Threshold-Free Symbol Decision Based on Spatial 4PPM Correlation for Uplink Screen Camera Communication Using Low-Luminance SDM Screen

In uplink screen camera communication from a smartphone screen to an indoor telephoto camera at a distance of 3.5 meters, low-luminance space division multiplexing screen is used to increase bitrate and enhance physical layer security (PLS) at wide angles. Since the low-luminance pixels on the screen are affected by ambient light, it is difficult to determine each cell symbol correctly using pre-trained pixel value threshold. To make symbol decisions without threshold learning, symbol decisions based on correlation of spatial 4 pulse position modulation (4PPM) pattern are demonstrated with numerical models and experiments. To increase bitrate further, threshold-free symbol decision methods combining 4PPM and inverted 4PPM cells are demonstrated. In addition, to enhance PLS at wide angles while maintaining error-free transmission in the front direction, threshold-free symbol decision methods are analyzed and experimentally verified at wide angles.