartphone in this video
This is the default camera application
Similar as human eyes
the camera requires a polarizing film
to reveal the modulated signal
After we attached the polarizing film in front of the camera
we can use our application to decode VLC messages
The VLC transmitter
is currently transmitting word “Hello”
repeatedly in baud rate of 14 Hz
As each character is encoded in 8 bits
and occupies one packet
it take about 1 second to receive one character
The client perform localization algorithm
to determine its location in 3D space
Our VLC design
can be seamlessly applied in indoor localization application
When broadcasting messages of a VLC transmitter represent the transmitter’s location
mobile client can localize itself
through comparing transmitters locations in the image
and the corresponding locations in real coordinates
To demonstrate this
four transmitters are deployed as left
They are broadcasting the assigned IDs
which can be mapped to real locations
Our receiving client
due to mobility
has opportunity to receive signals
from one or multiple transmitters
As expected, it can successfully decode them in all the scenarios
Once it successfully decoded messages
from three or more transmitters in the same image
the client perform localization algorithm to determine its location in 3D space.
We appreciate your time for viewing our video
Thank you