Master of Computer Science & IT (MCIT) Fall Semester 2013 Elec1ve Course (CT531) Wireless & Mobile Communica:on Lecture 3 Dr. Muhammad Mubashir Khan [email protected]Department of Computer Science & IT, NED University of Engineering & Technology, Karachi September 2013
Department of Computer Science & IT, NED University of
Engineering & Technology, Karachi
September 2013
Cellular Network Mobile radio telephone system was based on:
High power transmiSer/receivers Could support about 25 channels in
a radius of 80 Km
To increase network capacity: Mul1ple low-power transmiSers
(100W or less) Small transmission radius -> area split in cells
Each cell with its own frequencies and base sta1on Adjacent cells
use different frequencies The same frequency can be reused at
sufficient distance Implica1ons on mobile handsets
Cell Design Op1ons Simplest layout Adjacent antennas not
equidistant how do you handle users at the edge of the cell?
Ideal layout But we know signals travel whatever way they fell
like
The Hexagon PaSern A hexagon paSern can provide equidistant
access to
neighboring cell towers
d = R
In prac1ce, varia1ons from ideal due to
topological reasons Signal propaga1on Tower placement
3
Cell PaSern Constraints Each cell features one base transceiver
Through power control cover the cell area while limi1ng the power
leaking to other co-frequency Cells
Frequency reuse not possible for adjacent towers! The number of
frequency bands assigned to a cell dependent on its traffic
Cellular Network (Frequency Reuse)
4 3
5 1
2
7 4
2 6 3 5
3
1 2
6 7
4 3 4
3
1 2
D
Handover: Automa1c channel change when moving from one cell to
another during ongoing conversa1on.
D minimum distance between centers of co-channel cells R radius
of cell d distance between centers of adjacent cells N number of
cells in a repe11ous paSern, i.e. reuse factor
Following rela1onship holds
Capacity of a Cellular Network S Total # of duplex channels
available for use k Total # of duplex channels per cell N Cluster
of cells which collec1vely use the complete set of available
frequencies
If a cluster is replicated M 1mes within the system, the total #
of duplex channels C can be used as a measure of capacity
