Cellular network

1. Wireless Application Protocol
Cellular Network

2. Introduction
Early mobile system objective was to achieve a large
coverage using single high power antenna
Impossible to reuse the same frequencies in the same
coverage area.
For example, Bell mobile system in 1970 could support
maximum of 12 simultaneous calls over a thousand square
mile.
The Govt regulatory could not make spectrum allocation
proportion to the increasing demand
Became imperative to restructure the telephone system to
achieve high capacity with limited radio spectrum.

3. Cellular Concept
Cellular concept was a major breakthrough in solving problem of spectrum
and user capacity Offers high capacity without any major change in
technology.
Definition
A cellular mobile communications system uses a large number of low-power
wireless transmitters to create cells—the basic geographic service area of a
wireless communications system. Variable power levels allow cells to be
sized according to the subscriber density and demand within a particular
region. As mobile users travel from cell to cell, their conversations are
handed off between cells to maintain seamless service. Channels
(frequencies) used in one cell can be reused in another cell some distance
away. Cells can be added to accommodate growth, creating new cells in un
served areas or overlaying cells in existing areas.

4. Cellular System Architecture
To improve the quality of service and to support more users
in their systems. Because the amount of frequency spectrum
available for mobile cellular use was limited, efficient use of
the required frequencies was needed for mobile cellular
coverage.
Rural and urban regions are divided into areas according to
specific provisioning guidelines. Deployment parameters,
such as amount of cell-splitting and cell sizes, are
determined by engineers experienced in cellular system
architecture.

5. Cellular System Architecture
Cells:- A cell is the basic geographic unit of a cellular system.
Shape of the areas into which a coverage region is divided.
Cells are base stations transmitting over small geographic areas that are
represented as hexagons.
Each cell size varies depending on the landscape. Because of constraints
imposed by natural terrain and man-made structures
Clusters
A cluster is a group of cells No channels are reused within a cluster.

6. Cellular System Architecture
Example of Cluster

10. Frequency Reuse
Small number of radio channel frequencies were available for mobile systems,
The solution the industry adopted was called frequency planning or frequency
reuse. Frequency reuse was implemented by restructuring the mobile telephone
system architecture into the cellular concept.
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

14. Frequency Reuse

18. Frequency
Reuse
Patterns

19. The hexagonal shape representing a cell is
conceptual and simplistic model of coverage.
The actual radio coverage is known as the footprint
and is determined from field measurement,
propagation prediction models
–However a regular shape is needed for systematic system
design and adaptation to future growth.
It might be natural to choose a circle to represent
coverage but adjacent circles cannot be overlaid upon
a map without leaving gaps or creating overlapping.

20. Gaps
Overlapping
Case A
Case B

21. Three possible choices of shapes: square, equilateral
triangle and hexagon.
For a give distance between the center of a polygon
and its farthest perimeter points, the hexagon has the
largest area of the three
Thus by using hexagon geometry, the fewest number
of cells can cover a geographic region and it closely
approximates circle.

30. Capacity of System
When using hexagon to model coverage areas
–Center-excited Cell: BS depicted as being either in the center
of the cell
Omni-directional antenna is used
–Edge-excited Cell: on three of the six cell vertices
Sectored direction antenna is used
Consider a cellular system
–which has S duplex channels available for reuse.
–Each cell allocated group of k channels (k < S)
–S channels divided among N cells (unique and disjoint) then
S = kN

31. Cluster: N cells, which collectively use the complete set
of available frequencies
If a cluster is replicated M times in the system, the
number of duplex channels C as a measure of capacity
is
C = MkN = MS
So capacity is directly proportional to the replication
factor in a fixed area.
Factor N is called cluster size and is typically equal to
4, 7, 12.

32. If cluster size N is reduced while cell size is kept
constant
–more clusters are required
–More capacity is achieved
Large cluster size indicates that co-channel cells are
far from each other
Conversely, small cluster size means co-channel cells
are located much closer together
The value of N is a function of how much interference a
mobile or BS can tolerate

33. Clusters are inversely proportion to N
–Capacity is directly proportional to Clusters
–Thus frequency reuse factor is given by 1/N.
In last fig, each hexagon has exactly six
equidistant neighbors and that the lines joining
the centers of any cell and its neighbors are
separated by multiple of 60 degrees.
–There are only certain cluster sizes and layouts
possible

34. Locating co-channel neighbors
To connect hexagons without gaps,
–The geometry of hexagon is such that the number of cells per
cluster N can only have values
N = i
2
+ ij + j
2

where i and j are non-negative integers.
To find out the nearest co-channel neighbors of a
particular cell, do the following
–Move I cells along any chain of hexagon
–Then turn 60 degree counter clockwise and move j cells

35. Example: Locating co-channel cells
In this example N= 1 9, i= 3, j= 2

36. Example
BW = 33 MHz allocated to particular FDD cellular
system, where two 25 KHz simplex channel to
provide full-duplex for voice/data.
Compute the number of channels per cell if a system
uses
–Four-cell reuse
–Seven-cell reuse
–Twelve-cell reuse.
If 1 MHz is dedicated to control channels, determine
equitable distribution of control and voice channels
per cell for above three systems?

37. Solution: Part I
TotalBW = 33 MHz,
ChannelBW = 25 KHz x 2 = 50 KHz/duplex channel
S = 33,000 / 50 = 660 channels
For N = 4
k = 660 / 4 ≈ 165 channels
For N = 7
k = 660 / 7 ≈ 95 channels
For N = 12
k = 660 / 12 ≈ 55 channels

38. Solution: Part II
Sc = 1000 / 50 = 20 channels
Sv = S – Sc = 660 – 20 = 640 channels
For N=4,
5 control channels + 160 voice channel.
For N=7,
4 cells with 3 control + 92 voice channels
2 cells with 3 control + 90 voice channels
1 cell with 2 control + 92 voice channels
In practice, 1 control/cell and 4x91 + 3x92 voice channels
For N = 12,
8 cells with 2 control + 53 voice channels
4 cells with 1 control + 54 voice channels
In practice, 1 control and 8x53 + 4x54 voice channels

39. Operation of Cellular Systems
Base station (BS) at center of each cell
–Antenna, controller, transceivers
Controller handles call process
–Number of mobile units may in use at a time
BS connected to mobile telecommunications switching office (MTSO)
–One MTSO serves multiple BS
–MTSO to BS link by wire or wireless
MTSO:
–Connects calls between mobile units and from mobile to fixed
telecommunications network
–Assigns voice channel
–Performs handoffs
–Monitors calls (billing)
Fully automated

40. Overview of Cellular System

41. Channels
Control channels
–Setting up and maintaining calls
–Establish relationship between mobile unit and
nearest BS
Traffic channels
–Carry voice and data

42. Typical Call in
Single MTSO Area (1)
Mobile unit initialization
–Scan and select strongest set up control channel
–Automatically selected BS antenna of cell
Usually but not always nearest (propagation anomalies)
–Handshake to identify user and register location
–Scan repeated to allow for movement
Change of cell
–Mobile unit monitors for pages (see below)
Mobile originated call
–Check set up channel is free
Monitor forward channel (from BS) and wait for idle
–Send number on pre-selected channel
Paging
–MTSO attempts to connect to mobile unit
–Paging message sent to BSs depending on called mobile number
–Paging signal transmitted on set up channel

43. Typical Call in
Single MTSO Area (2)
Call accepted
–Mobile unit recognizes number on set up channel
–Responds to BS which sends response to MTSO
–MTSO sets up circuit between calling and called BSs
–MTSO selects available traffic channel within cells and notifies BSs
–BSs notify mobile unit of channel
Ongoing call
–Voice/data exchanged through respective BSs and MTSO
Handoff
–Mobile unit moves out of range of cell into range of another cell
–Traffic channel changes to one assigned to new BS
Without interruption of service to user

44. Other Functions
Call blocking
–During mobile-initiated call stage, if all traffic channels busy, mobile
tries again
–After number of fails, busy tone returned
Call termination
–User hangs up
–MTSO informed
–Traffic channels at two BSs released
Call drop
–BS cannot maintain required signal strength
–Traffic channel dropped and MTSO informed
Calls to/from fixed and remote mobile subscriber
–MTSO connects to PSTN
–MTSO can connect mobile user and fixed subscriber via PSTN
–MTSO can connect to remote MTSO via PSTN or via dedicated lines
–Can connect mobile user in its area and remote mobile user

45. Call Stages