CELLULAR NETWORK

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

Coverage range of base station also known as cell.

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

Cellular Network

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.

Types of Hand-off

Cellular Network

Hard handoff

Soft handoff

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

Cluster Size

Cellular Network

Calculate Size of Cluster ?

i=3j=2N=19

Cluster Size

Cellular Network

Calculate Size of Cluster ?

i=1j=2N=7

A

A

A

A

A

A

A

i

j

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

Frequency Reuse distance

Cellular Network

R

Frequency Planning

Cellular Network

Static Frequencies assignment

Dynamic Frequencies assignment

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.