EC8652 WIRELESS COMMUNICATION UNIT II -CELLULAR ARCHITECTURE

UNIT – 1
The Cellular Concept-System Design Fundamentals
Introduction, Frequency Reuse, Channel Assignment Strategies, Handoff Strategies- Prioritizing
Handoffs, Practical Handoff Considerations, Interference and system capacity — Co channel
Interference and system capacity, Channel planning for Wireless Systems, Adjacent Channel
interference , Power Control for Reducing interference, Trunking and Grade of Service, Improving
Coverage & Capacity in Cellular Systems- Cell Splitting, Sectoring.
INTRODUCTION TO WIRELESS COMMUNICATION
Wireless:
✓ Wireless is used to describe types of devices and technologies that use space as a signal propagating
medium, and are not connected by a wire or cable.
✓ Wireless communication may be defined as the transmission of user information without the use of wires.
✓ The user information could be in the form human voice, digital data, e-mail messages, video and other
services.
✓ The wireless communications covering radio, television,, radar, satellite, wireless and mobile, cellular and
other wireless networks.
Advantages of wireless communications:
1. Mobility: The users have freedom to move about while remaining connected, as compared with the
network with its coverage area.
2. Increased reliability: Use of wireless technology eliminate cable failures, so overall reliability
3. Ease of installation: Wireless communications and networks make it easier for any office to be modified
with new cubicles or furniture, without worrying about providing network connectivity through cables.
4. Rapid disaster recovery: Accidents may happen due to fire, etc., the organization hot prepared to recover
such natural disasters. So, disaster recovery plan is must for business.
5. Low cost: Eliminating the need to install cabling and using wireless communications results in significant
cost savings.
Disadvantages of wireless communications:
1. Radio signal interference: Signals from other wireless devices can disturb its radio transmission.
2. Security: Data transmitted between the wireless device and the access point can also be encrypted in such
a way that only the intended recipient can decode the message.
3. Health hazards: High powered levels of RF energy can produce biological damage.
4. Radio transmitters in wireless communications devices emit radio frequency (RF) energy cause adverse
health effects.
Wireless network generations:
The cellular systems have been classified into three generations
i) First generation analog cellular systems
ii) Second generation digital cellular systems
iii) Third generation digital cellular systems
1G analog cellular systems:
✓ Amps (Advance Mobile Phone System) and ETACS (Enhanced Total Access Communication
System)
2G digital cellular systems:

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✓ IS-136, GSM (Global System for Mobile or Group Special for Mobile) and PDC (Personal Digital
Cellular)
3G digital cellular systems:
✓ It aims to combine telephony, internet and multimedia into a single device.
Applications of wireless communications:
1. Office and household environments
2. Industry control
3. Education sector and Health service.
4.
Cellular concept:
If a given set of frequencies or radio channels can be reused without increasing the interference, then
the large geographical area covered by a single high power transmitter can be divided into a number of small
areas, each allocated power transmitters with lower antennas can be used.
THE CELLULAR CONCEPT

1. Explain the concept of cellular topology and cell fundamentals with examples. (Nov - 2015)

✓ If a given set of frequencies or radio channels can be reused without increasing the interference, then the
large geographical area covered by a single high power transmitter can be divided into a number of small
areas, each allocated power transmitters with lower antennas can be used.
✓ The Hexagon shape was chosen for cell because it provides the most effective transmission by
approximating a circular pattern while eliminating gaps present between adjacent circles.
✓ Each cellular base station is allocated a group of radio channels to be used with a small geographic area
called a cell.
✓ A group of cells that use a different set of frequencies in each cell is called a cell cluster.
Types of cell
✓ The physical size of a cell varies, depending on user density and calling patterns.
▪ Macro cells are large cells typically have a radius between 1 mile and 15 miles with base station
transmit powers between 1W and 6W.
▪ Microcells are the smallest cells typically have a radius between of 1500 feet or less with base station
transmit powers between 0.1W and 1W.
✓ Microcells are used in high-density areas such as in large cities and inside the buildings.
✓ Cellular radio signals are too weak to provide reliable communication at indoor, especially in well-
shielded areas or areas with high levels of interference. To overcome this, very small cells, called picocells
are used in same frequencies as regular cells in the same areas.
Location of base station
✓ When designing a system using hexagonal-shaped cells, main consideration is the location of the base
station transmitters.
▪ Center-excited cell- Base station transmitters can be located in the center of the cell and uses Omni
directional antennas which radiate and receive signals equally well in all directions.
▪ Edge- excited cell- Base station transmitters can be located in the edge of the cell and uses sectored
antennas which radiate for a particular direction.
▪ Corner- excited cell- Base station transmitters can be located in the corner of the cell and uses
sectored directional antennas.
Cellular system

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✓ Figure shows a basic cellular system which consists of mobile stations, base stations and a mobile
switching center (MSC).
✓ The Mobile Switching Center is sometimes called a mobile telephone switching office (MTSO), since it
is responsible for connecting all mobiles to the PSTN in a cellular system.
✓ Each mobile communicates via radio with one of the base stations and may be handed off to any number
of base stations throughout the duration of a call.
✓ The mobile station contains a transceiver, an antenna, and control circuitry, and may be mounted in a
vehicle or used as a portable hand-held unit.

Figure: Cellular system

Base station:
✓ The base stations consist of several transmitters and receivers which simultaneously handle full duplex
communications.
✓ The base stations generally have towers which support several transmitting and receiving antennas.
✓ The base station serves as a bridge between all mobile users in the cell and connects the simultaneous
mobile calls via telephone lines or microwave links to the MSC.
Mobile Switching Center:
✓ The MSC coordinates the activities of all of the base stations and connects the entire cellular system to
the PSTN.
✓ A typical MSC handles 100,000 cellular subscribers’ and 5,000 simultaneous conversations at a time, and
accommodates all billing and system maintenance functions.
➢ The channels used for voice transmission from the base station to mobiles are called forward voice
channels (FVC).
✓ The channels used for voice transmission from mobiles to the base station are called reverse voice
channels (RVC).
✓ The two channels responsible for initiating mobile calls are the forward control channels (FCC) and
reverse control channels (RCC).
✓ Control channels are often called setup channels because they are only involved in setting up a call and
moving it to an unused voice channel.
FREQUENCY REUSE
1. Discuss in detail about cellular concept and frequency reuse. [8m] [Nov 2014]
2. Give the concept of cellular and explain with an example. [8m] [April 2010]

✓ Each cellular base station is allocated a group of radio channels to be used with a small geographic area
called cell. Cells are grouped into clusters. Each cluster utilizes the entire available radio spectrum.
Frequency reuse or Frequency planning

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✓ Frequency reuse is the process in which the same set of frequencies can be allocated to more than one cell
and the cells are separated by sufficient distance.
✓ The ability to reuse the frequencies offers a means to expand the total system capacity without the need
to employ high power transmitters.
✓ Figure shows a geographic cellular radio coverage area containing three groups of cell called clusters.
Each cluster has seven cells in it and all cells are assigned the same number of full duplex cellular
telephone channels.
✓ Spatially reusing the available spectrum so that the same spectrum can support multiple users separated
by a distance is the primary approach for efficiently using the spectrum.
✓ Cells with the same letter use the same set of frequencies.
✓ The letters A, B, C, D, E, F and G denote the seven sets of frequencies.
✓ A cell cluster is outlined in bold and replicate over the coverage area.
✓ The actual radio coverage of a cell is known as the foot print. It is determined from field measurement or
propagation prediction models.

Figure: Illustration of the cellular frequency reuse concept.

Capacity expansion by frequency reuse

✓ Consider a cellular system which has a total of S duplex channels available for use.
✓ Let N be the cluster size in terms of the number of cells within it and each cells is allocated a group of K
channels (K<S).
✓ The N cells which collectively use the complete set of available frequencies is called cluster. The cluster
can be replicated many times to form the entire cellular communication systems.
✓ The N cells in the cluster would utilize all K available channels.
✓ For the total number of Channels C, available in the cluster is given
S = KN
where S → Number of full duplex cellular channels available in the cluster.
K → Number of channels in a cell
N → Number of cells in the cluster

✓ Let M be the number of times the cluster is replicated and C be the total number of channels used in the
entire cellular system with frequency reuse. C is then the system capacity and is given by
C=MKN

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C=MS
where C → Total channel capacity in a given area
M → Number of clusters in a given area
✓ The capacity of a cellular system is directly proportional to the number of times a cluster is replicated in
a fixed service area.
✓ The cluster size factor N= 4, 7, or 12.If the cluster size N is reduced while the cell size is kept constant,
more clusters are required to cover a given area and hence more capacity is achieved.

✓ The number of subscribers who can use the same set of frequencies in non-adjacent cells at the same area
is dependent on the total number of cells in the area.
✓ The number of users use the same set of frequencies is called the frequency reuse factor (FRF) and is
defined as
N
FRF =
C
where N → Total number of full duplex in an area
C → Total number of full duplex in an a cell.

Rules for determining the nearest co-channel neighbors

✓ To find the nearest co-channel neighbors of a particular cell, one must do the following:
▪ Step 1: Move I cells along any chain of hexagons;
▪ Step 2: Turn 60 degrees counter clockwise and more j cells.

Figure: Method of locating co-channel cells in a cellular system.

In this example, N = 19 (i.e., i = 3, j = 2).
✓ The method of locating co-channel cells in a cellular system using the preceding rule is shown in figure
for i = 3 and j= 2.
✓ The parameters i and j measure the number of nearest neighbor between co-channel cells, N is related to
I and j by the equation
N = i 2 + ij + j 2
✓ Frequency reuse needs to be structured so that co-channel interference is kept at an acceptable level.
✓ As a distance between co-channel cell increases, co-channel interference will decrease.
✓ If cell size is fixed, the average signal-to-co-channel interference ratio will beindependent of the
transmitted power of each cell.
✓ Co-channel reuse ratio,

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D
Q= = 3N
R
where Q → Co channel reuse ratio
D → Distance to the nearest co-channel cells
R → Radius of the cell
N → Number of cells in the cluster
✓ The advantages of Cellular Systems are
▪ The use of low power transmitter and
▪ It allows frequency reuse for capacity improvement.

CHANNEL ASSIGNMENT (OR) ALLOCATION TECHNIQUES

1. Briefly discuss the process of channel assignment in cellular networks.

2. Explain channel assignment in detail. [April/May 2018]
✓ A scheme for increasing capacity and minimizing interference is required.
✓ For efficient utilization of the radio spectrum, a frequency reuse scheme is used. So that capacity is
increased, interference is reduced.
✓ Channel assignment strategy improves the performance of the system.
• Used to manage calls when handoff is done.
• Minimize connection set-up time
• Adapt to changing load distribution
• Fault tolerance
• Scalability
• Low computation and communication overhead
• Minimize handoffs
• Maximize number of calls that can be accepted concurrently
Call Admission Control
✓ The function of call admission control is to determine whether or not to grant radio resources to a new
incoming/handoff call based on information such as the current channel occupation, the bandwidth and
✓ QoS requirements of calls in service, and the characteristics of the call that requests admission
– Call rejection (reject the admission of new call)
– Call dropping (forcing an ongoing call to premature termination)
✓ Channel assignment strategies can be classified
(a) Fixed Channel assignment
(b) Dynamic Channel assignment
(c) Hybrid Channel Allocation schemes (HCA schemes: combining both FCA and DCA techniques)
✓ The choice of the channel assignment strategy impacts the performance of the system, particularly how a
call is managed when a mobile user is handoff from one cell to another.
(a).Fixed Channel assignment
✓ Channels are pre-allocated to the cells during planning phase.
✓ Each cell is allocated a predetermined set of voice channels.
✓ Any call attempt within the cell can only be served by the unused channels in that particular cell.
✓ If all the channels in that cell are occupied, the call is blocked and the subscriber does not receive service.

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✓ Due to short term fluctuations in the traffic, FCA schemes are often not able to maintain high quality of
service and capacity attainable with static traffic demands.
✓ One approach to address this problem is to borrow free channels from neighboring cells.
(b).Dynamic Channel assignment
✓ No pre-allocation:
o In a dynamic channel assignment strategy, voice channels are not allocated to different cells
permanently.
✓ Each time a call request is made, the serving base station requests a channel from the MSC.
✓ MSC then allocates a channel to the requested cell using an algorithm that takes into account
▪ The likelihood of future blocking within the cell
▪ The frequency of use of the candidate channel
▪ The reuse distance of the channel
✓ To ensure minimum quality of service, the MSC only allocates a given frequency if that frequency is not
currently in use in the cell or any other cell which falls within the limiting reuse distance.
✓ Dynamic channel assignment reduces the likelihood of blocking increasing the capacity of the system.
✓ Dynamic channel assignment strategies require the MSC to collect real-time data on channel
occupancyand traffic distribution on a continuous basis.
✓ Advantage of dynamic Channel assignment are
▪ Increased channel utilization
▪ Decreased probability of a blocked call.
✓ Disadvantage of dynamic Channel assignment are
▪ Increases the storage
▪ Increases computational load on the system
(c). Hybrid Channel Allocation (HCA)
✓ HCA schemes are the combination of both FCA and DCA techniques.
✓ In HCA schemes, the total number of channels available for service is divided into fixed and dynamic
sets.
✓ The fixed set contains a number of nominal channels that are assigned to cells as in the FCA schemes and,
in all cases, are to be preferred for use in their respective cells.
✓ The dynamic set is shared by all users in the system to increase flexibility.
✓ Example: When a call requires service from a cell and all of its nominal channels are busy, a channel
from the dynamic set is assigned to the call.
✓ Request for a channel from the dynamic set is initiated only when the cell has exhausted using all its
channels from the fixed set.
✓ Optimal ratio: ratio of number of fixed and dynamic channels.
✓ 3:1 (fixed to dynamic), provides better service than fixed scheme for 50% traffic.
✓ Beyond 50% fixed scheme perform better.
✓ For dynamic, with traffic load of 15% to 32%, better results are found with HCA.

COMPARISON FCA AND DCA

Compare FCA and DCA.

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Attribute Fixed Channel Allocation Dynamic Channel Allocation

Traffic load Fixed Channel Allocation is better Dynamic Channel Allocation is
under heavy traffic load better under light/moderate traffic
load
Flexibility of Fixed Channel Allocation is less Dynamic Channel Allocation is
channel allocation flexible more flexible

Reusability of Fixed Channel Allocation has a Dynamic Channel Allocation has a

channels maximum possibility. limited possibility.

Temporal and Fixed Channel Allocation are very Dynamic Channel Allocation are
spatial changes sensitive very insensitive

Grade of service Fixed Channel Allocation is Dynamic Channel Allocation is

fluctuating stable.

Forced call Large probability in Fixed Channel Low/ Moderate probability in

termination Allocation Dynamic Channel Allocation

Suitability of cell Fixed Channel Allocation uses macro Dynamic Channel Allocation uses
size cellular system micro cellular system

Radio Equipment Fixed Channel Allocation covers Dynamic Channel Allocation has to
only the channels allotted to the cell. cover all possible channel that could
be assigned to the cell

Computational In Fixed Channel Allocation, In Dynamic Channel Allocation,

effort Computational effort is low. Computational effort id high

Call setup delay Low in Fixed Channel Allocation Moderate/High in Dynamic

Channel Allocation

Implementation Low in Fixed Channel Allocation Moderate/High in Dynamic

complexity Channel Allocation

Frequency Laborious and complex in Fixed None in Dynamic Channel

planning Channel Allocation Allocation

Signaling load Low in Fixed Channel Allocation Moderate/High in Dynamic

Channel Allocation

Control Centralized in Fixed Channel Centralized, decentralized or

Allocation distributed in Dynamic Channel
Allocation

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HAND OFF STRATEGIES

1. Explain the principle of cellular networks and various types of handoff techniques.
[16M-May 2016, 16M-May 2013]
2. Explain in detail a handoff scenario at cell boundary. [6M-Nov 2014]
3. Explain hand off strategies in detail. [April/May 2018][April/May 2019]

✓ When a mobile moves into a different cell while a conversation is in progress, the MSC automatically
transfers the call to a new channel belonging to the new base station.
✓ The handoff operation not only involves a new base station, but also requires that the voice and control
signals be allocated to channels associated with the new base station.
✓ Handoff calls can be admitted at a higher priority than new calls.
✓ To manage the admission of requests based on priority, it is necessary to reserve capacity for admitting
handoff requests.
✓ A particular signal level is specified as the minimum usable signal for acceptable voice quality at the base
station receiver.
✓ A sligthy stronger signal level is used as threshold at which a handoff is made.
✓ The time over which a call may be maintained within a cell, without handoff, is called the dwell time.
✓ Dwell time depends on
▪ Propagation
▪ Interference
▪ Distance between the subscriber
▪ Speed
✓ Handoff Margin △
▪ Margin △ = Phandoff threshold– P minimumusable signaldB
▪ △ is carefully selected
▪ △ too large → unnecessary handoff → MSC loaded down
▪ △ too small → not enough time to transfer → call dropped.

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Figure: Illustration of a handoff scenario at cell boundary.

Hand off methods
✓ Depending on the information used and the action taken to initiate the handoff, the methods for handoff
can be
▪ Mobile Controlled Hand off (MCHO)
▪ Network Controlled Hand off (NCHO) and
▪ Mobile Assisted Hand off (MAHO)
✓ MCHO
▪ MCHO is a desirable method because it reduces the burden on the network.
▪ However it increases the complexity of the mobile terminal.
✓ NCHO
▪ In NCHO, the BSs or Access Points (APs) monitor the signal quality from the mobile and report the
measurements to the MSC.
▪ The MSC is responsible for choosing the candidate AP and initiating the handoff.
▪ The mobile plays a passive role in the handoff process.
✓ MAHO
▪ In MAHO, the mobile measures the signal levels from the various APs using periodic beacon
generated by the APs.
▪ The mobile collects a set of power levels from different APs and feeds it back to the MSC via the
serving AP, for handoff decision making.
Prioritizing Handoffs
✓ Method for giving priority to handoffs are
▪ Guard channel concept
▪ Queuing of handoff requests

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Guard channel concept

✓ Guard channel concept is a fraction of the total available channels in a cell is reserved exclusively for
handoff requests from ongoing calls which may be handed off into the cell.
✓ Disadvantage of guard channel concept is reducing the total carried traffic as fewer channels are allocated
to originating calls.
✓ Advantage of guard channelis efficient spectrum utilization during dynamic channel assignment
strategies.
Queuing of handoff requests
✓ Queuing of handoff requestsdecreases the probability of forced termination of a call due to lack of
available channels.
✓ Queuing of handoffs is possible due to the fact that there is a finite time interval between the time the
received signal level drops below the handoff threshold and the time the call is terminated due to
insufficient signal level.
✓ The delay time and size of the queue is determined from the traffic pattern of the particular service area.
Practical Handoff Considerations
✓ In practical cellular systems, several problems arise when attempting to design for a wide range of mobile
velocities.
✓ High speed vehicles pass through the coverage region of a cell within a matter of seconds, whereas
pedestrian users may never need a handoff during a call.
✓ The MSC can quickly become burdened if high speed users are constantly being passed between very
small cells.
Umbrella cell approach
✓ To solve the above problem, Umbrella cell approach is used.
✓ By using different antenna heights (same building or tower) and different power levels, it is possible to
provide “large” and” small” cells which are co-located at a single location.
✓ 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.

Figure: The umbrella cell approach.

Cell Dragging
✓ Another practical handoff problem in microcell systems is known as cell dragging.
✓ Cell dragging results from pedestrian users that provide a very strong signal to the base station.
✓ Such a situation occurs in an urban environment when there is a line-of-sight (LOS) radio path between
the subscriber and the base station.
✓ As the user travels away from the base station at a very slow speed, the average signal strength does not
decay rapidly.

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✓ Even when the user has traveled well beyond the designed range of the cell, the received signal at the base
station may be above the handoff threshold, thus a handoff may not be made.
Intersystem handoff:
During a call if a mobile moves from one cellular system to a different cellular system controlled by a different
MSC, type of handoff is called intersystem handoff.
Types of Handoff
✓ Hard handover (hard handoff)- If the MSC monitors the strongest signal base station and transfer the
call to that base station then it is called hard handoff.
✓ Soft handoff: Mobile communicates with two or more cells at the same time and find which one is the
strongest signal base station then it automatically transfers the call to that base station is called soft
handoffs.
✓ Softer hand over: In this instance a new signal is either added to or deleted from the active set of
signals.
1. Hard handover
✓ The definition of a hard handover or handoff is one where an existing connection must be broken before
the new one is established.
✓ Intersystem handoff: During a call if a mobile moves from one cellular system to a different cellular
system controlled by a different MSC. This type of handoff is called intersystem handoff.
✓ The connection must be broken before it can move to the new channel where the connection is re-
established.
✓ Intra-frequency hard handovers where the frequency channel remains the same.
✓ Although there is generally a short break in transmission, this is normally short enough not to be noticed
by the user.
✓ In UMTS- Universal Mobile Telecommunication System most of the handovers that are performed are
intra-frequency soft handovers.
2. Soft handover
✓ The new 3G technologies use CDMA, it is not necessary to break the connection. This is called soft
handover.
✓ Soft handoff is defined as a handover where a new connection is established before the old one is
released.
3. Softer handover
✓ The third type of hand over is termed a softer handover, or handoff.
✓ In this instance a new signal is either added to or deleted from the active set of signals.
✓ It may also occur when a signal is replaced by a stronger signal from a different sector under the same
base station.
✓ This type of handover or handoff is available within UMTS as well as CDMA2000.
✓ Cellular handover or cellular handoff is performed by all cellular telecommunications networks, and
they are a core element of the whole concept of cellular telecommunications.

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✓ Soft handover is also less efficient than hard handover, but again more reliable as the connection is
never lost.
✓ It is therefore necessary for the cellular telecommunications network provider to arrange the network to
operate in the most efficient manner, while still providing the most reliable service.

Features of Handoff:

✓ Fast and lossless

✓ Minimal number of control signal exchanges.
✓ Scalable with network size.
✓ Capable of recovering from link failures.
✓ Efficient use of resources.

INTERFERENCE AND SYSTEM CAPACITY

1. Describe various interferences and increasing the system capacity of wireless cellular networks

✓ Interference is the major limiting factor in the performance of cellular radio.

✓ It limits capacity and increases the number of dropped calls.
✓ Sources of interference include
▪ Another mobile in the same cell
▪ Call inprogress in a neighboring cell,
▪ other base stations operating in the same frequency band,
▪ Any noncellular system which leaks energy intothe cellular frequency band.
✓ Interference is more severe in urban areas due to
▪ Greater RF noise floor
▪ large number of base stations and mobiles
✓ The two major types of interferences:
▪ Co-channel interference (CCI)
▪ Adjacent channel interference. (ACI)
✓ Adjacent channel interference is caused due to the signals that are adjacent in frequency.
Co-channel Interference and System Capacity
✓ Co-channel interference is caused due to the cells that reuse the same frequency set.
✓ The cells using the same frequency set are called co-channel cells.
✓ The interference between signals from the co-channel cells is called co-channel interference.
✓ Unlike thermal noise, co-channel interference cannot be overcome byincreasing the carrier power of a
transmitter
✓ This is because an increase in transmitter power increases the interference to neighboring co-channel cells.
✓ For similar sized cells, the co-channel interference is independent of the transmitted power and depends
on the radius of the cell and the distance to the nearest co-channel cells.
✓ Toreduce co-channel interference, co-channel cells must be physically separated.

✓ Co channel reuse ratio, Q=D/R

where Q → Co channel reuse ratio
D → Distance to the nearest co-channel cells
R → Radius of the cell

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✓ It determines the spatial separation relative to the coverage distance of the cell.
✓ For a hexagonal geometry
D
Q = = 3N
R
✓ Thus, a small value of Q provides larger capacity but higher co-channel interference.
✓ Hence there is a trade-off between capacity and interference.
Calculation of signal-to-interference ratio(S/I or SIR)
✓ The signal-to-interference ratio for a mobile is
S S
= i0
I
 Ii i =1

where S → Desired signal power

Ii → Interference power caused by the ithco-channel cell
i0 → Number of co-channel interfering cells
✓ The average received powerat a distance d is
−n
d 
Pr = P0  
 d0 
d 
Pr ( dBm ) = P0 ( dBm ) − 10n log  
 d0 
where P0 → Power received at a close-in reference point in the far field regionof the antenna
d0 → Small distance from the transmitting antenna
n →Path loss exponent.
✓ If Di is the distance of the ithinterferer, the receivedpower is proportional to (Di)-i0.
✓ The path loss exponent, nranges between 2 and 4.
✓ Thus the S/I for a mobile can be written as
S R −n
= i0

 (D )
I −n
i
i =1

✓ For only the first layer of equidistant interferers

S (D R )
= =
n
( 3N )
n

Q =
D
= 3N
I i0 i0 R
✓ S / I is usually the worst casewhen a mobile is at the cell edge
S R −4
=
I 2(D − R )−4 + 2(D + R )−4 + 2 D −4
S 1
=
I 2(Q − 1) + 2(Q + 1)−4 + 2Q −4
−4

✓ For a hexagonal cluster of cells

( )
i
S 1D0 1 i0
=   = 3N
I 6 R  6
✓ Hence, S/I is independent of the cell radius.

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Figure: Illustration of the first tier of co-channel cells for a cluster size of N=7. When the mobile is at the cell
boundary (point A), it experiences worst case co-channel interference on the forward channel. The marked
distances between the mobile and different co-channel cells are based on approximations made for easy
analysis.
Adjacent Channel Interference
✓ Interference resulting from signals which are adjacent in frequency to the desired signal is called adjacent
channel interference.
✓ Adjacent channel interference results from imperfect receiverfilters that allow nearby frequencies to leak
into the passband.
✓ The problem can be severe if an adjacent channel user is transmitting in very close range to a subscriber's
receiver.
✓ The near-far effect occurs when a mobile close to a base station radiates in the adjacent channel, while
the subscriber is far away from the base station.
✓ Adjacent channel interference can be reduced by
▪ Careful filtering
▪ Careful channel assignments.
✓ The frequency separation between each channel in a cell should be made as large as possible.
✓ If the subscriber is at a distance d1 and the interferer is at d2, then signal-to-interference ratio is
−n
S  d1 
= 
I  d 2 
✓ The frequency separation between each channel in a cell should be made as large as possible while
assigning them.
Power Control to Reduce Interference
✓ In practical systems, the power levels of every subscriber are under constant control by the serving base
stations.
✓ Power control
▪ Reduces interference levels
▪ Prolongs battery life

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✓ In CDMA spread spectrum systems, power control is a key feature to ensure maximal utilization of the
system capacity.
✓ Reduced interference leads to higher capacity.

TRUNKING AND GRADE OF SERVICE

1.Describe the various terms involved in trunking and grade of service

2. Write short notes on i) Trunking ii) Grade of service of cell system. [Nov/Dec 2017][April/May
2019]

✓ Cellular radio systems rely on trunking to accommodate a large number of users in a limited radio
spectrum.
✓ In a trunked radio system, each user is allocated a channel on a per call basis.
✓ Upon termination of the call, the previously occupied channel is immediately returned to the pool of
available channels.
✓ The time required to allocate a trunked radio channel to a requesting user is called Set-up Time.
✓ Call which cannot be completed at time of request due to congestion is called Blocked Call or lost call.
✓ Average duration of a typical call is called Holding Time, H.
✓ Request Rate is the average number of call requests per unit time. It is denoted by λ seconds-1.

✓ Traffic Intensity is themeasure of channeltime utilization, which is the average channel occupancy
measured in Erlangs.
✓ Load is the Traffic intensity across the entire trunked radio system, measured in Erlangs.
✓ A channel kept busy for one hour is defined as having a load of one Erlang.

✓ Grade of Service (GOS) is measure of congestion which is specified as the probability

▪ Probability of a call being blocked (Erlang B)
▪ Probability of a call being delayed beyond a certain amount of time (Erlang C)
✓ The grade of service (GOS) is a measure of the ability of a user to access a
trunked system during the busiest hour.
✓ The grade of service is used to define the desired performance of a particular trunked system by specifying
a desired likelihood of a user obtaining channel access given a specific number of
channels available in the system.
✓ In order to obtain proper GOS, it is the necessary to estimate
▪ Maximum required capacity
▪ To allocate the proper number of channels.
✓ Each user generates a traffic intensity of AuErlangs given by
Au = H

where Au → Traffic intensity

λ → Average number of call requests per unit time
H → Average duration of a call
✓ Total offered traffic intensity A, is given as
A = UAu
where U → Number of users in the system.
A → Total offered traffic.
✓ Traffic intensity per channel is given as

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Ac = UAu C
where C → Number of trunked channels offered by a trunked radio system
✓ The Erlang B formula is given by
AC
Pr blocking  = C C! k = GOS
A

k = 0 k!

✓ The likelihood of a call not having immediate access to a channel is determined by the Erlang C formula
Ac
Pr delay  0 =
 A  C −1 A k
Ac + c!1 − 
 C  k = 0 k!

IMPROVING CAPACITY IN CELLULAR SYSTEMS

1. Explain some techniques intended to improve the coverage area and capacity of cellular
system. (8M-Nov 2015) (or)
2. Explain in detail how to improve coverage and channel capacity in cellular systems. (16M-
May 2016) (or)
3. Define the methods of increasing the capacity of wireless cellular networks. (10M-May 2013)
4. Explain the capacity improvement techniques used in cellular system. (10M – May
2010][April/May 2019]

✓ As demand for service increases, system designers have to provide more channels per unit coverage area.
✓ Common Techniques used to expand the capacity of cellular systemsare
▪ Cell splitting
▪ Sectoring
▪ Microcell Zoning
✓ Cell splitting increases the number of base station deployed and allows an orderly growth of the cellular
system.
✓ Sectoring uses directional antennas to further control the interference and frequency reuse of channels.
✓ Microcell Zoning distributes the coverage of a cell andextends the cell boundary to hard-to-reach places.

Cell Splitting
Q. Write short notes on cell splitting [April/May 2019]
✓ Cell splitting is the process of subdividing a congested cell into smaller cellswith
▪ its own base station
▪ Corresponding reduction in antenna height
▪ Corresponding reduction in transmitter power.
✓ Splitting of cells reduces the cell size and thus more number of cells has to be used.
✓ More number of cells => More number of clusters =>More Channels => Higher capacity
✓ By defining new cells which have a smaller radius than the original cells and by installing these smaller
cells (called microcells) between the existing cells, capacity increases due to the additional number of
channels per unit area.
✓ Cell splitting allows a system to grow by replacing large cells by small cells, without upsetting the channel
allocation.
✓ Cells are split to add channels with no new spectrum usage

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✓ Depending on traffic patters the smaller cells may be activated/deactivated in order to efficiently use cell
resources.
✓ InthefigurethattheoriginalbasestationAhasbeensurroundedbysix newmicrocellbasestations.
✓ Thesmallercellswereaddedinsuchawayastopreservethefrequencyreuseplanofthesystem.
✓ Cellsplittingscalesthegeometryofthecluster.

Figure: Illustration of cell splitting

✓ When new cell radius is half the original cell radius,

Pr [at old cell boundary]  Pt1R − n
Pr [at new cell boundary]  Pt 2 (R / 2)
−n

Where Pr → Received power

Pt1 → Transmit power of larger cell base station
Pt 2 → Transmit power of smaller cell base station
N → Path loss exponent
✓ Transmit power must be reduced by 12db in order to fill in the original coverage area with microcell while
maintaining the S/I requirement.

Figure: Illustration of cell splitting within

a 3 km square centered base station A.

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Sectoring
✓ The technique for decreasing co-channel interference and thus increasing system capacity by using
directional antennas is called sectoring.
✓ The factor by which the co-channel interference is reduced depends on the amount of sectoring used.
✓ Cell Sectoring keeps R untouched and reduces D/R.
✓ Capacity improvement is achieved by reducing the number of cells per cluster, thus increasing frequency
reuse.
✓ It is necessary to reduce the relative interference without decreasing the transmitter power.
✓ The co-channel interference may be decreased by replacing the single omni-directional antenna by several
directional antenna, each radiating within a specified sector.
✓ A directional antenna transmits to and receives from only a fraction of the total number of co-channel
cells. Thus co-channel interference is reduced.
✓ A cell is normally partitioned into three 1200 sectors or six 60° sectors.

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(a) 1200 sectoring.(b) 600 sectoring.

Figure: Illustration of how 120° sectoring reduces interference from co-channel cells.Out of
the 6 co-channel cells in the first tier, only 2 of them, interfere with the center cell. If omni-
directional antennas were used at each base station, all 6 co-channel cells would interfere
with the center cell.
Advantages
✓ it improves Signal-to-interference ratio.
Disadvantages
✓ Disadvantages of cell sectoring includes
▪ Increased number of antennas at each base station.
▪ Decrease in trunking efficiency
▪ Increased number of handoffs.
Repeaters for Range Extension
✓ Repeaters are useful for hard to reach areas
▪ Within buildings and basement
▪ Tunnels
▪ Valleys
✓ Radio transmitters called repeaters are used to provide coverage in these areas.
✓ Repeaters are bidirectional.
▪ Receive signals from the base station
▪ Amplify the signals
▪ Reradiates the signals.
✓ Received noise and interference is also reradiated.
A Novel Microcell Zone Concept
✓ Zone Concept

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▪ A cell is divided into microcell or zones.

▪ Each microcell (Zone) is connected to the same basestation by coaxial cable, fiberoptic
cable, or microwave link.
▪ Each Zone uses a directional antenna
▪ As mobile travels from one zone to another, it retains the same channel. i.e. Without
handoff.
▪ The base station simply switches the channel to the next zone site.
▪ Mobile is served by the zone with the strongest signal.
✓ While the cell maintains a particular coverage area, the co-channel interference is reduced
because:
▪ The large central base station is replaced by several low powertransmitters.
▪ Directional Antennas are used.
✓ Decreased co-channel interference improves
▪ Signal Quality
▪ Capacity

Figure:The microcell concept

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