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How does a cellular phone work?
Question Date: 1999-04-20
Answer 1:

Good question. Let me start by asking you this question: do you know how a telephone works? If not, check an encyclopedia for more information. The basics of any telephone is that it turns your voice into an electrical signal which can travel over wires (telephone lines). Then, the other telephone turns that electrical signal into sound again. It is a lot like your walkman or stereo which turns the music coming from the tape or CD into an electrical signal (or "current") which travels up through the wires of your headphones to the speakers next to your ears. Get it?

O.k., now you know the basics of a regular telephone. So on to the next question... how does a cell phone send your voice as a "signal" if there's no wire to carry the signal? Well, it needs to use a carrier that doesn't need a wire, something that can transmit your voice "signal" through the air. People have been using radio waves for many years to send voices and music over the air to your radio. Cellular phones do the same thing, except they use microwaves (instead of radio waves) to transmit your voice.

Someday when you're old enough to drive, you will probably have a cell phone in your car. You'll be driving along and you will feel like calling your best friend from 7th grade, so you will pick up your phone and dial the number. That number will be transmitted by microwave to a RECEIVER STATION, where computers detect the microwave signal and dials the number for you using the old, standard telephone wires to send the call. Your best friend will be sitting in their living room and the call will come through. You two will be chatting away, but because you are driving a long way away from the first RECEIVER STATION, your call starts to break up. At that point, another computer detects that your signal is breaking up and your call is transferred to another RECEIVER STATION, one which is closer to you. So, you and your friend keep chatting away, as your voices are converted between electrical signals to microwave signals and then back to voices. Pretty amazing, right? That's how people can keep talking on the phone, even if they've travelled hundreds or thousands of miles in one day.

Did you know that the first cellular phone system began operation in Japan in 1979?

Answer 2:

ok -- there are actually two kinds of cellular phone -- digital and analog. Analog phones use 960-980 MHz and have about 60 channels to transmit on. They use narrow-band FM, which is a modulation very similar to that used in FM radio, except it is a voice band (3kHz) channel instead of 15kHz. This high frequency has both advantages and problems-- an advantage is the transmitting antenna can be very small (a few inches) and still have good gain. Another advantage is that there is less noise on the air at those frequencies -- (although there are other phones!) However, a disadvantage is that at those frequencies, the radio energy penetrates the earth's ionosphere and the e-layer and departs for places unknown. Thus the phone is limited to about line of sight -- over the horizon -- it needs a repeater.

The cell trick is to put a computer in each phone, and to predefine a set of shared repeaters (called cell trancievers) which send and recieve calls in each cell. (The cell is just the area covered by a particular repeater). When you turn on the phone -- it hunts for a clear channel to contact the local cell. When you call, the phone number and the cell phone ID (called a NAM) are transmitted to the repeater which places the call on either land lines or a dedicated link. The repeater moniters the signal strength and if it goes too low -- with re negotiate a channel (possibly with a different repeater). That way -- you can drive through several cells and the connection is not broken. (you can hear the background noise change as the channels are switched).

The cells themselves are roughly hexagonal (each repeater has three antennas in an equilateral triangle). This makes for good coverage.
Finally, at least one company has a contract with the Catholic Church to provide a site for their repeaters in church steeples. (They are high, and tend to be close to the prime urban cell areas). Typically, cells in the city are much closer than in the country -- and the phones transmit with a variable amount of power. Can you think of why this is done?

Digital cell phones use a similar cell scheme, but have more channels due to a clever modulation trick. Instead of a channel allocation, a digital phone encodes its output digitally with a key signature. All of the digital phones transmit on the same frequency band -- but with different signatures. The result is much more efficient and automatic channel allocation, and since noise is not likely to have the same signature -- low noise as well.

There is soon to be a third service -- direct satellite systems such as Iridium. In these systems, you talk directly to a low orbit satellite -- so they work anywhere on the planet. -- can yu guess why Motorola chose to use 70+ low orbit satellites versus a few high orbit ones?

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