upena dalal 002 542 33 powerpoint-slideschapter 2

7/24/2019 UPENA DALAL 002 542 33 Powerpoint-slidesChapter 2

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Oxford University Press 2014. All rights

Upena Dalal

Wireless

Communicationand Networks


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Cellular Theory

Chapter 2


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Why cellular technology

Cells clusters and cell s!litting "re#uency reuse conce!t and reuse distance

calculation

Cellular syste$ co$!onents

Antennas for %ase station

O!erations of cellular syste$s and hando&

Channel assign$ent ' (xed and dyna$ic

Cellular interferences'co)channel and ad*acentchannel

+ectori,ation

-o%ile trac calculation

+!ectru$ eciency of cellular syste$s

/ocation $anage$ent

ey o!ics


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he conce!t of truning 3resource sharing

%et5een t5o central tele!hone exchanges 5asused in conventional landline tele!hone syste$s.

A truned radio syste$ is a co$!lex centrallycontrolled full du!lex radio syste$ that allo5s

sharing of li$ited radio fre#uency 36" channelsa$ong a large grou! of users. 7t uses control orsignalling channels as 5ell.

7ntroduction


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he conce!t of cellular technology enco$!assesvarious as!ects of infrastructure for instancedivision of area fre#uency $anage$ent or

allocations and call handling.

"ollo5ing are the t5o $a*or co$!onents of $o%ileteleco$$unication syste$ in general8

Central station-- This is common for many

subscribers and includes the switchingequipment and an 6" trans$itter andreceiver.

obile telephone-- !"ery subscriber must

ha"e a mobile telephone that includes amicrophone# a s!eaer dialling facility aradio trans$itter and a receiver.


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7n $odern cellular tele!hony rural and ur%an

regions are divided into areas according to s!eci(c!rovisioning guidelines or standard !rotocols.

9e!loy$ent !ara$eters such as a$ount of cells!litting and cell si,es are deter$ined

ex!eri$entally in the cellular syste$ architecture. Provisioning for each region is !lanned according to

an engineering !lan 5hich includes cells clustersfre#uency reuse and handovers.

oday soft5are tools are also availa%le to !lan anddesign cellular architecture.

Cellular infrastructure


7/59 Oxford University Press 2014. All rights

A cell is the basic geographic unit of a cellular system.

he ter$ cellular comes from the honeycomb(hexagonal) shape of the areas into 5hich a coverageregion is divided theoretically.

Cell5ise one %ase station !rovides trans$ission overa s$all geogra!hic area.

Cell si,es vary de!ending on the landsca!e3to!ogra!hic locations such as $ountains valleysand !lains.

9ue to the constraints i$!osed %y natural terrain and

$an)$ade structures !ractically the cells are not!erfect hexagons.

Cells



A grou! of cells for$ a cluster , the size of which is

denoted as N."igure illustrates a seven)cell cluster. :ach cluster isallocated a set of fre#uencies.N is restricted %y thefollo5ing e#uation8

Cluster



non)negative integers i

andj determine the relative locationsof co-channel cells, as shownin "igure.i ; vertical $ove * ; slant hori,ontal$ove

A relationshi! exists %et5een thecluster design e#uation andfre#uency reuse as 5ell as reusedistance.

6eaching to co)channel cell



he si,e of a cell de!ends on the density of

su%scri%ers in an area.

he ca!acity of a net5or in a densely !o!ulatedarea can %e i$!roved %y reducing the si,e of thecells or %y increasing the nu$%er of cells along 5ithinstalling lo5)!o5er %ase stations. his 5ille&ectively increase the nu$%er of channels in thatarea %ecause of $ore fre#uency reuse.

On the %asis of their si,e cells $ay %e categori,edas macrocells, microcells , picocells, and femtocells .

Cell +i,e

tt



e ! tt ng' or $ax $ , ng nu$ erof users



A radio channel consists of a !air of fre#uencies for full

du!lex o!eration. he conce!t of fre#uency reuse is %ased on assigning to

each cluster the sa$e grou! of radio channels used5ithin a s$all geogra!hic area. A set of N dierentfre!uency groups



/et $ % total number of duplex channels available for reuse (i.e.,fre!uencies !er cluster& % number of duplex channels allocated to each cell of a cluster (&

' $)N % cluster size (in which there are N cells) % number of times the cluster is repeated % total eective number of duplex channels available in the areahus $ % & * N % * $ % &N

Cluster of seven cells



"or :xa$!le8

calculation



Cellular !lanning can %edone %y using varioussoft5ares and !lanningtools

screenshot of :ricsson :-+

6eal 5orld cells



There are three types of cellular systems

$% &nalog Circuit-switched Cellular 'ystem

Mobile Unit or mobile subscriber unit (+)

Cell Site or base station Mobile Telecommunication Switching

Ofce

(or connection management and billing)




2. Digital Circuit-switched Cellular'ystem Mobile Station

Base Station or Base Transceiver Station

Base Station Controller

Switching Subsystems

(% )acket-switched Cellular 'ystem 7t has six ele$ents8 -+ 3user e#ui!$ent %ase

station radio net5or controller36?C servicesu!!ort node 3++? gate5ay su!!ort node 3@+?and charging gate5ay function 3C@".


: l f di it l i it it h d ll l t



o %e studied in Cha!ter 117n detail.

:xa$!le of digital circuit s5itched cellular syste$exa$!le 3@+- syste$



5o $ain ty!es of antennas are usedin the 5ireless industry for a +'

omnidirectionaland directional

O$nidirectional 9irectional 5ith B

sectors

syste$s


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O$nidirectional antennas are nothing %ut the di!oles and

they radiate in all lateral directions.

A directional antenna sha!es and !ro*ects a %ea$ of radioenergy in a s!eci(c direction and receives radio energy onlyfro$ a s!eci(c direction e$!loying various hori,ontal

%ea$5idths.

ere the ter$ beamwidth refers to the conical si,e of theradiated %ea$.

9o5ntilting)))he radiation !attern of a do5ntilt antenna iselectrically or $echanically tilted do5n5ards at a s!eci(ednu$%er of degrees to avoid shado5ing under and near theantenna ,one.

syste$s


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A %ase station antenna is chosen according to the

situation de!ending u!on $any factors such asthe follo5ing8

+i,e of the area to %e covered

Con(gurations of the neigh%ouring cell sites

y!e of antenna used'o$nidirectional ordirectional

AntennaDs %ea$5idth in case of a directionalantenna

Allotted 6" s!ectru$ the antenna can utili,e

syste$s


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he !ro%a%le adverse e&ects of the radiation fro$ the %asestations are a $a*or concern es!ecially a$ong residents inur%an areas. 7nde!endent $onitoring of 6" ex!osure levelsaround $o%ile !hone %ase stations has %een conducted for$any years under the control of govern$ent agencies.

6adiation !attern fro$ %ase station 3/ighter shades of grey indicatedecreasing !o5er strength

+tations

ntennas or o e a o re#uency


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Currently P7"A 3!lanar inverted)" antenna'a shorted !atch

5ide%and antennas are used 5idely and so$eti$es fractalantennas are also used. P7"A is the $odi(ed version of the#uarter)5avelength !atch antenna.

As the !atch is shorted at the end the current at the end of

the !atch antenna is no longer forced to %e ,ero. hisresults in the sa$e currentEvoltage distri%ution as that of ahalf)5ave !atch antenna. he antenna is resonant at a#uarter)5avelength thus reducing the s!ace needed on thetele!honeF $oreover it ty!ically has good s!eci(c

a%sor!tion rate 3+A6 !ro!erties. 7t has a lo5 !ro(le and ano$nidirectional !attern.

ntennas or o e a o re#uency"ront :nd


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'tages for obile-originated Call

here are t5o !ossi%ilities in a $o%ile)originated call8mobile-to-mobile call and mobile-to-landline call .

As an exa$!le the call set)u! se#uences for $o%ile)originatedcalls in a @+- syste$ are given in "ig. 11.B in Cha!ter 11

$% obile-originatedCall

syste$s


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here are t5o !ossi%ilities8mobile-to-mobile call and landline-to-mobile call

2% obile-TerminatedCall

'tages for obile-terminated Call

syste$s


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-o%ile)originated calls are routed to the + (rst and then

to the +C. he +C for5ards this call to the -+C.

he -+C authenticates and routes the call to the calledsu%scri%er as !er the dialled digits.

7f the called su%scri%er is in $o%ility the !rocess to

receive the call on his $o%ile device is descri%ed in+ection 2.G.2

-o%ile)ter$inated calls co$e to the -+C (rst 5here the/6=H/6 en#uiry is carried out and as !er the

infor$ation the -+ is !aged in the suita%le +C. he +C for5ards this !age to all +s 5here the actual

!aging is done.

After a + gets a res!onse fro$ the $o%ile it allocatesa channel for this call.

On ending the call the + infor$s the +C and -+C.

+te!s

et5or or g nate or an ne


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A feature called registration is used for landline-

originated calls. landline user dials a $o%ile unit nu$%er.

he tele!hone co$!any ,one oce recogni,es thatthe call is for a $o%ile nu$%er and for5ards it to the

-+O=-+C. he -+O=-+C sends a !aging $essage to certain

cell sites %ased on the $o%ile unit nu$%er and thesearch algorith$. :ach cell site trans$its the !age

on its o5n set)u! channel. 7f the $o%ile unit is registered the registered site

!ages the $o%ile. he $o%ile unit recogni,es its o5nidenti(cation

on a strong set)u! channel locs onto it andres onds to the cell site.

et5or )or g nate or an ne)originated Call

C ll i i


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When a $o%ile user turns o& the trans$itter a

!articular signalling tone is trans$itted to the cellsite and the voice channel is freed %y %oth thesides.

he $o%ile unit resu$es $onitoring !ages through

the strongest set)u! channel.

Call er$ination

d & P d


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*ando+ is alsoknown ashandover

!t can becategori"ed as#

hard handoversot handoversoter handover

ard andover scenario

ando& Procedure

"e5 ter$s associated 5ith the hando&


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*ando+ probability 7t is the !ro%a%ility that a hando& is executed

%efore call ter$ination.

,ate of hando"er 7t is the nu$%er of handovers !er unit ti$e.

nterruption duration 7t is the duration of ti$e during a handover

!rocedure in 5hich a $o%ile is not connected to any %ase station.

*ando+ delay 7t is the distance %et5een the !oint at 5hich thehando& should occur and the !oint at 5hich it does occur.

)robability of unsuccessful hando+ 7t is the !ro%a%ility that ahando& is executed 5hile the rece!tion conditions are inade#uate.

*ando+ blocking probability 7t is the !ro%a%ility that a handovercannot %e co$!leted successfully.

"e5 ter$s associated 5ith the hando&!rocedure

d i


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he hando& occurs only if the ne5 %ase station is sucientlystronger than the current one %y handover $argin .

ard handover sche$e !revents theping-pong eect.

ard handover !rocedure exhi%its hysteresis .

*ando"er strategies The following are the two di+erentstrategies forhandover8

3a Centrali,ed $ethods 3used in @+-3% 9ecentrali,ed $ethods Iused in 9:C 3W//J

andover $argin

- th d f h d &


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Mobile$assisted hando% /he mobile measures the power

received from the surrounding %ase stations and continuouslyre!orts the results of the $easure$ents to the serving %asestation. A hando& is initiated 5hen the !o5er received fro$ the%ase station of a neigh%ouring cell %egins to exceed that fro$the current %ase station %y a certain level or a certain !eriod ofti$e. his $ethod is currently %eing used in the $o%ile syste$s.

Base station$assisted hando% 0n the 1rst-generationsystems, the strength measurements are $ade %y the %asestations and su!ervised %y the -+O. ere the %ase station$easures the signals fro$ the $o%iles served %y it as 5ell as

fro$ the $o%iles in the neigh%ouring cells and re!orts to the-+O. he -+O decides 5hether a hando& is necessary as 5ellas 5ho needs it. ere the load of the -+O is $ore co$!ared tothe $o%ile)assisted $ethod and hence hando&s are slo5er.

-ethods of hand)o&

+ ft d


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7t allo5s the -+C to $ae a soft decision regarding

the version of the userDs signal to !ass.

he a%ility to select %et5een the instantaneous

received signals fro$ a variety of %ase stations is

called soft handover. /he techni!ue is discussed in

detail in Cha!ter 11.

-ore suita%le for C9-A %ased syste$s.

+oft andover


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"igure for :xa$!le 2.B

Ch l A i t


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&i'ed Channel ssignment 7n the (xed channel assign$ent $ethod each cell isallocated a !redeter$ined set of voice channels. Only the unused channels in a

!articular cell can serve any call atte$!t 5ithin that cell. 7f all the channels areoccu!ied then the call 5ill %e %loced and the su%scri%ers 5ill not get any service.

Borrowing Strategy7n the %orro5ing strategy $ethod a cell is allo5ed to %orro5 achannel fro$ a neigh%ouring cell if all of its o5n channels are occu!ied. he -+Csu!ervises the %orro5ing !rocedure and ensures that the %orro5ing of the channeldoes not disru!t or interfere 5ith any of the calls in !rogress in the donor cell.

ynamic Channel ssignment 7n the dyna$ic channel assign$ent $ethod voicechannels are not allocated !er$anently in any of the cell. When a call re#uest is$ade the serving %ase station re#uests a channel fro$ the -+C 5hich thenallocates a channel to the re#uested cell.

Advantages of the dyna$ic channel assign$ent ))the lielihood of call %locing reduces co)channelinterference reduces and channel utili,ation increases.

9isadvantage is that the -+C $ust %e fast and ca!a%le of collecting real)ti$e data on channeloccu!ancy trac distri%ution and radio signal strength indications of all channels on a continuous

%asis.

Channel Assign$ent

C ll l i t f


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5o $a*or ty!es ofcellular interferencesare co-channelinterference andadjacent channel

interference.

o2channel 0nterferencedue to this scenario

Cellular interferences


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he co)channel cells surrounding a !articular cell exist in a circular3or rather hexagonal !attern. here 5ill %e $axi$u$ interferencefro$ the nearest circle of cells 5hich are also called 1rst-tier cellsand are always six in number. 3ased on that the signal)to)interference ratio 3+76 for a $o%ile receiver

5here + is the desired signal power from the desired base stationand 0i is the interference power of the base station of the ithinterfering co-channel cell (neglecting noise). n is the path lossexponent. /he e$!irical value of n ranges from two to 1ve.

ased on the distance of the interfering %ase stations to thedesired $o%ile receiver 4i, 456 ratio and cluster size N

)calculations


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5here & is the proportionality constant, which depends on thepower radiated by the base station at the centre. 0 representsthe power radiated by the other base stations in the 1rst tier,

5hich are six in nu$%er.

ence

"ro$ a%ove t5o e#uations

"ro$ the e#uation it is clear that N should be designed

critically for the desired +06.

)calculations


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Ad*acent channel

interference can %e$ini$i,ed throughcareful (ltering at 7"stages and carefulchannelassign$ents.

interference

+ectori,ation


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+ectori,ed %ase stations are created %y su%dividing an o$nicellinto sectors that are covered using directional antennas $ounted

in the sa$e %ase station location. "ollo5ing is the exa$!le of Bsector division and each sector is treated as di&erent cell.

o sectori,e a cell ahori,ontal e#uilateral

!latfor$ rese$%ling atriangle is de!loyedon a to5er. :ach sideof the !latfor$ iscalled a face. /hree,four, or six directional

antennas are installedon the !latfor$de!ending on thenu$%er of sectors.

+ectori,ation

Why +ectori,ation


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+ectori,ation facilitates 5ireless engineering and

o!erations in the follo5ing 5ays8 7t $ini$i,es or eli$inates co)channel

interference.

7t o!ti$i,es the fre#uency reuse !lan. his is

facilitated through another conce!t no5n as thefront-to-bac& ratio .

At a $ini$u$ it tri!les the ca!acity of any givencoverage area 5hen co$!ared to the ca!acityo&ered %y de!loying o$ni)antennas.

Why +ectori,ation



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rac calculation is essential to %oth $o%ile !hone

and landline syste$s. On the %asis of the trac condition the tele!hone

syste$ $ust %e designed such that only a $ini$u$nu$%er of calls are %loced and uninterru!ted

service is !rovided to the su%scri%ers. he trac varies considera%ly throughout a day %ut

$ost syste$s are designed to handle the tracduring the !ea %usy hour in a day.



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rac variation in a s$all %usiness (r$ as !er nor$alactivities in a day fro$ K a.$. to 10 !.$. is sho5n in (gure%y average trac $easured every hour

7n teletrac engineering trun& describes any entity that will carry

one call. /he number of truns connecting one -+C 5ith another isthe nu$%er of voice !airs used in the connectionF hence it isi$!ortant to deter$ine the nu$%er of truns re#uired %et5een the-+Cs.

runing eciency refers to the decrease in call %locing and

increase in coverage 5ithout call dro!s at the cost of $anage$entoverhead.

+o$e de(nitions


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Call bloc*ing 0t is the non-availability of any trun& or call propagationpath.

Busy hour or +ea* busy hour 0t is the 78-min interval in which thetra9c or the number of call atte$!ts is the highest in a day. 7t usuallyvaries fro$ day to day or over a nu$%er of days.

Time$consistent busy hour0t is the one hour period starting at thesame time each day for 5hich the average trac volu$e or thenu$%er of call atte$!ts is the highest over the days underconsideration.

Call com+letion rate 0t is the ratio of the number of successful callsto the number of call atte$!ts. 7t can %e re!resented in !ercentage.

Busy hour call attem+ts 0t is the number of call attempts in thebusy hour and it decides the net5or ca!acity.

Average %usy hour calls 3co$!leted ; usy hour call atte$!ts L callco$!letion rate

Busy hour calling rate 0t is the average number of calls originated bya subscriber during the %usy hour or the call intensity !er trac !athduring the %usy hour.

ay to busy hour trafc ratio 0t is the ratio of busy hour calling rateto the average calling rate for the day. 7t indicates ho5 $uch of the

+o$e de(nitions

ContDd+

o$e de(nitions


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Cont d Centum call second (CCS) 0t represents the call:time product and is a

tra9c measurement unit. 1 CC+ is one call for a duration of 100 s or 100

calls for 1 s duration each or any other co$%ination. Other $easures arecall seconds 3C+ and call $inutes 3C-.

verage call arrival rate 0t is the average number of calls that arriveduring the speci1ed ti$e duration.

verage call holding time 0t is the average duration of calls that arise

within the speci1ed ti$e duration or the average duration of occu!ancyof a trac !ath %y a call.

,rlang 0t is the unit of tra9c that represents the total use of onechannel or one call per hour that lasts for one hour.

Set$u+ time 0t is the time re!uired to allocate a trun& (or trun&ed radio)channel.

Bloc*ed call or lost call 0t is the call that cannot be completed due tolac& of channels.

+o$e de(nitions

rac Characteri,ation


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rac intensity $ore co$$only called the tra9c, is de1ned asthe average number of calls in !rogress. Although this is adi$ensionless #uantity the unit of trac has %een given ana$e'erlang 3a%%reviated as :.

7n a grou! of channels the average nu$%er of calls in !rogressde!ends on %oth the nu$%er of calls that arrive and theirduration. he duration of a call is often called its holding time .

he trac carried %y a grou! of truns is given %y

Where is the tra9c in erlangs, c is the average number of callarrivals during time / , and h is the average call holding time.

rac Characteri,ation


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When all the channels %eco$e occu!ied the syste$ cannotacce!t further calls. his state is no5n as congestion .

/he arrival of a new call can then %e handled %y either of thefollo5ing t5o $ethods8

3a loced 3for a lost call syste$ %ased on circuit s5itching

3% Mueued 3for a delayed syste$ %ased on $essage or!acet s5itching

rac carried ; rac o&ered ) rac lost

+ervice

he !ro!ortion of calls lost or delayed due to congestion is


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he !ro!ortion of calls lost or delayed due to congestion isused to $easure the service #uality called the grade ofservice (;


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A si$!le $athe$atical $odel is %ased on the follo5ing

assu$!tions8 Pure chance trac))7f the trac is !ure chance all

call arrivals and call ter$inations are inde!endentrando$ events therefore so$eti$es calledmemoryless tra9c . A co$$only used $odel forrando$ $utually inde!endent $essage 3here callarrivals is the >oisson process .

+tatistical e#uili%riu$))According to the statisticale#uili%riu$ assu$!tion the generation of trac is

a stationary rando$ !rocessF that is the!ro%a%ilities do not change during the !eriod %eingconsidered. Conse#uently the $ean nu$%er ofcalls in !rogress re$ains constant.

rac


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-arov $odel for a nu$%er of occu!ied channels


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>("), > (@), A, > ( N ) are the state probabilities .>8,", >",@,and so on are the !ro%a%ilities of a state incre$ent.

!in a net5or 5it ? channels

:rlang "or$ula


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:rlang deter$ined the @O+ 3i.e. the loss

!ro%a%ility of a lost call syste$ having Ntruns 5hen the o&ered trac is . /hesolution5formula was obtained on the basis ofthe following assu$!tions.8

Pure chance trac +tatistical e#uili%riu$

"ull availa%ility

/oss of calls encountering congestion

he for$ula is

:rlang "or$ula

:rlang C "or$ula


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7n an :rlang C tele!hone syste$ N channelsare available. New calls are assigned a channeluntil all channels are full. When all the channelsare occu!ied a ne5 call is #ueued until it can%e served. his is in contrast to an :rlang syste$ in 5hich ne5 calls are %loced.

he assu$!tions $ade here are the sa$e as inan :rlang syste$ exce!t the fourth one.

he for$ula is

:rlang C "or$ula

!ec ru$ e c ency o ce u ar


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/et

& % number of channels per cell

N % cluster size Bchannel % total bandwidth for the cellular net

Bsignal % occupied bandwidth per channel

hen Bchannel % &NBsignal

he s!ectru$ eciency +C of a cellular net can be de1ned as the

carried tra9c per cell c, ex!ressed in erlangs divided %y the%and5idth of the total syste$ Bchannel and the area of the cell+u. c is mostly computed from the Crlang 3 formula, with ce!ual to the atte$!ted trac $ulti!lied %y the !ro%a%ility ofsuccess 3; 1 E %locing !ro%a%ility. +!ectru$ eciency isex!ressed in erlang=-,=

!ec ru$ e c ency o ce u arsyste$s

/ocation $anage$ent


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he study of location $anage$ent ai$s at tracing asu%scri%er 35ith an active $o%ile unit on $ove andat reducing the overhead incurred in locating thatsu%scri%er in a cellular environ$ent though he=she$oves continuously fro$ cell to cell. Actually$o%ility $anage$ent in cellular syste$s consists of

t5o co$!onents8 handover $anage$ent

location $anage$ent.

/ocation $anage$ent

/ocation $anage$ent


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7n literature location u!date sche$es are classi(ed into t5o $aingrou!s8

'tatic or global scheme /ocation u!date is triggered %ased on theto!ology of the net5or.

Dynamic or local scheme A $o%ile sends a location u!date

$essage according to the ti$e ela!sed 3ti$e)%ased $ethod thenu$%er of cells visited 3$ove$ent)%ased $ethod or the distance inter$s of cells travelled 3distance)%ased $ethod.

he follo5ing are a fe5 ter$s related to location $anage$ent8

Centre cell 0t is the cell where the last location update occurred. -esiding area 0t is the area in which the mobile unit can be located.

.olling cycle 0t is the process performed by the networ& when a callarrives at a mobile terminal.he net5or sends a !olling signal tothe target cell in the residing area and 5aits for the res!onse.

/ocation $anage$ent

upena dalal 002 542 33 powerpoint-slideschapter 2

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