cellular network. early mobile system cellular network use of one powerful transmitter located on...
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Early mobile system
Cellular Network
Use of one powerful transmitter located on high location.
Range of signals was upto 50km.
These system achieve good coverage.
Frequency can’t be reused throughout the system
Transmitter need to be very powerful to cover entire area.
Cellular Network
Cellular Network
Instead of using single powerful transmitter, use multiple low power transmitter.
Multiple Transmitters
Cellular Network
Cellular Network
These transmitters known as Base Station (BTS).
Each base station allocated portion of allocated channels of entire system.
Example: if a telephone system has bandwidth of 3MHz. One channel needs 30KHz.
Channel per BTS =Available bandwidth
no. of station X per channel capacity
Cellular Network
Cellular Network
Channels of neighboring cells are different in cellular network.
If same frequencies used in neighboring cells then interference will be there.
Cellular Network
Cellular Network
Instead of using single powerful transmitter, use multiple low power transmitter.
These transmitters known as Base Station (BTS).
Each base station allocated portion of allocated channels of entire system.
Coverage range of base station also known as cell.
Channels of neighboring cells are different in cellular network.
Cellular Network- Cell
Cellular Network
In Cellular system land area is divided into regular shaped cells, Which can be hexagonal, square, circular or some other irregular shapes.
What will be shape of cell?
Criteria for shape of cell are
.
Area without overlap
Geometric shape
Area of cell
Shape of cell
Cellular Network
Area overlap No overlapping
No overlapping
No overlapping
Criteria not fulfilled
Area to circle 17.7%
Area to circle83%
Area to circle63.7%
Why Cellular ?
Cellular Network
Scalability: As the demand for service increases the number of base stations may be increased thereby providing additional radio capacity
This enables a fixed number of channels to serve an arbitrarily large number of subscribers by reusing the channel throughout the coverage region
Same frequencies can be used.
Why Cellular ?
Cellular Network
Distributed throughout the geographic region
frequency channels can be re-used.
Cellular Network- Points
Cellular Network
possible radio coverage of the cell
idealized shape of the cell
cell
segmentation of the area into cells
– use of several carrier frequencies– not the same frequency in adjoining cells– cell sizes vary from some 100 m up to 35 km depending on user
density, geography, transceiver power etc.– hexagonal shape of cells is idealized (cells overlap, shapes depend
on geography)– if a mobile user changes cells
handover of the connection to the neighbor cell
Cellular Network
Cellular Network
Problems:
Fixed network needed for the base stations
Handover necessary
Interference with other cells
Advantage:
Higher capacity, higher number of users
Less transmission power needed
More robust, decentralized.
Base station deals with interference, transmission area etc.
Hand Off
Cellular Network
A handoff refers to the process of transferring an active call or data session from one cell in a cellular network to another or from one channel in a cell to another.
Hard Hand Off
Cellular Network
Characterized by an actual break in the connection while switching from one cell or base station to another. The switch takes place so quickly that it can hardly be noticed by the user. Because only one channel is needed to serve a system designed for hard handoffs, it is the more affordable option.
Before Hand off
Hard Hand Off
Cellular Network
First connection will be broken.Then new connection will be established.
X
During Hand off
Break before make
Soft Hand Off
Cellular Network
Entails two connections to the cell phone from two different base stations. This ensures that no break ensues during the handoff. Naturally, it is more costly than a hard handoff.
Soft Hand off
make before break
Problem with traditional approach
Cellular Network
100 Khz
130 Khz
110 Khz
120 Khz
Every user needs 2Khz
Total 15 Users can use the service at a time.
Cellular Network
Cellular Network
Total frequency = 30 Khz
10Khz
10Khz
10Khz
Every user needs 2Khz
Per cell 5 users
Tota users= 15
10Khz
10Khz
10Khz
+15
10Khz
10Khz
10Khz
+15
10Khz
10Khz
10Khz
+15
Cellular Network
Cellular Network
Total frequency = 30 Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
The N cells which collectively use the complete set of available frequencies is called a Cluster.
Cluster
Cellular Network
N Cell which utilize complete frequency of system.
Certain value of N are possible, which allow repeating of those cluster without repeating.
N=i2+ij+j2
i and j are non negative integers.
Cluster
Cellular Network
Find nearest co-channel.
move i cells along any chain of hexagons then,
turn 60 degrees counter-clockwise and move j cells.
i=2j=1
N=7
Frequency Reuse Factor
Cellular Network
it is portion of total frequency allotted to one cell.
Total frequency = 30 Khz
10Khz
10Khz
10Khz
Total frequency
Frequency in one cell
Frequency reuse factor=
1
Size of Cluster
Frequency reuse factor=
Frequency Reuse
Cellular Network
Frequency reuse is a method used by service providers to improve the efficiency of a cellular network and to serve millions of subscribers using a limited radio spectrum
A transmitter transmitting in a specific frequency range will have only a limited coverage area
Beyond this coverage area, that frequency can be reused by another transmitter
Static Frequency Assignment
Cellular Network
50KHz 50KHz
Certain frequency assigned to certain cell.
Different cells have different traffic load.Ex. first cell having only two users, so allotted frequencies to first cell is not fully utilized.
Second cell having high traffic.
Dynamic Frequency Assignment
Cellular Network
• base station chooses frequencies depending on the frequencies already used in neighbor cells
• more capacity in cells with more traffic
• assignment can also be based on interference measurements
Increasing Capacity-Cell Splitting
Cellular Network
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
Cell can be again divided into smaller cell. Capacity will be increased.
Problem with this approach is increased load for hand-off. Need more infrastructure (base stations)
Increasing Capacity-Cell Splitting
Cellular Network
Femto Cell: Indoor, Home Pico Cell: In buildings, range:10-200 meters
Micro cell:200-2KM
Macro cell:1KM-30KM
http://www.wirelesscommunication.nl/reference/chaptr04/cellplan/cellsize.htm
Increasing Capacity-Cell Sectorization
Cellular Network
Omni directional antenna radiate in all directions. Instead of omni direction antenna, single direction antenna is used to divided cell into sector.
Omni Direction antenna. Neighboring cell can’t use same set of frequency because of interferences.
Cell sectorization allow use of same set of frequency in adjacent cell.
Frequency Reuse
Cellular Network
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
Reuse Distance: The closest distance between the centers of two cells using the same frequency (in different clusters)
Also known as frequency re-use distance
How to calculate frequency reuse distance?
Cellular Network
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz 10Khz
reuse= sqrt(3 N)RN : size of cluster
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz 10Khz
10Khz
10Khz
10Khz
10Khz
10Khz
10Khz 10Khz
Traffic intensity
Cellular Network
a =λ ·T
traffic intensity is the average channel occupancy used to measure the time of channel utilization . Unit is Erlang.
λ = Average number of callsT = average time of call
Traffic intensity- Numericals
Cellular Network
Q. In a cellular mobile system, each mobile subscriber average two calls per hour at an average call duration of three minutes. Determine the traffic intensity per mobile subscriber.
Numerical
Cellular Network
Q. If a total of 33MHz bandwidth is allocated to particular cellular telephone system which uses two 25kHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system uses (a) 4-cell reuse, (b) 7-cell reuse (c) 12-cell reuse. If 1 MHz of the allocated spectrum is dedicated to control channels, determine an equitable distribution of control channels and voice channels in each cell for each of the three systems.
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