Introduction to Radio Transmission

Wire, cable, and fiber are well-behaved transmission media, and they display little variability in performance. The radio medium, on the other hand, displays notable variability in performance. The radio-frequency spectrum is shared with others and requires licensing. Metallic and fiber media need not be shared and do not require licensing (but often require right-of-way).

A major factor in the selection process is information bandwidth. Fiber optics seems to have nearly an infinite bandwidth. Radio systems have very limited information band-widths. It is for this reason that radio-frequency bands 2 GHz and above are used for PSTN and private network applications. In fact, the U.S. Federal Communications Commission (FCC) requires that users in the 2-GHz band must have systems supporting 96 digital voice channels where bandwidths are still modest. In the 4- and 6-GHz bands, available bandwidths are 500 MHz, allocated in 20- and 30-MHz segments for each radio-frequency carrier.

One might ask, Why use radio in the first place if it has so many drawbacks? Often, it turns out to be less expensive compared with fiber-optic cable. But there are other factors such as

• No requirement for right-of-way

• Less vulnerable to vandalism

• Not susceptible to "accidental" cutting of the link

• Often more suited to crossing rough terrain

• Often more practical in heavily urbanized areas

• As a backup to fiber-optic cable links

Fiber-optic cable systems provide strong competition with LOS microwave, but LOS microwave does have a place and a good market.

Satellite communications is an extension of LOS microwave. It is also feeling the "pinch" of competition from fiber-optic systems. It has two drawbacks. First, of course, is limited information bandwidth. The second is excessive delay when the popular geostationary satellite systems are utilized. It also shares frequency bands with LOS microwave.

One application showing explosive growth is very small aperture terminal (VSAT) systems. It is very specialized and has great promise for certain enterprise networks, and there are literally thousands of these networks now in operation.

Another application that has promise of wide deployment is large families of low earth orbit (LEO) satellites. Motorola developed and fielded Iridium that could provide worldwide cellular/PCS coverage. Because of LEO's low-altitude orbit (about 785 km above the earth's surface), the notorious delay problem typical of GEO (geostationary satellite) is nearly eliminated. Iridium worked perfectly. It was the marketing effort that failed. The system was sold at a fire sale at pennies on the dollar. Our opinion is that Iridium was too early for its time.

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