Wireless

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Wireless Mobile Networking Cellular Concept

• Azizol Bin Abdullah
• azizol_at_fsktm.upm.edu.my (A2.04)

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Chapter 2 Cellular Concept

1. Cell
2. Cell Structure
3. Cell Splitting VS Cell Sectoring
4. Frequency Reuse
5. Cochannel Interference
6. Handoff Region
7. Some Computations

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Terminology
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Terminology contd
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Cellular Concept Basics

• - Early mobile telephony systems were not
  cellular. Coverage over a large area was provided
  by a high powered transmitter mounted on a tall
  tower. Frequency reuse was not employed. That
  resulted in very low capacity.
• - The cellular concept arose in the 1970s from
  the need to restructure the radio telephone
  system with the increase in demand. The increase
  in demand could not be satisfied just by
  additional spectrum allocations.

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Infrastructure Cellular System Back then

• Early cellular system had a high-power
  transmitter to cover whole service area.

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Infrastructure Cellular System These Days

• Then, cellular system replaced a large zone with
  a number of smaller cells, with as single BS
  covering a fraction of the area.

Neighboring base stations (BS) are assigned
different sets of channels. Capacity can be
increased by additional partitions.
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Cellular Concept
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Cellular Concepts Basics
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• This is a modern tower with three different
  cell-phone providers riding on the same
  structure. If you look at the base of the tower,
  you can see that each provider has its own
  equipment, and you can also see how little
  equipment is involved today (older towers often
  have small buildings at the base)

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Cellular System Definition
A cellular mobile communication system uses a
large number of low-power wireless transmitters
to create cells- the basic geographic service
area of a wireless communications system.
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Cellular System Cells

• In each cell area, users are served by a single
  BS(Base Station).
• Mobile devices in this cell area are known as
  MSs(Mobile Stations).
• Ideally, cell should be in circular shape.
• For convenience, the cells are shown with a hex
  pattern. A hex pattern is the simplest pattern
  that can translate into an area.
• In practice, cells are not hexagonal and BS are
  not exactly in the center of the cell.

R
Circular
Hexagon
BS
R
BS
R Radius
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Cellular System Cells (2)

• The most important factor in cellular system is
  the size and the shape of cell.
• Factors that cause reflections and refractions
  of the signal
• Elevation of the terrain
• Presence of a hill or tall building
• Presence of particles in the air.

R
Circular
Hexagon
BS
R
BS
R Radius
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Cellular System Clusters
A cluster is a group of cells. No channels are
reused within a cluster. Figure 1 illustrates a
seven-cell cluster.
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Cell Splitting vs Cell Sectoring

• Using Cell-Splitting and Sectoring to Improve
  Capacity/Coverage of Cellular Systems
• Cell splitting is achieved by installing smaller
  cells (microcells) in saturated macrocellular
  regions. More channels become available to users
  that appear in the saturated region (based on
  demand and economic consideration)

As a service area becomes full of users, this
approach is used to split a single area into
smaller ones. In this way, urban centers can be
split into as many areas as necessary to
provide acceptable/sound service levels, while
larger, less expensive cells can be used to
cover remote rural regions
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Cell Splitting

• subdivide a congested cell into smaller cells
• each with its own base station, reduction in
• antenna and transmitter power
• more cells -gt more clusters-gt higher capacity
• achieves capacity improvement by essentially
  rescaling the system.

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Cell splitting from radius R to R/2 and R/4
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Cell Sectoring
-Sectoring improves capacity by using sectorized
antennas (120 degrees, 60 degrees) that reduce
the co-channel interference. Reduction in
co-channel interference means that the cluster
size can be reduced which in turn leads to more
channels per cell.
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Cell Sectoring (contd)

• In basic form, antennas are omnidirectional
• Replacing a single omni-directional antenna at
  base station with several directional antennas,
  each radiating within a specified sector.

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Cell Sectoring (contd)

• achieves capacity improvement by essentially
  rescaling the system.
• less co-channel interference, number of cells in
  a cluster can be reduced
• Larger frequency reuse factor, larger capacity

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Micro Cell Zone Concept

• Large control base station is replaced by several
  lower powered transmitters on the edge of the
  cell.
• The mobile retains the same channel and the base
  station simply switches the channel to a
  different zone site and the mobile moves from
  zone to zone.
• Since a given channel is active only in a
  particular zone in which mobile is traveling,
  base station radiation is localized and
  interference is reduced.

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Increasing Capacity in Cellular Systems

• As demand for wireless services increases, the
  number of channels assigned to a cell is not
  enough to support the required number of users.
• Solution ???
• increase channels per unit coverage area.

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Approaches to Increasing Capacity

• Frequency borrowing frequencies are taken from
  adjacent cells by congested cells
• Cell splitting cells in areas of high usage can
  be split into smaller cells
• Cell sectoring cells are divided into a number
  of wedge-shaped sectors, each with their own set
  of channels
• Microcells antennas move to buildings, hills,
  and lamp posts

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Cellular Concept Cell Co-channel Interference
-Cell A and B of a conventional, analog system
are using the same frequency. The area of
overlap, area C, has a frequency conflict and
interference. -This is similar to what you
experience when you are driving between the
broadcast zones of two radio stations
transmitting at the same frequency.
co-channel interference Interference resulting
from two or more simultaneous transmissions on
the same channel. Cell Sectoring is said able to
reduce this type of interference.
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Cellular Concept Frequency Reuse
-Because only a small number of radio channel
frequencies were available for mobile systems,
engineers had to find a way to reuse radio
channels to carry more than one conversation at a
time. The solution the industry adopted was
called frequency planning or frequency reuse.
-The concept of frequency reuse is based on
assigning to each cell a group of radio channels
used within a small geographic area. Cells are
assigned a group of channels that is completely
different from neighboring cells. The coverage
area of cells is called the footprint. This
footprint is limited by a boundary so that the
same group of channels can be used in different
cells that are far enough away from each other so
that their frequencies do not interfere
-Cells with the same number have the same set of
frequencies. Here, because the number of
available frequencies is 7, the frequency reuse
factor is 1/7. That is, each cell is using 1/7 of
available cellular channels.
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Cellular Concept Handoff

• Handoff is the process of transferring a call
  from one base station to another when a user's
  radio signal becomes weaker at the first and
  stronger at the second base station. "Weaker" and
  "stronger" is quantified by a signal threshold
  level, which is above the minimum signal level
  for acceptable voice communications. Selecting
  this threshold level is critical to 1) ensure
  unnecessary handoffs do not occur and 2) call
  dropping does not occur.

- In early cellular, signal strength measurements
were made by base stations and relayed to MSC.
The MSC decided on handoff and performed the
necessary communication between base stations to
allow for a successful handoff. In modern
cellular, handoffs are mobile assisted, called
mobile-assisted handoffs (MAHO). MAHO 1) leads to
faster handoff and 2) allows handoff decisions
to be based on metrics other than signal
strength, e.g., SIR.
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• Capacity Computations
• Assume there are N cells, each allocated k
  different frequency channels. These N cells are
  said to form a cluster. Total number of channels
  per cluster is given by
• S kN
• Total capacity associated with M clusters
• C MkN MS
• A cluster may be replicated more times in a given
  area if the cells are made smaller (note that
  power needs to be reduced accordingly).

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• Capacity Computations
• Total number of users
• where
• W total bandwidth
• N frequency reuse factor
• B channel bandwidth
• m number of cells required to cover an area

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E1
A total of 33 MHz are allocated to a system
which uses 2x25 kHz for full duplex (i.e., each
channel is 50 kHz). What is the number of
channels per cell? Number of channels per
system Since 4 and 7 are popular number
of cells per cluster/system a. For reuse
N 4 b. For reuse N 7
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E
A total of 33 MHz are allocated to a system
which uses 2x25 kHz for full duplex (i.e., each
channel is 50 kHz). What is the number of
channels per cell? Number of channels per
system a. For reuse N 4
For reuse N 7
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E2
Now assume 1 MHz of the 33 MHz is allocated
to control channels. Each control channel is
still 50 kHz Total number of voice
(traffic) channels is now ? excluding
control traffic Again, we take N4 and
N7 For N 4 gt For N 7 gt
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E2
Now assume 1 MHz of the 33 MHz is allocated
to control channels. Each control channel is
still 50 kHz Total number of voice
(traffic) channels is now For N 4 gt
640/4 160 voice channels control
channels. For N 7 gt 640/7 91
channels control.