Lecture 2 The Cellular Concept Prof. Shamik Sengupta Office 4210 N Fall 2010

Introduction  Why Cellular? –Increase the number of users that can simultaneously be supported –Use the concept of cell to offer services in small area –Low power transmitter is sufficient –Divide and reuse the frequency channels in different areas simultaneously –Reduce interference among the users

What is interference?  Weakening of one signal strength due to transmission of another signal in the close proximity  When a BS transmits, the signal strength reduces rapidly over distance –Receiving radius –Interference radius  Interference majorly of two types: –Co-channel interference –Adjacent channel interference

Interference Screenshot

The concept of cell  A cell is controlled by a BS –Under the BS, there are several mobile stations (MS)  BS controls the on-air activity of these MSs  The cell radius is usually the receive radius (also known as footprint)  The interference radius is usually greater than receive radius (the extent to which a signal can travel)

The concept of cell (contd.)  The shape of a cell –Intuitively a circle –But there are several disadvantages of this shape in analysis –Creation of holes –Creation of overlapping region  What can be the best possible alternative of a circle without the disadvantages? –Square –Hexagon –perfect for fitting purpose –Better than squares in estimating a circle

The concept of cell (contd.)  In a hexagon cell, the location of BS can be important –Center of the cell (center excited cell) –Applicable for BS with omni-directional antenna –On the cell vertices (edge excited cell) –Applicable for BSs with sectored directional antenna –Usually 3 of the 6 vertices are used (alternative vertices) –Center excited cells are mostly considered due to their design simplicity –In reality, BSs can be located in any place depending on the physical location and convenience

The concept of frequency reuse  What is frequency reuse? –Neighboring cells use different frequencies –Frequency is reused in different cells beyond a certain distance (reuse distance)

In-Class analysis on frequency reuse

Channel assignment strategies  Channel assignments among the cells in a cluster –Fixed assignment –Dynamic assignment  Fixed assignment –A new call arrives –It will be served only if there is any unoccupied channel –Else the call is blocked  Dynamic assignment –Common pool of channels at the higher level (Mobile Switching center, MSC) –BSs request for channels from MSC when needed

Handoff strategies  What is handoff?  Processing handoffs strategies  When to handoff?  Hard handoff vs. soft handoff  Prioritizing handoff using guard channels concept

Co-channel interference & System Capacity  In-class notes and analysis

The concept of dB and dBm  In-class notes and analysis  What is dB? –Decibel (dB) – relative unit of measurement to describe power gain or less  What is dBm?  Why are the concept of dB and dBm used in wireless? –Capable of expressing both large and small power values in short convenient form

Numerical examples

Improving capacity in Cellular systems  Cell splitting

Improving capacity in Cellular systems  Cell sectoring