Basic Theory and Operation
Cell phone theory is simple. Executing that theory is extremely complicated. Each cell site has a base station with a computerized 800 or 1900 megahertz transceiver and an antenna. This radio equipment provides coverage for an area that's usually two to ten miles in radius. Even smaller cell sites cover tunnels, subways and specific roadways. The area size depends on, among other things, topography, population, and traffic.
When you turn on your phone the mobile switch determines what cell will carry the call and assigns a vacant radio channel within that cell to take the conversation. It selects the cell to serve you by measuring signal strength, matching your mobile to the cell that has picked up the strongest signal. Managing handoffs or handovers, that is, moving from cell to cell, is handled in a similar manner. The base station serving your call sends a hand-off request to the mobile switch after your signal drops below a handover threshold. The cell site makes several scans to confirm this and then switches your call to the next cell. You may drive fifty miles, use 8 different cells and never once realize that your call has been transferred. At least, that is the goal. Let's look at some details of this amazing technology, starting with cellular's place in the radio spectrum and how it began.
The FCC allocates frequency space in the United States for commercial and amateur radio services. Some of these assignments may be coordinated with the International Telecommunications Union but many are not. Much debate and discussion over many years placed cellular frequencies in the 800 megahertz band. By comparison, PCS or Personal Communication Services technology, still cellular radio, operates in the 1900 MHz band. The FCC also issues the necessary operating licenses to the different cellular providers.
Although the Bell System had trialed cellular in early 1978 in Chicago, and worldwide deployment of AMPS began shortly thereafter, American commercial cellular development began in earnest only after AT&T's breakup in 1984. The United States government decided to license two carriers in each geographical area. One license went automatically to the local telephone companies, in telecom parlance, the local exchange carriers or LECs. The other went to an individual, a company or a group of investors who met a long list of requirements and who properly petitioned the FCC. And, perhaps most importantly, who won the cellular lottery. Since there were so many qualified applicants, operating licenses were ultimately granted by the luck of a draw, not by a spectrum auction as they are today.
The local telephone companies were called the wireline carriers. The others were the non-wireline carriers. Each company in each area took half the spectrum available. What's called the "A Band" and the "B Band." The nonwireline carriers usually got the A Band and the wireline carriers got the B band. There's no real advantage to having either one. It's important to remember, though, that depending on the technology used, one carrier might provide more connections than a competitor does with the same amount of spectrum. [See A Band, B Band
Mobiles transmit on certain frequencies, cellular base stations transmit on others. A and B refer to the carrier each frequency assignment has. A channel is made up of two frequencies, one to transmit on and one to receive.]
[A Band, B Band] Actually, the strange arrangement of the expanded channel assignments put more stringent filtering requirements on the A band carrier, but it's on the level of annoying rather than crippling. Minor point.