Cellular concepts

04132023 1

UNIT-3

THE CELLUAR CONCEPT

DrVrince Vimal MIT MIET GROUP MEERUT

04132023 DrVrince Vimal MIT MIET GROUP MEERUT 2

Cellular Systems--Cellular Concepts The cellular concept was a major breakthrough in solving the

problem of spectral congestion and user capacity It offered very high capacity in a limited spectrum allocation without any major technological changes

The cellular concept has the following system level ideas

Replacing a single high power transmitter with many low power transmitters each providing coverage to only a small area

Neighboring cells are assigned different groups of channels in order to minimize interference

The same set of channels is then reused at different geographical locations


When designing a cellular mobile communication system it is important to provide good coverage and services in a high user-density area

Reuse can be done once the total interference from all users in the cells using the same frequency (co-channel cell) for transmission suffers from sufficient attenuation Factors need to be considered include

Geographical separation (path loss)

Shadowing effect


Cell Footprint

The actual radio coverage of a cell is known as the cell footprint

Irregular cell structure and irregular placing of the transmitter may be acceptable in the initial system design However as traffic grows where new cells and channels need to be added it may lead to inability to reuse frequencies because of co-channel interference

For systematic cell planning a regular shape is assumed for the footprint



Frequency reuse Cellular system depends upon intelligent allocation and reuse

of channels Each BS is allocated separate group of channels to be used in

small geographic region called as CELL Adjacent cells are allocated separate group of channels BS antenna are designed to provide coverage to particular

cell By doing this same group of channels can be used again in

separate cells physically at large distance from cell containing those channels by very well keeping interference within tolerable limits

This design process of selecting and allocating the channels of CBS within system is called as frequency reuse


A cellular system which has a total of S duplex channels

S channels are divided among N cells with each cell uses unique and disjoint channels

If each cell is allocated a group of k channels then S = k N


Terminologybull Cluster size The N cells which collectively use the

complete set of available frequency is called the cluster size

bull Co-channel cell The set of cells using the same set of frequencies as the target cell

bull Interference tier A set of co-channel cells at the same distance from the reference cell is called an interference tier The set of closest co-channel cells is call the first tier There is always 6 co-channel cells in the first tier


Co-ordinates for hexagonal cellular geometry

bull With these co-ordinates an array of cells can be laid out so that the center of every cell falls on a point specified by a pair of integer co-ordinates



Designing a cellular system

bull N=19bull (i=3 j=2)



bull The cluster size must satisfy N = i2 + ij + j2 where i j are non-negative integers


bull Can also verify that

where Q is the co-channel reuse ratio

Problem

bull Total 33 MHz bw allocated to a FDD cellular system which uses 25 kHz simplex channels to provide full duplex voice and control channels Find the number of channels available per cell if a system uses a) four-cell reuse b) 7-cell reuse If 1 MHz of the allocated spectrum is dedicated to control channels find an equitable distribution of control channels amp voice channels

bull Total bw = 33 MHzndash Channel bw = 2 X 25khz = 50 khz ndash Total available channels S = 33 00050 = 660 channels

bull For N=4ndash Total no of ch per cell k = 6604 = 165 channels

bull For N = 7ndash Total no of ch per cell k = 6607 = 95 channels

bull 1MHz for control channels ie 100050 = 20 control channels So only 640 channels (660-20) would be allotted for voice

bull For N = 4 ndash 5 control ch + 160 voice ch per cell

bull For N =7ndash 4 cells with (3 control ch + 92 voice ch) amp 2 cells with (3 control + 90 voice ch) amp 1 cell with (2 control

ch + 92 voice channels)ndash Each cell with 1 control ch and 4 cells with 91 voice ch and 3 cells with 92 voice ch


Handover Handoff

Occurs as a mobile moves into a different cell during an existing call or when going from one cellular system into another

It must be user transparent successful and not too frequent

Not only involves identifying a new BS but also requires that the voice and control signals be allocated to channels associated with the new BS


Once a particular signal level Pmin is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level PHO is used as a threshold at which a handover is made


bull =handoff threshold -Minimum acceptablesignal to maintain the callbull too smallndash Insufficient time to complete handoff before call is lostndash More call losses

bull too largendash Too many handoffsndash Burden for MSC


Dwell Time

The time over which a user remains within one cell is called the dwell time

The statistics of the dwell time are important for the practical design of handover algorithms

The statistics of the dwell time vary greatly depending on the speed of the user and the type of radio coverage


Handover indicator

Each BS constantly monitors the signal strengths of all of its reverse voice channels to determine the relative location of each mobile user with respect to the BS This information is forwarded to the MSC who makes decisions regarding handover

Mobile assisted handover (MAHO) The mobile station measures the received power from surrounding BSs and continually reports the results of these measurements to the serving BS

Practical handover

bull The Umbrella Cell approach will help to solve this problems High speed users are serviced by large (macro) cells while low speed users are handled by small (micro) cells

Practical handoverbull A hard handover does ldquobreak before makerdquo

ie The old channel connection is broken before the new allocated channel connection is setup This obviously can cause call dropping

bull In soft handover we do ldquomake before breakrdquo ie The new channel connection is established before the old channel connection is released This is realized in CDMA where also BS diversity is used to improve boundary condition

Interference and System Capacity

bull In a given coverage area there are several cells that use the same set of frequencies These cells are called co-channel cells The interference between signals from these cells is called co-channel interference

bull If all cells are approximately of the same size and the path loss exponent is the same throughout the coverage area the transmit power of each BS is almost equal We can show that worse case signal to co-channel interference is independent of the transmitted power It becomes a function of the cell radius R and the distance to the nearest co-channel cell Drsquo

bull On control channel If leads to missed or block calls


In urban areas more severe due high RF noise floor

The 2 major types are

Co-Channel interference

Adjacent channel interfernce



Q- Co-Channel reuse ratio is given by-

Let i0 is the no of co- channel nterfering cells than

ndash Received power at a distance d from the transmitting antenna is approximated by

ndash Useful signal at the cell boundary is the weakest given by Pr (R) Interference signal from the co-channel cell is given to be Pr (D ) prime

Interference and System Capacityndash Drsquo is normally

approximated by the base station separation between the two cells D unless when accuracy is needed Hence


bull For the forward link a very general case

where Di is the distance of the ith interfering cell from the mobile i0 is the total number of co-channel cells exist


bull If only first tier co-channel cells are considered then i0 = 6

Unless otherwise stated normally assuming Di asymp D for all i

Outage probabilitybull The probability that a mobile station does not receive a

usable signalbull For GSM this is 12 dB and for AMPS this is 18 dB If

there is 6 co-channel cells then

bull Exercise please verify thisndash For n=4 a minimum cluster size of N=7 is needed to meet the

SIR requirements for AMPSndash For n=4 a minimum cluster size of N=4 is required to meet the

SIR requirements for GSM

Outage probability

Outage probability

bull Approximation in distance has been made on the 2nd tier onwards

Outage probability

bull More accurate SIR can be obtained by computing the actual distance

bull Our computation of outage only based on path loss For more accurate modeling shadowing and fast fading need to be taken into consideration This will not be covered in this course

Coverage Problemsbull Revision

ndash Recall that the mean measured value

ndash Measurement shows that at any value of d the path loss PL(d) at a particular location is random and distributed log-normally (normal in dB) about this mean value

Pr (d)dB = Pr (d)dB + Xσwhere Xσ is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ(in dB)

Boundary coveragebull There will be a proportion of locations at distance R (cell radius) where a

terminal would experience a received signal above a threshold γ (γ is usually the receiver sensitivity)

bull where Q(x) is the standard normal distribution

Cell coveragebull Proportion of locations within the area defined by the cell

radius R receiving a signal above the threshold γ

Cell coverage Solution can be found using the graph provided (n path loss exponent)

Cell coveragebull Example if n=4 σ=8 dB and if the boundary is to have

75 coverage (75 of the time the signal is to exceed the threshold at the boundary) then the area coverage is equal to 94

bull If n=2 σ=8 dB and if the boundary is to have 75 coverage then the area coverage is equal to 91

bull 1048713 An operator needs to meet certain coverage criteria This is typically the ldquo90 rulerdquo ndash 90 of a given geographical area must be covered for 90 of the time

Cell coveragebull The mean signal level at any distance is determined by path

loss and the variance is determined by the resulting fading distribution (log-normal shadowing Rayleigh fading Nakagami-m etc) In this course we will deal with log-normal shadowing only

bull The proportion of locations covered at a given distance (cell boundary for example) from BS can be found directly from the resultant signal pdfcdf

bull The proportion of locations covered within a circular region defined by a radius R (the cell area for example) can be found by integrating the resultant cdf over the cell area

UNIT-3

Cellular Systems--Cellular Concepts

Slide 3

Cell Footprint

Slide 5

Frequency reuse

Slide 7

Terminology


Slide 10


Designing a cellular system (2)

Slide 13

Problem

Handover Handoff

Slide 16

Dwell Time

Handover indicator

Practical handover

Practical handover (2)


Slide 23

Interference and System Capacity (2)

Slide 25




Outage probability

Outage probability (2)



Coverage Problems

Boundary coverage

Cell coverage

Cell coverage (2)

Cell coverage (3)

Cell coverage (4)


Cellular Systems--Cellular Concepts The cellular concept was a major breakthrough in solving the

problem of spectral congestion and user capacity It offered very high capacity in a limited spectrum allocation without any major technological changes

The cellular concept has the following system level ideas

Replacing a single high power transmitter with many low power transmitters each providing coverage to only a small area

Neighboring cells are assigned different groups of channels in order to minimize interference

The same set of channels is then reused at different geographical locations





Shadowing effect


Cell Footprint


































Problem










Handover Handoff










Dwell Time





Handover indicator



Practical handover


































Outage probability

Outage probability


Outage probability





















UNIT-3


Slide 3

Cell Footprint

Slide 5

Frequency reuse

Slide 7

Terminology


Slide 10



Slide 13

Problem

Handover Handoff

Slide 16

Dwell Time

Handover indicator

Practical handover



Slide 23


Slide 25




Outage probability




Coverage Problems

Boundary coverage

Cell coverage

Cell coverage (2)

Cell coverage (3)

Cell coverage (4)





Shadowing effect


Cell Footprint


































Problem










Handover Handoff










Dwell Time





Handover indicator



Practical handover


































Outage probability

Outage probability


Outage probability





















UNIT-3


Slide 3

Cell Footprint

Slide 5

Frequency reuse

Slide 7

Terminology


Slide 10



Slide 13

Problem

Handover Handoff

Slide 16

Dwell Time

Handover indicator

Practical handover



Slide 23


Slide 25




Outage probability




Coverage Problems

Boundary coverage

Cell coverage

Cell coverage (2)

Cell coverage (3)

Cell coverage (4)


Cell Footprint


































Problem










Handover Handoff










Dwell Time





Handover indicator



Practical handover


































Outage probability

Outage probability


Outage probability





















UNIT-3


Slide 3

Cell Footprint

Slide 5

Frequency reuse

Slide 7

Terminology


Slide 10



Slide 13

Problem

Handover Handoff

Slide 16

Dwell Time

Handover indicator

Practical handover



Slide 23


Slide 25




Outage probability




Coverage Problems

Boundary coverage

Cell coverage

Cell coverage (2)

Cell coverage (3)

Cell coverage (4)































Problem










Handover Handoff










Dwell Time





Handover indicator



Practical handover


































Outage probability

Outage probability


Outage probability





















UNIT-3


Slide 3

Cell Footprint

Slide 5

Frequency reuse

Slide 7

Terminology


Slide 10



Slide 13

Problem

Handover Handoff

Slide 16

Dwell Time

Handover indicator

Practical handover



Slide 23


Slide 25




Outage probability




Coverage Problems

Boundary coverage

Cell coverage

Cell coverage (2)

Cell coverage (3)

Cell coverage (4)


Handover Handoff










Dwell Time





Handover indicator



Practical handover


































Outage probability

Outage probability


Outage probability





















UNIT-3


Slide 3

Cell Footprint

Slide 5

Frequency reuse

Slide 7

Terminology


Slide 10



Slide 13

Problem

Handover Handoff

Slide 16

Dwell Time

Handover indicator

Practical handover



Slide 23


Slide 25




Outage probability




Coverage Problems

Boundary coverage

Cell coverage

Cell coverage (2)

Cell coverage (3)

Cell coverage (4)







Dwell Time





Handover indicator



Practical handover


































Outage probability

Outage probability


Outage probability





















UNIT-3


Slide 3

Cell Footprint

Slide 5

Frequency reuse

Slide 7

Terminology


Slide 10



Slide 13

Problem

Handover Handoff

Slide 16

Dwell Time

Handover indicator

Practical handover



Slide 23


Slide 25




Outage probability




Coverage Problems

Boundary coverage

Cell coverage

Cell coverage (2)

Cell coverage (3)

Cell coverage (4)


Handover indicator



Practical handover


































Outage probability

Outage probability


Outage probability





















UNIT-3


Slide 3

Cell Footprint

Slide 5

Frequency reuse

Slide 7

Terminology


Slide 10



Slide 13

Problem

Handover Handoff

Slide 16

Dwell Time

Handover indicator

Practical handover



Slide 23


Slide 25




Outage probability




Coverage Problems

Boundary coverage

Cell coverage

Cell coverage (2)

Cell coverage (3)

Cell coverage (4)

Practical handover


































Outage probability

Outage probability


Outage probability





















UNIT-3


Slide 3

Cell Footprint

Slide 5

Frequency reuse

Slide 7

Terminology


Slide 10



Slide 13

Problem

Handover Handoff

Slide 16

Dwell Time

Handover indicator

Practical handover



Slide 23


Slide 25




Outage probability




Coverage Problems

Boundary coverage

Cell coverage

Cell coverage (2)

Cell coverage (3)

Cell coverage (4)

































Outage probability

Outage probability


Outage probability





















UNIT-3


Slide 3

Cell Footprint

Slide 5

Frequency reuse

Slide 7

Terminology


Slide 10



Slide 13

Problem

Handover Handoff

Slide 16

Dwell Time

Handover indicator

Practical handover



Slide 23


Slide 25




Outage probability




Coverage Problems

Boundary coverage

Cell coverage

Cell coverage (2)

Cell coverage (3)

Cell coverage (4)

Outage probability


Outage probability





















UNIT-3


Slide 3

Cell Footprint

Slide 5

Frequency reuse

Slide 7

Terminology


Slide 10



Slide 13

Problem

Handover Handoff

Slide 16

Dwell Time

Handover indicator

Practical handover



Slide 23


Slide 25




Outage probability




Coverage Problems

Boundary coverage

Cell coverage

Cell coverage (2)

Cell coverage (3)

Cell coverage (4)

Outage probability





















UNIT-3


Slide 3

Cell Footprint

Slide 5

Frequency reuse

Slide 7

Terminology


Slide 10



Slide 13

Problem

Handover Handoff

Slide 16

Dwell Time

Handover indicator

Practical handover



Slide 23


Slide 25




Outage probability




Coverage Problems

Boundary coverage

Cell coverage

Cell coverage (2)

Cell coverage (3)

Cell coverage (4)



















UNIT-3


Slide 3

Cell Footprint

Slide 5

Frequency reuse

Slide 7

Terminology


Slide 10



Slide 13

Problem

Handover Handoff

Slide 16

Dwell Time

Handover indicator

Practical handover



Slide 23


Slide 25




Outage probability




Coverage Problems

Boundary coverage

Cell coverage

Cell coverage (2)

Cell coverage (3)

Cell coverage (4)

Cellular concepts

Technology

Transcript of Cellular concepts