Cellular phone system
In 1983, the analog cell-phone standard called AMPS (Advanced Mobile Phone
System) was approved by the FCC i.e.-Federal communication commission and
first used in Chicago. AMPS uses a frequencies between 824 megahertz (MHz)
and 894 MHz for analog cell phones
Digital cell phones use the same radio technology as analog phones, but they use it in a
different way. Analog systems do not fully utilize the signal between the phone and the
cellular network -- analog signals cannot be compressed and manipulated as easily as a
true digital signal. This is the reason why many cable companies are switching to digital
-- so they can fit more channels within a given bandwidth. It is amazing how much
more efficient digital systems can be. Digital phones convert your voice into binary
information (1s and 0s) and then compress it allows between three and 10 digital cellphone calls to occupy the space of a single analog call.
2. Cell Approach
Cellular radio provides mobile telephone service by employing a network of cell sites
distributed over a wide area. Cell is a network of small geographical.Each cell site has a
base station with a computerized 800 or 1900 megahertz transceiver and an antenna. This
radio equipment provides coverage for an area that's usually two to ten miles in radius.
Even smaller cell sites cover tunnels, subways and specific roadways. An area's size
depends on, among other things, topography, population, and traffic. A cell site contains
a radio transceiver and a base station controller which manages, sends, and receives
traffic from the mobiles in its geographical area to a cellular telephone switch. It also
employs a tower and its antennas, and provides a link to the distant cellular switch called
a mobile telecommunications switching office. This MTSO places calls from land based
telephones to wireless customers, switches calls between cells as mobiles travel across
cell boundaries, and authenticates wireless customers before they make calls.
Because cell phones and base stations use low-power transmitters, the same frequencies can be
reused in non-adjacent cells. The two purple cells can reuse the same frequencies.
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Figure for Physical Cell, Tower structure
Each cell has a base station that consists of a tower and a small building
containing the radio equipment (more on base stations later).
Cell phones have low-power transmitters in them. Many cell phones have two
signal strengths: 0.6 watts and 3 watts. The base station is also transmitting at
low power. Low-power transmitters have two advantages:
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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 batteryoperated, is relatively low. Low power means small batteries, 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
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also runs one central office called the Mobile Telephone Switching Office
(MTSO). This office handles all of the phone connections to the normal landbased phone system, and controls all of the base
stations in the region.
Let's say you have a cell phone, you turn it on and
someone tries to call you. Here is what happens to
From Cell to Cell
the call:
All cell phones have
special codes associated
 When you first power up the phone, it listens
with them. These codes
for an SID (see sidebar) on the control
are used to identify the
channel. The control channel is a special
phone, the phone's owner
frequency that the phone and base station use and the service provider.
to talk to one another about things like call setCell Phone Codes
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"
 Electronic Serial
Number (ESN) - a
message.
unique 32-bit number
 When it receives the SID, the phone
programmed into the
compares it to the SID programmed into the
phone when it is
phone. If the SIDs match, the phone knows
manufactured
that the cell it is communicating with is part of
 Mobile Identification
its home system.
Number (MIN) - a 10digit number derived
 Along with the SID, the phone also transmits a
from your phone's
registration request, and the MTSO keeps
number
track of your phone's location in a database - System Identification
this way, the MTSO knows which cell you are
Code (SID) - a unique
in when it wants to ring your phone.
5-digit number that is
assigned to each carrier
 The MTSO gets the call, and it tries to find
by the FCC
you. It looks in its database to see which cell
you are in.
While the ESN is considered a
 The MTSO picks a frequency pair that your
permanent part of the phone,
phone will use in that cell to take the call. That both the MIN and SID codes are
means that you use one frequency for talking programmed into the phone
when you purchase a service
and a second, separate frequency for
plan and have the phone
listening. Both people on the call can talk at
activated.
once.
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Figure depicting ttwo transmitters use different frequencies, (frequency pair) so both
parties can talk at the same time.
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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. As you travel, the signal is passed from
cell to cell.
In nutshell a mobile phone runs a self diagnostic when it's powered up. Once completed it
acts like a scanning radio, it picks one with the strongest signal, the nearest cell or sector
usually providing that. Not making a call but still on? The mobile re-scans every seven
seconds or when signal strength drops before a pre-determined level. The mobile sends
its phone number, its electronic serial number, and its home system ID. Among other
things. The cell site relays this information to the mobile telecommunications switching
office. The MTSO, in turn, communicates with different databases, switching centers and
software programs.
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3. Cellular Access Technologies
Since radio spectrum is a limited resource shared by all users, a method must be devised
to divide up the bandwidth among as many users as possible. The method chosen by
GSM is a combination of Time- and Frequency-Division Multiple Access
(TDMA/FDMA). The FDMA part involves the division by frequency of the (maximum)
25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart. One or more
carrier frequencies are assigned to each base station. Each of these carrier frequencies is
then divided in time, using a TDMA scheme. GSM: Probably the most useful thing to
know about the Global System for Mobile communications (GSM) is that it is
an international standard. If you travel in Europe and many other parts of the
world, GSM is the only type of cellular service available. Instead of using analog
service, GSM was developed as a digital system using TDMA technology . GSM
systems provide a number of useful features:
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Uses encryption to make phone calls more secure
Data networking
Group III facsimile services
Short Message Service (SMS) for text messages and paging
Call forwarding
Caller ID
Call waiting
Multi-party conferencing
GSM is the international standard in Europe, Australia and much of Asia and Africa. In
covered areas, cell-phone users can buy one phone that will work anywhere where the
standard is supported. To connect to the specific service providers in these different
countries, GSM users simply switch subscriber identification module (SIM) cards.
SIM cards are small removable disks that slip in and out of GSM cell phones. They store
all the connection data and identification numbers you need to access a particular
wireless service provider.
TDMA is used as the access technology for Global System for Mobile
communications
The first word tells you what the access method is. The second word, division,
lets you know that it splits calls based on that access method.

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FDMA puts each call on a separate frequency.
TDMA assigns each call a certain portion of time on a designated
The last part of each name is multiple access. This simply means that more
than one user can utilize each cell.
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FDMA separates the spectrum into distinct voice channels by splitting it into
uniform chunks of bandwidth. To better understand FDMA, think of radio
stations: Each station sends its signal at a different frequency within the available
band.
In FDMA, each phone uses a different frequency.
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Figure for Time Division mulplexing.
Above figure show analog voice signal is converted into digital then send to
mobile istrument on analog FTDMA carrier and vice versa. These digital signal is
muliplexed ( mixed) with digital signal of other many users as TDMA techniques
on single voice channel ( single media e.g. single cable otherwise it require as
much nos. of cables as nos. of users to talk simultanouesly).
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4. 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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5. Future trend :
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3G (third-generation wireless) phones may look more like PDAs( Palm Desk Accessories
containing mobile & PC features), with features such as video-conferencing, advanced
personal calendar functions and multi-player gaming. It may reppace GSM in near future
and TDM will be replaced by CDMA(code division multiplexing as advance and
advantegoeus techniqe).
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