Video Semaphore Decoding for Free-Space Optical Communication

Using real-time image processing we have demonstrated a low bit-rate free-space optical communication system at a range of more than 20km with an average optical transmission power of less than 2mW. The transmitter is an autonomous one cubic inch microprocessor-controlled sensor node with a laser diode output. The receiver is a standard CCD camera with a 1-inch aperture lens, and both hardware and software implementations of the video semaphore decoding (VSD) algorithm. With this system sensor data can be reliably transmitted 21 km from San Francisco to Berkeley. Intelligent encoding and video processing algorithms are used to reject noise and ensure that only valid message packets are received. Dozens of independent signals have been successfully received simultaneously. A software implementation of the VSD algorithm on a Pentium computer with a frame grabber was able to achieve an effective frame rate of 20 fps, and a corresponding bit rate of 4bps. This bit rate is adequate to transmit real-time weather sensor information. The VSD algorithm has also been implemented in a custom hardware board consisting of a video ADC, RAM, Xilinx FPGA, and a serial output to PC. This system successfully operated at 60 fps with a bit rate of 15 bps.