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Lecture 2 Lecture 2 The Cellular ConceptThe Cellular Concept
Prof. Shamik Sengupta
Office 4210 N
ssengupta@jjay.cuny.edu
http://jjcweb.jjay.cuny.edu/ssengupta/
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 onIn-Class analysis onfrequency reusefrequency 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 examplesNumerical examples
Improving capacity in Cellular systems
Cell splitting
Improving capacity in Cellular systems
Cell sectoring