16-02-2013, 09:53 AM
CELLULAR MOBILE COMMUNICATIONS
CELLULAR MOBILE.pdf (Size: 110.88 KB / Downloads: 33)
Abstract:
GSM stands for Global System for
Mobile communications, and CDMA
stands for Code Division Multiple
Access. The GSM is built around the
TDMA, or the Time Division Multiple
Access concept. Basically, the two
technologies address differently the
same fundamental problem of mobile
communication, how to divide the finite
frequency of airwaves between multiple
users at the same time, or, how to make
more than one person to carry on a
conversation on the same frequency
without causing mutual interference..
HISTORY & DEVELOPMENT
Cell phones have become the modern
fastest means of Communication on
move especially for the Business
Magnets & Professional job holders. The
roots of the cellular systems belong to
the years of 1940’s but the trial service
did not begin until 1970. Initially there
were only 2000 consumers could use
them, as there were only a few channels
& People who really needed Mobile
Communications capability installed
radio-telephones in their cars. In the
radio telephone system, there was one
central antenna tower per city, with
about 25 channels. So the phone in the
car needs a powerful Transmitter—big
enough to transmit up to 80km.But
currently in this 21st century there are
around 1400 million subscribers all over
the world.
THE CELL & CLUSTER:
Cells are base stations (BS)
communicates with mobiles via a
channel & broadcast at very low power
levels (typically 200 mW to 1W). The
channel is made of two frequencies, one
for transmitting to the base station and
one to receive information from the base
station. Cell Size varies depending on
the landscape.
ROAMING
Roaming was the most challenging
issues the cell phone manufactures
faced. The goal was simple: the phone
could be used anywhere in the world
where compatible technology is used.
When the user turns his cell phone on in
a roaming area, the cell phone identifies
itself to the switch. The switch looks up
the information & discovers that the
phone is not a local phone. It now
attempts to find the home switch based
on the exchange. When the Home switch
is located, the switch determines if
roaming is possible. If roaming is
possible, the switch (referred to as
roaming switch) sets up the Visitor
Location Register (VLR) registering the
phone in the locality. The home switch is
notified about the change so that it can
route call to the switch in the roaming
location.
GLOBAL SYSTEM FORMOBILE
COMMUNICATION(GSM)
GSM was the first and is still the most
popular cellular network standard in use
established in 1982. The GSM standard
transmits data on one of three frequency
bands: 900MHz,1800MHz& 1900 MHz.
The GSM 900MHz and 1800MHz
standards are used in India. Initially
GSM was the modified and more
efficient version of TDMA it enabled
Voice & Data services. Today’s GSM
consists of consists of mainly 3 parts
(systems) to supports its network, they
are:
1. GPRS
2. EDGE
3. GPS
1. General Packet Radio Service
(GPRS):
GPRS supports Video conferencing,
streaming audio & video & continuous
connectivity to internet for the GSM
subscribers.
Enhanced Data Rates For GSM
Evolution(EDGE):
EDGE Provides Multimedia
applications for mobile devices and third
generation GSM services . Here now
instead of cells satellites are used.
3. Global Positioning System(GPS):
GPS mainly provides it’s users with the
information consisting of time, position
& velocity of the user at any point on the
surface of the earth, in any climatic
condition at any instant of time.
Principles of GSM
Frequency Concepts -For Mobile
communication a Duplex Channel is
required which mean that two way both
at a time, it uses two frequencies: One to
the Mobile Station (MS) and one from
the Mobile Station. The direction from
the Mobile Station to the network is
referred to as uplink and the direction
from the network to the Mobile Station
is referred to as down link. Speech
coding-GSM is a digital system, so
speech that is inherently analog, has to
be digitized. The method of a Regular
Pulse Excited -- Linear Predictive Coder
(RPE--LPC) with a Long Term Predictor
loop is employed, where in, information
from previous samples, which does not
change very quickly, is used to predict
the current sample. Speech is divided
into 20 millisecond samples, each of
which is encoded as 260 bits, giving a
total bit rate of 13 kbps. This is the socalled
Full-Rate speech coding.
Access Method
IS-95 (CDMA)uses a multiple access
spectrum spreading technique called
Direct Sequence (DS) CDMA. Each user
is assigned a binary, Direct Sequence
code during a call. The DS code is a
signal generated by linear modulation
with wideband Pseudorandom Noise
(PN) sequences. As a result, DS CDMA
uses much wider signals than those used
in other technologies. Wideband signals
reduce interference and allow one-cell
frequency reuse. There is no time
division, and all users use the entire
carrier, all of the time.
Modulation
The type of modulation followed is
spread spectrum that scatters data
transmissions across the available
frequency band in a pseudorandom
pattern. Spreading the data across the
frequency spectrum makes the signal
resistant to noise, interference, and
snooping. In general, Spread Spectrum
communications is distinguished by
three key elements: (1) the signal
occupies a bandwidth much greater than
that which is necessary to send the
information. This results in many
benefits, such as immunity to
interference and jamming and multi-user
access. (2) The bandwidth is spread by
means of a code that is independent of
the data. The independence of the code
distinguishes this from standard
modulation schemes in which the data
modulation will always spread the
spectrum somewhat. (3) The receiver
synchronizes to the code to recover the
data. The use of an independent code
and synchronous reception allows
multiple users to access the same
frequency band at the same time.
CONCLUSION
The battle between GSM and CDMA
technologies in India is hotting up!
While the initial tilt was towards the
GSM with players like Hutch, Spice,
BPL and BSNL preferring it, the TATA
and the Reliance have now entered the
market putting their bets on CDMA.
The final conclusion is that it is not so
much over technology that a customer
decides which network to adopt. It is
basically performance! Whichever
mobile communication provider puts in
better efforts in giving a higher
performance, the subscriber is bound to
follow that path!