maju cell 2009 fall week 3 4 5 cellular concept mobility rfplanning trunking

8/2/2019 MAJU Cell 2009 Fall Week 3 4 5 Cellular Concept Mobility RFPlanning Trunking

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Muhammad Ali Jinnah University, Islamabad Campus, Pakistan

Cellular Concept Cellular Concept Cellular Concept Cellular Concept

Lecturer: Associate Prof. Dr Noor M Khan Department of Electronic Engineering,

Dr. Noor M KhanDr. Noor M KhanDr. Noor M KhanDr. Noor M KhanEE, MAJUEE, MAJUEE, MAJUEE, MAJU

EE6733 Cellular Mobile CommunicationsWeek 3Week 3Week 3Week 3- ---4 ; Fall4 ; Fall4 ; Fall4 ; Fall - --- 2009200920092009

Cellular Concept 1

PSTNMSC

Muhammad Ali Jinnah University, Islamabad Campus, Islamabad, PAKISTAN Ph: +92 (51) 111-878787, Ext. 147 Fax: +92 (51) 2822743email: [email protected]

,

[email protected]


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Cellular Concept

Simple Solution Single high powered transmitter on a tall tower Good coverage but very low capacity



Cellular Concept 2

No frequency reuse High Capacity Solution

Cellular concept solves problem of low capacity

Replaces a single high power transmitter (large cell)with many low power transmitters (small cells)

Much smaller and more efficient mobile units


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Operation

The Cellular Concept is a system level idea: Each base station is allocated a portion of the total

number of channels available to the entire system



Cellular Concept 3

Nearby base stations are assigned different groups of channels All channels are assigned to a relatively small number

of neighboring base stations

The level of interference between base stations(and the mobile users) is controlled


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Scalability

Frequency can be re-used as many times asnecessary as long as interference between co-channel stations can be kept within acceptable



Cellular Concept 4

m ts. As the demand increases, the number of base

stations can be increased (with a corresponding

decrease in transmitter power) This fundamental principal is the foundation of all

modern wireless communication systems.


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Frequency Re-use The process of selecting and allocating channel

groups for all base stations within a system isknown as frequency re-use or frequency planning



Cellular Concept 5

AG

FE

C

D

B

G

FE

C

DA

G

F

E

C

DA

B


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Comments on HexagonalCells

Hexagon geometry approximates omni-directional base station with free spacepropagation



Cellular Concept 6

When hexagons are used base stations caneither be in the center (center excited) - omni directional

antennas or on 3 of the six cell vertices (edge excited) -

sectored directional antennas


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On Hexagonal Cells..? Hexagon geometry approximates omni-directional base

station with free space propagation



Cellular Concept 7


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Simple Calculation

Let S be the total number of duplex channels Let k be the number of channels in each cell N cells collectivel use the com lete set of S



Cellular Concept 8

available channels.

N is the cluster size ( N =4,7 or 12), then S = kN If a cluster is replicated M times Total number of duplex channels = MS= MkN 1/N is called the frequency re-use factor


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More About Cellular Structure

Each cell has exactly six equidistantneighbors Thus there is onl certain cluster sizes and



Cellular Concept 9

cell layouts possible

It can be shown that the number of hexagonal cells per cluster is given by

N = i 2 + ij + j 2

i & j are non negative integers


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i & j, Co-Channel Neighbors

To find the nearest co-channel neighbors Move i cells along any chain of hexagons



Cellular Concept 10

Turn 60 degrees counter clockwise, and

move j cells


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Example

i= 3, (move 3 cells alongany chain of hexagons)



Cellular Concept 11

j=2 (turn 60 degreescounter clockwise andmove 2 cells);

i=3

j=2 This is the way to find thecentral cell of the new cluster

Number of cells in thecluster; N=9+3*2+4=19.


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Channel Assignment

Channel assignment (Frequency reuse) efficient utilization of radio spectrum increased capacity



Cellular Concept 13

Channel assignment can be fixed dynamic

Affects performance especially handover (handoff )


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Fixed Channel Assignment

Each cell is allocated a predetermined set of channels An call attem t within the cell can onl be



Cellular Concept 14

served by the unused channels in that cell Variations that allow channel borrowing

exist A cell is allowed to borrow from its neighbor MSC supervises the borrowing procedure


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Dynamic Channel Assignment

Each time a call is attempted, the servingBS request a channel from the MSC The lendin al orithm take into account



Cellular Concept 15

likelihood of future blocking frequency re-use of candidate channel other cost functions

Dynamic schemes reduces the call blockingprobability and increases system capacity


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Implications

Dynamic schemes require the MSC tocollect real time data on all channels hannel occu anc



Cellular Concept 16

Traffic distribution Radio signal strength indications (RSSI)

Makes the MSC more complex, andincreases its storage and computational load


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Handover (Handoff)

Need to be performed successfully andinfrequently as possible and be transparent tousers



Cellular Concept 17

Need to decide the optimum signal level toperform handover

Generally the level is decided the handover

level is set slightly above it = P r handover - P r minimum usable


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Example



Cellular Concept 18


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Dwell Time

The time a call may be maintained within a cell,without handover, is called the dwell time . Dwell time is dependent on a number of factors



Cellular Concept 19

propagation interference distance from BS etc.

Therefore even a stationary subscriber may have arandom and finite dwell time


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Mobile Assisted Handover(MAHO)

In analog systems the signal strength was measuredby the base station and supervised by the MSC,

The MSC decides if a handover is necessary or not



Cellular Concept 20

assisted Mobile measures signal strength and reports to the

serving BS

Handover is initiated when power received from the BSof a neighboring cell begins to exceed the power receivedfrom the current BS - certain level & duration


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Handover 2

MAHO is faster and more suited for micro cellularenvironments It is also possible to have intersystem handover,



Cellular Concept 21

Handover from a cell of one MSC to a cell of anotherMSC

Numerous issues

A local call (initially) may become a long distance call Need to deal with incompatibility of the MSC


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Handover Policy Ways of handling handover requests

Same as all initial call requests Give it higher priority Queue requests



Cellular Concept 22

in mid conversation than being blocked on a newcall attempt

Fraction of the total available channels in a cell isreserved for handover requests from ongoing calls- guard channel


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Practical HandoverConsiderations

Cell dragging - pedestrian users that provide avery strong signal to the BS (LOS), but moved toa close range of another base station causing



Cellular Concept 23

Difficulty in obtaining physical cell sites Zoning laws (no high rise structures) Public protest (radiation concerns) e.t.c

Too many handoffs for high speed mobiles (on avehicle)


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Handoff Improvements

First generation mobile 10s to make handoff and = 6-12 dB. ( = P r handover - P r minimum usable )

GSM II eneration 1-2s to make handoff with



Cellular Concept 24

= 0-6 dB.

Better system efficiency and handling high speedvehicles


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Soft Handoff

Channelized wireless systems (such as GSM) haveto switch channels in the process of handoff There is always risk of losing the connection



Cellular Concept 25

- , o e v s on u t p e ccesssystem provides Soft Handoff . Soft Handoff does not mean changing the channel

but rather deciding which base station will handlethe connection This is a unique property of CDMA concept.


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CDMA Frequency ReusePattern

G CA

B

CDM



Cellular Concept 26

AG

FE

C

D

BF

ED

G

FE

C

DA

B


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Co-channel Interference

There are several cells that use the same setfrequencies co-channel cells


EE6733 Cellular Mobile CommunicationsWeek 3Week 3Week 3Week 3- ---4 ; Fall4 ; Fall4 ; Fall4 ; Fall - --- 2009200920092009Cellular Concept 27

Interference between signals from these cells iscalled co-channel interference

Unlike thermal noise, this cannot be overcome by

increasing the signal power The co-channel cells must be physically separated

h d l h l b d k


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Co-channel Interference

Because the cell size is same, co-channelinterference is independent of transmitted power It is a function of the radius of the cell ( R), and



the distance to the center of the nearest co-channel cell ( D ) For hexagonal geometry, co-channel reuse ratio

Q is given by N

DQ 3==

M h d Ali Ji h U i i I l b d C P ki


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Carrier to Interference Ratio

Carrier to Interference ratio (SIR or S/I) is alsoindependent of transmitted power It is a function of the radius of the cell ( R), and



the distance to the center of the nearest co-channel cell ( D ) For the first tier in hexagonal geometry, Carrier

to Interference ratio is usually taken as( )

421 1

36 6

S D N

I R

=

free space

E d i t e

d b y F

ox i t R

e a d er

C o p y r i gh

t ( C ) b y F

ox i t

S of t w

ar e C om

p an

y ,2 0 0 5 -2

0 0 8

F or E v

al u a t i on

Onl y .

M h d Ali Ji h U i it I l b d C P ki t


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Some Arithmetic Again

Let i 0 be the number of co channelinterfering cells then carrier signal to interference ratio (SIR)



S - desired signal power from desired BS; I i -interference power caused by the ithinterfering co-channel cell BS

0

1

i

ii

I I

=

=

Muhammad Ali Jinnah University Islamabad Campus Pakistan


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Received Power Ave. received signal strength at any point decays

as a power law of the distance of separation ( d )and is given byn

d



P 0 - Power received at a close-in reference point inthe far field region of the antenna at a smalldistance; n - path loss component (2-4)

r

d =

00

=

00 log10)()( d

d ndBmPdBmPr



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SIR

If D i is the distance of the ith

interferer, thereceived power at a given mobile due to the i th

interfering cell will be proportional to (D i)-n



When the transmit power of each BS is equaland the path loss exponent is the same

=

= 0

1

)(i

i

ni

n

D

R

I

S



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Simpler SIR

Considering only the first layer of interferingcells & if all these BS are equidistant



i0 - number of neighboring/interfering co-channel cells

00 ii I ==



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Interference LimitationExample

i0 =6 number of co-channel cells N =7 n = 2

Dr. Noor M KhanDr. Noor M KhanDr. Noor M KhanDr. Noor M KhanEE, MAJUEE, MAJUEE, MAJUEE, MAJUEE6733 Cellular Mobile Communications

Week 3Week 3Week 3Week 3- ---4 ; Fall4 ; Fall4 ; Fall4 ; Fall - --- 2009200920092009Cellular Concept 34

Considering only the first layer of interferingcells & if all these BS are equidistant

26 I ==



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1 6

2 12

Dr. Noor M KhanDr. Noor M KhanDr. Noor M KhanDr. Noor M KhanEE, MAJUEE, MAJUEE, MAJUEE, MAJUEE6733 Cellular Mobile Communications

Week 3Week 3Week 3Week 3- ---4 ; Fall4 ; Fall4 ; Fall4 ; Fall - --- 2009200920092009Cellular Concept 35

3 18

k 6k



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Interference Limitation

n

=

= 0

1

)(i

i

ni

n

D

R I S

N kR D K 31

0 !1!

]0Pr[ C

k

k C

C

k A

C AC A

Adelay



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Grade of Service for DelayedCalls

]Pr[ t delayGOS >=



( ) H

t AC

edelay

>= ]0Pr[

]0|Pr[]0Pr[ >>>= delayt delaydelay



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Average Delay in a QueuedSystem

The average delay D for all calls in a queuedsystem is: H



Where the average delay in the queue is H/(C-A) H average duration of a call

C number of channels A total offered traffic

AC

maju cell 2009 fall week 3 4 5 cellular concept mobility rfplanning trunking

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