Reliable visibile light communication with dark light synchronization

Patent metadata

Field	Value
Publication number	US-9264138-B2
Application number	US-201313895400-A
Country	US
Kind code	B2
Filing date	May 16, 2013
Priority date	May 16, 2013
Publication date	Feb 16, 2016
Grant date	Feb 16, 2016

Abstract

LEDs that transmit or receive data in a VLC channel may create a flickering effect which is caused by the human eye being able to perceive the fluctuations of the light intensity in a LED. To prevent flickering such that the LED is perceived as being illuminated at a constant intensity, the LEDs may emit light based on a pattern of dark and energy slots. During the dark slots, the LEDs may be reversed bias to determine how much light is received. Based on comparing the light received, each VLC system may synchronize respective clocks such that the dark slots align. The VLC systems may then transmit a data frame preamble instructing the other VLC systems to cease synchronization and use the dark slots to receive the data from the data frame. While transmitting the data frame, the VLC systems may continue to use the energy slots to prevent flicker.

First claim

What is claimed is:

1. A method, comprising:
detecting, using a first visible light system, an amount of light received during first and second dark slots of a predefined synchronization pattern;
comparing the amount of light received during the first dark slot to the amount of light received during the second dark slot;
adjusting, based on the comparison, a clock in the first visible light system such that the first and second dark slots align with corresponding dark slots of a second visible light system, wherein the first and second visible light systems use the same synchronization pattern;
emitting light from a light source of the first visible light system during an energy slot in the predefined synchronization pattern,
wherein respective time periods of the first and second dark slots and energy slot are set to mitigate an ability of a human eye to detect a change from one of the first and second dark slots to the energy slot in the predefined synchronization pattern; and
after adjusting the clock of the first visible light system, transmitting a predefined start frame delimiter (SFD) preamble, wherein the SFD preamble is configured to maintain the synchronization between the first and second visible light systems,
wherein the second visible light system is configured to record the data received during the corresponding dark slots in a data window and evaluate the data window to determine if the first visible light system is transmitting the SFD preamble.

2. The method of claim 1 , wherein the two dark slots are consecutive slots in the predefined synchronization pattern, and wherein the second visible light system is configured to adjust a respective clock at the same time the clock in first visible light system is adjusted such that the corresponding dark slots align with the first and second dark slots.

3. The method of claim 2 , wherein the first dark slot occurs before the second dark slot, and wherein adjusting the clock in the first visible light system further comprises:
if the light detected during the first dark slot is greater than the light detected in the second dark slot, shifting the clock forward in time; and
if the light detected during the second dark slot is greater than the light detected in the first dark slot, shifting the clock backward in time.

4. The method of claim 1 , wherein the light source is a light-emitting diode (LED), and wherein the LED is reversed biased during the first and second dark slots and forward biased during the energy slot.

5. The method of claim 1 , further comprising, before comparing the amount of light received during the first dark slot to the amount of light received during the second dark slot, identifying an equalization value based on ambient light,
wherein the equalization value is applied to one of the amount of light measured during the first dark slot or the amount of light measured during the second dark slot to equalize the comparison.

6. The method of claim 1 , further comprising, before transmitting a data frame from the first visible light system to the second visible light system:
randomly selecting a value for a timer from a range of values; and
upon determining that the timer has expired, transmitting the data frame.

7. The method of claim 1 , further comprising, after adjusting the clock, transmitting a data frame from the first visible light system to the second visible light system, wherein the first visible light system transmits coded data using a predefined code schema that mitigates flicker,
wherein the second visible light system decodes the coded data into logical bits based on the predefined code schema.

8. The method of claim 7 , further comprising, after transmitting the data frame:
switching into a receive mode, wherein the first visible light system follows the predefined synchronization pattern;
upon determining that the second visible light system has transmitted an acknowledgement frame, adjusting the clock in the second visible light system; and
upon determining that the second visible light system has not transmitted the acknowledgment frame, retransmitting the data frame.

9. The method of claim 1 , wherein time periods of the first dark slot, the second dark slot, and the energy slot are less than or equal to 30 ms.

10. A first visible light system, comprising:
a light source; and
a controller coupled to the light source, the controller configured to:
detect an amount of light received during first and second dark slots of a predefined synchronization pattern,
compare the amount of light received during the first dark slot to the amount of light received during the second dark slot,
synchronize, based on the comparison, a clock in the visible light system such that the first and second dark slots align with corresponding dark slots of a second visible light system, wherein the first and second visible light systems use the same synchronization pattern,
emit light from the light source during an energy slot in the predefined synchronization pattern,
wherein respective time periods of the first and second dark slots and energy slot are set to mitigate an ability of a human eye to detect a change from one of the first and second dark slots to the energy slot in the predefined synchronization pattern, and
after adjusting the clock of the first visible light system, transmit a predefined start frame delimiter (SFD) preamble, wherein the SFD preamble is configured to maintain the synchronization between the first and second visible light systems,
wherein the second visible light system is configured to record the data received during the corresponding dark slots in a data window and evaluate the data window to determine if the first visible light system is transmitting the SFD preamble.

11. The system of claim 10 , wherein the light source is a light-emitting diode (LED), and wherein the LED is reversed biased during the first and second dark slots and forward biased during the energy slot.

12. The system of claim 10 , wherein the controller is configured to, before transmitting a data frame to the second visible light system using the light source:
randomly select a value for a timer from a range of values; and
upon determining that the timer has expired, transmit the data frame.

13. The system of claim 10 , wherein the controller is configured to, after synchronizing the clock, transmit a data frame to the second visible light system using the light source, wherein the controller transmits coded data using a predefined code schema that mitigates flicker,
wherein the second visible light system is configured to decode the coded data into logical bits based on the predefined code schema.

14. The system of claim 13 , wherein the controller is configured to, after transmitting the data frame:
switch into a receive mode, wherein the controller follows the predefined synchronization pattern;
upon determining that the second visible light system has transmitted an acknowledgement frame, synchronize the clock in the second visible light system; and
upon determining that the second visible light system has not transmitted the acknowledgment frame, retransmit the data frame.

15. A first visible light system, comprising:
a light source; and
a controller coupled to the light source, the controller configured to:
detect an amount of light received during first and second dark slots of a predefined synchronization pattern,
compare the amount of light received during the first dark slot to the amount of light received during the second dark slot,
synchronize, based on the comparison, a clock in

Assignees

• Disney Entpr Inc
• Eth Zurich Eidgenoessische Tech Hochschule Zurich

Inventors

• Schmid Stefan Matthias
• Corbellini Giorgio
• Gross Thomas R
• Mangold Stefan

Classifications

• H04B10/116 Primary

  Visible light communication · CPC title

• H04L2007/045

  Fill bit or bits, idle words · CPC title

• H04L7/04

  Speed or phase control by synchronisation signals {(H04L7/0075 takes precedence)} · CPC title