Modulating a light source in a light based positioning system with applied DC bias
In one aspect, the present disclosure relates to a self-identifying optical transmitter for broadcasting a one-way authentication code using light-based communication. The transmitter may include a memory for storing an identifier of the transmitter, a processor for generating a data signal including an identifier of the transmitter, a modulator for receiving the data signal and generating an electrical signal, the modular generating the electrical signal by modulating the data signal. The transmitter may also include a light source for receiving the electrical signal, converting the electrical signal into an optical signal, and continuously broadcasting the optical signal as an optical data transmission stream. The optical data transmission stream may be used to verify that a receiving mobile device is near the transmitter. The transmitter may also include an optical surface for dispersing the optical data transmission stream as the optical data transmission stream is emitted from the transmitter.
1. A method for modulating a light source in a light based positioning system, the method comprising:
selecting a modulation scheme for the light source, wherein the modulation scheme periodically varies an output of the light source from a peak output level to a bottom output level;
adding a DC bias to the modulation scheme; and
applying the modulation scheme with the DC bias to the light source, wherein:
the DC bias reduces a required power for the peak output level to achieve a given luminous output by increasing the bottom output level of the modulation scheme, and
an average power of the light source when modulated is equal to an average power of the light source to achieve the given luminous output without application of the modulation scheme.
2. The method of claim 1 , comprising reducing the required power for the peak output level by the DC bias to maintain thermal heating of the light source below a threshold.
3. The method of claim 2 , wherein the threshold comprises a thermal heating level determined based on a maximum allowable droop.
4. The method of claim 1 , wherein the peak output level and the bottom output level comprise electrical current levels.
5. The method of claim 1 , wherein the peak output level and the bottom output level comprise voltage levels.
6. The method of claim 1 , wherein the light source comprises a light-emitting diode (LED).
7. The method of claim 1 , further comprising:
setting a modulating period of the modulation scheme based on an image sampling time of a rolling shutter camera of a device intended to receive the given luminous output of the light source.
8. The method of claim 7 , wherein the applying the modulation scheme with the DC bias to the light source is based on the modulating period.
9. The method of claim 8 , wherein the modulating period is set based on a plurality of rolling shutter camera image sampling times.
10. A light source for use in a light based positioning system, the light source comprising:
an emitter with a luminosity broadcasting a modulated signal; and
a modulator for receiving a modulation scheme and driving the luminosity of the emitter by applying the modulation scheme with a DC bias to the emitter, wherein:
the modulation scheme periodically varies the luminosity of the emitter from a peak output level to a bottom output level,
the DC bias reduces a required power for the peak output level to achieve a given luminous output by increasing the bottom output level of the modulation scheme, and
an average power of the emitter when modulated is equal to an average power of the emitter to achieve the given luminous output without application of the modulation.
11. The light source of claim 10 , wherein the peak output level is reduced by the DC bias to maintain thermal heating of the light source below a threshold.
12. The light source of claim 11 , wherein the threshold comprises a thermal heating level determined based on a maximum allowable droop.
13. The light source of claim 10 , wherein the peak output level and the bottom output level comprise electrical current levels.
14. The light source of claim 10 , wherein the peak output level and the bottom output level comprise voltage levels.
15. The light source of claim 10 , wherein the emitter comprises a light-emitting diode (LED).
16. The light source of claim 10 , wherein:
the modulation scheme includes a modulation period based on an image sampling time of a rolling shutter camera of a device intended to receive the given luminous output of the light source.
17. The light source of claim 16 , wherein:
the modulator applies the modulation scheme with the DC bias to the emitter further based on the modulating period.
18. The light source of claim 17 , wherein the modulating period is set based on a plurality of rolling shutter camera image sampling times.
19. A mobile device, comprising:
a rolling shutter camera;
a processor coupled to the rolling shutter camera and the wireless interface;
a memory; and
software in the memory to be run by the processor, wherein running of the software by the processor configures the mobile device to implement functions, including functions to:
operate the rolling shutter camera to capture an image including a visible light signal transmitted from a visible light source, the visible light signal comprising visible light modulated in accordance with a modulation scheme having an added DC bias, wherein:
the modulation scheme periodically varies luminosity of the visible light output from a peak output level to a bottom output level, and
the DC bias reduces a required power for the peak output level to achieve a given luminous output by increasing the bottom output level of the modulation scheme, and
an average power of the light source when modulated is equal to an average power of the light source to achieve the given luminous output without application of the modulation scheme; and
demodulate the periodic variations of the luminosity of the visible light from the captured image to obtain data carried in the visible light signal.
20. The mobile device of claim 19 , wherein the running of the software by the processor configures the mobile device to implement further functions, including functions to:
determine an identification of the visible light source from the data; and
process the identification to determine a position of the mobile device.