|How does a cellular phone work?
|Question Date: 1999-04-20|
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
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
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
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
The cells themselves are roughly
hexagonal (each repeater has three antennas in an
equilateral triangle). This makes for good
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?
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
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
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