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Mobile Computing COE 446

Network PlanningTarek Sheltami

KFUPMCCSECOE

http://faculty.kfupm.edu.sa/coe/tarek/coe446.htm

Principles of Wireless Networks

K. Pahlavan and P. Krishnamurth

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Outline

Network topology Cell hierarchy Cell Fundamentals Signal to interference ratio

calculation

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Wireless Network Topology The main benefit and elements of a cellular

network planning by relating the bandwidth, number of cells, frequency of reuse factor and capacity of the network

The number of simultaneous users using the system is given by:

A cellular topology reduces the coverage of both of MTs and BS

The reduction of the size of coverage lowers the required transmitted power, which increases the battery lifetime and reduces the sizes of MTs and BSs

The larger the number of cells, the larger the capacity and the smaller the size of mobile devices

m(W / N)n

B

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Wireless Network Topology.. We need a fixed network infrastructure to

interconnect the cells and ensure that the entire system works in a coordinated manner

The more number of cells, the more number of handoffs and the more complex the design of network

We need to address the following technical issues for planning a cellular network:

Selection of frequency reuse pattern Physical deployment and radio coverage

modeling The growth of the network Analysis of the relationship between the

capacity, cell size and cost of infrastructure

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Wireless Network Topology.. Cellular Hierarchy

Three reasons for hierarch1. To extend the coverage to the areas that are

difficult to cover by a large cell2. To increase the capacity of the network for

those areas that have a high density of users3. Some applications need certain coverage The cellular hierarchy is classified as follows:1. Femtocells: the smallest unit of the cellular

hierarchy used for connection of personal equipments (notebooks, notepads,..). They cover few meters, where all these devices in physical range of the user

2. Picocells: small cells inside a building that support indoor network such as WLANs. They cover few tens of meters

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Wireless Network Topology.. Cellular Hierarchy

3. Microcells: cells cover the inside streets with antennas mounted at heights lower than the rooftop of building along the streets. They cover hundreds of meters and are used in urban areas

4. Macrocells: traditional cellular network. They cover areas on the order of kilometers and their antenna are mounted above the rooftop of typical buildings in the coverage area.

5. Megacells: cover nationwide areas with range of hundreds of kilometers and mainly used for satellites

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Cell Fundamentals In practice cells are of arbitrary shape (close to

circle) Mathematical analysis is easier to analyze the

topology For cell of the same shape to form tessellation

so that there are no ambiguous areas that belong to multiple cells or to no cell

The cell shape can be only of three types regular polygons: equilateral triangle, square, or regular hexagon

A hexagon cell is the closest approximation to circle of these three and has been used traditionally for system design

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Triangular and rectangular cells

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Arranging regular hexagons that can cover a given area without creating ambiguous regions

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Cell Fundamentals..

In order to investigate the effects of interference, which changes with distance, there is a need to come up with a way of determining distances and identify cells

In order to maximize the capacity, cochannel cells must be places as far apart as possible for a given cluster size

It can be shown there only that six cochannel cells for a given reference cell at this distance

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Cell Fundamentals..

L

L

D3N

R

DL = distance between cochannels, N = cluster size, and RL = cell radius

This quantity is also referred to as cochannel reuse ratio

Values of N are taken from: i2 + ij + j2, i and j are integers

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If we take i = 2 and j = 1, we see that N = 4 + 2 + 1 = 7. Selecting a cell A, we can determine its cochannel cell by moving two units along one face of the hexagon and more unit in a direction 60o or 120o to this direction. Clusters of size N = 7 can be created as in figure page 13 .

Example

Cell Fundamentals..

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Hexagonal cellular architecture with a cluster size of N =7

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Cell Fundamentals..

The number of cells in a cluster N determines the amount of cochannel interference and also the number of frequency channels available per cell.

With fixed-channel allocation, each cluster uses Nc channels and each cell uses Nc/N, where Nc= channel available for the entire system

It desirable to maximize the number of channels allocated to a cell, therefore, N should be as small as possible

But, reducing N increases the Sr

Actual deployment is far more complication because of irregular differing cell sizes and propagation mechanisms

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Signal-to-Interference Ratio Calculation

It is given as:

Where, Pdesired is the strength of the signal from the desired BS and PPinterference, i is the signal strength from the ith interference BS

The signal strength falls as some power of the distance α called the power-distance gradient or path loss gradient

desiredr

int erference, ii

PS

P

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Signal-to-Interference Ratio Calculation..

The transmitted power Pt, after a distance d in meters, the signal ratio will be proportional to Ptd-α

In most simple case, the signal strength falls as the square of the distance in free space (α=2)

If we have BS1 and BS2 located in an area with same transmit power Pt and an MT is at distance of d1 from BS1 and d2 from BS2

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Signal-to-Interference Ratio Calculation..

Let us assume that the MT is trying to communicate with BS1, then the signal from BS2 is interference

The larger the ratio d2/d1, the better the performance

)(1

2

2

1

d

d

dKP

dKPS

t

tr