Reading Time: 6 minutes
Why Cellular?-Frequency Reuse.
How do you call your friends?
Research in this domain.
Microwaves are widely used
for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can be pointed directly at the receiving antenna.
Less attenuation to water vapour,oxygen molecules,etc.
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 spectrum.
}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 network
• 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 network
•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 cable
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 preference)
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 cloned.
-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 agents.
-dedicated channel for sending ack,tracking call,tracking messages,other control signals
-only control data ;no voice
-for only voice transmission
CALLING & RECEIVING :-
-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 receiver
-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 BTS
-the communication with the receiver’s cell phone is established
-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
-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
-then the BTS authenticates the cell phone and sends an acknowledgement to the cell phone
-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
-the MSC queries HLR and VLR and based on the information obtained it routes the call to the receiver’s BSC and BTS
the cell phone uses the voice channel and time slot assigned to it by the BTS to communicate with the receiver.
Calling & Receiving Continued:-
-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
– Based on this record, the MSC notes the location of the VLR and indicated the corresponding BSC about the incoming call
-The BSC routes the call to the particular BTS which uses the paging channel to alert the phone.
-The receiver cell phone monitors the paging channel periodically and once it receives the call alert from the BTS it responds to the BTS
-The BTS communicates a channel and a time slot for the cell phone to communicate
– Now the call is established
-How to do Hand-off?
-What factors determine handoff?
-Types of hand-off
First generation Hand-off
MAHO-In 2g phones
First generation Hand-off:-
-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.
MAHO-In 2g phones:-
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.
is 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 balancing.
Why green Communication?
-Use of solar panels in place of diesel in base station.
-Dynamic Power Assignment based on load detection,
-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.
RESEARCH IN THIS DOMAIN:-
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 zones.
THINGS YOU NEED TO KNOW:-
Basic electronics.(1st year).-Websearch.
Signal & System.(2nd year).-Prof.S.C Dutta Roy IITD NPTEL
Analog Modulation.(2rd year)-Prof. Surendra Prasad IITD NPTEL
Digital Modulation.(3rd Year)-Taub –Communication Engineering.
Speech Coding(no year)-Rabiner;Rappaport;Youtube-E amberkairajah-video.
Wireless Communication(Final Year).-Rappaport;Prof.Ranjan K Bose-IITD NPTEL
Advanced 3G 4g NPTEL-IIT KANPUR
Wireless Communication “Rappaport”-Pearson Publications.
NPTEL Videos-Prof.Ranjan K Bose-IIT Delhi.
Types of Hand-offs:-