03-07-2013, 04:52 PM
Video Semaphore Decoding for Free-Space Optical Communication
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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.
INTRODUCTION
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.
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 fullwidth,
half-maximum interference filter the noise power from the sun may still be unacceptable. The use of an imaging receiver
can further alleviate this problem.
SYSTEM DESCRIPTION
Three optical communication systems developed in our lab are presented in this section. Each system consists of a laser
transmitter subsystem and a receiver subsystem. The following two sections describe the differences between the systems.
Laser transmitter
A miniature weather station incorporating light intensity, humidity, temperature, and barometric pressure sensors was built2,3. A
microcontroller chip on this board controlled sensor data acquisition, filtering and packetization of the acquired data, and the lowlevel
control of the laser diode. The sensor data was acquired once per second. Each packet consisted of a training sequence,
followed by four eight-bit sensor values, followed by a sixteen-bit cyclic redundancy check (CRC) that was added to the end of
each packet to ensure the validity of received data. In order to ensure a nearly constant average optical power level, 3 bit
transitions are introduced into each byte by inverting the 2nd, 4th, and 6th bit as shown in Figure 2.
Integrated Laser Transceiver
In order for free-space laser transmission to become a practical method for communication over any distance, fast and robust
automatic link acquisition is essential. In section 3.1, a manually aimed laser transmitter is described. The targeting process
described in that section highlights the main drawback to narrow-beam communication, namely, that in order to receive any
signal at all, the transmitter needs to be aligned very precisely to the receiver. This is tedious to set up initially, and is subject to
long-term drift as environmental conditions change. The use of an automatic link acquisition and optimization system allows for
very coarse manual alignment (+/-15 degrees) and maintenance-free operation henceforth; essentially a “fire and forget” link
management strategy.