The “Cells” in Cell Phones

Posted by jns on 30 December 2007

Isaac & I returned home yesterday, flying from Kansas City (central standard time) to Washington, DC (eastern standard time). As we arrived at the gate in DC, I overheard this conversational exchange from the seats in front of mine:

Mother: Oh, look! My cell phone has changed back to eastern time.
Teen-age son: That’s because they work with satellites, and they know where you are.

I was a bit surprised that, contrary to common belief, young people still don’t know everything. There is at least one surprising misconception in the son’s mind that I should have cleared up on the spot, but I didn’t. Y’all are so lucky.

Modern mobile phones, known also as cell phones, do not communicate through satellites, and they never have. There have existed satellite telephones that do, but they’re a much different beast, not to mention much larger and much heavier.

Cell phones communicate with cell towers–or, more generally, cell sites, since not all cellular antennae are on towers; many are hidden on building tops, for instance. If the cell sites are visible they are easily recognizable, most often triangular structures with vertical “bars” on each face of the triangle. Each of the “bars” is actually an antenna for transmitting to mobile phones or else receiving signals from them. The antennae are used in pairs so that they send and receive signals directionally.

The whole idea of “cells” was originally the way to provide coverage over a wide area without requiring a large amount of power in the handset, and also as a way to use restricted amounts of radio-frequency bandwidth efficiently and provide for a number of users.

In some area over which the cellular provider wants coverage, the area is divided into hexagonal “cells” that cover the area. (Look at a bathroom floor sometime that has 6-sided tiles and you will see that the area can be completely covered without gaps.) At the center of each cell is a cell site. The cell site has the three-sided shape so that it can hear in all directions. A cell site is responsible for all the cellular phones in its cell.

The imagined boundaries of the cells overlap a bit, so each cell actually operates on a slightly different frequency^# from all of its neighbors. Because of that, cell sites over a wider area can reuse frequencies, but there is the added technical challenge of tracking a particular cell phone between the ranges of neighboring cell sites and switching an active conversation from being routed through one cell site to being routed to another without dropping the call. That process is call “handover”. When a particular phone switches cells it also shifts the frequency that it uses for the radio link by a small amount.

The size of each cell varies depending on terrain and obstructions and such things, but in denser areas cell sites will be about 5 to 8 miles apart, so that’s the furthest that you cell phone usually has to transmit its signal, which is something it can manage to do with the relatively tiny batteries that it carries.

Cell sites do not continuously track a cell phone unless the phone is engaged in a conversation. If your phone has been off, it will always talk with the nearest cell site when you turn it back on and the phone network takes note of your position. Occasionally every cell sites will query phones with broadcast messages; the phones respond, and that way your cellular network can quickly find which cell site to use to contact your phone when you are receiving a call. The cell sites also broadcast timing signals, which is how cell phones always seem to know the right time. Note this: the cell site doesn’t have to know where your phone is to get the time correct, instead your phone simply takes the local time of the cell site that it can hear.

Now, one last note about cell phones talking through satellites. Most communications satellites are in geostationary orbits, which means they are in orbits where they appear stationary in the sky. This is where you will find the satellites that broadcast satellite radio and television, too.

Anyway, to be in a geosynchronous orbit requires that the satellite be at an altitude of about 22,000 miles. That’s a long way compared to the 8-mile distance to the nearest cell site. In fact, it’s about 2,750 times as far away. Other things being equal^* that means that the cell phone would require 7.6 million times the power for its signal to reach the satellite. As you might guess, the would require a much bigger battery than your cell phone has in it.

By the way, before I finish, I have to chastise the FCC for poor technical writing. I was looking around for a few details about cell-phone networks and I found this page of “Cell Phones FAQs” from “The FCC Kids [sic] Zone”. You may wish to look at the answer for “How Does a Cell Phone Work” and count how many errors and imprecise statements you can find in one paragraph. (For extra credit, read the other answers if you can stand it.) There are many irritants, but I could start with the absurdity of using the copper-wire based, home phone system as a conceptual basis, since kids don’t use that archaic communication system anymore as a conceptual referent.

There there’s this statement:

A cell phone turns your voice into a special type of electricity and sends it over the air to a nearby cell tower; the tower sends your voice to the person you are calling.

Calling propagating electromagnetic waves a “special type of electricity” is incorrect and unnecessary, an egregious error. Saying the tower “sends your voice” is no better. Despite what this FCC author seemed to think, it’s entirely possible not to go into pages of detail about the time-slice multiplexing and analog-signal digitization (most cell networks these days are digital) used to “send your voice” over the network and still get it right without the stupid and inaccurate “send your voice” gambit.

These are just the type of gratuitous and imprecise over-simplifications about science and technology that drive me into a frenzy and that I have vowed that Ars Hermeneutica will combat. If any of my four regular readers happen to know someone at the FCC, have them get in touch and we can straighten out these things before any more bad ideas get into kids’ (note the apostrophe) heads.
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^# Cell phones also use a different frequency to transmit from the frequency they use for receive, but that’s a needless conceptual complication at this stage.

^* There are details, naturally. The 7.6 million number is the square of 2,750, because radiated electromagnetic power diminishes as the square of the distance. However, satellite communications is possible with these geosynchronous satellites because their receivers have much higher gain (i.e., can hear much weaker signals) than terrestrial cell sites. They also are much too far away to be able to break an urban region up into cells and distinguish calls from different cells, let alone transmit in different cells, but that’s a whole other story.