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 = I2 + J2 + (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 =

N3N

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 (con’t) 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 (con’t)

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 (con’t) 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 (con’t) 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.