Mobile Phones - The Basics

V. Cellular frequency and channel discussion

Mobile phone frequencies start at 824.04 MHz and end at 893.7 MHz. That's 69.66 megahertz worth of radio frequency spectrum. Quite a chunk. By comparison, the AM broadcast band takes up only 1.17 megahertz of space. That band, however, provides only 107 frequencies to broadcast on. Cellular may provide thousands of frequencies to carry conversations and data. This large number of frequencies and the large channel size required account for the large amount of spectrum used. The most common system, AT&T's Advanced Mobile Phone Service or AMPS, for example, uses 832 channels that are 30 kHz wide. Years ago Motorola and Hughes each tried making more spectrum efficient systems, cutting down on channel size or bandwidth, but these never caught on. Motorola's system, NAMPS, standing for Narrowband Advanced Mobile Service provided 2412 channels, using channels 10 kHz wide instead of 30kHz. While voice quality was poor and technical problems abounded, NAMPS died because digital and its inherent capacity gain came along, otherwise, as Mark puts it, "We'd have all gone to NAMPS eventually, poor voice quality or not."

I mentioned that a typical cell channel is 30 kilohertz wide compared to the ten kHz allowed on an AM radio station. How is it possible, you might ask, that a one to three watt mobile phone call can take up a path that is three times wider than a 50,000 watt broadcast station? Well, power does not necessarily relate to bandwidth. A high powered signal might take up lots of room or a high powered signal might be narrowly focused. A wider channel helps with audio quality. An FM stereo station, for example, uses a 150 kHz channel to provide the best quality sound. A 30 kHz channel for cellular gives you great sound almost automatically, nearly on par with the normal telephone network.

I also said that the cellular band runs from 824.04 MHz to 893. 97 MHz. In particular, cell phones or mobile phones use the frequencies from 824.04 MHz to 848.97 and the base stations operate on 869.04 MHz to 893.97 MHz. These two frequencies in turn make up a channel. 45 MHz separates each transmit and receive frequency within a cell or sector, a part of a cell. That separation keeps them from interfering with each other. Getting confusing? Let's look at the frequencies of a single cell for a single carrier. For this example, let's assume that this is one of 21 cells in an AMPS system:

Cell#1 of 21 in Band A (The non-wireline carrier)

Channel 1 (333) Tx 879.990 Rx 834.990

Channel 2 (312) Tx 879.360 Rx 834.360

Channel 3 (291) Tx 878.730 Rx 833.730

Channel 4 (270) Tx 878.100 Rx 833.100

Channel 5 (249) Tx 877.470 Rx 832.470

Channel 6 (228) Tx 876.840 Rx 831.840

Channel 7 (207) Tx 876.210 Rx 831.210

Channel 8 (186) Tx 875.580 Rx 830.580 etc., etc.,

The number of channels within a cell or within an individual sector of a cell varies greatly, depending on many factors. As Mark van der Hoek writes, "A sector may have as few as 4 or as many as 80 channels. Sometimes more! For a special event like the opening of a new race track, I've put 100 channels in a temporary site. That's called a Cell On Wheels, or COW. Literally a cell site in a truck."

Cellular network planners assign these frequency pairs or channels carefully and in advance. It is exacting work. Adding new channels later to increase capacity is even more difficult. Channel layout is confusing since the ordering is non-intuitive and because there are so many numbers involved. Speaking of numbers, check out the sidebar. Channels 800 to 832 are not labeled as such. Cell channels go up to 799 in AMPS and then stop. Believe it or not, the numbering begins again at 991 and then goes up to 1023. That gives us 832. Why the confusion and the odd numbering? The Bell System originally planned for 1000 channels but was given only 666 by the FCC. When cellular proved popular the FCC was again approached for more channels but granted only an extra 166. By this time the frequency spectrum and channel numbers that should have gone to cellular had been assigned to other radio services. So the numbering picks up at 991 instead of 800.

You might wonder why frequencies are offset at all. It's so you can talk and listen at the same time, just like a regular telephone. Cellular is not like CB radio. Citizen's band uses the same frequency to transmit and receive. What's called "push to talk" since you must depress a microphone key or switch each time you want to talk. Cellular, though, provides full duplex communication. It's more expensive and complicated to do it this way. That's since the mobile unit and the base station both need circuitry to transmit on one frequency while receiving on another. But it's the only way that permits a normal, back and forth, talk when you want to, conversation. Take a look at the animated .gif below to visualize full duplex communication. See how two frequencies, a voice channel, lets you talk and listen at the same time?

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