Coded Light Messaging for Rolling-Shutter Camera Decoding
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Solution Overview
Problem
Existing coded light communication systems using rolling shutter cameras face challenges in capturing and decoding messages due to short temporal views of the coded light source, conflicts with automatic exposure control, limited driver technology for high-frequency signaling, and inter-symbol interference (ISI) caused by the rolling shutter process.
Innovation Solution
A signal format is developed where a message is cyclically repeated and timed such that different parts of the message are seen by the camera in each frame, with specific idle periods and packet lengths to mitigate ISI, allowing for longer message capture and decoding, and a robust Wiener filter is used for equalization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a message is transmitted continuously in coded light, then the communication rate is improved, but the rolling shutter camera can only capture short temporal views causing message fragmentation
Solution Approach 1:
The continuous message is divided into multiple packets, where each packet can be independently captured by the rolling shutter camera. This segmentation allows the receiver to reconstruct the complete message from multiple partial captures, resolving the contradiction between continuous transmission and fragmented capture.
Solution Approach 2:
The message packets are transmitted periodically with specific timing intervals that synchronize with the camera's frame rate. This periodic transmission ensures that different packets are captured in different frames, allowing complete message reconstruction while maintaining continuous communication.
2Loss of information
If the message duration is extended to capture longer information, then the information capacity is improved, but the message may not fit within a single frame's temporal footprint
Solution Approach 1:
The solution transitions from a single-dimension temporal capture to a multi-dimensional approach by distributing message packets across multiple frames in the time dimension. This allows the effective message duration to exceed any single frame's temporal footprint while maintaining complete information capture through temporal aggregation.
3Productivity
If high-frequency signaling is used to increase data rate, then the communication speed is improved, but driver technology limitations and visible flicker effects occur
Solution Approach 1:
The coded light signal is transmitted continuously at a moderate frequency that remains imperceptible to human vision. This continuous transmission maintains effective communication without the harmful visible flicker effects that would result from high-frequency pulsing, resolving the contradiction between data rate and visual comfort.
4Device complexity
If the rolling shutter process is used for camera capture, then the camera hardware simplicity is maintained, but inter-symbol interference is introduced
Solution Approach 1:
The message is pre-segmented into packets with timing designed to account for the rolling shutter's line-by-line capture process. This preliminary structuring of the transmission to anticipate the camera's scanning behavior minimizes inter-symbol interference while maintaining hardware simplicity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables the capture and decoding of messages longer than the footprint in a single frame, reduces ISI, and ensures reliable communication without visible flicker or stroboscopic effects, even at sensitive frequencies.
Implementation Method 1
data is modulated into the visible illumination emitted by a light source, e.g. by an LED based luminaire
Implementation Method 2
a rolling-shutter camera which captures frames by exposing a plurality of lines of each frame in sequence
Data Source
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AI summary
A coded light signal is embedded into visible light emitted from the light source, to be received by a rolling-shutter camera which captures frames (16) by exposing a plurality of lines (18, 24) of each frame in sequence, the camera having an exposure time with each line being exposed for the exposure time. The coded light signal is formatted according to a format whereby the coded light signal comprises at least one message and the message is repeated multiple times with a timing such that, when samples of the coded light signal are obtained from a substantially smaller number of lines (24) than exposed by the camera in each frame and the message is longer than this number of lines, a different part of the message is seen by the camera in each of a plurality of different ones said frames.