August 21, Mobile Computing COE 446 Network Planning Tarek Sheltami KFUPM CCSE COE Principles of Wireless Networks K. Pahlavan and P. Krishnamurth

August 21, Outline Network topology Cell hierarchy Cell Fundamentals Signal to interference ratio calculation

August 21, 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

August 21, 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

August 21, Wireless Network Topology.. Cellular Hierarchy Three reasons for hierarch 1. To extend the coverage to the areas that are difficult to cover by a large cell 2. To increase the capacity of the network for those areas that have a high density of users 3. 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

August 21, 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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8 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

August 21, Triangular and rectangular cells

August 21, Arranging regular hexagons that can cover a given area without creating ambiguous regions

August 21, 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

August 21, Cell Fundamentals.. D L = distance between cochannels, N = cluster size, and R L = cell radius  This quantity is also referred to as cochannel reuse ratio  Values of N are taken from: i 2 + ij + j 2, i and j are integers

August 21, If we take i = 2 and j = 1, we see that N = = 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 60 o or 120 o to this direction. Clusters of size N = 7 can be created as in figure page 13. Example Cell Fundamentals..

August 21, Hexagonal cellular architecture with a cluster size of N =7

August 21, 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 N c channels and each cell uses N c /N, where N c = 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 S r  Actual deployment is far more complication because of irregular differing cell sizes and propagation mechanisms

August 21, Signal-to-Interference Ratio Calculation  It is given as: Where, P desired is the strength of the signal from the desired BS and P Pinterference, i is the signal strength from the i th interference BS  The signal strength falls as some power of the distance α called the power-distance gradient or path loss gradient

August 21, Signal-to-Interference Ratio Calculation..  The transmitted power P t, after a distance d in meters, the signal ratio will be proportional to P t d -α  In most simple case, the signal strength falls as the square of the distance in free space (α=2)  If we have BS 1 and BS 2 located in an area with same transmit power P t and an MT is at distance of d 1 from BS 1 and d 2 from BS 2

August 21, Signal-to-Interference Ratio Calculation..  Let us assume that the MT is trying to communicate with BS 1, then the signal from BS 2 is interference  The larger the ratio d 2 /d 1, the better the performance