“The concept of transmitting information through the air by means of a modulated light signal is quite old; and although significant advances have been made over the past 10 years, the concept remains relatively simple: a narrow beam of light is launched at a transmission station, transmitted through the atmosphere, and subsequently received at the receive station. The advances, which have led to what we now refer to as free-space optical communications, or FSO, have come about in response to a need for greater bandwidth and improved communications systems. Inasmuch as FSO and fiber-optic transmission systems use similar infrared (IR) wavelengths of light and have similar transmission bandwidth capabilities, FSO is often referred to as “fiberless optics” or “optical wireless” transmission. Furthermore, given the fact that the optical spectrum is unlicensed with frequencies of the order of hundreds of terahertz, most FSO systems use simple ON–OFF keying (OOK) as a modulation format, the same standard modulation technique that is used in digital fiberoptics systems, wherein data are typically transmitted in a digital format with light “ON” representing a “1” and light “OFF” representing a “0.” This simple modulation scheme allows FSO systems to be designed as bandwidth- and protocol-transparent physical layer connections.
In examining FSO performance, it is important to take several system parameters into consideration. In general, these parameters can be divided into two different categories: internal parameters and external parameters. Internal parameters are related to the design of a FSO system and include optical power, wavelength, transmission bandwidth, divergence angle, and optical loss on the transmit side and receiver sensitivity, bit-error rate (BER), receive lens diameter, and receiver field of view (FOV) on the receive side. External parameters, or non-system-specific parameters, are related to the environment in which the system must operate and include visibility and atmospheric attenuation, scintillation, deployment distance, window loss, and pointing loss.
It is important to understand that many of these parameters are not independent but are linked together in specifying overall system performance. For example, system availability is a function of not only the deployment distance but also of local climate and transceiver design. In addition, a system optimized for long-range performance (>1 km) may not be optimally designed for high-availability (>99.9%) short-range performance. Overall, optimum FSO system design is highly dependent upon the intended application, required availability, and cost point.
The performance of a FSO link is primarily dependent upon the climatology and the physical characteristics of its installation location. In general, weather and installation characteristics that impair or reduce visibility also effect FSO link performance. A typical FSO system is capable of operating at a range of two to three times that of the naked eye in any particular environmental condition. The primary factors affecting performance include atmospheric attenuation, scintillation, window attenuation, alignment or building motion, solar interference, and line-of-sight obstructions.
Excerpts from “Understanding the performance of free-space optics”
WCA Technical Symposium, San Jose, CA. January 14, 2003. Scott Bloom , CTO