Wireless Communication

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 Cellular Concept
In earlier stage only few people used mobile radio systems for
particular area.
Earlier only 300 to 400 people used the mobile
communication.
A single high powered transmitter antenna
mounted on a tall tower is used to achieve a large coverage area
Day today life, the demand of mobile service is increased.
Increasing in demand for mobile service result in need for high
capacity with limited radio spectrum, while at the same time covering
very large area.

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 Cellular Concept
 To over come this problem, we are introducing a concept
called Cellular concept.
 The cellular concept offer very high
capacity in a limited spectral allocation
without any major technological change.
 The cellular concept is a system-level
idea which the high power transmitter (large cell) is
replaced with many low power transmitters (small cells).

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• In communication systems, a channel is a medium
through which information is transmitted from a
sender to a receiver.
• Channel is a duplex one
• N number of channel together are called frequency
band.
Guard Band

• 2G, 3G Spectrum
• Spectrum is limited because only range of frequency is
allotted for everything.

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 Cellular Concept

High power Antenna Low power Antenna

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• Cell: Each base station is allotted a group of radio
channel to be used within a small area is called cell.
• Footprint: The actual coverage area of a cell is known
as footprint.
• It is determined from the field measurement or
propagation prediction models.

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 Cellular Concept

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 Cell Shapes
 We can’t split the cell with any shape, a particular pattern
should be used to split the area.
 The cell shape should be uniform.
 Parameters to choose the geometric shape of the cell
 Should cover the entire region
 There should not be overlap
 All area should be in equal size
 The cell must be designed to serve the weakest
mobile located within the edge of the cell

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 Cell shapes

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 Cell shapes
• The circle will cover the all area.
• Why we are not using circle here, because
it will produce gap or overlap.

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 Cell shapes
• The experts suggested to choose Hexagon shape for
splitting the cell
Why we choose Hexagon?
• It has large coverage area
• It closely resembles the circular pattern which occur for
an omni directional transmission.
• Few cells can cover a large geometrical area.
How many antenna’s required to cover a cell?
• Omni directional Antenna = 1
• Sector / directional Antenna – Minimum 3, Maximum 6
corners of the cell.

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 Drawbacks of cellular concept

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 Drawbacks of cellular concept

 Each base station is allocated a group of radio channels

to be used within a cell
 Base stations in adjacent cells are assigned different
channels
 In case if the spectrum is unlimited, then we may be
provided with infinite channels
 But in a reality spectrum is limited

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 Frequency Reuse

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 Frequency Reuse
• By using cellular concept we cant allocate

the channel, because the spectrum is limited.

• To overcome this we are going for a

technique called Frequency reuse technique

• The same group of channels can be used to cover
different cells that are separated from one another by
distance large enough to keep interference level within
tolerable limits.

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 Frequency Reuse

 This process of selecting and allocating channel group

for all the base stations within the system is called
frequency reuse

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 Frequency Reuse
• Consider a cellular system which has a total duplex
channels S available for use.
• It is assumed there are N number of cells
(i.e) N = 3
• Each cell is allocated a group of K channel
(K = 1000)
• The total number of available radio channel can be
expressed as S = NK
S = 3 x 1000
S = 3000
• If the cluster is replicated M times within the system, the
total number of duplex channel is used to measure the
capacity C
05/09/2025 C = MS 18
 Frequency Reuse

C = MKN
M = 3, C = 3 x 3000
C = 9000
• The capacity of the cellular system is directly
proportional to the number of times a cluster is replicated
in a fixed area
C∝M
• If cluster size increase then the capacity C increases
• The number of times cluster gets repeated in a system
depends on the number of cells in one cluster.

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• The cluster size should be carefully chosen so that
interference is within the tolerable limit and as the same
time the capacity also get increases.
Standard cluster size

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 Frequency Reuse
 To find the nearest co channel neighbor of any particular cell
 Move i cells along any chain of the hexagon
 Turn 60 degree counter clockwise or anti counter
clockwise and move j cells

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 Advantages of Frequency Reuse
Technique
 Large coverage area
 Efficient spectrum utilization
 Enhanced system capacity
Improved Quality of Service (QoS)
Reduced Cost
Reduced Interference
Improved Spectral Efficiency

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 Channel Assignment

Strategies

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 Channel Assignment Strategies
• In cellular concept, channel assignment strategy are used
for efficient utilisation of available radio spectrum
Advantages:
All the users must be served completely
To improve the capability of the cell
To minimise the interference
To meet the traffic demand effectively
Types of Channel Assignment

Fixed Channel Dynamic Channel

Assignment Assignment

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 Fixed Channel Assignment

• Each cell is allocated a predetermined set of voice

channels.
• If all the channel in the cells are occupied then the new
call will be blocked.
Borrowing strategies:
• In this, a cell is allowed to borrow channels from
neighbouring cell if all of its own channel or already
occupied.
• The borrowing procedures are monitored by mobile
switching centre (MSC) which ensures that the borrowing
of the channel does not interfere with any of the cells in
progress.
 The fixed channel Assignment is not flexible with users. 25
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 Dynamic Channel Assignment
 In this approach channels are not allowed to different cells
permanently.
 when a call request is made the Base Station request a
channel from Mobile Switching Centre and the MSC will
allocate at channel to the requested cell.
 All available channels are accessible to all of the cells
Advantage:
 Increased channel utilization and reduce the probability of
blocked calls.
 This approach the reduce the likelihood of blocking, increase
the capacity of the system.
Disadvantage:
 MSC storage and load is high.
 The work of MSC is traffic distribution and signal strength
indication.
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HANDOFF TECHNIQUES

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 Handoff Techniques
 Handoff is the process of transferring a mobile call from
one cell to another cell without dropping call when a
mobile moves into a different cell
 Handoff is a technique that allows the call to continue
uninterrupted service when the user move from one cell
to another
 MSC – Mobile Switching Center

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 Handoff Techniques

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 Handoff Techniques
∆ = Pr.h – Pr.m
 Pr.h - received handoff signal power
 Pr.m - Minimum usuable handoff signal power
 Threshold should not be to large or too small
 Handoff must be performed successfully
 Dropped call due to excess delay of MSC due to traffic.
Dwell Time:
Minimum acceptable time to transfer call from one
Base station to another Base station is called Dwell Time.

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 Inter-system handoff
 Occurs when a mobile unit moves from one cellular system
to another
 Used to prevent a call from dropping
 Involves multiple MTSOs (Mobile Telephone Switching
Offices)
 May result in a local call becoming a long-distance call

Inter System Handoff

Intra System Handoff

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 Intra-system handoff
 Occurs when a user's call is transferred to a new radio
channel within the same cell
 Can be used to balance load
 When a call moves from one cell to another within the
same base station controller (BSC)

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 MAHO – Mobile Assisted Handoff

MAHO:
1st generation – MSC & BS did the handoff decision
• Receiver in MSC is used to scan and determine
the signal strength and give information to MSC.
From this information MSC decide the Handoff.
2nd generation – Mobile Assisted Handoff (MAHO)
• Every mobile station measure the received
power from base station and report it to the
serving base station.
• The mobile phone decide everything.
• Practical handoff will be faster.
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 Cell Dragging
 Mobile network terminology refers to a situation where a
mobile phone call remains connected to a cell tower even
when it is moving out of that cell's coverage area.
 Essentially "dragging" the call beyond its intended range,
often happening due to a handoff mechanism where the
call isn't transferred to a neighbouring cell tower.
 Resulting in poor call quality and potential network
congestion.
Factors contributing to cell dragging:
• Poor cell site placement
• Incorrect handoff threshold settings
• Interference from neighbouring cells
 To solve this problem, the handoff threshold and radio
coverage parameter must be adjusted carefully.
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 Umbrella Cell Pattern
• 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.
• Macro cells: Large cells used in remote areas with a
high reach
• Micro cells: Small cells used in densely populated areas
with a low reach

Macro cells

Micro cells

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 Umbrella Cell Pattern
Benefits:
• Coverage: Umbrella cells provide coverage for large
areas
• Interference reduction: Umbrella cells reduce
interference
• Handoff: Umbrella cells minimize handoffs for high-
speed users

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 Prioritizing Handoff
 The method of giving priority to the handoff is called the
guard channel concept.
Guard channels
 Reserve a portion of the available channels for
handoffs. This method can improve the probability of
successful handoffs.
Queuing
 Put handoff requests in a queue and serve them in the
order they arrive. This method can reduce the number of
failed handoffs.
Improved Call Admission Control (CAC) mechanism
 Prioritizes handoff calls and can reduce the probability of
dropping a call.
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 Benefits of Prioritizing Handoff
• Reduces the rate of handoff failures,
• Improves network performance,
• Provides higher bandwidth for end users,
• Reduces new call blocking, and
• Decreases the handoff blocking probability.

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 Types of Handoff
Hard Handoff:
 When there is an actual break in the connectivity while
switching from one Base Station to another Base
Station.
 The connection quality is not that good.
Soft Handoff:
 Soft Handoff is a mechanism in which the device gets
connected with two or more base stations at the same
time.
 Soft Handoff adopted the ‘make before break’ policy.

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 Types of Handoff
Delayed Handoff:
 Delayed handoff occurs when no base station is
available for accepting the transfer.
 The call continues until the signal strength reaches a
threshold, and after that, the call is dropped.
 Generally, it happens when the user is out of the
network coverage area, or at some dead spots where
network reach is very low.
Mobile-Assisted Handoff:
 It generally used when a mobile phone helps a base
station to transfer the call to another base station with
better-improvised connectivity and more signal
strength.
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 Interference and System
Capacity

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 Source of Interference
• Interference due to signal generated from
other mobile
• A call that is in progress in neighbouring cell.
• Some other based station operating in the
same frequency range
• Cross talk

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 Effect of interference
 Interference in cellular zone will definitely
degrade the signal quality.
 In some extreme cases interference main lead
to blocked call status.
 Interference is the bottle neck that limits the
capacity of cellular system.

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 Type of Interference
Type of Interference

Co-Channel Interference Adjacent Channel Interference

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 Co-Channel Interference (CCI)
• It is a type of interference that occurs when
two or more wireless communication systems,
operating on the same frequency channel,
transmit signals that overlap and interfere
with each other.
• The CCI can be reduced by increasing S/N
(signal to noise) ratio.
• It can also be reduced by increasing the
distance between two cells.

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 Co-Channel Interference (CCI)
• Co-Channel Reuse Ratio:

• N = Cluster size
• D = Distance between the cell
• R = Radius of the cell

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 Co-Channel Interference (CCI)

• = Number of interfering co-channel cell

• n = Path loss Exponent

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 Co-Channel Interference (CCI)
• Y is a mobile at the cell boundary

• If frequency reuse scheme is carefully planned and

implemented in the design of cellular communication
the overall capacity of the system can be enhanced.

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 Adjacent channel Interference (ACI)
• Adjacent channel interference is a kind of interference arising
due to signals that are adjacent in frequency.
• This interference is due to Imperfect receiver side filters
which permits nearby signal frequency to mix with the actual
passband.

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 Near-Far Effect
• The near-far effect occurs when two radios with equal power
transmit signals at the same time, but from different distances
from a service antenna.
• The signal from the radio closer to the antenna will be stronger
than the signal from the radio further away.
• This can make it difficult to detect or filter out the weaker
signal.

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 Factor to reduce ACI
 Carefully designing receiver filters with good
out-of-band rejection.
 Proper channel allocation to avoid assigning
adjacent channels to nearby cells,
 Using modulation schemes with low out-of-
band radiation, and
 Implementing advanced signal processing
techniques like orthogonal frequency division
multiplexing (OFDM)
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Channel Planning for Wireless Systems
• Channel planning for wireless systems involves allocating radio
frequency (RF) channels to optimize network performance,
minimize interference, and ensure reliable communication.
Objectives:
1. Maximize capacity: Allocate channels to support the required
number of users and data throughput.
2. Minimize interference: Reduce co-channel interference (CCI)
and adjacent channel interference (ACI) to maintain signal
quality.
3. Ensure coverage: Provide adequate coverage and signal
strength throughout the network.

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 Steps involved in Channel Planning
1. Network planning: Define network architecture, cell
layout, and frequency reuse patterns.
2. Frequency allocation: Assign frequencies to cells,
sectors, or channels based on network requirements.
3. Channel assignment: Allocate specific channels to each
cell, sector, or user equipment (UE).
4. Power control: Adjust transmit power levels to
minimize interference and optimize coverage.
5. Monitoring and optimization: Continuously monitor
network performance and adjust channel planning as
needed.
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 Techniques used
in Channel Planning
1. Frequency reuse: Reuse frequencies in non-
adjacent cells to increase capacity.
2. Channel borrowing: Borrow channels from
adjacent cells to handle traffic fluctuations.
3. Dynamic channel allocation: Allocate channels
dynamically based on traffic demand.
4. Interference coordination: Coordinate
transmission powers and channels between
cells to minimize interference.
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 Power Control for Reducing Interference

• Power control at base stations: Base stations

adjust the power of downlink control channels.
• Power control at mobile stations: Mobile
stations adjust the power of uplink control
channels.
• Power control algorithms: Algorithms like
iterative power control adjust power based on
signal-to-interference ratio (SIR) requirements.

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 Benefits
• Reduced interference: Power control reduces
interference between users and networks.
• Extended battery life: Reduced power
consumption means longer battery life for
devices.
• Improved quality of service: Power control helps
maintain an acceptable quality of service (QoS).
• Increased cell capacity: Power control can help
maximize cell capacity.
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 Trunking
and
Grade of Service

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 Trunking
• Trunking allows users to share a limited number of
channels in a cell.
• Channels are allocated to users on demand and
returned when the call ends.
• There are two types of trunking systems: blocked call
cleared (BCC) and blocked call delayed (BCD).
• In BCC, users who are denied service terminate their
request and return to inactive status.
• In BCD, a queue buffers denied calls, and service is
restored when busy channels are freed up.

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 Trunking
• The amount of traffic intensity that is carried
by one channel is expressed as “one Erlang”
• For example, if a mobile radio channel was
occupied for 15 minutes of an hour, the it is
said as
Traffic Intensity, Tu = 15/60 = 0.25 Erlang

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 Grade of service (GoS)
• GOS is the probability of a call being blocked or delayed for
more than a specified interval.
• GOS is expressed as a Decibel (db)
• GOS is calculated during the busy hour, when traffic intensity
is the highest.
• GOS is used to determine the minimum number of channels
needed.
Traffic Intensity, Au = ʎ. H
Where, ʎ - Average number of call request or Incoming call
h – Average call duration

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 Grade of service (GoS)
 In case if the traffic is distributed equally among all the channels
then the traffic intensity for a single channel A c is
Ac = NAu / C
Where,
N = Number of users in a truncked system within unspecified
number of channels in it.
 There are two category of trunking system is available
1. Trunking system with no queuing for the call request.
2. Trunking system with queue provision for holding calls
that are blocked.
 The two broad classes of the trunk radio system is also called as
Blocked call clear (BCC) and Blocked call delayed (BCD) system.
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 Grade of service (GoS)
1. GoS: The grade of service is a measure of congestion
which is a probability of a call which is being blocked.
2. Request rate: It represents the average number of the
call requests made in a unit time and it is denoted by
λ/sec.
3. Load: The available traffic density across the complete
trunked radio system.
4. Setup time: It represents the time needed to allocate a
trunked channel for a user who has made a request.
5. Holding time: It represents the average time duration
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 Blocked Call Clear (BCC)
• Blocked call cleared refers to a situation where a call
attempt is initially denied access to the network due to all
available channels being occupied.
• But the caller is then able to try again later when a channel
becomes available.
• Essentially meaning the call is "blocked" until a channel is
cleared and the call can proceed.
• This is often used in cellular networks and is calculated
using the Erlang B formula.

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 Blocked Call Delayed (BCD)
 Blocked call delayed refers to a situation where a call
attempt is initially unable to connect due to all available
channels being occupied.
 Instead of completely reject the call, it is placed in a
queue and waits for a free channel to become available.
What is Erlang C?
 Erlang C is a traffic modelling formula, primarily used to
calculate delays and to predict waiting times for callers.
 This mathematical equation enables to predict their load
and calculate the number of service/support agents
required to service the desired number of calls to achieve a
targeted service level.
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 Improving Coverage and
Capacity in Cellular
System

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 Improving Coverage and Capacity in
Cellular System
• To increase the coverage area in a cellular
system it is very important to assign more
number of radio channels to a cell so as to
meet the mobile traffic.
• For enhancing the cellular coverage capacity
there are many techniques available and some
important cellular techniques are
i.Cell Splitting
ii.Cell Sectoring
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 Cell splitting
 Cell splitting involves dividing a congested
cell into smaller cells, each with its own
base station and frequency allocation.
This process allows more users to be
accommodated within the same geographic
area.

Benefits of Cell Splitting

1. Increased Capacity: Cell splitting enables more users to be supported
within the same area, increasing the overall capacity of the network.
2. Improved Coverage: Smaller cells provide better coverage and reduced
interference, resulting in improved signal quality and reliability.
3. Enhanced Quality of Service (QoS): Cell splitting enables better QoS, as
more resources can be allocated to each user.

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 Cell sectoring
 Cell sectoring involves dividing a
cell into multiple sectors, each
with its own antenna and frequency
allocation. This allows multiple
users to share the same frequency
band while reducing interference.

Benefits of Cell Sectoring

1. Increased Capacity: Cell sectoring enables more users to be supported
within the same cell, increasing the overall capacity of the network.
2. Improved Coverage: Sectoring provides better coverage and reduced
interference, resulting in improved signal quality and reliability.
3. Enhanced Quality of Service (QoS): Cell sectoring enables better QoS, as
more resources can be allocated to each user.

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