How do Cell-phone
In the dark ages before cell phones, people
who really needed mobile-communications
In the radio-telephone system, there was one
central antenna tower per city, and perhaps
available on that tower. This
meant that the phone in your car needed a
powerful transmitter -- big enough to
transmit 40 or 50 miles (about 70 km). It
also meant that not many people could use
telephones -- there just were not enough
The genius of the cellular system is the
division of a city into small
This allows extensive
across a city, so that millions of people
can use cell phones simultaneously.
A good way to understand the sophistication
of a cell phone is to compare it to a
or a walkie-talkie.
Full-duplex vs. half-duplex
- Both walkie-talkies and CB radios are
devices. That is, two people communicating
on a CB radio use the same
so only one person can talk at a time. A
cell phone is a
device. That means that you use one
frequency for talking and a second, separate
frequency for listening. Both people on the
call can talk at once.
- A walkie-talkie typically has one channel,
and a CB radio has 40 channels. A typical
cell phone can communicate on 1,664 channels
- A walkie-talkie can transmit about 1 mile
(1.6 km) using a 0.25-watt transmitter. A CB
radio, because it has much higher power, can
transmit about 5 miles (8 km) using a 5-watt
transmitter. Cell phones operate within
and they can switch cells as they move
around. Cells give cell phones incredible
range. Someone using a cell phone can drive
hundreds of miles and maintain a
conversation the entire time because of the
In half-duplex radio, both
transmitters use the same
frequency. Only one party can
talk at a time.
In full-duplex radio, the two
transmitters use different
frequencies, so both parties can
talk at the same time.
Cell phones are full-duplex.
In a typical analog cell-phone system in the
United States, the cell-phone carrier
to use across the city. The
carrier chops up the city into cells. Each
cell is typically sized at about
10 square miles
(26 square kilometers). Cells are normally
thought of as hexagons on a big
Each cell has a
that consists of a tower and a small
building containing the radio equipment.
We'll get into base stations later. First,
let's examine the
"cells" that make up a cellular system.