Cellular comn

1. WIRELESS NETWORKS….
6.2.1 Fundamentals of Cellular Comns

2. Learning Objectives
• Frequencies in use in Wireless Communication
Networks
• Wireless Communication Environment
• Evolution of Mobile Communication Networks
• Concept of a Cell
• Types of Cells
• Why Hexagonal Cells?
• Frequency Reuse Techniques.

3. Cellular System Definitions
• Mobile Station
• A station in the cellular radio service intended for use while in
motion at unspecified locations. They can be either hand-held
personal units (portables) or installed on vehicles (mobiles)
• Base station
• A fixed station in a mobile radio system used for radio
communication with the mobile stations. Base stations are located at
the center or edge of a coverage region. They consists of radio
channels and transmitter and receiver antennas mounted on top of a
tower.

4. Cellular System Definitions
• Mobile Switching Center
• Switching center which coordinates the routing of calls in a
large service area. In a cellular radio system, the MSC
connections the cellular base stations and the mobiles to the
PSTN (telephone network). It is also called Mobile Telephone
Switching Office (MTSO)
• Subscriber
• A user who pays subscription charges for using a mobile
communication system
• Transceiver
• A device capable of simultaneously transmitting and receiving
radio signals

5. Cellular System Definitions
• Control Channel
• Radio channel used for transmission of call setup, call request,
call initiation and other beacon and control purposes.
• Downlink
• Radio channel used for transmission of information from the
base station to the mobile
• Uplink
• Radio channel used for transmission of information from
mobile to base station

6. DL
UL
UL
DCC
DCC
UCC
UCC
Base Station - Mobile Network
UCC : Uplink Control Channel
DLL : Downlink Control Channel

7. Control Channels
• Control channels are usually called as setup channels.
• The Channels that are responsible for initiating mobile calls and which
are involved only in setting up a call and moving it to unused voice
channels are called as Control Channels. They are also called as setup
channels.
• The two channels responsible for initiating mobile calls UCC & DCC.
• Control Channels transmit and receive data messages that carry call
initiation and service requests

8. Basic Cellular System

9. Cellular Networks

10. Cellular
Networking technology
that breaks geographic
area into cells shaped
like honey comb
Cell
is the radio coverage
area of one base
transceiver station
1
2
3
4
5
6
7
6
7
2
1
5
GSM Concepts -
Cellular Structure

11. Basic principles of cellular systems
• Communication is always between mobile and base
station (not directly between mobiles)
• Each cellular base station is allocated a group of radio
channels within a small geographic area called a cell.
• Neighboring cells are assigned different channel
groups.
• By limiting the coverage area to within the boundary
of the cell, the channel groups may be reused to cover
different cells.
• Keep interference levels within tolerable limits.
• Frequency reuse or frequency planning

12. Rationale behind cellular
systems
• Solves the problem of spectral congestion
and increases user capacity.
• Offer very high capacity in a limited
spectrum
• Reuse of radio channel in different cells.
• Enable a fixed number of channels to serve
an arbitrarily large number of users by
reusing the channel throughout the coverage
region.

13. Consequences
• Transmit frequencies are re-used across these
cells and the system becomes interference
rather than noise limited
– the need for careful radio frequency planning
– a mechanism for handling the call as the user
crosses the cell boundary - call handoff (or
handover)
– increased network complexity to route the call and
track the users as they move around
• But one significant benefit: very much
increased traffic capacity, the ability to service
many users

14. Cells with the same letter, use
the same set of frequencies.
A cell cluster is outlined
in bold, and replicated over
the coverage area. A cluster is a
group of adjacent cells among which
the available frequencies are
distributed.
In this example, the
cluster size, N, is equal to 7;
and the frequency reuse factor is 1/7,
since each cell contains 1/7 of the
total number of available channels.
A
A
C
C
B
B
G
G
F
F D
D
G
G
E
E
D
D
B
B
B
B
F
F
F
F
E
E
E
E
C
C
A
A
A
A
G
G
C
C
D
D
Cellular Frequency Reuse Concept

15. An Example of a Cellular Cluster

16. – clusters of cells
• contain small number of cells e.g. 4, 7, 12, 21 or
more
• the smaller the number of cells per cluster, the
more channels per cell
• controlled by a single Base Station Controller

17. – types of cell
• macrocells : large higher power cells for remote and sparsely
populated areas
• microcells : small lower power cells used for densely populated areas
• selective cells : cells where the coverage does not need to be 360°
– e.g. entrances to tunnels, transmitters at junctions of adjacent cells
• umbrella cells : covers several microcells
– used for fast moving traffic crossing several adjacent microcells
– causes multiple handovers in quick succession
– e.g. a motorway going through a city
– power level of umbrella cell higher than microcells it covers
– when speed of mobile is too high, handed off to the umbrella cell
– avoids many further handovers
A
B
C

19. The Umbrella Cell Approach
The umbrella cell approach is used to provide large area coverage to high
speed users while providing small area coverage to users travelling at low
speeds. This ensures that the number of handoffs are minimized for high
speed users while at same time additional micro cell channels are
provided for pedestrian users.

20. Umbrella cell approach:
• Minimizes the number of hand offs for high speed users,
• Provides additional micro-cell users for pedestrians.
• A High speed user converting into a low speed user may be
switched to smaller micro-cell by the Base Station, without
MSC intervention.
• Speed Estimation can be performed by slope of short term
average calculations of the received power, or by some more
more sophisticated algorithms.
20

21. Coverage Area
• Pico-cell – O(10m)
– covers a room
• Micro-cell – O(100m)
– covers a floor/street
• Macro-cell – O(10 mi.)
– big towers
• Satellites
– regions/countries

22. Choices:
Factors:
Factors:
Choices of Hexagonal Cell
• No overlap or gap between
cells
A1
A1
S
S
S
S
S
S
A
A2
2 A
A3
3

23. For a given S
A3 > A1
A3 > A2
Here, A3 provides maximum coverage area
for a given value of S.
Actual cellular footprint is determined by the
radiation pattern of a given transmitting
antenna and ideal footprint is circular.
By using hexagon geometry, the fewest
number of cells covers a given geographic
region, approximating a circular footprint.

24. Coverage Patterns

25. Cellular Coverage Representation

26. Channel Capacity
Let a cellular system have total of
S duplex channels for use.
If S channels are divided among N cells
(in a cluster) as unique and disjoint channel
groups, each having the same number of
channels, total number of available radio
channels is:
S = KN
Where K is the number of channels / cell.

27. …Channel Capacity
If a cluster is replicated M times within the
system,
the total number of duplex channels, C, or
the capacity, is
C = MKN = MS.
Cluster size N = 4, 7 or 12

28. Capacity Improvement
• Initially, cellular systems are often noise
limited:
– The main deployment concern is coverage
• As traffic increases, systems become
interference limited:
– The main deployment concern is capacity

29. How to increase capacity?
• Use sectorised antenna
• Cell splitting
• Discontinuous Transmission (DTX)
• Use speech detection / silence suppression
• Power control
• Adapt transmission power to what is just needed
(given the position of the mobile)
• Dynamic Channel Allocation
• Adding new channels – often expensive or impossible

30. Sectoring
• Cell sectorisation is a technique in wireless communication where
a cell site is divided into multiple sectors, each served by its own
antennas and radio equipment, to increase network capacity and
coverage.
• By reusing the same frequency in different sectors, this method
reduces congestion, enhances signal quality, and minimizes
interference, leading to improved service quality.
• While this approach boosts capacity and coverage, it requires
careful planning to manage interference and involves higher
infrastructure costs.
• Despite these challenges, sectorisation is crucial in urban areas
for handling high traffic and in rural regions for targeted coverage,
ultimately providing a better user experience.

34. • Cell splitting in mobile communication is the process in which the
bigger cells split up into smaller ones. In mobile communication we
use cell splitting so that we can expand the capacity of the system.
Cell splitting also helps to increase the number of channels.
• Cell Splitting is the process of subdividing a cell into smaller cells
each with its own Base Station. On splitting, new cells with smaller
radius are added called microcells.
• Each new cell created is independent and has reduced antenna height
and transmitter power. The creation of new smaller cells increases the
capacity of the system as a whole. Cell Splitting increases the
frequency reuse factor. A higher frequency reuse factor increases the
capacity of the cellular system in Cell Splitting.

35. Design Tradeoff
• Smaller cell means higher capacity
• However, smaller cell also results in
higher handoff probability, which also
means higher overhead
• Moreover, cell splitting should not
introduce too much interference to users
in other cells

36. How to increase capacity?
• Use sectorised antenna
• Cell splitting
• Discontinuous Transmission (DTX)
• Use speech detection / silence suppression
• Power control
• Adapt transmission power to what is just needed
(given the position of the mobile)
• Dynamic Channel Allocation
• Adding new channels – often expensive or impossible

37. Cell Splitting. Dividing large cells into smaller cells (microcells, picocells, femtocells) to
serve more users and reduce congestion. Smaller cells can provide higher capacity and
better signal quality.
Cell Sectorisation. Dividing a cell into multiple sectors, each served by its own
antennas and radio equipment. Allows for frequency reuse within the same cell,
increasing overall capacity.
Frequency Reuse. Efficiently reusing frequencies in non-adjacent cells to maximize
spectrum utilization. Requires careful planning to minimize interference.
Carrier Aggregation. Combining multiple frequency bands to increase the available
bandwidth for users. Enhances data rates and network capacity.
Advanced Antenna Technologies. Implementing Multiple Input Multiple Output
(MIMO) systems, which use multiple antennas at both the transmitter and receiver to
improve communication performance. Beamforming technology focuses the signal in
specific directions to enhance coverage and capacity.
How to increase capacity?

38. Higher Frequency Bands. Utilizing higher frequency bands (such as
millimeter waves) which offer more bandwidth but have shorter ranges.
Often used in urban areas with high user density.
Enhanced Modulation and Coding Schemes. Using higher-order
modulation schemes (e.g., 256-QAM) to transmit more bits per symbol.
Improves spectral efficiency and increases data rates.
Network Optimization. Employing advanced algorithms and AI to optimize
network resources dynamically. Adjusting parameters like power levels,
frequency assignments, and handovers to improve capacity and
performance.
How to increase capacity?

39. Hand off
• Process of transferring a mobile telephone call from
one cell to another without dropping the call.
Cellular users may traverse several cells during a
conversation, sometimes requiring a high-speed
handoff in a moving vehicle.
• HANDOFF: The process of transferring a call across
the cell boundaries.
– Handoffs are prioritized over new calls.
– Handoffs need to be performed infrequently.

40. Handoffs – the basics

41. Inter-System Hand Off
• Definition: “-----When a mobile moves from one cellular
system to another, with different MSC”.
• The situation occurs when
“Signal in the resident cell becomes weak, and no other cell
within the system can take the call”.
• Issues:
– Local Call becomes long-distance call
– Compatibility between two MSCs.

42. Prioritizing Hand off
• Systems differ in methods and policies of hand off:
– Some systems take hand off like a new call initiation: user will be more
annoyed in case of call drop than call blocking for some time.
– Various Methods of Prioritizing Hand off have been devised and
implemented.
Guard Channel Concept
• Reserve some channels exclusively for hand off--- do not use them
for call initiation.
• Advantage is increased probability of successful hand off.
Disadvantage is lower capacity because of less number of channels
for call initiation.