The Cellular Concept
Introduction:

AT & T and Bell Labs offered first mobile telephone service in 1973.
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This system’s high powered BS Txs with elevated antennas provided
a larger coverage area and enough signal for urban settlements.
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Typically 250 watt FM transmitter paged mobiles when there was an
incoming call for the mobile.

Limitation - limited users, no frequency reuse, cell congestion, high
power requirement.

Improvement: The main objective of cellular concept is to allocate
more users in a limited allocated spectrum.
The Cellular Concept
Introduction:
The basic system characteristics are

Area divided into Cells, each served by base station BS with lower
power transmitter covers a few hundred meters in some cities.
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Each cell gets a portion of total no. of channels.

Neighboring cells assigned different groups of channels in order to
reduce the interference.

Multiple lower-power base stations that service mobile users within
their coverage area and handoff users to neighboring base stations
as users move.
The Cellular
Concept
The Cellular Advantage

Large capacity.

Efficient use of spectrum resources.

Nationwide coverage & Adaptability to traffic density.

Telephone service to both vehicle and portable user terminals.

Affordability, which could eventually make it a mass-market service.

Power requirement for mobile is less due to smaller cell and low
power transmitter.

Longer battery life and smaller mobile station form factors.
The Cellular Concept
Cellular Limitation: Initial implement cost is large due to:
 Deployment of large no. of low power stations.

Acquisition of lands for cell sites.

The associated hardware like RBS Txr-Rxr, controller, Antennas and
towers.
Implementation of basic cellular architecture:
 Entire city is divided into cells, or smaller coverage areas.

Each cell had its own lower-power base station.

The radio channels must be allocated to each cell in such a way, it
minimize interference and provide necessary system performance to
handle the traffic load within the cells.
The Cellular Concept
Implementation of basic cellular architecture

Cluster : It is a group of cell that makes use of all the available radio
spectrum.

Cluster has N cells with unique and disjoint channel.

Since adjacent cell cannot use the same frequency channels, the total
frequency allocation is divided up over the cluster and then repeated
for other clusters in the system.

The no. of cells in a cluster is known as the cluster size / frequency
reuse factor (1/N)
Cellular Hierarchy
 It is created in the cellular system based on the cell size, as
shown in the below table.
The Cellular Concept
Illustration of cellular system capacity: An Example

Consider service provider wants to provide cellular communications
to a particular geographic area. The provider is licensed = 5MHz. Each
system subscriber bandwidth (channel B.W) = 10 KHz. If the service
provider was to provide coverage from only one transmitter site, the
total theoretical number of possible simultaneous users = Total B.W/
Channel B.W = 5 MHz/ 10kHz / user = 500 users. If, however, the
service provider implements a cellular system with 35 transmitter
sites, located to minimize interference and provide total coverage of
area, determine the new system capacity?
The Cellular Concept
Illustration of cellular system capacity: Solution:
 Assume the cluster size N = 7

The allocated B.W/cell= System B.W/ Number of cells in a cluster
=5*106/7=714kHz

Bandwidth per cell = 714 kHz.

No. of cluster 35/7= 5.

Each cell has a capacity =714kHz/10kHz/user = 71 users

Total system capacity =35 cells*71 users/cell = 2485 users.

This is a system capacity increase of =5 times.
CELL STRUCTURE

The Footprint: The actual radio coverage of a cell and is
determined from field measurements or propagation prediction
models.
Why circle cannot be used to represent the coverage area of a
base station?

Because adjacent circles cannot be overlaid upon a map without
leaving gaps or creating overlapping regions.

Thus, when considering geometric shapes which cover an entire
region without overlap and with equal area, there are three
sensible choices: a square; an equilateral triangle; and a hexagon.
CELL STRUCTURE
Why Hexagonal model?

A cell must be designed to serve the weakest mobiles within the
footprint, and these are typically located at the edge of the cell.

For a given distance between the center of a polygon and its
farthest perimeter points, the hexagon has the largest area of the
three.

By using the hexagon geometry, the fewest number of cells can
cover a geographic region.

The hexagon closely approximates a circular radiation pattern
which would occur for an Omni-directional base station antenna and
free space propagation.
CELL STRUCTURE
Frequency Reuse

The key characteristic of a cellular network is the ability to reuse
frequencies to increase both coverage and capacity.

Extensive frequency reuse allows for many users to be supported at
the same time.

Total spectrum allocated to the service provider is broken up into
smaller bands.

A cell is assigned one of these bands. All communications in this cell
occur over these frequencies only.

Neighboring cells are assigned a different frequency band to ensure
that nearby transmissions do not interfere with each other.

The same frequency band is reused in another cell that is far away.
Large distance limits the interference caused by this co-frequency
cell.
Frequency Reuse
Reuse Number

To gain the maximum reuse of the frequencies for a cellular system,
cells are arranged in clusters.

Interference levels generated by the co-channel cells is used to
determine the minimum size cluster that can be used.

D = reuse distance, N = cluster size N, R = cell radius R.
Reuse Number

The frequency reuse distance can be calculated by:
D = R (3N) 1/2

Values of N can only take on numbers calculated from the following
expression: N = i2 + ij + j2, where i and j are integers.
Frequency Reuse
CELLULAR INTERFERENCE ISSUES (S/I)

S/I ratio gives an indication of the quality of the received signal.

Smaller cluster sizes
larger no. of users
lowered S/I ratio
decrease in the radio link quality.

Trade-off between no. of users and signal quality.

AMPS system : S/I ratio = 18dB is possible for a cluster of size => 7.

Thus typical AMPS system
was deployed with a
cluster size of N=7.
CAPACITY EXPANSION TECHNIQUES





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Cellular capacity is a number of users in a cell.
Some of the capacity expansion techniques are :
1. Cell splitting
2. Cell sectoring
Cell splitting - process of subdividing a congested cell into smaller
cells. (each with its own base station and a corresponding reduction
in antenna height and transmitter power).
The increased no. of cells would increase the no. of clusters which in
turn would increase the no. of channels reused and capacity.
Each cell is divided into six new smaller cells with approximately
one-quarter the area of the larger cells and use the same channel.
To preserve the overall system frequency reuse plan, the transmit
power of these cells must be reduced by a factor of approximately
16 or 12dB.
CAPACITY EXPANSION TECHNIQUES
Cell splitting
Conclusion:
Cell splitting effectively increases system capacity by reducing the cell
size and therefore reducing the frequency reuse distance thus
permitting the use of more channels.
CAPACITY EXPANSION TECHNIQUES
Cell splitting
Advantages:
 Increases the system capacity.
 Reduces the cell size, frequency reuse distance.
 Increases the number of channels.
Disadvantages:
 Co channel interference increases.
 Difficult to acquire appropriately located cell sites.
 No. of base station increases.
 Trunking efficiency decreases and Handoff process increases.
CAPACITY EXPANSION TECHNIQUES
2. Cell Sectoring - Uses directional antennas to effectively split a cell into
3 or sometimes 6 new cells.
½
 Reuse Factor/ frequency reuse ratio : Q= D/R = (3N)

3 directional antennas
with 120o beamwidth to
illuminate the entire area
previously services by
omnidirectional antenna
CAPACITY EXPANSION TECHNIQUES

Cell Sectoring provides interference reduction, hence S/I ratio
increases. co-channel interference.

It does not require new cell sites and additional antennas and
triangular mounting only.

Demerits: Increased network system architecture complexity
Table below tabulates these new values for a three-sector scheme for
some common values
of cluster size.

Co-Channel Interference (CCI)

For the efficient use of available spectrum, frequency is reused over
relatively small geographical areas.

Increasing frequency reuse
increases interference
system capacity and service quality.

The cells using the same set of frequencies are co-channel cells.

Co-channel interference is the cross talk between 2 different radio
transmitters using the same radio frequency as in co-channel cells.

CCI occurs due to : either adverse weather conditions or poor
frequency planning or overcrowded radio spectrum.

If D/R ratio is increased
channel cells will increase
decreases
effective distance between the cointerference will decrease.
CAPACITY EXPANSION TECHNIQUES