Millions of people all
over the world use mobile phones. They are such a useful
gadgets that with their use one can talk to anywhere
from just about everywhere. In modern world these mobile
phones provide wide array of functionality, and every
now and then new functions are added at frenetic pace.
The typical functionality available in most of the mobile
phones includes provision for contact information storage,
built-in calculator and currency converter, simple games
interface, small digital camera, TV-tuner card, WAP
services etc.
But have you ever thought about the operating fundamentals
of mobile phones and its technology? What makes it different
from the standard land-line telephone? In this article
the fundamentals of mobile phones technology is succinctly
discussed.
In fact a mobile phone is an extremely sophisticated
radio, but a radio nonetheless. The mobile phone technology
is the confluence of telephone technology and wireless
communication, which were invented by Alexander Graham
Bell in 1876 and Nikolai Tesla in 1880 respectively.
In the years before the advent of mobile phones, people
used to install radio-telephones in their vehicles.
This old system consisted of one central powerful antenna
for any specific region, and a limited number of available
channels for that tower. This system needed a very powerful
transmitter to transmit signals even up to 100 kilometers.
This old system couldn’t facilitate many users,
as there were not enough channels. The existing mobile
phone system divides every region into small cells.
This allows extensive frequency reuse across that region
which eventually provides millions of channels for users.
Frequency reuse means the use of every frequency after
every alternate cell. This means even for a small frequency
band there is huge number of channels for communication
facilitating more and more users.
But there comes a question of interference during frequency
reuse operation. But you will be amazed to know that
the cell are divided in such a way that there is requirement
of only low power transmitter for each cell. It should
be clear that each cell has a base station, which consists
of a tower and small equipments that are part of transmitters
and receivers. The low power transmitter is capable
of only transmitting signals with a specified power,
which do not interfere the adjacent cells. Moreover,
every cell uses only 1/7th of the available communication
channels so it has a unique set of frequencies, which
prevents any collision among the signals from different
cells. Thus the cynicism of interference and collision
is avoided with the clever use of wireless communication
technology.
As the city is divided into numerous number of cells,
which consequently necessitates equal number of base
stations. Logically it should increase the cost operation
but in fact it is not because of the exponential increase
in the number of mobile phones user. The higher the
user the lower will be the costs per user. Each carrier
in each city also operates one central control tower
called Mobile Telephone Switching Office (MTSO). The
function of this MTSO is to handle all of the phone
connections to the land phone system and to control
all the base stations in the region.
Now we are ready to understand what happens when we
switch on our mobile phone. It should be understood
that all the mobile phones have special codes associated
to them for identification purposes. When you first
power up the phone, it listens for an SID (system identification
code; its stored in the SIM card) on the control channel.
The control channel is a special frequency that the
mobile phone and base station use to communicate to
one another about things like call set-up and channel
changing. If the phones can’t find any control
channel to communicate to, it knows it is out of range
and displays a “no service” message. When
your phone receives the SID, it compares the received
SID to the SID programmed into the phone. If the SIDs
matches, the phone knows that the cell it is communicating
is part of your subscriber’s system. During the
course of communication the phone also sends one registration
request to the MTSO. The MTSO thus keeps track of your
phone in their database, this way the MTSO knows the
location of your phone and rings you accordingly when
it wants to connect to you. Now when your friend calls
you, the MTSO quickly searches your phone’s location
from its database and tries to reach you immediately.
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.
Now, 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.
It is also interesting to know what happens when you
leave coverage area of your mobile service provider
(eg. Vodafone) to the coverage area of some other mobile
service provider (eg. Orange). This situation is called
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 mobile phone provider,
which then checks its database to confirm that the SID
of the phone you are using is valid. Your mobile phone
provider verifies your phone to the MTSO of the cell
you are roaming in, which then tracks your phone as
you move through its cells. And the amazing thing is
that all of this happens within seconds. But the most
insane thing of this process is the exorbitantly high
cost. So better be careful while using the roaming services.
Contributor: Mohammad Shabi Hashmi
has contributed this article. He holds a B.Tech from
Aligarh Muslim University, India; and MS from Darmstadt
University of Technology, Germany. Currently he is a
DHPA Scholar of UK Research Council and is working at
Agilent Center for High Frequency Engineering, Cardiff.
He is simultaneously working towards his Doctor of Engineering
from Cardiff University, UK.
