Electromagnetic Spectrum.
Why Micro-waves?
Why Cellular?-Frequency Reuse.
Basic Structure.
How do you call your friends?
Spectrum crunch
Cognitive radio
Cell breathing
Green communication.
Research in this domain.
Microwaves are widely used
 for point-to-point communications because
their small wavelength allows convenientlysized antennas to direct them in narrow
beams, which can be pointed directly at the
receiving antenna.
 Less attenuation to water vapour,oxygen
 Range roughly from 800MHz-30GHz.
Method used by service providers to improve
the efficiency of a cellular network and to serve
millions of subscribers using a limited radio
Based on the fact that after a distance a radio
wave gets attenuated and the signal falls
bellow a point where it can no longer be used
or cause any interference.
Transmitter transmitting in a specific frequency
range will have only a limited coverage area
Beyond coverage area frequency is reused by
another transmitter.
the entire network coverage area is divided
into cells based on the principle of
frequency reuse
• basic geographical unit of a cellular
• Represented graphically as a hexagonal
shape, but in reality it is irregular in shape
• Width of cells changes as demand changes.
• is a group of adjacent cells, usually 7 cells;
no frequency reuse is done within a cluster.
BTS (Base Transceiver Station)
Main component of a cell
Connects the subscribers to the cellular
Transmission/reception of information; it uses
several antennas spread across the cell
BSC (Basic Station Controller)
Interface between BTSs and it is linked to BTSs
by cable or microwave links; it routes calls
between BTSs; it is also connected to the MSC
MSC (Mobile Switching Center)
Coordinator of a cellular network
Connected to several BSCs,
Routes calls between BSCs;
Links the cellular network with other networks like
PSTN through fiber optics, microwave or copper
Home Location Register (HLR)
- maintained by the service provider
- permanent data about each subscriber (i.e.
location, activity status, account status, call
forwarding preference, caller identification
Visitor Location Register(VLR)
–stores temporary data about a subscriber; it is
kept in the MSC of the of the area the
subscriber is located in; when the subscriber
moves to a new area the new MSC requests this
VLR from the HLR of the old MSC
Equipment Identity Register(EIR) –
-database located near the MSC and
containing information identifying cell
phones which are stolen or which are
Authentication unit:-This unit holds the security concerns that
hover around the all channels, authenticating
even the correct base station not the ones
held for data recovery by some external
Control channel
-dedicated channel for sending ack,tracking
call,tracking messages,other control signals
-only control data ;no voice
 Voice channel
-for only voice transmission.
the subscriber dials the receiver’s number and
sends it to the BTS
the BTS sends to its BSC the ID, location and
number of the caller and also the number of the
the BSC forwards this information to its MSC
the MSC routes the call to the receiver’s MSC which
is then sent to the receiver’s BSC and then to its
the communication with the receiver’s cell phone is
when the receiver’ phone is in an idle state it
listens for the control channel of its BTS
if there is an incoming call the BSC and BTS
sends a message to the cells in the area where
the receiver’s phone is located
the phone monitors its message and compares
the number from the message with its own
if the numbers matches the cell phone sends
an acknowledgement to the BTS
after authentication, the communication is
established between the caller and the receiver
1. when the phone needs to make a call it
sends an access request (containing phone
identification, number) ; if another cell
phone tries to send an access request at the
same time the messages might get
corrupted, in this case both cell phones wait
a random time interval before trying to
send again
2. then the BTS authenticates the cell phone
and sends an acknowledgement to the cell
3. the BTS assigns a specific voice channel and time
slot to the cell phone and transmits the cell phone
request to the MSC via BSC
4. the MSC queries HLR and VLR and based on the
information obtained it routes the call to the
receiver’s BSC and BTS
5. the cell phone uses the voice channel and time
slot assigned to it by the BTS to communicate with
the receiver.
1. When a request to deliver a call is made in
the network, the MSC or the receiver’s
home area queries the HLR; if the cell phone
is located in its home area the call is
transferred to the receiver; if the cell phone
is located outside its home area, the HLR
maintains a record of the VLR attached to
the cell phone
2. Based on this record, the MSC notes the
location of the VLR and indicated the
corresponding BSC about the incoming call
3. The BSC routes the call to the particular
BTS which uses the paging channel to alert
the phone.
4. The receiver cell phone monitors the
paging channel periodically and once it
receives the call alert from the BTS it
responds to the BTS
5. The BTS communicates a channel and a
time slot for the cell phone to communicate
6. Now the call is established
Why Hand-off?
How to do Hand-off?
What factors determine handoff?
Types of hand-off
First generation Hand-off
MAHO-In 2g phones
In this signal strength measurements are made
by base station & supervised by MSC.
Each base station monitors the signal strengths
of all the control channels to determine the
relative location of each mobile user with respect
to the base station tower.
The locator receiver is controlled by the MSC & is
used to monitor the signal strength of users in
need of hand-off.
Based on the locator receiver signal strength info
from each base station ,the MSC decides whether
hand-off is necessary or not.
Every mobile station measures the received
power from surrounding base stations &
continually reports the results to these
measurements to the serving base station.
A hand-off is initiated when power received
from base station of a neighbouring cell
begins to exceed the power received from
current base station by a certain level or for a
certain period of time.
Types of hand-offs
Who’s this?
Generally the spectrum is sold to private
companies by govt. & they provide connectivity.
However, at one point of time the probability that
the spectrum is fully utilized in low & medium
density zones remains low; thus a loss of
spectrum takes place.
A method called Software defined radio-cognitive
radio was figured out by Dr.Joseph Mitola.
This method is useful for determining which
radio frequency band is unused.
Function of Cognitive radio: Spectrum sensing: Detecting unused
spectrum and sharing it, without harmful
interference to other users; an important
requirement of the cognitive-radio network
to sense empty spectrum.
Cell breathingis a mechanism which allows overloaded cells
to offload subscriber traffic to neighbouring
cells by changing the geographic size of their
service area. Heavily loaded cells decrease in
size while neighbouring cells increase their
service area to compensate. Thus, some
traffic is handed off from the overloaded cell
to neighbouring cells, resulting in load
-Why green Communication?
-Use of solar panels in place of diesel in base
-Dynamic Power Assignment based on load
-A fixed control channel scans number of
mobile users in an area of a cell & tells
neighbouring bts whether or not to expand.
-if the minimum requirement fails then bts
expand & area is served by that neighbouring
bts. While the bts of that cell remains off.
Which company has record of
highest number of functional
towers in India?
Methods for Efficient Hand-off with same
quality of service. (QOS)
Tackling Spectrum Crunch – Software defined
Radio-cognitive radio.
Maintaining the QOS within a cell at specified
power level.
Security in software defined radio.
Green wireless communication-focussing cell
breathing techniques even in high traffic
Basic electronics.(1st year).-Websearch.
Signal & System.(2nd year).-Prof.S.C Dutta Roy
Analog Modulation.(2rd year)-Prof. Surendra
Prasad IITD.
Digital Modulation.(3rd Year)-Taub –
Communication Engineering.
Speech Coding(no year)Rabiner;Rappaport;Youtube-E amberkairajahvideo.
Wireless Communication(Final Year).Rappaport;Prof.Ranjan K Bose-IITD
Wireless Communication “Rappaport”-Pearson
NPTEL Videos-Prof.Ranjan K Bose-IIT Delhi.
IEEE Papers.
E-handoffs-Mr.Rohit Trehan
-basic building block call transfer Mr.
Vishwendra Choudhary.
w- types of hand-off-VIT,Mmbai
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