Concept of & Cell Sectoring and Micro Cell: by Kundan Kumar

CONCEPT OF & CELL
SECTORING AND MICRO

CELL
By Kundan Kumar
Shape of Cells
Square
Width d cell has four neighbors at distance d and four at distance
d
Better if all adjacent antennas equidistant
Simplifies choosing and switching to new antenna
Hexagon
Provides equidistant antennas
Radius defined as radius of circum-circle
Distance from center to vertex equals length of side
Distance between centers of cells radius R is R
Not always precise hexagons
Topographical limitations
Local signal propagation conditions
Location of antennas
2
3
Cellular Geometries
Frequency Reuse
Power of base transceiver controlled
Allow communications within cell on given frequency
Limit escaping power to adjacent cells
Allow re-use of frequencies in nearby cells
Use same frequency for multiple conversations
10 50 frequencies per cell
E.g.
N cells all using same number of frequencies
K total number of frequencies used in systems
Each cell has K/N frequencies
Advanced Mobile Phone Service (AMPS) K=395, N=7 giving 57
frequencies per cell on average
Characterizing Frequency
Reuse
D = minimum distance between centers of cells that use the same
band of frequencies (called cochannels)
R = radius of a cell
d = distance between centers of adjacent cells (d = R)
N = number of cells in repetitious pattern
Reuse factor
Each cell in pattern uses unique band of frequencies
Hexagonal cell pattern, following values of N possible
N = I
2
+ J
2
+ (I x J), I, J = 0, 1, 2, 3,
Possible values of N are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21,
D/R=
D/d =

N 3
N
Frequency Reuse Patterns
Frequency Reuse Patterns
Principles of Cellular Frequency Reuse
Typical frequency reuse plan for 7 different radio
frequencies, based on hexagonal cells.
In fact some problems in cellular frequency
assignment are solved using map coloring
theory.
Principles of Cellular Frequency Reuse
(cont)
Frequency 're-use' distance is the closest
distance between the centers of two cells using
the same frequency (in different clusters) is
determined by the choice of the cluster size C
and the lay-out of the cell cluster.
Co-Channel Interference (CCI)
CCI arises in cellular systems where the available
frequency channels are divided into different sets.

Each set being assigned to a specific cell and with
several cells in the system using the same set of
frequencies.

CCI limits the system capacity

This interference generally happens in places where
population is high.

The Capacity of Cellular
Network
Why do we need more capacity?

Reach more users at the same time

Share more information throughout the network.

New technologies will require more complex
solutions and these solutions can be achieved
with maximum space available.
The Capacity of Cellular Network
(cont)
The capacity of cellular systems can be
increased by;

Frequency borrowing

Cell splitting

Cell sectoring

Microcells
Frequency Borrowing
RF bandwidth is the most important constraint
in wireless systems.
So to increase the capacity, frequency of
Radio Signals and wireless systems shall be
increased.
To do this, frequencies are taken from adjacent
cells by congested cells.
Cell Splitting
The unit area of RF coverage for cellular
network is called a cell.
In each cell, a base station transmits from a
fixed cell site location, which is often centrally
located in the cell.
In base stations where the usage of cellular
network is high, these cells are split into
smaller cells.

Cell Splitting (cont)
The radio frequencies are reassigned, and
transmission power is reduced.

A new cell site must be constructed when a cell is
split

Cell splitting is one of the easy and less costly
solution when increasing the capacity of cellular
network.

Splitting the cells into smaller ones also lead to a
new solution called cell sectoring.
Cell Sectoring
Sectorization consists of dividing an
omnidirectional (360 degree) view from the cell
site into non-overlapping slices called sectors.
When combined, sectors provide the same
coverage but they are considered to be
separate cells.
Also considered as one of easy and
inexpensive capacity increasing solution.
Sectoring
In basic form, antennas are omnidirectional
Replacing a single omni-directional antenna at base station
with several directional antennas, each radiating within a
specified sector.
achieves capacity improvement by essentially rescaling the
system.
less co-channel interference, number of cells in a cluster can
be reduced
Larger frequency reuse factor, larger capacity

Sectoring methods
Sectoring Examples
Only two cochannel cell
S/I improvement 7.2dB
Capacity 12/7
First type handoff
Trunking efficiency low
Urban area not good
Example 3.9

DAYANANDA SAGAR
COLLEGE OF ENGINEERING,
BANGALORE
Repeater
Extend coverage range
Directional antenna or distributed antenna
systems

Microcells
As the splitting of cell idea evolves, the usage
of smaller cells become efficient and it leads
the creation of microcells.

The aim of creating microcells are increasing
the capacity of cellular network in areas where
population is high.
Microcells (cont)
Typical comparison can be made like this;

Cells typically range in size from two to twenty
kilometers in diameter.

Microcells range from about a hundred meters to
a kilometer in diameter.
Micro Cell Zone Concept
Large control base station is replaced by
several lower powered transmitters on the
edge of the cell.
The mobile retains the same channel and the
base station simply switches the channel to a
different zone site and the mobile moves from
zone to zone.
Since a given channel is active only in a
particular zone in which mobile is traveling,
base station radiation is localized and
interference is reduced.

Micro Cell Zone
Superior to sectoring, any base
station channel may be assigned to
any zone by the base station
Same channel
No handoff
Only the active zone
DAYANANDA SAGAR
COLLEGE OF ENGINEERING,
BANGALORE
Example
2.33 times
capacity gain
Questions
Question1
How can Cellular network capacities will be
improve in the future?

A: There are lots of solutions for improving the
capacity of the Network. But the one of the
most logical one is, using the logical solution
cell in the sector with adaptive antennas. And
using more cells where the number of
subscriber is bigger.

Question 2
Why we need the frequency reuse? What are
the reasons?

A: We need frequency reuse because we
have a bandwidth. If we use same frequency
in every cell, the other cells make interference.
Hence the specific frequency is trying to not
use by the other cells.

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CONCEPT OF & CELL

SECTORING AND MICRO

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