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Unsynchronized Visual Communication

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As smartphone penetration continues to grow and displays are widely available, there are many opportunities to transmit information over screen-camera links in the form of a video of 2D barcodes. A key challenge for smartphone based visual communication over screen-camera links is imperfect frame synchronization. The difficulty arises from frame rate diversity and variability due to camera capability, lighting conditions, and system factors. If the transmit frame rate is too high, the receiver might lose original frames or capture mixed frames, which are normally not decodable. Previous systems simply reduce the effective screen frame rate to be half the camera frame capture rate. This under-utilizes the transmitter side capacity and is inefficient.

We achieve frame synchronization with LightSync, which features in-frame color tracking to decode imperfect frames and a linear erasure code across frames to recover lost frames. LightSync allows smooth communication between the screen and the camera at any combination of the transmit and receive frame rates, as long as the receive rate is at least half the transmit rate. This means that each receiver can scale the decoding performance with its own camera capability. Across several phones, our system can more than double the average throughput compared to previous approaches.

Bio: Wenjun Hu is an assistant professor of Electrical Engineering and Computer Science at Yale University, based in the newly launched Yale Institute for Network Science. Until the end of 2013, she was a researcher at Microsoft Research Asia. Previously, she was a postdoctoral research associate at the University of Washington. She did her PhD with Prof. Jon Crowcroft at the University of Cambridge, though spending about 7 months each at MIT CSAIL and Microsoft Research Cambridge during her PhD. Her work has mainly focused on building various wireless systems, including smartphone based visual communication, wireless video, MIMO , and network coding in wireless mesh networks.

This talk is part of the Computer Laboratory Systems Research Group Seminar series.

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