Video Semaphore Decoding for Free-Space Optical Communication M Last, B Fisher, C Ezekwe, S Hubert, S Patel, S Hollar, B Leibowitz, KSJ Pister Berkeley Sensor and Actuator Center 497 Cory Hall University of California, Berkeley Berkeley, CA 94720-1774 ABSTRACT 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. Keywords: Free Space, Laser, Wireless, Optical, Communication, Long Range, San Francisco, Low Power 1. INTRODUCTION 1.1 Free Space Laser Communication For applications where line of sight is available between a transmitter and a receiver, free-space optical communications systems can present tremendous advantages over their RF counterparts. Perhaps most important, optical power can be collimated in tight beams even from small apertures. Diffraction enforces a f undamental on the divergence of a beam, whether it be from an antenna or a lens. Laser pointers are cheap examples demonstrating milliradian collimation from a millimeter aperture. To get similar collimation for a 1 GHz RF signal would require an antenna 100 meters across, due to the difference in wavelength of the two transmissions. As a result, optical transmitters of millimeter size can get antenna gains of one million or more, while similarly sized RF antennas are doomed by physics to be mostly isotropic. With this kind of transmitter gain, microwatt signals can be sent over multi-kilometer distances with a strong SNR. Micro Electro-Mechanical Systems (MEMS) technology has the potential to integrate a laser diode, collimating optics and a beamsteering mirror into a package with a volume of only a few cubic millimeters1. Using a MEMS steered laser transmitter, the next generation of infrared ports may have ranges of kilometers instead of the current centimeters, or data rates of a few gigabits per second instead of only a few megabits per second. The high antenna gains possible using optical frequencies have advantages for the receiver end of the link as well. Since each pixel in an imaging optics system has a very narrow field of view, and high levels of integration allow many optical transducers on a chip, an imaging receiver such as a CCD or photodiode array can function as a collection of independent optical receivers. In this scenario, each pixel in an imaging array stares at a very narrow field of view, limiting background noise and focusing most of the received optical power onto a very small receiver area. If done properly, this can result in an extremely high signal-to-noise ratio limited only by the noise in the receiver electronics. Additionally, since each pixel in the array receives light from a Correspondence: M Last; Email: [email protected]; Telephone: (510)643-2236. KSJ Pister; Email: [email protected]; Telephone: (510)643-9268different portion of the field of view of the receiver, spatially separated transmitters image onto different pixels in the array, forming a type of spatial division multiple access (SDMA), and can therefore transmit freely without interfering with one another. In the next section of this paper, the feasibility of low-power, long-range optical communication links is discussed. To experimentally verify these conclusions, three optical communication systems were built and tested (Section 3). The results of these tests are described and discussed in Section 4. 2. THEORY 2.1 Signal Power The single largest factor determining how much signal power will arrive at a receiver in a long-range optical link is the divergence angle of the transmitted beam. For a diffraction-limited optical system, the divergence full-angle of the output beam in an ideal system is given by: ddivλθ22.1= [ 1] where λ is the center wavelength of the laser and d is the limiting aperture of the system. The power density D in Watts/Steradian is given by: −=2cos12divoPDθπ [ 2] The intensity of signal at range R is the power/area: 2RDIs= . This describes a spherical cap of constant power. The receiver aperture defines the portion of the transmitted power that is focused onto the photodetector. The power at the photodiode is given by rspdAIP = , where Ar is the area of the aperture of the receiver. 2.2 Noise Sources The main source of noise in this type of link derives from the presence of reflected sunlight in the field of view of the receiver. This ambient light power causes two problems. First, it adds a large DC offset. Ideally this does not degrade the signal, but in any practical receiver with finite output swing it reduces the usable dynamic range of the receiver. Second, since the photons arrive with a Poisson distribution, this DC offset contributes shot noise power directly proportional to the ambient light power. When this optical input is integrated as collected charge over some exposure period, the result is a noise variance that is equal to the total amount of charge collected. Therefore the optical SNR is the square of the received signal charge divided by the received charge due to ambient light. Although the use of narrow-band filters alleviates this ambient light problem, even with a 15nm full-width, half-maximum interference filter the noise power from the sun may still be unacceptable. The use