Cellular Systems 1

8/18/2019 Cellular Systems 1

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Lecture 11: Cellular Networks

• Introduction

• Principle of wireless networks

• The principle of frequency reuse

• Cellular system overview

Ben Slimane

[email protected]


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Cellular Networks

• The purpose of wireless networks is to provide

wireless access to the fixed network (PSTN)


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Cellular Networks

• ultiple low!power trans"itters (1## $ orless) are used

• The service area is divided into cells

% &ach cell is served ' its own antenna % &ach 'ase station consists of a trans"itter a

receiver and control unit

% *ase station placed in the "iddle or at the 'order

of the cell % &ach 'ase station is allocated a certain fre+uenc

'and (fre+uenc allocation)


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Cellular ,eo"etries


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Cellular ,eo"etries

Rd -=

• The "ost co""on "odel used for wireless networks

is unifor" hexa.onal shape areas

% / 'ase station with o"ni!directional antenna is placed in

the "iddle of the cell


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Cellular ,eo"etries

• Cells are classified 'ased on their si0es

• acrocells with radius of 1k" or "ore (widearea) % exa.onal shape cells

• icrocells with radius of 1##" or "ore (cities) % exa.onal shape cells

% anhattan (cit) tpe cell structure

• Picocells with radius in the "eters (indoor) % Shape depends on the roo"


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2esi.n of $ireless Networks

• The desi.n is done in two steps

% /rea covera.e plannin.

% Channel (3re+uenc) allocation

• uta!e area• Covera!e area


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3re+uenc 4euse

• /n efficient wa of "ana.in. the radio spectru" is ' reusin. the sa"e

fre+uenc within the service area as often as possi'le

• This fre+uenc reuse is possi'le thanks to the propa.ation properties of

radio waves


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3re+uenc 4euse

• $e for" a cluster of cells

% 2ivide the total nu"'er of channels (fre+uencies) 'etween

the cells of the cluster5

% /ll the channels within the cluster are ortho.onal

• No interference 'etween cells of the sa"e cluster

• $e repeat the cluster over the service area

• The distance 'etween the clusters is called the reusedistance "

• The desi.n reduces to findin. "#


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3re+uenc 4euse

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• 3or hexa.onal cells the nu"'er of cells in the cluster is .iven

'


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3re+uenc 4euse Pattern

• 3re+uenc reuse pattern for $&'


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3re+uenc 4euse Patterns

• 3re+uenc reuse pattern for $&(


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Capacit of $ireless Networks

• The capacit of a wireless network is "easured asthe avera.e of si"ultaneous radio links supported

' the sste"s

=&C)$* users)cell

• The area capacit is defined as

=&C)+$ x ,cell-* users)unit area

• ,cell is the cell area


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/pproaches of >ncreasin.

Capacit• /ddin. new channels

• 3re+uenc 'orrowin. % fre+uenciesare taken fro" ad?acent cells 'con.ested cells

• Cell splittin. % cells in areas of hi.husa.e can 'e split into s"aller cells

• 2irectional antennas % cells aredivided into a nu"'er of wed.e!

shaped sectors each with their ownset of channels

• icrocells % antennas "ove to 'uildin.s hills and la"p posts


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Cellular Sste" @verview


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Cellular Sste"s Ter"s

• *ase Station (*S) % includes an antenna a controllerand a nu"'er of transceivers

• o'ile teleco""unications switchin. office (TS@)

% connects calls 'etween "o'ile units• Two tpes of channels availa'le 'etween "o'ile unit

and *S % Control channels % used to exchan.e infor"ation havin. to

do with settin. up and "aintainin. calls % Traffic channels % carr voice or data connection 'etween

users


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Steps in an TS@

Controlled Call 'etween

o'ile Asers

• o'ile unit initiali0ation

• o'ile!ori.inated call• Pa.in.

• Call accepted

• @n.oin. call

• andoff


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Examples of Mobil Cellular

Calls


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Examples of Mobile Cellular

Calls


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Examples of Mobile Cellular

Calls


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/dditional 3unctions in an

TS@ Controlled Call• Call 'lockin.

• Call ter"ination

• Call drop• Calls toBfro" fixed and re"ote "o'ile

su'scri'er


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o'ile 4adio Propa.ation

&ffects• Si.nal stren.th

% ust 'e stron. enou.h 'etween 'ase station and "o'ile

unit to "aintain si.nal +ualit at the receiver

% ust not 'e so stron. as to create too "uch cochannelinterference with channels in another cell usin. the sa"e

fre+uenc 'and

• 3adin.

% Si.nal propa.ation effects "a disrupt the si.nal and causeerrors


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4adio 4esource /llocation pro'le"

i

To each active ter"inal assi.n

! *ase station ! Channel (3re+uencD)

! Trans"itter power

such that Link Eualit F power constraints are satisfied

for as "an ter"inals as possi'le


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andover Perfor"ance

etrics• Cell 'lockin. pro'a'ilit % pro'a'ilit of a new call

'ein. 'locked

• Call droppin. pro'a'ilit % pro'a'ilit that a call is

ter"inated due to a handover • Call co"pletion pro'a'ilit % pro'a'ilit that an

ad"itted call is not dropped 'efore it ter"inates

• Pro'a'ilit of unsuccessful handover % pro'a'ilit

that a handover is executed while the receptionconditions are inade+uate


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andover Perfor"ance

etrics• andoff 'lockin. pro'a'ilit % pro'a'ilit that a handoff

cannot 'e successfull co"pleted

• andoff pro'a'ilit % pro'a'ilit that a handoff occurs 'efore call ter"ination

• 4ate of handoff % nu"'er of handoffs per unit ti"e• >nterruption duration % duration of ti"e durin. a handoff

in which a "o'ile is not connected to either 'ase station

• andoff dela % distance the "o'ile "oves fro" the

point at which the handoff should occur to the point atwhich it does occur


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andover Strate.ies Ased to

2eter"ine >nstant of andover

• 4elative si.nal stren.th

•4elative si.nal stren.th with threshold

• 4elative si.nal stren.th with hsteresis

• 4elative si.nal stren.th with hsteresis and

threshold• Prediction techni+ues


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andover decision


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Trans"itter Power Control

• $h trans"itter power controlG

% 4educe ter"inal power consu"ption

% 4educe interference within the cellular sste" and

i"prove +ualit

% &fficient handlin. of "o'ilit

% >n SS sste"s usin. C2/ itHs desira'le to

e+uali0e the received power level fro" all "o'ileunits at the *S

• 4educe near!far pro'le"


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Tpes of Power Control

• @pen!loop power control % 2epends solel on "o'ile unit

% No feed'ack fro" *S

% Not as accurate as closed!loop 'ut can react +uicker tofluctuations in si.nal stren.th

• Closed!loop power control % /d?usts si.nal stren.th in reverse channel 'ased on "etric

of perfor"ance

% *S "akes power ad?ust"ent decision and co""unicates to"o'ile on control channel


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Traffic &n.ineerin.

• >n cellular sste"s the nu"'er of active users(calls) is rando"5

• >deall availa'le channels would e+ual

nu"'er of su'scri'ers active at an ti"e % Not possi'le in practice

• 3or $ channels per cell and active

su'scri'ers per cell we have• I $ non!'lockin. sste"

• J $ 'lockin. sste"


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Perfor"ance Euestions

• *lockin. Pro'a'ilit

% Pro'a'ilit that a call re+uest is 'lockedG

• Sste" capacit for a .iven 'lockin.

pro'a'ilitG

• $hat is the avera.e delaG

• Sste" capacit for a certain avera.e delaG


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Traffic >ntensit

• >n cellular sste"s calls are Poissondistri'uted with λ callsBs

• The traffic load of the sste" is

∀λ is the nu"'er of calls per seconds

• h is the avera.e call duration in seconds

• , K avera.e nu"'er of calls arrivin. durin. avera.eholdin. period (in &rlan.s)

h , λ =


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3actors that 2eter"ine the

Nature of the Traffic odel• anner in which 'locked calls are handled

% Lost calls delaed (LC2) % 'locked calls put in a +ueue

awaitin. a free channel

% *locked calls re?ected and dropped• Lost calls cleared (LCC) % user waits 'efore another atte"pt

• Lost calls held (LC) % user repeatedl atte"pts callin.

• Nu"'er of traffic sources

% $hether nu"'er of users is assu"ed to 'e finite or infinite

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Cellular Systems 1