CELLULAR AND MOBILE

COMMUNICATIONS

by
VIDYA SAGAR POTHARAJU
Associate Professor,
Dept of ECE,
VBIT.

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 TEXT BOOKS

1.Mobile and Cellular Telecommunications-W.C.Y.Lee 2nd Edn, 1989.

2. Wireless Communications-Theodre.S.Rapport, Pearson education,2nd Edn.,2002.
3. Mobile Cellular Communications-Gottapu sashibushan rao,pearson,2012.
REFERENCES:
1.Principles of mobile communications-Gordon L.Stuber,Springer intl,2nd Edn.,2001.
2. Modern Wireless Communications-Simon Haykin,Michael moher,Pearson Edu,2005.
3 Wireless Communications theory and techniques,Asrar U.H.Sheikh,Springer,2004.
4. Wireless Communications and networking,Vijay Garg,Elsevier Publications,2007.
5. Wireless Communications-Andrea Goldsmith,Cambridge University Press,2005.

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 Syllabus

INTRODUCTION TO CELLULAR MOBILE RADIO SYSTEMS:

Limitations of conventional mobile telephone systems, Basic Cellular Mobile System,
First, second, third, and fourth generation cellular wireless systems, Uniqueness of mobile
radio environment. Fading-Time dispersion parameters, Coherence bandwidth, Doppler
spread and coherence time.

FUNDAMENTALS OF CELLULAR RADIO SYSTEM DESIGN

Concept of frequency reuse, Co-channel interference, co-channel interference reduction
factor, Desired C/I from a normal case in a omnidirectional antenna system, system
capacity, trunking and grade of service, Improving coverage and capacity in cellular
systems- Cell splitting, Sectoring, Microcell zone concept.

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 Conventional Mobile System

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 Limitations of Conventional Mobile Telephone Systems
One of many reasons for developing a cellular mobile telephone system and deploying it
in many cities is the operational limitations of conventional mobile telephone systems:
 Limited service capability,
 Poor service performance,
 Inefficient frequency spectrum utilization.
Limited service capability :
 Each area is allocated with one or more channels.
 Which is large autonomous geographic zone.
 The transmitted power should be as high as the federal specification allows.
 The user who starts a call in one zone has to reinitiate the call when moving into a new zone because the
call will be dropped.

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 Poor Service Performance
In the past, a total of 33 channels were all allocated to three mobile telephone systems.
o Mobile Telephone Service (MTS)-40MHz
o Improved MTS (IMTS) MJ-150MHz
o Improved MTS (IMTS) MK -450MHz
6 channels of MJ serving 320 customers, with another 2400 customers on a waiting list.
6 channels of MK serving 225 customers, with another 1300 customers on a waiting list.

The large number of subscribers created a high blocking probability during busy hours.

Although service performance was undesirable, the demand was still great.

A high-capacity system for mobile telephones was needed.

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 Inefficient frequency spectrum utilization

The frequency utilization measurement (Mo), is defined as the maximum number of

customers that could be served by one channel at the busy hour.
Mo = Number of customers/channel
Mo = 53 for MJ system,37 for MK system
The offered load can then be obtained by
A = Average calling time (minutes) x total customers / 60 min (Erlangs)
Assume average calling time = 1.76 min.
A1 = 1.76 x53 x 6 / 60 = 9.33 Erlangs (MJ system)
A2 = 1.76 x 37 x 6 / 60 = 6.51 Erlangs (MK system)

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 If the number of channels is 6 and the offered loads are A1 = 9.33 and A2 =
6.51, then from the Erlang B model the blocking probabilities,

B1 = 50 percent (MJ system)

B2 =30 percent (MK system),

It is likely that half the initiating calls will be blocked in the MJ system, a very high
blocking probability.

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 If the actual average calling time is greater than 1.76 min, the
blocking probability can be even higher.

To reduce blocking probability we must decrease Mo.

As far as frequency spectrum utilization is concerned, the conventional

system does not utilize the spectrum efficiently since each channel can
only serve one customer at a time in a whole area.

This is overcome by the new cellular system.

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 A Basic Cellular System

Fig 1 : Basic Cellular system

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  A basic cellular system consists of three parts:

 a mobile unit, a cell site, and a mobile telephone switching office (MTSO)
 Mobile units: A mobile telephone unit contains a control unit, a transceiver, and an antenna system.
 Cell site: The cell site provides interface between the MTSO and the mobile units. It has a control
unit, radio cabinets, antennas, a power plant, and data terminals.
 MTSO: The switching office, the central coordinating element for all cell sites, contains the cellular
processor and cellular switch. It interfaces with telephone company zone offices, controls call processing,
and handles billing activities.
 Connections: The radio and high‐speed data links connect the three subsystems. Each mobile unit
can only use one channel at a time for its communication link.
 The MTSO is the heart of the cellular mobile system. Its processor provides central coordination and
cellular administration.

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 Uniqueness of Mobile Radio Environment

The mobile radio channel places fundamental limitations on the performance of

wireless communication systems.
The transmission Paths can vary from simple line-of-sight to ones that are severely
obstructed by buildings, mountains, and foliage.
Radio channels are extremely random and difficult to analyze.

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 Mobile Communication Operation

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 Evolution of Mobile Radio Communications
• Major Mobile Radio Systems
• 1934 - Police Radio uses conventional AM mobile communication system.
• 1935 - Edwin Armstrong demonstrate FM
• 1946 - First public mobile telephone service - push-to-talk
• 1960 - Improved Mobile Telephone Service, IMTS - full duplex
• 1960 - Bell Lab introduce the concept of Cellular mobile system
• 1968 - AT&T propose the concept of Cellular mobile system to FCC.
• 1976 - Bell Mobile Phone service, poor service due to call blocking
• 1983 - Advanced Mobile Phone System (AMPS), FDMA, FM
• 1991 - Global System for Mobile (GSM), TDMA, GMSK
• 1991 - U.S. Digital Cellular (USDC) IS-54, TDMA, DQPSK
• 1993 - IS-95, CDMA, QPSK, BPSK

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 Examples of Mobile Communication Systems
Pagers-Simplex
Hand held Walkie-Talkies-Half duplex
Cordless phones-Full duplex
Cellular telephones-Full duplex

pager Walkie-Talkie Cordless phone Cellular telephone

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 Forw ard C hannel

R everse C hannel

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 DUPLEXING
In wireless communication systems ,it is often desirable to allow the user to send
simultaneously information to the base station while receiving information from
the base station.
Duplexing is done either using frequency or time domain techniques:
Frequency division duplexing (FDD)
Time division duplexing (TDD)
FDD - is more suitable for radio communication systems,
TDD- is more suitable for fixed wireless systems

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 Overview
Data Rates
2 Mbps

1 Mbps 3G
(144Kbps to 2Mbps)

100 Kbps
2.5G
(10-150Kbps)
10 Kbps
2G
(9.6Kbps)
1 Kbps
1G
(<1Kbps)

1980 1990 2000 2010

Years
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 Cellular networks: From 1G to 5G

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 Introduction to Radio Wave Propagation

• The mobile radio channel places fundamental limitations on the

performance of wireless communication systems
• Paths can vary from simple line-of-sight to ones that are severely obstructed
by buildings, mountains, and foliage
• Radio channels are extremely random and difficult to analyze
• The speed of motion also impacts how rapidly the signal level fades as a
mobile terminals moves about.

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 Problems Unique to Wireless systems
• Interference from other service providers
• Interference from other users (same network)
• CCI due to frequency reuse
• ACI due to Tx/Rx design limitations & large number of users sharing finite BW
• Shadowing : Obstructions to line-of-sight paths cause areas of weak received signal strength

• Fading :
• When no clear line-of-sight path exists, signals are received that are reflections off
obstructions and diffractions around obstructions
• Multipath signals can be received that interfere with each other
• Fixed Wireless Channel → random & unpredictable
• must be characterized in a statistical fashion
• field measurements often needed to characterize radio channel performance
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 Mechanisms that affect the radio propagation ..

• Reflection
• Diffraction
• Scattering
• In urban areas, there is no direct line-of-sight path between:
• the transmitter and the receiver, and where the presence of high- rise buildings causes
severe diffraction loss.
• Multiple reflections cause multi-path fading

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 Reflection, Diffraction, Scattering
• Reflections arise when the plane waves are incident upon a surface with dimensions
that are very large compared to the wavelength
• Reflection - occurs when signal encounters a surface that is large relative to the
wavelength of the signal
• Diffraction occurs according to Huygens's principle when there is an obstruction
between the transmitter and receiver antennas, and secondary waves are generated
behind the obstructing body
• Diffraction - occurs at the edge of an impenetrable body that is large compared to
wavelength of radio wave. (Waves bending around sharp edges of objects)
• Scattering occurs when the plane waves are incident upon an object whose dimensions
are on the order of a wavelength or less, and causes the energy to be redirected in
many directions.
• Scattering – occurs when incoming signal hits an object whose size is in the order of
the wavelength of the signal or less

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 Ground Reflection (2-Ray) Model
•a model where the receiving antenna sees a direct path signal as well as a signal reflected off
the ground.
•In a mobile radio channel, a single direct path between the base station and mobile is rarely
the only physical path for propagation
− Hence the free space propagation model in most cases is inaccurate when used alone
• Hence we use the 2 Ray GRM
− It considers both- direct path and ground reflected propagation path between transmitter and receiver
This was found reasonably accurate for predicting large scale signal strength over distances of several
kilometers for mobile radio systems using tall towers ( heights above 50 m )

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 Ground Reflection (2-Ray) Model
• Good for systems that use tall towers (over 50 m tall)
• Good for line-of-sight microcell systems in urban environments
• ETOT is the electric field that results from a combination of a direct line-of-
sight path and a ground reflected path
The maximum T-R separation distance ( In most mobile communication systems ) is
only a few tens of kilometers, and the earth may be assumed to be flat.

• ETOT =The total received E-field,

• ELOS=The direct line-of-sight component
• Eg =The ground reflected component,

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 Diffraction
Occurs when the radio path between sender and receiver is obstructed by an impenetrable body and by a
surface with sharp irregularities (edges)
The received field strength decreases rapidly as a receiver moves deeper into the obstructed (shadowed)
region, the diffraction field still exists and often has sufficient strength to produce a useful signal.
Diffraction explains how radio signals can travel urban and rural environments without a line-of-sight
path
The phenomenon of diffraction can be explained by Huygen's principle, which states that all points on a
wave front can be considered as point sources for the production of secondary wavelets, and that these
'wavelets combine to produce a new wave front in the direction of propagation

The field strength of a diffracted wave in the

shadowed region is the vector sum of the electric
field components of all the secondary wavelets
in the space around the obstacle.

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 Scattering
• The medium which the wave travels consists of objects with dimensions smaller than the
wavelength and where the number of obstacles per unit volume is large – rough surfaces,
small objects, foliage, street signs, lamp posts.
• Generally difficult to model because the environmental conditions that cause it are
complex
• Modeling “position of every street sign” is not feasible.

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 Illustration ..

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 Mobile Radio Propagation Environment
• The relative importance of these three propagation mechanisms depends on the
particular propagation scenario.
• As a result of the above three mechanisms, macro cellular radio propagation can be
roughly characterized by three nearly independent phenomenon;
• Path loss variation with distance (Large Scale Propagation )
• Slow log-normal shadowing (Medium Scale Propagation )
• Fast multipath fading. (Small Scale Propagation )

• Each of these phenomenon is caused by a different underlying physical principle and

each must be accounted for when designing and evaluating the performance of a
cellular system.

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 Path Loss: Models of "large-scale effects"

• location 1, free space loss (Line of Sight) is likely to give an accurate estimate of path
loss.
• location 2, a strong line-of-sight is present, but ground reflections can significantly
influence path loss. The plane earth loss (2-Ray Model) model appears appropriate.
• location 3, plane earth loss needs to be corrected for significant diffraction losses, caused
by trees cutting into the direct line of sight.
• location 4, a simple diffraction model is likely to give an accurate estimate of path loss.
• location 5, loss prediction fairly difficult and unreliable since multiple diffraction is
involved
•
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 Radio Propagation Mechanisms

Line Of Sight (LOS) Non Line Of Sight (NLOS)

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 Free Space Propagation Model
•Free space propagation model is used to predict:
•Received Signal Strength when the transmitter and receiver have a clear, unobstructed
LoS between them.
•The free space propagation model assumes a transmit antenna and a receive antenna to be
located in an otherwise empty environment. Neither absorbing obstacles nor reflecting
surfaces are considered. In particular, the influence of the earth surface is assumed to be
entirely absent.
Satellite communication systems and microwave line-of-sight radio links typically undergo free
space propagation.

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 Free Space Propagation Model

• Path Loss
• Signal attenuation as a positive quantity measured in dB and defined as the difference (in dB)
between the effective transmitter power and received power.
• Friis is an application of the standard “Free Space Propagation Model “
• It gives the Median Path Loss in dB ( exclusive of Antenna Gains and other losses )
• clear, unobstructed line-of-sight path → satellite and fixed microwave
• Friis Transmission Equation (Far field)

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 Friis Free Space Equation
• Pt Transmitted power,
• Pr(d) Received power
• Gt Transmitter antenna gain,
• Gr Receiver antenna gain,
• d T-R separation distance (m)
• L System loss factor not related to propagation system losses (antennas, transmission lines between equipment and
antennas, atmosphere, etc.)
• L = 1 for zero loss
• Signal fades in proportion to d2
• The ideal conditions assumed for this model are almost never achieved in ordinary terrestrial communications, due
to obstructions, reflections from buildings, and most importantly reflections from the ground.

• The Friis free space model is only a valid predictor for “Pr ” for values of “d” which are in the far-field of the
“Transmitting antenna

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 Example
• (a) If a transmitter produces 50 watts of power, express the transmit power in units of
dBm, and dBW.
• (b) If 50 watts is applied to a unity gain antenna with a 900 MHz carrier frequency, find
the received power in dBm at a free space distance of 100 m from the antenna, What is Pr
(10 km)? Assume unity gain for the receiver antenna.
Solution: (a) TX power in dBm = 10 log10 (Pt/1mW) = 10 log10 (50/1mW)=47 dBm
Tx power in dBW = 10 log10 (Pt/1W) = 10 log10(50)=17 dBW
(b)
Rx power = Pr(d) = Pt Gt Gr 2 / (4)2 d2 L
Wavelength,  = 0.3333333 , GT=Gr = 1, D=100 m, L=1
Pr(100 m) = 3.52167x10-06 W = 3.5x10-3 mW =10log (3.5*10-3) = -24.5 dBm
Pr(10*1000 m) = 3.5*10-3 /10^4 = 3.5*10-7 mW

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 Small Scale Multipath fading
• Multipath creates small scale fading effects:
• Rapid changes in signal strength over a small travel distance or time interval
• Random frequency modulation due to varying Doppler shifts on different multipath signals
• Time dispersion (echoes) caused by multipath propagation delays
• Factors influence small scale fading
• Multipath propagation – result in multiple version of transmitted signal
• Speed of mobile – result in random frequency modulation due to different Doppler shifts
• Speed of surrounding – if the surrounding objects move at a greater rate than the mobile
• The transmission bandwidth of the signal – if the transmitted radio signal bandwidth is greater than
the bandwidth of the multipath channel

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 Multipath Propagation.
The presence of reflecting objects and scatterers in the channel creates a constantly
changing environment that dissipates the signal energy in amplitude, phase, and time.
These effects result in multipath propagation.
The multipath propagation results fluctuations in signal strength, thereby inducing
small-scale fading, signal distortion, or both.
Multipath propagation often lengthens the time required for the baseband portion of
the signal to reach the receiver which can cause signal smearing due to intersymbol
interference.

38
 Speed Of The Mobile
The relative motion between the base station and the mobile results in random
frequency modulation.
Different Doppler shifts on each of the multipath components.
Doppler shift will be positive- moving toward BS.
Doppler shift will be negative-away from the BS.
The phase change in the received signal due to the difference in path and results in
change in frequency.
Doppler shift positive-increase in frequency.
Doppler shift negative-decrease in frequency.

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 Speed Of Surrounding Objects
If objects in the radio channel are in motion, they induce a time varying Doppler shift
on multipath components.
If the surrounding objects move at a greater rate than the mobile, then this effect
dominates the small-scale fading.
Otherwise, motion of surrounding objects may be ignored, and only the speed of the
mobile need be considered.

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 The Transmission Bandwidth Of The Signal
If the transmitted radio signal bandwidth is greater than the "bandwidth" of the
multipath channel, the received signal will be distorted, but the received signal strength
will not fade much over a local area.
The bandwidth of the channel can be quantified by the coherence bandwidth which is
related to the specific multipath structure of the channel.
The coherence bandwidth is a measure of the maximum frequency difference for
which signals are still strongly correlated in amplitude.
If the transmitted signal has a narrow bandwidth as compared to the channel, the
amplitude of the signal will change rapidly, but the signal will not be distorted in time.

41
 Parameters Of Mobile Multipath Fading
Many multipath channel parameters are derived from the power delay profile.
Power delay profiles are generally represented as plots of relative received power as a
function of excess delay with respect to a fixed time delay reference.
Power delay profiles are found by averaging instantaneous power delay profile
measurements over a local area in order to determine an average small-scale power
delay profile.

42
 Time Dispersion Parameters
Multipath channel parameters can be given as
Mean excess delay
RMS delay spread
Excess delay spread
These parameters can be determined from power delay profile.
The time dispersive properties of multipath channels are most commonly
quantified by their mean excess delay and rms delay spread .

43
Mean excess delay

RMS delay spread

where

44
Depends only on the relative amplitude of the multipath components.
Typical RMS delay spreads
Outdoor: on the order of microseconds
Indoor: on the order of nanoseconds
Maximum excess delay (X dB) is defined to be the time delay during which
multipath energy falls to X dB below the maximum.
excess delay =

45
46
 Coherent Bandwidth(Bc)
Coherent band width ,Bc , is a statistic measure of the range of frequencies over
which the channel can be considered to be “flat”.
A channel which passes all spectral components with approximately equal gain and
linear phase.
Two sinusoids with frequency separation greater than Bc are affected quite
differently by the channel.

47
If the coherent bandwidth is defined as the bandwidth over which the frequency
correlation function is above 0.9, then the coherent bandwidth is approximately.

If the frequency correlation function is above 0.5

48
 Doppler Spread and Coherence Time
Doppler spread and coherent time are parameters which describe the time varying
nature of the channel in a small-scale region.
When a pure sinusoidal tone of fc is transmitted, the received signal spectrum, called
the Doppler spectrum, will have components in the range fc-fd and fc+fd, where fd is
the Doppler shift.

49
• Coherent time Tc is the time domain dual of Doppler spread.
• Coherent time is used to characterize the time varying nature of the frequency
dispersiveness of the channel in the time domain.

50
Two signals arriving with a time separation greater than Tc are affected differently
by the channel.
A statistic measure of the time duration over which the channel impulse response is
essentially invariant.
If the coherent time is defined as the time over which the time correlation function
is above 0.5, then

51
 Doppler Shift Calculation

• Δl is small enough to consider

= 1
• v = speed of mobile, λ= carrier wavelength
• fd is +/-ve when moving towards/away the wave

2l 2 d cos( )
Phase difference,   
 
1  v
Doppler Shift, f d   cos( )
2 t 

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 • Doppler Effect: When a wave source and a receiver are moving towards each other, the frequency of
the received signal will not be the same as the source.
• When they are moving toward each other, the frequency of the received signal is higher than the
source.
• When they are opposing each other, the frequency decreases.
Doppler Shift in frequency:
f R  fC  f D
f R  fC  f D MS Moving
speed v
where v is the moving speed, v
f D  cos
 is the wavelength of carrier. 
Signal

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 Example
• Consider a transmitter which radiates a sinusoidal carrier frequency of 1850 MHz. For a vehicle moving
96 km/h, compute the received carrier frequency if the mobile is moving
(a) directly towards transmitter
(b) Directly away from the transmitter
(c) In a direction perpendicular to the direction of arrival of the transmitted signal

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 Solution:
fc = 1850 MHz
λ= c / f
λ = 0.162 m
v = 96 km/h= 26.67 m/s
(a) f = fc+ fd = 1850.00016 MHz
(b) f = fc – fd = 1849.999834 MHz
(c) In this case, θ =90o, cos θ = 0,
And there is no Doppler shift.
f = fc (No Doppler frequency)

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 Syllabus
FUNDAMENTALS OF CELLULAR RADIO SYSTEM DESIGN
Concept of frequency reuse, Co-channel interference, co-channel interference reduction
factor, Desired C/I from a normal case in a omnidirectional antenna system, system
capacity, trunking and grade of service, Improving coverage and capacity in cellular
systems- Cell splitting, Sectoring, Microcell zone concept.

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 Frequency Carries/Channels
The information from sender to receiver is carried over a well defined frequency band.
This is called a channel.
Each channel has a fixed frequency bandwidth (in KHz) and Capacity (bit-rate)
Different frequency bands (channels) can be used to transmit information in parallel and
independently.
Replacing a single, high power transmitter (large cell) with many low power transmitters
(small cells).
Each providing coverage to only a small portion of the service area.
Each base station is allocated a portion of the total number of channels available to the entire
system,
Nearby base stations are assigned different groups of channels.
All the available channels are assigned to a relatively small number of neighboring base
stations.
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 Example

Assume a spectrum of 90KHz is allocated over a base frequency b for communication

between stations A and B
Assume each channel occupies 30KHz.
There are 3 channels
Each channel is simplex (Transmission occurs in one way)
For full duplex communication:
Use two different channels (front and reverse channels)
Use time division in a channel

Channel 1 (b - b+30)
Station A Channel 2 (b+30 - b+60) Station B
Channel 3 (b+60 - b+90)

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 Frequencies for communication

twisted pair coax cable optical transmission

1 Mm 10 km 100 m 1m 10 mm 100 m 1 m
300 Hz 30 kHz 3 MHz 300 MHz 30 GHz 3 THz 300 THz

VLF LF MF HF VHF UHF SHF EHF infrared visible UV

light
VLF = Very Low Frequency UHF = Ultra High Frequency
LF = Low Frequency SHF = Super High Frequency
MF = Medium Frequency EHF = Extra High Frequency
HF = High Frequency UV = Ultraviolet Light
VHF = Very High Frequency

Frequency and wave length:  = c/f

wave length , speed of light c  3x108m/s, frequency f
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 Coverage Aspect of Next Generation Mobile
Communication Systems

Satellite
In-Building

Urban
Suburban

Global

Picocell Microcell Macrocell Global

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 Cellular Geometries

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 FREQUENCY REUSE
 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

 “The design process of selecting and allocating channel groups for all of the cellular base
station within a system is FREQUENCY REUSE/PLANNING”

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 Cell
 Cell is the small geographic area covered by the base station.
 The area around an antenna where a specific frequency range is used.
 Cell is represented graphically as a hexagonal shape, but in reality it is irregular in shape.

cell

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 Cell Shape
R
R
R
Cell
R R

(a) Ideal cell (b) Actual cell (c) Different cell models

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 Cellular Geometries

• The most common model used for wireless networks is uniform hexagonal
shape areas
– A base station with omni-directional antenna is placed in the middle of the cell

d  3R

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 Fundamentals of Cellular Systems

Ideal cell area

(2-10 km radius)

BS
Cell
MS

Alternative MS
Hexagonal cell area
shape of a cell
used in most models

Illustration of a cell with a mobile station and a base station

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 Why hexagon for theoretical coverage?
For a given 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 hexagon
closely approximates a circular radiation pattern which would occur for an omnidirectional BS antenna and
free space propagation
When using hexagons to model a coverage areas, BS transmitters are depicted as either being in the center of
the cell (center-excited cells) or on the three of the six cell vertices (edge-excited cells)
Normally omnidirectional antennas are used in center-excited cells and directional antennas are used in
corner-excited cells

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 Signal Strength
Signal strength
(in dB)

Cell i Cell j
-60 -60
-70 -70
-80 -80
-90
-90 -100
-100

Select cell i on left of boundary Select cell j on right of boundary

Ideal boundary
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 Frequency Reuse
An efficient way of managing the radio spectrum is by reusing the same frequency, within the
service area, as often as possible
This frequency reuse is possible thanks to the propagation properties of radio waves

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 How Often Are Frequencies Reused (Frequency Reuse Factor)?

Cells with the same number have the same set of frequencies

Frequency Reuse

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 Cluster:

A cluster is a group of adjacent cells.

No frequency reuse is done within a cluster.
Number of cells in cluster N=i2+ij+j2

2
1 7 3
1
3 1
3
2 6 4
2
4 5

3-cell cluster 4-cell cluster 7-cell cluster

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VBIT potharajuvidyasagar.wordpress.com
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 2

7 3 2
1 7 3

6 4 1

5 6 4

2 5

7 3 2

1 7 3

6 4 1
5 6 4

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VBIT potharajuvidyasagar.wordpress.com
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 Frequency Allocation Concepts
• Consider a cellular system which has a total of S duplex channels.
• Each cell is allocated a group of k channels, k  S .
• The S channels are divided among N cells.
• The total number of available radio channels S  kN
• The N cells which use the complete set of channels is called cluster.
• The cluster can be repeated M times within the system. The total number of channels, C, is used as a
measure of capacity
C  MkN  MS

• The capacity is directly proportional to the number of replication M.

• The cluster size, N, is typically equal to 4, 7, or 12.
• Small N is desirable to maximize capacity.
• The frequency reuse factor is given by 1 / N

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 Frequency Reuse
The frequency reuse concept can be used in the time domain and the space domain.
Frequency reuse in the time domain results in the occupation of the same frequency in
different time slots. It is called time-division multiplexing (TDM). Frequency reuse in the
space domain can be divided into two categories. F7 F2
1. Same frequency assigned in two different
geographic areas, such as AM or FM radio F7 F2 F6 F1
F1 F3
stations using the same frequency in different
cities. F6 F1
F1 F3 F5 F4 F7 F2
2. Same frequency repeatedly used in a same
general area in one system2—the scheme is used F5 F4 F7 F2 F6 F1
F1 F3
in cellular systems. There are many cochannel
cells in the system. The total frequency spectrum F6 F1
F1 F3 F5 F4
allocation is divided into K frequency reuse
patterns, as illustrated F5 F4
in Fig. for K = 4, 7, 12, and 19.
VBIT 7 cell reuse cluster
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Fx: Set of frequency
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 • Hexagonal geometry has
– exactly six equidistance neighbors
– the lines joining the centers of any cell and each of its neighbors are separated by
multiples of 60 degrees.
• Only certain cluster sizes and cell layout are possible.
• The number of cells per cluster, N, can only have values which satisfy
j
 N = 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, 28, …, etc.
The popular value of N being 4 and 7. 60o

N  i 2  ij  j 2 i
where i and j are integers.

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 j=1

j=1 i=2
i=2 j=1
j direction
i=2
60° i=2
i direction j=1
j=1
1 2 3… i i=2 i=2
j=1
(a) Finding the center of an adjacent cluster using integers i
and j (direction of i and j can be interchanged). (b) Formation of a cluster for N = 7 with i=2 and j=1

j=2 i=3 j=2 i=3

j=2 i=2 j=2 j=2

i=2 j=2
i=2 i=3
i=2 i=3
i=2 j=2
j=2
j=2 i=2 j=2

j=2 i=3 j=2 i=3

(c) A cluster with N =12 with i=2 and j=2 (d) A Cluster with N = 19 cells with i=3 and j=2
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 Reuse Distance

R Cluster
• For hexagonal cells, the reuse distance is given by

F7 F2 D  3N R

F6 F1
F1 F3
where R is cell radius and N is the reuse pattern
(the cluster size or the number of cells per cluster).
F5 F4 F7 F2
D
F6 F1
F1 F3
• Reuse factor is q   3N
R
F5 F4

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 Frequency Reuse
• For hexagonal cells, the number of cells in the cluster is given by

N  I 2  J 2  ( I  J ), I , J  1,2,3,4...
N  {1,3,4,7,9,12,16,19,21,...}
D  R 3N

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 Smaller N is greater capacity

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 D  R 3N

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 Geometry of Hexagonal Cells (1)
How to determine the DISTANCE between the nearest co-channel cells ?
Planning for Co-channel cells
D is the distance to the center of the nearest co-channel cell
R is the radius of a cell

3R
i
30o
R
3R
0

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where D = Distance between the cells using the same frequency,
R = Center to vertex distance,
N = Cluster size,
q = Reuse frequency.
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VBIT potharajuvidyasagar.wordpress.com
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 D2 = 3 * R2 * (i2 + j2 + i * j)
As N = i2 + j2 + i * j
D2 = 3 * R 2 * N
D2 /R2 = 3*N
D/R = √3 N
As q = D/R
q = √3N

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 Co-channel cells for 7-cell reuse

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 Cochannel Interference

First tier cochannel

Second tier cochannel Base Station
Base Station

R
D6
D5
D1

D4 Mobile Station
D2

D3

Serving Base Station

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VBIT potharajuvidyasagar.wordpress.com
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 Cochannel Interference

 Cochannel interference ratio is given by

C Carrier C
  M
 Ik
I Interference
k 1

where I is co-channel interference and M is the maximum

number of co-channel interfering cells.
For M = 6, C/I is given by

C C
 where g is the propagation path loss slope
g
I M  D  and g = 2~5.
  k 
 R 
k 1

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 Co-Channel Interference
Consider only the first tier of interfering cells,
if all interfering base stations are equidistant from the desired base station and if this
distance is equal to the distance D between cell centers,
then the above equation can be simplified to:
(i.e., r=R and assume Di=D and use q=D/R):

S r  R  ( D / R ) q 
 NI
 
 
I NI D NI NI
 Di
i 1


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 DESIRED C/I FROM A NORMAL CASE IN AN OMNIDIRECTIONAL ANTENNA SYSTEM
Assume that all Dk are the same for simplicity, as shown in Fig; then D = Dk , and q = qk , and

the value of C/I is based on the required system performance and the specified value of γ
is based on the terrain environment. With given values of C/I and γ , the cochannel
interference reduction factor q can be determined. Normal cellular practice is to specify
C/I to be 18 dB or higher based on subjective tests.
this acceptance implies that both mobile radio multipath fading and cochannel interference
become ineffective at that level. The path-loss slope γ is equal to about 4 in a mobile radio
environment
1/4
q = D/R = (6 × 63.1) = 4.41
The 90th percentile of the total covered area would be achieved by increasing the transmitted power at each cell;
increasing the same amount of transmitted power in each cell does not affect the result of Eq. (2.7-4). This is because
q is not a function of transmitted power. The computer simulation described in the next section finds the value of q
= 4.6, which is very close to Eq. The factor q can be related to the finite set of cells K in a hexagonal-shaped cellular
system by

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 Example:
Co-Channel Interference
If S/I = 15 dB required for satisfactory performance for forward
channel performance of a cellular system.

a) What is the Frequency Reuse Factor q (assume K=4)?

b) Can we use K=3?

Assume 6 co-channels all of them (same distance from the mobile), I.e. N=7

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 Example: Co-Channel Interference
a) NI =6 => cluster size N= 7, and when =4
The co-channel reuse ratio is q=D/R=sqrt(3N)=4.583

S q 1
  6 (4.583) 4  75.3
I NI
Or 18.66 dB  greater than the minimum required level  ACCEPT IT!!!
b) N= 7 and =3 
S q
  16 (4.583)3  16.04
I NI
Or 12.05 dB  less than the minimum required level  REJECT IT!!!

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 Example: co-Channel Interference
We need a larger N (thus q is larger). Use eq. N =i2+ij+j2, for i=j=2  next
possible value is N=12.

q=D/R=sqrt(3N) =6 and =3


S q
  111 (6)3  19.6
I NI

Or 15.56 dB  N=12 can be used for minimum requirement, but it decreases the capacity
(we already gave an example: when cluster size is smaller, the capacity is larger).

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 Worst Case Co-Channel Interference
i.e., mobile terminal is located at the cell boundary where it receives the weakest signal from its own cell but
is subjected to strong interference from all all the interfering cells.

We need to modify our assumption, (we assumed Di=D).

The S/I ratio can be expressed as
S r  R  D+R
 
I NI
2( D  R)   2 D   2( D  R) 
 Di 
R
i 1
D D+R
S 1

I 2(q  1) 4  2q   2(q  1) 4
D
D-R
Used D/R=q and  =4. D-R
Where q=4.6 for
normal seven cell reuse pattern.

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 • For hexagonal geometry with 7-cell cluster, with the mobile unit being at the cell
boundary, the signal-to-interference ratio for the worst case can be approximated as

S R 4

I 2( D  R) 4  ( D  R / 2) 4  ( D  R / 2) 4  ( D  R) 4  D 4

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 Example: Worst Case Cochannel Interference

A cellular system that requires an S/I ratio of 18dB. (a) if cluster size is 7, what is the worst-case
S/I? (b) Is a frequency reuse factor of 7 acceptable in terms of co-channel interference? If not, what
would be a better choice of frequency reuse ratio?

 Solution
(a) N=7  q = 3N  4.6 . If a path loss component of =4, the worst-case signal-to-interference ratio is
S/I = 54.3 or 17.3 dB.
(b) The value of S/I is below the acceptable level of 18dB. We need to decrease I by
increasing N =9. The S/I is 95.66 or 19.8dB.

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 Example: Worst Case Cochannel Interference
For a conservative estimate if we use the shortest distance (=D-R) then
S 1
 4
 1
6 ( 3.6 ) 4
 28
I 6(q  1)

Or 14.47 dB.
REMARK: In real situations, because of imperfect cell site locations and the rolling nature of
the terrain configuration, the S/I ratio is often less than 17.3 dB. It could be 14dB or lower
which can occur in heavy traffic.
Thus, the cellular system should be designed around the S/I ratio of the worst case.

REMARK:
If we consider the worst case for a 7-cell reuse pattern We conclude that a co-channel interference
reduction factor of q=4.6 is not enough in an omnidirectional cell system.
In an omnidirectional cell system N=9 (q=5.2) or N=12 (q=6.0) the cell reuse pattern would be a better
choice. These cell reuse patterns would provide the S/I ratio of 19.78 dB and 22.54 dB, respectively.

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 CAPACITY EXPANSION IN CELLULAR SYSTEM

Techniques to provide more channels per coverage area is by

Cell splitting
Cell sectoring
Coverage zone approches

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 CELL SPLITTING
Cell splitting increases the capacity of cellular system since it increases the
number of times the channel are reused
Cell splitting - defining new cells which have smaller radius than orginal cells by
installing these smaller cells called MICROCELLS between existing cells
Capacity increases due to additional number of channels per unit area

“Cell splitting is process of subdividing a congested cell into smaller cells

each with its own base station(with corresponding reduction in antenna
height and tx power)”

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 Cell Splitting

Large cell
(low density)

Small cell
(high density)
Smaller cell
(higher density)

Depending on traffic patterns the smaller

cells may be activated/deactivated in
order to efficiently use cell resources.

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 CELL SPLITTING
Split congested cell into smaller cells.
– Preserve frequency reuse plan. Reduce R to R/2
– Reduce transmission power.

microcell

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 • Transmission power reduction from Pt1 to Pt 2
• Examining the receiving power at the new and old cell boundary
Pr [at old cell boundary ]  Pt1R  n
Pr [at new cell boundary ]  Pt 2 ( R / 2)  n

• If we take n = 4 (path loss) and set the received power equal to each other
Pt1
Pt 2 
16
• The transmit power must be reduced by 12 dB in order to fill in the original coverage area.
• Problem:
if only part of the cells are splited
– Different cell sizes will exist simultaneously
• Handoff issues - high speed and low speed traffic can be simultaneously accommodated

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 CELL SPLITTING

•Splitting cells in each CELL

•Antenna downtiliting

Illustration of cell splitting within a 3 km by 3 km square

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 Sectoring
• Decrease the co-channel interference and keep the cell radius R unchanged
– Replacing single omni-directional antenna by several directional antennas
– Radiating within a specified sector

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 Cell Sectoring by Antenna Design

c
c
120o 120o
a
b a
b

(a). Omni (b). 120o sector (c). 120o sector (alternate)

d f
90o e 60o a
a
c
d b
b c

(d). 90o sector (e). 60o sector

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 Cell Sectoring by Antenna Design
 Placing directional transmitters at corners where three adjacent cells meet

C
X
A

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 Microcell Zone Concept
• Antennas are placed at the outer edges of the cell
• Any channel may be assigned to any zone by the base station
• Mobile is served by the zone with the strongest signal.
• Handoff within a cell
– No channel re-assignment
– Switch the channel to a different zone site
• Reduce interference
– Low power transmitters are employed
Microcell
• Micro cells can be introduced to alleviate capacity problems
caused by “hotspots”.
• By clever channel assignment, the reuse factor is unchanged. As
for cell splitting, there will occur interference problems when
macro and micro cells must co-exist.
Microcells-Reduced power, Good for city streets, along roads and
inside large buildings

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 Terminology : Cellular traffic
 Trunking : Trunking is the concept that allows large number of users to use a smaller number of
channels(or phone lines, customer service representatives, parking spots, public bathrooms, …)
as efficiently as possible.
 Grade of service (GoS) : A user is allocated a channel on a per call basis. GoS is a measure of the
ability of a user to access a trunked system during the busiest hour. It is typically given as the
likelihood that a call is blocked (also known as blocking probability mentioned before).
 Trunking theory : is used to determine the number of channels required to service a certain
offered traffic at a specific GoS.
 Call holding time (H) : the average duration of a call.
 Request rate (λ) : average number of call requests per-unit time.

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 Trunking and Grade of Service
• Erlangs: One Erlangs represents the amount of traffic density carried by a channel that is
completely occupied.
– Ex: A radio channel that is occupied for 30 minutes during an hour carries 0.5 Erlangs of traffic.
• Grade of Service (GOS): The likelihood that a call is blocked.
• Each user generates a traffic intensity of Au Erlangs given by Au  H

H: average duration of a call.

 : average number of call requests per unit time
• For a system containing U users and an unspecified number of channels, the total offered traffic
intensity A, is given by
A  UAu

• For C channel trunking system, the traffic intensity, Ac is given as Ac  UAu / C

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 Traffic flow or intensity A
 Measured in Erlang, which is defined as the call minute per minute.
 Offered traffic for a single user is given as Au = λ ⋅H
 λ  average number of call request
 H  duration of a call
 For a system containing U user, total offered traffic A = U⋅ Au
 Exercise :
 There are 3000 calls per hour in a cell, each lasting an average of 1.76 min.
 Offered traffic A = (3000/60)(1.76) = 88 Erlangs
 If the offered traffic exceeds the maximum possible carried traffic, blocking occurs. There are
two different strategies to be used.
 Blocked calls cleared
 Blocked calls delayed
 Trunking efficiency : is defined as the carried traffic intensity in Erlangs per channel, which
is a value between zero and one. It is a function of the number of channels per cell and the
specific GoS parameters.
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 Capacity
S = no of duplex channels available
K = no of channel in one cell
N = no of cell/cluster
M = no of cluster in a given system
C = total no of duplex channel available in a cellular system (capacity)
C=M*K*N=M*S

Example:
For K = 100, N = 7, calculate system capacity for M = 6 and M = 4
i) C = 6 * 100 * 7 = 4200 channels
ii) C = 4 * 100 * 7 = 2800 channels

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 Example 1

If total of 44 MHz of bandwidth is allocated to a particular FDD cellular

radio system which uses two 25kHz simplex channels to provide full duplex voice
and control channel, calculate the number of channels available per cell, k for

(a) 3 cell reuse

(b) 7 cell reuse
(c) 12 cell reuse

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 Formula (Cellular Traffic)
i) Total number of channel per cell (NC)

NC = (Allocated spectrum) / (channel BW x Frequency reuse factor)

unit = channel / cell
ii) No.of cell in the service area = Total coverage area / area of the cell.
Unit = cell
Traffic intensity of each cell can be found from table or Erlang B chart. Depend on NC and GOS
Traffic capacity = # of cell x traffic intensity /cell ( Erlang )

Iii) Total no.of user (U) = total traffic (A) / traffic per user ( Au)
Iv) number of call that can be made at any time = NC x no.of cell

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 Example 2
How many users can be supported for 0.5% blocking probability for the
following number of trunked channels in a blocked calls cleared system.
(a) 1
(b) 5
(c) 10
(d) 20
(e) 100

Assume each user generates 0.1 Erlangs of traffic

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 Example 3
A city has an area of 3000 sq.km and is covered by a cellular system using 7 cell per cluster. The area of
a cell is 100 sq.km. The cellular system is allocated total bandwidth of 40 MHz of spectrum with full
duplex channel bandwidth of 200 KHz. For the GOS of 2 % and the offered traffic per user is 0.03
Erlangs, calculate;
a) The number of cell in the city
b) The number of channels per cell
c) Traffic intensity of each cell
d) Traffic intensity for the city
e) The total number of users that can be served in the city
f) The number of mobiles per channel
g) Number of call that can be made at any time in the city

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 Erlang B Trunking GOS

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 Erlang B

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125 Thank you………………

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