Clock Tracking Algorithm for VPPM Optical Camera Communication
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Solution Overview
Problem
In optical wireless communication systems, there is a need to track and compensate for the frequency offset between the symbol rate of incoming light and the frame rate of the camera, which is essential for accurate data extraction, especially in asynchronous variable pulse position modulation (VPPM) systems where synchronization through start frame delimiters (SFD) is not sufficient for long packet transmissions.
Innovation Solution
The implementation of a detection algorithm and apparatus that uses a composite waveform with a lower frequency time varying amplitude component and a high data rate VPPM component, allowing for region-of-interest subsampling and frequency clock offset tracking, enabling the camera to adjust its sampling rate to synchronize with the LED timing clock, thereby correcting positive or negative frequency offsets.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If SFD synchronization is used to align camera frame rate with symbol rate, then initial synchronization is achieved, but frequency offset accumulates during long packet transmission
Solution Approach 1:
The patent implements a feedback mechanism where the receiver continuously monitors the received signal for frequency offset and dynamically adjusts the sampling rate accordingly. The algorithm estimates frequency offset by analyzing phase differences between consecutive symbols and applies corrections in real-time, creating a closed-loop system that maintains synchronization throughout long packet transmissions.
Solution Approach 2:
The patent transitions from static SFD-based synchronization to dynamic frequency offset tracking. The receiver continuously adapts its sampling rate based on real-time frequency offset estimates, making the synchronization system dynamic rather than fixed. This allows the system to compensate for drift that occurs over time during long packet transmissions.
2Device complexity
If camera frame rate is fixed, then hardware simplicity is maintained, but frequency offset cannot be compensated
Solution Approach 1:
The patent introduces dynamic frequency offset tracking that allows the camera sampling rate to be adjusted in real-time based on estimated frequency offset. This dynamic adjustment mechanism compensates for frequency drift without requiring complex hardware changes, maintaining hardware simplicity while improving data extraction reliability through adaptive sampling rate modification.
Solution Approach 2:
The patent changes the operational parameters of the camera by dynamically adjusting the sampling rate based on frequency offset estimates. Instead of fixing the camera frame rate, the system modifies the sampling parameter adaptively, allowing frequency offset compensation while keeping the hardware itself simple and unchanged.
3Productivity
If asynchronous VPPM is used for high data rate transmission, then data rate is improved, but timing synchronization becomes difficult
Solution Approach 1:
The patent employs feedback-based frequency offset tracking where the receiver continuously monitors the asynchronous VPPM signal, estimates frequency offset from phase differences between symbols, and adjusts sampling rate accordingly. This feedback loop maintains timing synchronization despite the asynchronous nature of high-rate VPPM transmission.
Solution Approach 2:
The patent applies preliminary frequency offset estimation and compensation before data extraction. By estimating the frequency offset early in the reception process and applying corrections beforehand, the system prepares the sampling rate for accurate data extraction, reducing the complexity of real-time timing tracking during high-rate VPPM transmission.
Data Source
AI summary
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.