Free space transmission systems require a transducer to convert signal energy of one form into electromagnetic or optical energy for transmission. The transducer must also focus the energy so it may launch the energy in the desired direction. When air is the medium, particles in the air (such as water) may absorb or redirect (scatter) the transmitted signal.
Figure below shows two types of free space transmission systems: radio and optical. The microwave transmission system shows that some of the electromagnetic energy is absorbed by the water particles in the air. The optical transmission system uses a laser and photo-detector. The optical transmission system shows that some of the optical energy is scattered in other directions as it passes through smog and water particles.
In 1951, microwave radio transmission through free space became the backbone of the telecommunications infrastructure. Point-to-point microwave transmission systems have the data transmission capacity of hundreds of megabits per second. Although the extensive deployment of fiber optic cable has removed some of need for microwave radio systems, microwave radio is still used in places that are hard to reach or not cost effectively served by fiber cable.
In addition, microwave radio free space technology is the basis for satellite communications. In the commercial broadcast industry satellites are fed from terrestrial sites called “mother stations.” The mother station transmits up to the satellite on multiple frequencies called “uplinks”. Transceivers on the satellite, referred to as “transponders”, retransmit the signals back down to earth. These signals are known as “downlinks”. On the ground, satellite dishes (focusing antennas) receive the downlink signals and a radio receiver converts these signals back into television images and sound. Home Box Office (HBO) and ShowTime are examples of commercial broadcast companies that use satellite almost exclusively for distribution.
In developing countries and countries where the telecommunications infrastructure is of poor quality, radio, microwave, and satellite have been used to solve connectivity requirements in short order. Data radio and very small aperture satellite (VSAT) systems have allowed banks and other information-dependent companies to reliably connect to branch offices for the online exchange of information.
Since the mid-1980’s data radio has played a major role in the telecommunications industry in developing countries where the copper wire infrastructure is generally of substandard quality.
Modern optical transmission systems use infrared, and other laser optical signals to carry large amounts of information. Free space infrared and laser communication systems have been limited to span small distances of a few miles due to interference of the particles in air. Infrared systems have gained popularity because of their high bandwidth and ease of installation. Optical systems usually do not require government licenses or other authorization to use. These optical systems can be found connecting buildings on a campus and as supplements to wired LAN’s within an office or plant.
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