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(Page 3) Cellular telephone basics cont. . .
V. Cellular frequency and channel discussion
Cell phone frequencies start at 824.04 MHz and end at 893.7 MHz. [4] 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. [See NAMPS] 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."[NAMPS2]
I mentioned that a typical cell channel is 30 kilohertz wide compared to the ten kHz allowed an AM radio station. How is it possible, you might ask, that a one to three watt cellular 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 mobiles 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 nonwireline 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. [See Adding channels] 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. Arggh!
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?
Full duplex communication example. The two frequencies are paired and constitute a voice channel. Paths indicate direction of flow.
- Notes:
- [Adding channels] "The channels for a particular cell are assigned by a Radio Frequency Engineer, and are fixed. The mobile switch assigns which of those channels to use for a given call, but has no ability to assign other channels. In a Motorola (and, I think, Ericsson) system, changing those assigned channels requires manual re-tuning of the hardware in the cell site. This takes several hours. Lucent equipment allows for remote re-tuning via commands input at the switch, but the assignment of those channels is still made by the RF engineer, taking into account re-use and interference issues. Re-tuning a site in a congested downtown area is not trivial! An engineer may work for weeks on a frequency plan just to add channels to one sector. It is not unusual to have to re-tune a half dozen sites just to add 3 channels to one." Mark van der Hoek. Personal correspondence. (back to text)
[NAMPS] Macario, Raymond. Cellular Radio: Principles and Design, McGraw Hill, Inc., New York 1997 90. A good but flawed book that's now in its second edition. Explains several cellular systems such as GSM, JTACS, etc. as well as AMPS and TDMA transmission. Details all the formats of all the digital messages. Index is poor and has many mistakes. (back to text)
[NAMPS2] "Only a few cities ever went with NAMPS, and it didn't replace AMPS, it was used in conjunction with AMPS. We looked at it for the Los Angeles market (where I spent 7 years with PacTel/AirTouch) but it just didn't measure up. The quality just wasn't good, and the capacity gains were not the 3 to 1 as claimed by Motorola. The reason is that you cannot re-use NAMPS channels as closely as AMPS channels. Their signal to noise ratio requirements are higher due to the reduced bandwidth. (We engineered to an 18dB C/I ratio for AMPS, whereas we found that NAMPS required 22 dB.) [See The Decibel for more on carrier interference ratios, ed.] Also, market penetration of NAMPS capable phones was an issue. If only 30% of your customers can use it, does it really provide capacity gains? The Las Vegas B carrier loved NAMPS, though. At least, that's what Moto told us. . . though even under the best of conditions NAMPS doesn't satisfy the average customer, according to industry surveys. There's no free lunch, and you can't get 30 kHz sound from 10 kHz. But the point is moot - - NAMPS is dead." Mark van der Hoek. Personal correspondence. (back to text)
[Adding channels] "The channels for a particular cell are assigned by a Radio Frequency Engineer, and are fixed. The mobile switch assigns which of those channels to use for a given call, but has no ability to assign other channels. In a Motorola (and, I think, Ericsson) system, changing those assigned channels requires manual re-tuning of the hardware in the cell site. This takes several hours. Lucent equipment allows for remote re-tuning via commands input at the switch, but the assignment of those channels is still made by the RF engineer, taking into account re-use and interference issues. Re-tuning a site in a congested downtown area is not trivial! An engineer may work for weeks on a frequency plan just to add channels to one sector. It is not unusual to have to re-tune a half dozen sites just to add 3 channels to one." Mark van der Hoek. Personal correspondence. (back to text)
