cellular and mobile communicationsfundamentals of cellular radio system design concept of frequency...

CELLULAR AND MOBILE COMMUNICATIONS

by

VIDYA SAGAR POTHARAJU

Associate Professor,

Dept of ECE,

VBIT.

VBIT potharajuvidyasagar.wordpress.com 110/4/2018

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.


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.


Conventional Mobile System

10/4/2018 4VBIT potharajuvidyasagar.wordpress.com

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.


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.

Poor Service Performance


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)

Inefficient frequency spectrum utilization


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.


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.


A Basic Cellular System

Fig 1 : Basic Cellular system


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.


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.

Mobile Communication Operation


• 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

Evolution of Mobile Radio Communications


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



Forw ard Channel

Reverse Channel

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


1G

(<1Kbps)

1 Kbps

10 Kbps

100 Kbps

2 Mbps

1 Mbps

Data Rates

1980 1990 2000 2010

2G

(9.6Kbps)

2.5G

(10-150Kbps)

3G

(144Kbps to 2Mbps)

Years

Overview


Cellular networks: From 1G to 5G


Introduction to Radio Wave Propagation

• The mobile radio channel places fundamental limitations on theperformance of wireless communication systems

• Paths can vary from simple line-of-sight to ones that are severely obstructedby 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 amobile terminals moves about.


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


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


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


Ground Reflection (2-Ray) Model

•a model where the receiving antenna sees a direct path signal as well as a signal reflected offthe ground.•In a mobile radio channel, a single direct path between the base station and mobile is rarelythe 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 severalkilometers for mobile radio systems using tall towers ( heights above 50 m )


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,


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.


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.


Illustration ..


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.


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

•VBIT potharajuvidyasagar.wordpress.com 3110/4/2018

http://people.deas.harvard.edu/~jones/es151/prop_models/propagation.html#fsl

http://people.deas.harvard.edu/~jones/es151/prop_models/propagation.html#pel

http://people.deas.harvard.edu/~jones/es151/prop_models/propagation.html



Radio Propagation Mechanisms

1

2

3

4

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


Free Space Propagation Model

•Free space propagation model is used to predict:

•Received Signal Strength when the transmitter and receiver have a clear, unobstructedLoS between them.

•The free space propagation model assumes a transmit antenna and a receive antenna to belocated in an otherwise empty environment. Neither absorbing obstacles nor reflectingsurfaces are considered. In particular, the influence of the earth surface is assumed to beentirely absent.

Satellite communication systems and microwave line-of-sight radio links typically undergo freespace propagation.


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)


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


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


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


Multipath Propagation.

38

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.

Speed Of The Mobile

39

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.

Speed Of Surrounding Objects

40

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.

The Transmission Bandwidth Of The Signal

41

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.

Parameters Of Mobile Multipath Fading

42

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.

Time Dispersion Parameters

43

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 .

44

Mean excess delay

RMS delay spread

where

45

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 =

Coherent Bandwidth(Bc)

47

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.

48

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

Doppler Spread and Coherence Time

49

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.

50

• 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.

51

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

Doppler Shift Calculation

• Δl is small enough to consider

• v = speed of mobile, λ= carrier wavelength

• fd is +/-ve when moving towards/away the wave

2 2 cos( )Phase difference,

1Doppler Shift, cos( )

2d

l d

vf

t

= 1


• 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:

where v is the moving speed,

is the wavelength of carrier.

cos

vfD

DCR fff MS

Signal

Moving

speed v

DCR fff


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


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)


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.

VBIT potharajuvidyasagar.wordpress.com2-5610/4/2018

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.

Frequency Carries/Channels


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

Example

Channel 1 (b - b+30)

Channel 2 (b+30 - b+60)

Channel 3 (b+60 - b+90)

Station A Station B


VLF = Very Low Frequency UHF = Ultra High FrequencyLF = Low Frequency SHF = Super High FrequencyMF = Medium Frequency EHF = Extra High FrequencyHF = High Frequency UV = Ultraviolet LightVHF = Very High Frequency

Frequency and wave length: = c/f

wave length , speed of light c 3x108m/s, frequency f

Frequencies for communication

1 Mm300 Hz

10 km30 kHz

100 m3 MHz

1 m300 MHz

10 mm30 GHz

100 m3 THz

1 m300 THz

visible light

VLF LF MF HF VHF UHF SHF EHF infrared UV

optical transmissioncoax cabletwisted pair


Coverage Aspect of Next Generation Mobile

Communication Systems

Picocell Microcell Macrocell Global

Urban

Suburban

Global

Satellite

In-Building


Cellular Geometries


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”


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

cell


Cell Shape

Cell

R

(a) Ideal cell (b) Actual cell

R

R R

R

(c) Different cell models


Rd 3

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


Fundamentals of Cellular Systems

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

BS

MS

Cell

Hexagonal cell area

used in most models

Ideal cell area

(2-10 km radius)

Alternative

shape of a cell

MS


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

Why hexagon for theoretical coverage?


Signal Strength

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

Ideal boundary

Cell i Cell j

-60

-70

-80

-90

-100

-60-70

-80-90

-100

Signal strength

(in dB)


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

Frequency Reuse


Cells with the same number have the same set of frequencies

How Often Are Frequencies Reused (Frequency Reuse Factor)?

Frequency Reuse


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

1

3

2

1

4

3

2

2

7

5

4

3

1

6

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


2

7

5

4

3

1

6

2

7

5

4

3

1

6

2

7

5

4

3

1

6

2

7

5

4

3

1

6


• Consider a cellular system which has a total of S duplex channels.

• Each cell is allocated a group of k channels, .

• The S channels are divided among N cells.

• The total number of available radio channels

• 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

• 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

Sk

kNS

MSMkNC

N/1

Frequency Allocation Concepts


Frequency Reuse

F1

F2

F3

F4F5

F6

F7 F1

F2

F3

F4F5

F6

F7

F1

F2

F3

F4F5

F6

F7 F1

F2

F3

F4F5

F6

F7

F1

F1

F1

F1

Fx: Set of frequency 7 cell reuse cluster

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.

1. Same frequency assigned in two different

geographic areas, such as AM or FM radio

stations using the same frequency in different

cities.

2. Same frequency repeatedly used in a same

general area in one system2—the scheme is used

in cellular systems. There are many cochannel

cells in the system. The total frequency spectrum

allocation is divided into K frequency reuse

patterns, as illustrated

in Fig. for K = 4, 7, 12, and 19.


• 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

22 jijiN

where i and j are integers.

N = 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, 28, …, etc.

The popular value of N being 4 and 7.

i

j

60o


(c) A cluster with N =12 with i=2 and j=2

i=3

j=2

i=3 j=2 i=3

j=2

i=3

j=2

i=3j=2i=3

j=2

(d) A Cluster with N = 19 cells with i=3 and j=2

j=2

j=2

j=2

j=2j=2

j=2

i=2

i=2

i=2

i=2

i=2

i=2

(b) Formation of a cluster for N = 7 with i=2 and j=1

60°

1 2 3 … i

j direction

i direction

(a) Finding the center of an adjacent cluster using integers i

and j (direction of i and j can be interchanged).

i=2i=2j=1

j=1

j=1

j=1

j=1

j=1

i=2

i=2

i=2i=2


Reuse Distance

F1

F2

F3

F4F5

F6

F7

F1

F2

F3

F4F5

F6

F7

F1

F1

• For hexagonal cells, the reuse distance is given by

RND 3

R

where R is cell radius and N is the reuse pattern

(the cluster size or the number of cells per cluster).

NR

Dq 3• Reuse factor is

Cluster


NRD

N

JIJIJIN

3

,...}21,19,16,12,9,7,4,3,1{

...4,3,2,1,),(22

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


Smaller N is greater capacity


NRD 3


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

R

30o

R3

R3i

j

D

0


where D = Distance between the cells using the same frequency,

R = Center to vertex distance,

N = Cluster size,

q = Reuse frequency.


D2 = 3 * R2 * (i2 + j2 + i * j)

As N = i2 + j2 + i * j

D2 = 3 * R2 * N

D2 /R2 = 3*N

D/R = √3 N

As q = D/R

q = √3N


Co-channel cells for 7-cell reuse


Cochannel Interference

Mobile Station

Serving Base Station

First tier cochannel

Base StationSecond tier cochannel

Base Station

R

D1

D2

D3

D4

D5

D6


Cochannel Interference

Cochannel interference ratio is given by

M

k

kI

C

ceInterferen

Carrier

I

C

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

M

k

k

R

D

C

I

C

1

gwhere g is the propagation path loss slope

and g = 2~5.


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):

III

N

i

i

N

q

N

RD

DN

R

D

r

I

SI

)/(

1


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

q = D/R = (6 × 63.1) = 4.411/4

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


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


Example: Co-Channel Interferencea) NI =6 => cluster size N= 7, and when =4

The co-channel reuse ratio is q=D/R=sqrt(3N)=4.583

3.75)583.4( 4

61

IN

q

I

S

Or 18.66 dB greater than the minimum required level ACCEPT IT!!!

b) N= 7 and =3

04.16)583.4( 3

61

IN

q

I

S

Or 12.05 dB less than the minimum required level REJECT IT!!!


Example: co-Channel Interference

6.19)6( 3

111

IN

q

I

S

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).

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


Worst Case Co-Channel Interferencei.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

R

D-R

D-R

D+R

D+R

D

D

44

1

)1(22)1(2

1

)(22)(2

qqqI

S

RDDRD

R

D

r

I

SIN

i

i

Used D/R=q and =4. Where q=4.6 for

normal seven cell reuse pattern.


• 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

44444

4

)()2/()2/()(2

DRDRDRDRD

R

I

S


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-caseS/I? (b) Is a frequency reuse factor of 7 acceptable in terms of co-channel interference? If not, whatwould be a better choice of frequency reuse ratio?

Solution

(a) N=7 q = . 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.

6.43 N


Example: Worst Case Cochannel InterferenceFor a conservative estimate if we use the shortest distance (=D-R) then

28)1(6

14)6.3(6

1

4

qI

S

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.


Techniques to provide more channels per coverage area is by

Cell splitting

Cell sectoring

Coverage zone approches

CAPACITY EXPANSION IN CELLULAR SYSTEM


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

“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)”


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.


CELL SPLITTING

Split congested cell into smaller cells.

– Preserve frequency reuse plan.

– Reduce transmission power.

microcell

Reduce R to R/2


• Transmission power reduction from to

• Examining the receiving power at the new and old cell boundary

• If we take n = 4 (path loss) and set the received power equal to each other

• 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

1tP 2tP

n

tr RPP 1]boundary cell oldat [n

tr RPP )2/(]boundary cellnew at [ 2

16

PP 1t

2t


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

CELL SPLITTING

•Splitting cells in each CELL

•Antenna downtiliting


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


Cell Sectoring by Antenna Design

60o

120o

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

(e). 60o sector

120o

(c). 120o sector (alternate)

a

b

c

ab

c

(d). 90o sector

90o

a

b

c

d

a

bc

d

e

f


Cell Sectoring by Antenna Design

Placing directional transmitters at corners where three adjacent cells meet

A

C

B

X


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


Terminology : Cellular traffic

Trunking : Trunking is the concept that allows large number of users to use a smaller number ofchannels(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.


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 Erlangs given by

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

• For C channel trunking system, the traffic intensity, is given as

HAu

uUAA

cA CUAA uc /

uA


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.


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


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


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


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


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 ofa cell is 100 sq.km. The cellular system is allocated total bandwidth of 40 MHz of spectrum with fullduplex channel bandwidth of 200 KHz. For the GOS of 2 % and the offered traffic per user is 0.03Erlangs, 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


Erlang B Trunking GOS


Erlang B


Thank you………………

VBIT potharajuvidyasagar.wordpress.com

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