I/Q Demodulator Based Optical Camera Communications

Abstract

Visible light communications achieve wireless communications using light sources such as lighting fixtures and electric signboards. The light sources need to blink at high speed to avoid flickering. However, the blinking frequency is much higher than the frame rate of the general-purpose cameras that are integrated into consumer devices such as smartphones. This paper proposes a method where the principle of the I/Q demodulator as employed in radio frequency communications is used to estimate the phase and amplitude of modulated light blinking at a sufficiently higher speed than the frame rate. This is achieved by using two cameras with slightly different exposure start timing and estimating the phase and amplitude from the pixel values of the light-emitting diode (LED) image in one frame for each camera. In contrast to the conventional method with a very short exposure time, the exposure time can be increased, resulting in higher efficiency of light capture. Additionally, we propose another configuration employing one camera and two LEDs. An experiment to verify that communication is possible with the proposed method, which achieved 200 bits/s with 60 frames per second cameras. We also confirmed the proposed methods have similar signal-to-noise ratio to bit error ratio performance.