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