How Cell Phones Work
One of the most interesting things about a cell phone is that it is actually
a radio -- an
extremely sophisticated radio, but a radio nonetheless. The telephone was
invented by Alexander Graham Bell in 1876, and wireless communication can trace
its roots to the invention of the radio by Nikolai Tesla in the 1880s (formally
presented in 1894 by a young Italian named Guglielmo Marconi). It was only
natural that these two great technologies would eventually be combined!
In the dark ages before cell phones, people who really needed
mobile-communications ability installed radio telephones in their cars.
In the radio-telephone system, there was one central antenna tower per city, and
perhaps 25 channels available on that tower. This central antenna
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 radio telephones -- there just were not enough channels.
The genius of the cellular system is the division of a city into small
cells. This allows extensive frequency reuse across a city, so
that millions of people can use cell phones simultaneously. In a typical analog
cell-phone system in the United States, the cell-phone carrier receives about
800 frequencies
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).
Each cell has a base station that consists of a tower and a small
building containing the radio equipment (more on base stations later).
A single cell in an analog system uses one-seventh of the available duplex voice
channels. That is, each cell (of the seven on a hexagonal grid) is using
one-seventh of the available channels so it has a unique set of frequencies and
there are no collisions:
- A cell-phone carrier typically gets 832 radio frequencies to use in a
city.
- Each cell phone uses two frequencies per call -- a duplex channel
-- so there are typically 395 voice channels per carrier. (The other 42
frequencies are used for control channels.)
- Therefore, each cell has about 56 voice channels available.
In other words, in any cell, 56 people can be talking on their cell phone at
one time. With digital
transmission methods, the number of available channels increases. For
example, a TDMA-based digital system can carry three times as many calls
as an analog system, so each cell has about 168 channels available.
Cell phones have low-power transmitters in them. Many cell phones have
two signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios
transmit at 4 watts). The base station is also transmitting at low power.
Low-power transmitters have two advantages:
- The transmissions of a base station and the phones within its cell do
not make it very far outside that cell. Therefore, in the figure above, both of
the purple cells can reuse the same 56 frequencies. The same frequencies
can be reused extensively across the city.
- The power consumption of the cell phone, which is normally
battery-operated, is relatively low. Low power means small, and this
is what has made handheld cellular phones possible.
The cellular approach requires a large number of base stations in a city of
any size. A typical large city can have hundreds of towers. But
because so many people are using cell phones, costs remain low per user. Each
carrier in each city also runs one central office called the Mobile Telephone
Switching Office (MTSO). This office handles all of the phone connections to
the normal land-based phone system, and controls all of the base stations in the
region.
From Cell to Cell
All cell phones have
special codes associated with them. These codes are used to identify the
phone, the phone's owner and the service provider.
Let's say you have a cell phone, you turn it on and someone tries to call
you. Here is what happens to the call:
- When you first power up the phone, it listens for an SID (see
sidebar) on the control channel. The control channel is a special
frequency that the phone and base station use to talk to one another about
things like call set-up and channel changing. If the phone cannot find any
control channels to listen to, it knows it is out of range and displays a
"no service" message.
- When it receives the SID, the phone compares it to the SID programmed
into the phone. If the SIDs match, the phone knows that the cell it is
communicating with is part of its home system.
- Along with the SID, the phone also transmits a registration request,
and the MTSO keeps track of your phone's location in a database -- this way, the
MTSO knows which cell you are in when it wants to ring your phone.
- The MTSO gets the call, and it tries to find you. It looks in
its database to see which cell you are in.
- The MTSO picks a frequency pair that your phone will use in that cell
to take the call.
- The MTSO communicates with your phone over the control channel to
tell it which frequencies to use, and once your phone and the tower switch on
those frequencies, the call is connected. You are talking by two-way
radio to a friend!
- As you move toward the edge of your cell, your cell's base station
notes that your signal strength is diminishing. Meanwhile, the base
station in the cell you are moving toward (which is listening and measuring
signal strength on all frequencies, not just its own one-seventh) sees your
phone's signal strength increasing. The two base stations coordinate with each
other through the MTSO, and at some point, your phone gets a signal on a control
channel telling it to change frequencies. This hand off switches your
phone to the new cell.
Roaming
If the SID on the control
channel does not match the SID programmed into your phone, then the phone knows
it is roaming. The MTSO of the cell that you are roaming in contacts the
MTSO of your home system, which then checks its database to confirm that
the SID of the phone you are using is valid. Your home system verifies
your phone to the local MTSO, which then tracks your phone as you move through
its cells. And the amazing thing is that all of this happens within seconds!
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