Clock tracking algorithm for twinkle VPPM in optical camera communication systems
First Claim
1. A receiver circuitry to correct frequency offset between a receiving camera and a transmitting LED source, comprising:
- a shift register to receive incoming data from the LED source, the incoming data having a plurality of symbols and a symbol rate (fs), the shift register having a plurality of storage cells, each storage cell to receive and store one data bit per clock cycle, each bit of data representing a sampled pixel state and each bit of data sampled at a camera frame rate (fc);
a plurality of comparator logic gates to correspond to a first group of the shift register storage cells, the plurality of logic gates configured to receive a sampled bit of data from a corresponding shift register storage cell during a clock cycle; and
a controller to receive an output from the first plurality of comparator logic gates to identify a frequency offset between the symbol rate (fs) and the camera frame rate (fc) as one of a positive, negative or neutral frequency offset, the controller configured to compensate for the frequency offset if the frequency offset is one of the positive or negative frequency offset.
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Abstract
Optical signaling is implemented by modulating visible light with variable pulse position modulation (VPPM). VPPM is a composite waveform and its optical signal includes a Start Frame Delimiter (SFD) which indicates start of optical signaling. To identify modulated lights, the duty cycle is periodically changed in the waveform to induce an AM envelope at a frequency higher than the response of the human eye. The signal is then sampled via a camera producing an alias frequency that produces noticeable blinking. Because the communication is asynchronous, the desired camera frame rate (fc) in relationship to the modulation bit rate timing clock (or symbol rate, fs) is only approximate. Consequently, a frequency offset develops between the camera frame rate (fc) and the symbol rate (fs) in transmission of long packets. The disclosed embodiments provide a detection algorithm, system and apparatus to provide clock offset tracking and correction.
54 Citations
24 Claims
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1. A receiver circuitry to correct frequency offset between a receiving camera and a transmitting LED source, comprising:
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a shift register to receive incoming data from the LED source, the incoming data having a plurality of symbols and a symbol rate (fs), the shift register having a plurality of storage cells, each storage cell to receive and store one data bit per clock cycle, each bit of data representing a sampled pixel state and each bit of data sampled at a camera frame rate (fc); a plurality of comparator logic gates to correspond to a first group of the shift register storage cells, the plurality of logic gates configured to receive a sampled bit of data from a corresponding shift register storage cell during a clock cycle; and a controller to receive an output from the first plurality of comparator logic gates to identify a frequency offset between the symbol rate (fs) and the camera frame rate (fc) as one of a positive, negative or neutral frequency offset, the controller configured to compensate for the frequency offset if the frequency offset is one of the positive or negative frequency offset. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. A tangible machine-readable non-transitory storage medium that contains instructions, which when executed by one or more processors result in performing operations to correct frequency offset between a receiving camera and a transmitting LED source, the instructions comprising:
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receiving an incoming light at a pixel array of an optical receiving device; identifying a modulated light from the incoming light, the modulated light received at region of interest (ROI) of the pixel array, the modulated light carrying a data stream having a plurality of symbols and a symbol rate (fs); identifying a start frame delimiter (SFD) in the data stream and sampling the plurality of symbols at a camera frame rate (fc); determining a frequency offset between the camera frame rate (fc) and the symbol rate (fs) of the data stream; identifying the frequency offset as one of a positive, negative or neutral and compensating for the frequency offset if the frequency offset is one of the positive or negative frequency offset. - View Dependent Claims (10, 11, 12, 13, 14, 15, 16)
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17. A method to correct frequency offset between a receiving camera and a transmitting LED source, the method comprising:
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receiving an incoming light at a pixel array of an optical receiving device; identifying a modulated light from the incoming light, the modulated light received at region of interest (ROI) of the pixel array, the modulated light carrying a data stream having a plurality of symbols and a symbol rate (fs); identifying a start frame delimiter (SFD) in the data stream and sampling the plurality of symbols at a camera frame rate (fc); determining a frequency offset between the camera frame rate (fc) and the symbol rate (fs) of the data stream; identifying the frequency offset as one of a positive, negative or neutral and compensating for the frequency offset if the frequency offset is one of the positive or negative frequency offset. - View Dependent Claims (18, 19, 20, 21, 22, 23, 24)
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Specification