Proceedings Article | 28 January 2009
KEYWORDS: Free space optics, Telecommunications, Atmospheric particles, Laser scattering, Field programmable gate arrays, Receivers, Atmospheric optics, Scattering, Signal attenuation, Absorption
A new transceiving protocol is demonstrated for a Free Space Optical (FSO) communication system, and it's discussed
in two parts: the transmitting protocol and the receiving protocol. During the discussion of these two parts, the cooperation
of them is also discussed.
Different from wired communication, an FSO system modulates the data on a narrow beam of laser transmitting through
the free space or the atmosphere, and the protocol presented in this paper is mainly optimized for terrestrial Free Space
Optical links, in which the signal channel of the system is mainly the atmosphere. Due to the complex composition and
activity of the atmosphere, this signal channel brings in great influence on the transmitting laser in it, for example, the
absorption and scattering of the atmosphere molecules and aerosols, the scintillation of received laser power caused by
the turbulence of the atmosphere, all of which results in a much higher Bit Error Rate (BER) of the communication
system. Thus in designing a protocol for an FSO system, more effort should be taken in the encoding of the data stream,
the synchronization of the data stream, error checking and exception handling.
The main function of the transmitting protocol includes interfacing the outer input data with a parallel port, buffering the
input data, encoding the input data stream, serializing the parallel data and output the serialized data. It also has an output
management unit to manage the activity of each part of the transmitting protocol. The main function of the receiving
protocol includes filtering and synchronizing the input serial data stream, paralleling the serial data stream, decoding the
input data, error checking, exception handling and interfacing the outer receiver with a parallel port. The entire
transceiving protocol could be programmed into a single FPGA chip to improve system integrity and reduce the system
cost.
The presented protocol could be taken as "protocol transparent" for outer interfaces, meaning that when interfacing the
presented system to an outer system, you don't have to consider what protocol the outer system transceiving data stream
is under, for example, the TCP/IP protocol or anything else, in the case that its I/O interface is a parallel port.
Simulation and final experiment prove that the protocol presented is working fine at a certain bit rate scale.
