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 e-mail me Appendix: Early Bell System overview of IMTS and cellular Appendix: Call processing diagram
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(Page 2) Cellular telephone basics cont. . .

lII Cell and SectorTerminology

    Let's talk about cell terminology since the terms can get quite confusing. We'll need to know these terms for later. Most cells have been split into sectors or individual areas to make them more efficient and to let them to carry more calls. We illustrate a three sectored site below, the pie shaped pieces being the sectors. Notice the illustration on the right? The middle dot is the cell site. Antennas transmit inward to each cell. That's very important to remember. Each sector has its own set of channels. In this example the cell site transmits and receives on three different sets of channels, one for each part of the three cells it covers.
    WFI's Mark van der Hoek writes about cells and sectors and the kind of antennas needed: "These days most cells are divided into sectors. Typically three but you might see just two or rarely six. Six sectored sites have been touted as a Great Thing by manufacturers such as Hughes and Motorola who want to sell you more equipment. In practice six sectors sites have been more trouble than they're worth. So, typically, you have three antenna per sector or 'face'. You'll have one antenna for the voice transmit channel, one antenna for the set up or control channel, and two antennas to receive. Or you may duplex one of the transmits onto a receive. By sectorising you gain better control of interference issues (you're transmitting in one direction instead of broadcasting all around) and you can tighten up your frequency re-use" [See Cells and Sectors]

IV Basic Theory and Operation

Cell phone theory is simple. Executing that theory is extremely complex. Each cell site has a base station with a computerized 800 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. An area's 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 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.

Learn more about cellular switches


    [Cell and sector discussion] "In a really crowded site you'll have all 4 antennae transmitting and the two outer ones also receiving. Two receive antennas, by the way, are the most that are ever needed. And the receives will almost always be the two outer ones -- you want as much separation as reasonably possible. If you are using Motorola equipment, you'll have just one setup channel run through a 1 to 3 splitter and simulcasting out of each sector. Lucent and Ericsson do it right and have an individual setup channel per sector. Unlike Motorola, each sector has its own channels, they do not pool them.
    Mark then goes on to say there is a limit on how many channels can be added to a cell:
    "There are some limitations as to how many frequencies you can put on a single antenna. Some are frequency dependent, and then there is the power question. "Scotty! Give me more power!" "Ah canna do it, Captain! This antenna is only rated for 500 watts continuous duty. We've already got 50 channels doing 10 watts each. If I crank it up any more the dilithium crystals are gonna blow!"
    "Well, a bit of a digression there, but you get the idea. We're concerned about input power here - the gain of the antenna and the resulting ERP [effective radiated power] is another story. As to frequency limitations, that gets complicated. Suffice it to say that we can't just stuff any combination of frequencies into a multiplexer and squirt them all out of an antenna. There are certain spacing, or separation, requirements. So you may run into complications here before you run into power limitations, or you may run into both . . ." Mark van der Hoek. Personal correspondence: [back to text]

[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. (back to text)

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