00000- inter cell interference coordination techniques in hetnets

8/10/2019 00000- Inter Cell Interference Coordination Techniques in HETNETS

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Inter Cell Interference

Coordination techniques inHETNETS

Almost Blank Sub-Frames approach

Elena Rodrguez Martnez

June 2013


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Inter cell interference coordination techniques in HETNETSAlmost Blank Sub-frames approach

Date: ! "une !#$ %a&e of '$

ABSTRACT

The increasin& popularit( of connected de)ices and the au&mentation of users* number

are boostin& the mobile broadband traffic+ ,hich has &ro,n eponentiall( durin& the last

(ears. Also costumers/ epectations are on the rise+ users ha)e come to epect a consistent+

hi&h-qualit( and seamless mobile broadband eperience e)er(,here. To meet these

epectations+ the capacit( and the co)era&e of the current net,orks need to be impro)ed to

deli)er hi&h data throu&hput ,ith )er( lo, latenc(. Since spectrum has become a scarce

resource no,ada(s+ ne, ,a(s ha)e to be found to impro)e the net,ork performance. The ke(

options to achie)e this tar&et include impro)in& and densif(in& the eistin& macro la(er and

complementin& the macro la(er ,ith lo, po,er nodes0 that is to sa(+ deplo(in&

hetero&eneous net,orks.

The concept of hetero&eneous net,orkshas recentl( attracted considerable interest as

a ,a( to optimi1e the performance of the net,ork+ particularl( for unequal user or traffic

distribution situations. A hetero&eneous net,ork is composed of multiple radio access

technolo&ies+ architectures+ transmission solutions+ and base stations of )ar(in& transmission

po,er that can interoperate+ thus creatin& a multila(er structure. Due to the different

operatin& modes of the nodes+ some of them ,ork in open access mode but others ,ork in

closed access mode+ and the unbalanced transmission po,er of the different base stations of

the net,ork+ select the appropriate se)er station can be challen&in& for the users equipments.

A ,ron& cell selection process can lead to the under-utili1ation of lo, po,er nodes0 so that+

ran&e etension technique is proposed to allo, more users to be attached to lo, po,er

nodes.

2ana&e the interferences caused b( the macro station to the lo, po,er nodes and )ice

)ersa is one of the bi&&est challen&es in the deplo(ment of hetero&eneous net,orks.

Enhanced Inter 3ell Interference 3oordination 4eI3I35 schemes ha)e been proposed to deal

,ith this problem. These approaches can be di)ided into time domain techniques+ such as

Almost Blank Sub-frames 4ABS5+ and frequenc( domain techniques. Their implementation

rela(s in the use of some basic characteristics of the radio access technolo&( used in the

net,ork.


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This pro6ect is structured as follo,s: in Section # the concept of hetero&eneous net,orks

is defined+ deepenin& in to the factors that ha)e led to their apparition. Different techniques to

address the ma6or technical challen&es of hetero&eneous deplo(ments are described in

Section . In Section $+ the main techniques to deal ,ith intra-frequenc( interference are

introduced. 7inall(+ one of the methods described in the pre)ious section+ the time domain

technique+ is eplained in detail in Section 8.

Objectie!

The main &oal of this pro6ect is to ha)e a deeper understandin& about the no)el concept

of hetero&eneous net,orks+ comprehendin& the reasons that ha)e led to the need of them as

,ell as the different approaches to implement them and the technical challen&es in)ol)ed in

these sorts of deplo(ments.

Another ob6ecti)e is to make a thorou&h anal(sis of the inter cell interference

coordination time domain technique+ called almost blank sub-frames+ in the case ,hen macro

stations and femto nodes coeist in the same &eo&raphical area+ eaminin& carefull( different

,orks done in this field.


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"#$E%

#. Section #: 2obile S(stem E)olution .............................................................................. 9

#.# Traffic Eplosion ........................................................................................................ 9

#. Approaches To 2eet Increasin& Demand ................................................................

#..# Net,ork Elements .......................................................................................... #

. Section : Hetero&eneous Net,orks Techniques ........................................................ #'

.# 3ell selection ........................................................................................................... #'

. ;an&e Epansion ..................................................................................................... #9

..# ;elated


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8.# Track 2acro ?Es And @ictims Detection ................................................................ $=

8.#.# 3hannel ualit( Indicator 7eedback ............................................................... $=

8. Indentif( A&&ressors 7emto Nodes ......................................................................... $9

8.$ Acti)ation f ABS 2ode ......................................................................................... $

8.$.# ABS %atterns .................................................................................................... 8!

8.8 Trackin& of SIN; durin& ABS Transmission ............................................................. 8

8.' Deacti)ation of ABS mode ...................................................................................... 8$

8.> ;elated .# Instantaneous ;adio-3hannel ualit( ............................................................ 8$

8.>. SIN; Estimation ............................................................................................... 88

8.>.$ Throu&hput De&radation In 7emto 3ells ......................................................... 8'

3onclusions ....................................................................................................................... 89

Terms and abbre)iations .................................................................................................. 8

;eferences ........................................................................................................................ '#


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&"'(RES "#$E%

7i&ure #: Ce( facts ............................................................................................................... 9

7i&ure : Ce( options to epand net,ork capacit( ........................................................... #!

7i&ure $: HETNET architecture .......................................................................................... ##

7i&ure 8: Hetero&eneous net,ork t(pes of lo, po,er nodes ...................................... #$

7i&ure ': lobal small cell deplo(ment forecast+ b( cate&or( .......................................... #$

7i&ure >: Initial access ....................................................................................................... #'

7i&ure =: 3ell selection ...................................................................................................... #9

7i&ure 9: ;an&e epansion ................................................................................................ #9

7i&ure : Simulation results .............................................................................................. !

7i&ure #!: %ico cell embedded in a macro cell .................................................................. #

7i&ure ##: Tools discussed in $%% ...................................................................................

7i&ure #: ;esource partitionin& ....................................................................................... $

7i&ure #$: Soft-cell scheme ............................................................................................... 8

7i&ure #8: FTE &eneric frame structure ............................................................................. '

7i&ure #': 7requenc( reuse of # ....................................................................................... >

7i&ure #>: Intra-frequenc( interference ........................................................................... >

7i&ure #=: Hard frequenc( reuse ...................................................................................... 9

7i&ure #9: 7ractional frequenc( reuse ..............................................................................

7i&ure #: Soft frequenc( reuse ........................................................................................

7i&ure !: 2acro-pico scenario ........................................................................................ $#

7i&ure #: 2acro-femto scenario ..................................................................................... $#

7i&ure : 3ross-carrier schedulin& ................................................................................... $

7i&ure $: Illustration of ABSs used in different scenarios ............................................... $8


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7i&ure 8: 2acro-femto interference scenario ................................................................ $>

7i&ure ': Scheme of ABS procedure................................................................................ $=

7i&ure >: 7ast response and accurate trackin& of the SIN; usin& Calman filter ............. 8'

7i&ure =: Scenario considerate ........................................................................................ 8>

7i&ure 9: Associated reduced blankin& rates and offsets+ and effecti)e blankin& rate at

the macro cell ................................................................................................................... 8>

7i&ure : A&&re&ate throu&hput of the femto cells ........................................................ 8=


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1)

SECT"O# 1*MOB"+E S,STEM E-O+(T"O#

1)1

TRA&&"C E%.+OS"O#

In recent (ears+ mobile broadband traffic has &ro,n eponentiall(+ eceedin& )oice+

thanks to the ne, &eneration of mobile terminals+ such as smartphones+ tablets and laptops+

and to the ne, ser)ices and capabilities the( offer. 2obile users ha)e also increased and+ ,ith

them+ the number of connections. 7urthermore+ cellular operators ha)e in &eneral reported

non-uniform traffic distributions in their net,orks+ statin& that for instance '!G of the total

traffic )olume is carried on onl( $!G of the macrosites. Eact percenta&es of course )ar( from

net,ork to net,ork #. The required capacit( has au&mented faster than pro&ress in spectral

efficienc(. In addition+ the ser)ice is mi&ratin& from a )oice-centrali1ed model to a data-

centrali1ed model. Subscribers use connected de)ices not onl( to access the Internet+ but also

to access applications and cloud-based ser)ices+ includin& )ideo and other band,idth-intensi)e

content. As a result of these trends+ o)erall mobile data traffic is epected to &ro, tenfold b(

!#> .

7i&ure #: Ce( facts#

#%etaB(te J #!

#'B


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?sers* epectations for mobile broadband are &ro,in& parallel to traffic and+

increasin&l(+ users epect a robust+ hi&h-qualit( and seamless ser)ice. 7urther+ more and more+

customers are operatin& inside offices and buildin&s+ ,here about =! percent of toda(*s data

traffic is &enerated and ,here co)era&e represents a ma6or problem for mobile operators.

2eet the demand for mobile broadband is speciall( challen&in& in certain scenarios0 such as:

Far&e outdoor hotspots ,ith hi&h traffic demand and a dense macro net,ork+

impl(in& hi&h interference. E.&.+ to,n squares and commercial streets.

Far&e+ isolated indoor hotspots+ ,hich ma( be difficult to reach from an outdoor

macro net,ork. E.&.+ businesses and hotels

Far&e indoor hotspots+ ,here mobilit( demands and interference are hi&h. E.&.+

shoppin& centers+ airports and sub,a( stations.

Focali1ed+ indoor hotspots or minor co)era&e holes+ ,hich represent a challen&e of

implementation and cost to con)entional cellular net,orks. E.&.+ small offices and

restaurants.

In order to meet the &ro,in& demand for mobile broadband and users* epectations+ it

is necessar( impro)e data performance o)erall and at cell ed&es+ and+ to achie)e this+ more

resources are needed and also ne, ,a(s of acquirin&+ deplo(in&+ mana&in& and optimi1in&

these resources. Broadband ser)ices pro)iders use a )ariet( of technolo&ies in order to meet

customers* epectations0 namel(+ impro)e the eistin& net,ork+ densif( current macro cells

and+ the most important one+ add small cells to impro)e co)era&e+ capacit( and po,er si&nal

,hen necessar(.

1)2 A..ROAC/ES TO MEET "#CREAS"#' $EMA#$

Traditionall(+ the desi&n adopted for the implementation of ,ireless cellular net,orks is

a homo&enous approach. A homo&eneous cellular s(stem is a net,ork of base stations in a

planned la(out and a collection of user terminals+ in ,hich all the base stations ha)e similar

transmit po,er le)els+ antenna patterns+ recei)er noise floors+ and similar backhaul

connecti)it( to the 4packet5 data net,ork $. In these kind of net,orks+ the locations of the

macro stations has to be carefull( planned and the settin&s of each station should be properl(

confi&ured in order to maimi1e the co)era&e and control the interference bet,een ad6acent

stations.


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As the traffic demand &ro,s dramaticall(+ spectral efficienc( in point-to-point ,ireless

net,orks is reachin& its theoretical limit and the capacit( of the current net,ork is not

enou&h. Therefore+ it is necessar( to find solutions0 that is to sa(+ impro)e net,ork capacit(+ to

maintain uniform user eperience and+ thereb(+ customers satisfied. There are se)eral

approaches that can be taken to meet traffic and data rate demands. n a hi&h le)el+ the ke(

options to epand net,ork capacit( include impro)in& and densif(in& the macro la(er and

complementin& the macro la(er ,ith lo, po,er nodes+ thereb( creatin& a hetero&eneous

net,ork.

7i&ure : Ce( options to epand net,ork capacit( 8

Impro)in& the eistin& net,ork consists in enhancin& current macro cells. So that it is

necessar( pro)ide them ,ith more spectrum+ ad)anced antennas ,ith increased order of

di)ersit(+ and ad)anced baseband processin& capacit( ,ithin and bet,een nodes. Althou&h

the impro)ement achie)ed could not be enou&h at some point+ increasin& capacit( and data

rates in this ,a( is an attracti)e solution as it a)oids installin& ne, sites. Due to lar&e

attenuation bet,een terminals and the base station caused b( the distance and the radio

propa&ation+ the recei)ed po,er is relati)el( lo,. That is kno,n as po,er limitation and is the

cause of limited data rates.

Densif(in& the macro net,ork consists in reducin& the co)era&e area of each cell and

increasin& the total number of macro-cell sites0 that is+ addin& cells strate&icall( located. As a

result+ the traffic per square meter can be increased and the distance bet,een the base

station and the terminal ,ill be shorter+ impl(in& an impro)ement in achie)able data rates.


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The densification+ if the number of macro stations is lo,+ keeps the number of sites relati)el(

lo, ,hile net,ork performance becomes less sensiti)e to traffic location0 all this+ ,ithout

se)erel( increasin& the interference bet,een cells. Ho,e)er+ in deplo(ments alread(

presentin& hi&h densit( of macro stations+ the densification of the net,ork ma( be limited b( a

hi&h inter-cell interference and+ in dense urban areas+ site acquisition for macro base stations

,ith to,ers is quite difficult.

Althou&h these t,o strate&ies+ impro)e and densif(+ ma( sli&htl( enhance the capacit(

of the net,ork+ no,ada(s ,ireless cellular s(stems ha)e e)ol)ed to the point ,here an

isolated s(stem+ ,ith 6ust one macro base station+ achie)es near optimal performance.

3onsequentl(+ future &ains of ,ireless net,orks ,ill be obtained from a more fleible andad)anced deplo(ment model+ ,hich ,ill allo, impro)in& broadband user eperience

e)er(,here and in a cost effecti)e ,a(. This ne, deplo(ment model is based on densifi(n& the

current net,ork ,ith complementar( lo, po,er nodes0 that is to sa(+ addin& small cells to

create a hetero&eneous net,ork 4HETNET5.

HETNETS in)ol)e the use of different t(pes of radio technolo&( and emplo( lo, po,er

nodes ,orkin& to&ether ,ith the current macro cells0 that is to sa(+ the( ma( coeist in the

same &eo&raphical area sharin& the same spectrum+ so it is not necessar( that the( pro)ide full

area co)era&e. 7or this reason+ ,hile the location of the macro stations is &enerall( carefull(

planned+ the lo, po,er nodes are t(picall( deplo(ed in a relati)el( unplanned manner.

?suall(+ the main aim of lo, po,er nodes is to eliminate co)era&e holes in the macro net,ork+

impro)e capacit( in hot-spots and impro)e cell ed&e throu&hput0 that is ,h(+ the location

chosen for their deplo(ment is based on the kno,led&e of co)era&e issues and traffic densit(

in the net,ork.

7i&ure $: HETNET architecture lo, po,er node # is used capacit( impro)ement in a hot-spot+ lo,

po,er node- and lo, po,er node-$ are used for impro)in& ed&e throu&hput

2acro - 2acro -##

$


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Deplo(in& lo, po,er nodes can be challen&in&+ as performance depends on close

proimit( to ,here traffic is &enerated and+ due to their reduced co)era&e ran&e+ a lot of them

ma( be needed. Ne)ertheless+ o,in& to theirlo,er transmit po,er and smaller ph(sical si1e+

lo, po,er stations can offer fleible site acquisitions. 7urthermore+ HETNETS allo, impro)in&

spectral efficienc( per unit area andoffer )er( hi&h capacit( and data rates in areas co)ered b(

the lo, po,er nodes. Therefore+ it is an attracti)e solution in scenarios ,here users are hi&hl(

clustered.

1)2)1#ETOR E+EME#TS

Elements that compose a HETNET can be di)ided into t,o main &roups. n one hand+

macro stations+ ,hich are con)entional operator installed base stations 4BSs5 that pro)ide

open public access and ,ide area co)era&e t(picall( on the order of a fe, tens of kilometers.

Their transmit po,er )aries bet,een ' < and 8!


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2icrocells: The( are usuall( deplo(ed in outdoor urban areas that are capacit(

constrained+ but ma( be also useful in rural areas+ ,here population is concentrated

in a limited area. The( ha)e a co)era&e area of less than kilometers and their

t(pical transmit po,er is the same as picocells. 2icrocells are also ,idel( deplo(ed.

2etrocells: The( are deplo(ed in outdoor urban areas that are capacit( constrained+

as microcells. Their t(pical ran&e is of !! meters or less and their transmit po,er

ran&es from '! m< to approimatel( =.

The net fi&ure sho,s the prediction of the &lobal number of small cells for !#>. It is

important to note the leadin& role pla(ed b( femto cells in HETNETS deplo(ments.

7i&ure ': lobal small cell deplo(ment forecast+ b( cate&or( =


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In order to find the ri&ht combination bet,een the three approaches discussed -

impro)e+ densif( and add small cells- to meet future capacit( and co)era&e demands+ it ,ill be

necessar( to consider the nature of the eistin& net,ork+ the a)ailabilit( of backhaul and

spectrum+ estimated traffic )olumes and required data rates+ as ,ell as some technical and

economic considerations.


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Inter cell interference coordinationAlmost Blank Sub-frames approach

Date: ! "une !#$

Cell search

2)SECT"O# 2*

As stated in the pre)i

radio technolo&( and the us

important impro)ements in

emphasi1ed that HETNETS c

;elease 9+ + #!+ K5 and ,it

epansion+ concept that ,ill

challen&es for HETNETS de

coordination bet,een cells s

the most used in HETNETS.

In con)entional sin&le

mobile terminal connects to

stron&est+ ,hile the undesire

Althou&h this &i)es the opti

best strate&( for HETNETS si

resources coordination bet

ad)anced techniques for ef

HETNETS+ needed in order to

2)1

CE++ SE+ECT

A terminal+ before bei

such as searchin& the possib

and deri)in& some s(stem inf

techniques in HETNETS

Cell selectionDerive system

informationUser d

/

ETERO'E#EO(S #ETORS TEC

us section+ HETNETS+ that is to sa(+ the mi of

of macro cells ,ith lo, po,er nodes ,orkin&

terms of increased data rates and cell co)e

an be deplo(ed ,ith the eistin& technolo&ie

out standards chan&es+ althou&h them ,ill en

be further eplained belo,. In this section+ t

plo(ment ,ill be discussed as ,ell as ser)

trate&ies ,hen the radio access technolo&( us

la(er net,orks+ in other ,ords+ homo&eneou

the node from ,hich the do,nlink 4DF5 si&n

d si&nals from the other base stations are treat

um ser)er selection methodolo&( for these net

ce this principle can lead to sub-optimal perfor

een base stations and ser)er selection strat

icient interference mana&ement+ are )er( im

achie)e &ains in throu&hput and user-eperienc

"O#

n& able to recei)e or transmit data+ must perf

le cells to ,hich it can be connected+ selectin

ormation.

7i&ure >: Initial access

%a&e #' of '$

ata Rxx

/#"(ES

different t(pes of

to&ether+ pro)ide

a&e. It should be

s 4e.&. HS%A+ FTE

ble further ran&e

e main technical

er selection and

d is FTE+ ,hich is

s net,orks+ each

al stren&th is the

d as interference.

orks+ it is not the

ance. Therefore+

e&ies+ as ,ell as

portant issues in

e.

rm certain steps+

the suitable one


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The cell search procedure that a ?ser Equipment 4?E5 ,ishin& to access the FTE s(stem

follo,s includes a series of s(nchroni1ation sta&es b( ,hich the ?E determines time and

frequenc( parameters that are necessar( to demodulate DF si&nals+ to transmit ,ith correct

timin& and to acquire some critical s(stem parameters. There are three s(nchroni1ation

requirements in FTE: s(mbol timin& acquisition+ carrier frequenc( s(nchroni1ation+ and

samplin& clock s(nchroni1ation. There are t,o cell search procedures in FTE+ one for initial

s(nchroni1ation and another for detectin& nei&hbor cells in preparation for hando)er. In both

cases+ the ?E uses t,o special si&nals broadcast on each cell: %rimar( S(nchroni1ation

Sequence 4%SS5 and Secondar( S(nchroni1ation Sequence 4SSS5. The detection of these si&nals

allo,s the ?E to complete time and frequenc( s(nchroni1ation and to acquire useful s(stem

parameters such as cell identit(+ c(clic prefi len&th+ and access mode 47DDLTDD5. nce ?E

kno,s the ph(sical cell id 4%3I5 for a &i)en cell+ it also kno,s the location of cell reference

si&nals that are used in channel estimation+ cell selection L reselection and hando)er

procedures.

In homo&eneous deplo(ments+ ,here all the cells transmit on similar po,er le)els+ the

cell selection process is based on the comparison of the ;eference Si&nal ;ecei)ed %o,er

4;S;%5 and the ;eference Si&nal ;ecei)ed ualit( 4;S;5 of do,nlink si&nalin& transmitted

from nei&hborin& cells. That is to sa(+ each ?E selects its ser)in& cell ID accordin& to the cell

from ,hich the lar&est ;S;% is pro)ided 9:

Cell IDserving = argmax{i} {RSRP{i}}

;S;% is defined as the linear a)era&e o)er the po,er contributions 4in


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7ollo,in& this principle+ the cell ,hich pro)ides lar&er ;S;% ,ill be selected as the

ser)in& cell. As a consequence of the lar&e disparit( bet,een the transmit po,er le)el of

macro and lo, po,er stations+ DF co)era&e of lo, po,er nodes is much smaller than that of

macro stations and+ that is ,h(+ con)entional cell selection method cannot be applied in

HETNETS. Therefore+ the assi&nment of user equipments 4?Es5 to appropriate ser)er stations

becomes a non-tri)ial task. 7or the uplink 4?F5+ the stren&th of the recei)ed si&nal depends

onl( on the terminal transmit po,er ,hich is the same either to the lo, po,er node or to the

macro station+ hence the ?F co)era&e of all the stations is similar.

In HETNETS+ follo, the ser)er selection strate&( eplained before+ that is to sa(+ the one

based on DF recei)ed si&nal stren&th+ causes an inefficient use of lo, po,er nodes in different,a(s. n one hand+ most users* terminals ,ill be connected to,ards macro stations+ based on

the si&nal stren&th.


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7i&ure =: 3ell selection

2)2 RA#'E E%.A#S"O#

In order to epand the co)era&e of lo, po,er nodes+ it is possible to increase their

transmission po,er0 in that ,a(+ the cell si1e of lo, po,er nodes ,ill be etended. Ho,e)er+

this strate&( affects the cost and si1e of the node+ so it is not a desirable option. Another

approach to increase lo, po,er*s DF co)era&e footprint is the one kno,n as ran&e epansion.

In this proposal+ the decision of ,here is the cell border is based on the path loss. This

technique consist in addin& an offset to the recei)ed DF si&nal stren&th in the con)entional cell

selection mechanism+ the one eplained before0 as a consequence+ the cell border le)el chosen

,ill be the addition of the ;S;% and the offset. 7or the macro cell+ biasiJ !dB ,ill be chosen

and for the lo, po,er node biasiM!.

Cell ID serving = argmax{i} {RSRP{i}+bias{i}}

7i&ure 9: ;an&e epansion

; po,er

4path loss5-#

%F based border

;an&e epansin 1one

; po,er

;S;% based border

4path loss5-#


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;an&e epansion allo,s more users to be associated ,ith lo, po,er stations+ enablin& a

more equitable distribution of resources b( offloadin& traffic from the macro station. This fact

is especiall( beneficial in situations ,here users are clustered around a lo, po,er node. The

ad)anta&es of this technique are:

Enhanced ?F data rates: takin& into account ?F path loss ,hen associatin& terminals

,ith a lo, po,er node and si&nificantl( miti&atin& cross-tier interference.

Increased capacit(: transmission resources can be reused b( recei)in& DF traffic from

the lo, po,er node e)en if the recei)ed si&nal stren&th from the macro is hi&her.

Impro)ed robustness: sensiti)it( to ideal placement in a traffic hotspot can be

reduced b( enlar&in& the co)era&e area of a lo, po,er node.

A hetero&eneous deplo(ment+ ,ith a modest ran&e epansion some,here in the re&ion

of $-8dB+ is alread( possible in FTE ;elease 9. The benefits &ained from ran&e epansion are

hi&hl( dependent on the indi)idual scenario and are at the epense of reducin& the DF si&nal

qualit( of those users in the epanded re&ion0 so that+ in man( cases+ modest ran&e epansion

is best. Ne)ertheless+ $rd eneration %artnership %ro6ect 4$%%5 has recentl( discussed the

applicabilit( of ecessi)e ran&e epansion ,ith cell selection offsets up to dB #!. These

deplo(ments are particularl( problematic. As terminals in the ran&e epansion 1one ma(eperience )er( lo, DF si&nal to interference ratio+ interferences ma( kill DF control si&nalin&+

,hich is essential for the lo, po,er node to control transmission acti)it(. It is important to

emphasi1e that it is the DF interference at ?Es associated ,ith the lo, po,er node that needs

additional protection from the macro0 the ?F interference at the lo, po,er station can be

miti&ated usin& the same po,er control principle than in a macro-onl( net,ork. In order to

allo, more epansion and+ ,ith that+ more uptake area for lo, po,er nodes and a hi&her total

throu&hput+ DF control si&nalin& must be protected+ that is ,h(+ enhanced inter-cell

interference coordination 4eI3I35 techniques are so important. In summar(+ a tradeoff

bet,een offloadin& and interference from the macro station is required.

2)2)1RE+ATE$ ORS

In 9 a simulation for compare the t,o cell selection strate&ies described+ the

con)entional one+ based onl( in ;S;%+ and the one based in biased ;S;% 4ran&e epansion5+ is

reali1ed )erif(in& the most important ad)anta&es and disad)anta&es of the ran&e epansion


20/53


Date: ! "une !#$ %a&e ! of '$

technique described pre)iousl(. In this papers+ different bias )alues 4bias i J' L#! L ! dB5are

adopted.;esults are summari1ed in the net fi&ure:

7i&ure : Simulation results - a5 The spectral efficienc( &ain compare to con)entional cell selection

scheme. b5 ?E association statistics 9

As it can be seen in fi&ure 9 b+ the simulation results demonstrate that usin& biased ;S;%

selection ends in more ?Es bein& ser)ed b( the small cells. The &reater the bias+ more users

are associated ,ith the lo, po,er node. In fi&ure 9 a+ it is e)idenced that there is onl( a minor

impro)ement on ,hole cell efficienc( 4first column5+ onl( about #.!G for ' dB bias.


21/53


Date: ! "une !#$ %a&e # of '$

2)2)2A#OT/ER TEC/#"CA+ C/A++E#'ES

Because of the reduction of the DF si&nal qualit(+ users on the ed&e of the lo, po,er

node cell ,ill suffer hi&her DF interference+ ,hich ne&ati)el( affects backhaul capacit(+ needed

in order to connect the small cells to the core net,ork+ internet and other ser)ices. Althou&h

backhaul net,ork desi&n ,ill be a ma6or issue in HETNETS because of the comple topolo&( of

the )arious t(pes of coeistin& cells+ this pro6ect is not focused on this point.

Besides backhaul+ another important technical challen&e to consider is the hando)er.

Hando)ers are essential in order to pro)ide a seamless uniform ser)ice ,hen users mo)e in or

out of the cell co)era&e. 7urthermore+ hando)ers are efficient for traffic load balancin&+ b(

shiftin& users at the border of ad6acentLo)erlappin& cells from the more con&ested cells to the

less con&ested ones ##. T(picall(+ a hando)er is done ,hen recei)e si&nal stren&th of the

tar&et cell eceeds the one of the source cell. This is still true for a bias )alue of ! dB. Ho,e)er+

if a hi&her bias )alue is chosen for the tar&et cell 4i.e. the pico cell5 the hando)er takes place

earlier+ namel( ,hen the recei)ed si&nal stren&th of the tar&et cell plus the bias )alue eceeds

the recei)ed si&nal stren&th of the source cell #.

7i&ure #!: %ico cell embedded in a macro cell

In brief+ ran&e epansion is not onl( effecti)e for optimi1in& the use of resources in the

s(stem+ but also for reducin& the frequenc( of hando)ers+ hence impro)in& s(stem throu&hput

and user eperience.Despite these benefits this feature presents a challen&e to interference

mana&ement for HETNETS+ but the deplo(ment of appropriate Inter-3ell Interference

3oordination 4I3I35 al&orithms can further boost s(stem performance.

2acro station 2acro ?E 3ell ed&e

pico ?E

3enter

pico ?E

%ico station

%ico ;S%S

2acro

Bias


22/53



2)3 /ETERO'E#EO(S $E.+O,ME#TS

There are t,o different approaches to hetero&eneous deplo(ment both of ,hichpro)ide support for ecessi)e ran&e epansion. n one hand+ the resource partitionin& in

frequenc( or time domain+ and on the other hand the shared cells+ also kno,n as soft-cells

schemes or multi-sector cells. The net fi&ure sho,s the three hetero&eneous deplo(ments -

the basic one+ the resources partitionin& and the shared cells- and summari1es their main

characteristics+ ,hich ,ill be eplained more detailed belo,.

7i&ure ##: Tools discussed in $%% 8

2)3)1RESO(RCE .ART"T"O#"#'

Throu&h resource coordination amon& base stations+ DF interference caused b( hi&h

po,er macro base stations to the user terminals ser)ed b( lo, po,er base stations can be

miti&ated. This is the concept on ,hich eI3I3 techniques are based. Therefore+ it ,ill be sli&htl(

introduced here and+ throu&h the net section+ it ,ill be eplained in more detail.

To enable resource coordination amon& base stations+ t,o different sets of resources

ma( be allocated for the t,o classes of nodes0 namel( hi&h po,er and lo, po,er base

stations. The resources can be time domain 4slots or sub-frames5 in a s(nchronous s(stem or

frequenc( domain 4&roups of sub-carriers5. The resource partitionin& deplo(ment consists in

restrictin& macro cell transmissions from usin& the same time-frequenc( resources as lo,

po,er nodes in order to protect the control si&nalin& from the lo, po,er nodes.


23/53


Date: ! "une !#$ %a&e $ of '$

In the frequenc( domain+ DF control si&nals from the macro and lo, po,er nodes are

placed in separate carriers. Due to that fact+ and assumin& that transmissions from lo, po,er

nodes are time s(nchroni1ed ,ith the o)erl(in& macro+ the control si&nalin& in the ran&e

epansion 1one ,ill not be epose to ma6or interference from the macro node because it

,ould be placed in another frequenc(. Throu&h the use of carrier a&&re&ation+ data

transmissions can still benefit from the full band,idth of both carriers.

In the time domain+ DF control si&nals from the lo, po,er node are protected b(

reducin& macro transmission acti)it( in certain sub-frames. The lo, po,er node is pro)ided

,ith data about the protected sub-frames and can use this information ,hen schedulin& users

,ho are in the ran&e epansion 1one.

In both+ frequenc( and time domain partitionin& schemes+ lo, po,er nodes create ne,

cells+ ,ith indi)idual cells identities that differ from the macro cell identit(. As a consequence+

each lo, po,er node transmits unique s(stem information and s(nchroni1ation si&nals0 that is

to sa(+ each of these cells has separate broadcast channel 4B3H5+ cell-specific reference si&nal

43;S5+ %SS and SSS.

7i&ure #: ;esource partitionin& #!


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Date: ! "une !#$ %a&e 8 of '$

2)3)2S/ARE$ CE++S

In the shared cells approach+ lo, po,er nodes and the macro station do not create ne,

cells0 therefore+ the( are all part of the same cell. This fact leads to a cell ,ith a unique cell

identit( and s(nchroni1ation si&nals but ,ith more than one transmission points. As a result+

different t(pes of information can come from different sites+ or in other ,ords+ different

transmission point+ ,hich are transparent to the ?E. So coordination bet,een lo, po,er

nodes and the macro station is one of the most important issues in this approach.

7i&ure #$: Soft-cell scheme #!

This technique has some important benefits in front of the resource partitionin&

technique. Since there is onl( one cell formed b( lo, po,er nodes and the macro site+ the

deplo(ment is easier because careful cell plannin& is not needed. i)en that lo, po,er nodes

can turn off their transmissions ,hen the( are not necessar(+ it is possible to sa( that the

technique is ener&eticall( efficient. The soft cell scheme also allo,s an efficient use of the

spectrum as there is no problem ,ith 3;S interference. In addition to these ad)anta&es+ due to

the fact that transmission nodes are transparent to ?E+ soft cells can pro)ide &reater mobilit(

robustness than deplo(ments ,ith separate cells. Traditional hando)er procedure is not

required ,hen mo)in& bet,een macro and lo, po,er nodes+ so the probabilit( of dropped

connections is lo,er.


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Date: ! "une !#$ %a&e ' of '$

3)

SECT"O# 3*"#TERCE++ "#TER&ERE#CE MA#A'EME#T

No,ada(s+ the e)olution of the ph(sical la(er has reached a le)el+ ,here operation )er(

close to information theoretic bounds on achie)able spectral efficienc( for a &i)en si&nal to

interference and noise ratio 4SIN;5 is feasible #8. 7or this reason+ impro)in& the SIN; b(

reducin& or+ e)en better+ a)oidin& the inter-cell interference is the onl( ,a( to achie)e

si&nificant increases in spectral efficienc(. Therefore+ researchers are focusin& on inter-cell

interference coordination 4I3I35 ,hen definin& net &eneration mobile communication

standards+ such as FTE-Ad)anced 4FTE-A5.

3)1 BAC'RO(#$

7or the DF+ FTE uses ortho&onal frequenc( di)ision multiple access 47D2A5+ ,hich is a

combination of ortho&onal frequenc( di)ision multiplein& 47D25 and time di)ision multiple

access 4TD2A5+ unlike for the ?F that Sin&le-carrier frequenc(-di)ision multiple access 4S3-

7D2A5 is used. DF and ?F transmissions are or&ani1ed into frames of #! ms+ each one di)ided

into #! sub-frames. Each sub-frame di)ides into slots of !.' ms. Each slot consist in > or =

7D2 s(mbols+ dependin& on ,hich kind of c(clic prefi is used+ normal or etended. In the

time domain+ a slot is eactl( one ;esource Block lon&.

7i&ure #8: FTE &eneric frame structure

# frame 4#! ms5

# sub-frame #slot

! # $ K. #

! # $ 8 ' >

= 7D2 S(mbols

c(clic prefi


26/53


Date: ! "une !#$ %a&e > of '$

7D2A identifies different subscribers in the same cell b( different time and

subcarriers. To maimi1e spectrum efficienc(+ FTE is desi&ned for a frequenc( reuse of #+ or in

other ,ords+ uses intra-frequenc( net,orkin&+ meanin& that e)er( base station uses the ,hole

s(stem band,idth for transmission. Since all nei&hbor cells are usin& same frequenc( channels+

there is no frequenc( plannin& amon& cells to deal ,ith interference issues. Hence+ FTE macro

cell deplo(ments eperience hea)( interference at the boundaries of the cells #'.

7i&ure #': 7requenc( reuse of # #'

There is a hi&h probabilit( that different subscribers in the nei&hborin& cells+ especiall(

those at the cell ed&e+ recei)e t,o or more si&nals of the same frequenc( at the same time. If

these co-frequenc( si&nals from )arious cells are )er( stron&+ subscribers ma( suffer hi&h

interference and the qualit( of their communication is affected+ leadin& in to e)entuall( lo,

throu&hput or call drops. 3areful consideration need to be implemented on the control

channel miti&atin& the inter-cell interference+ as the ?E could consider the radio link as failed

in case of se)ere interference resultin& in ser)ice outa&e due to the unreliable control

channels.

7i&ure #>: Intra-frequenc( interference #=


27/53


Date: ! "une !#$ %a&e = of '$

Due to co-frequenc( si&nals+ HETNETS require some sort of interference miti&ation+

since lo, po,er cells and macro cells are o)erlappin& in man( scenarios. Besides intra-

frequenc( interference+ there are some other factors inherent to HETNETS that &enerate

interference+ such as the unplanned deplo(ment of lo, po,er nodes or the po,er difference

bet,een nodes. I3I3 and eI3I3 schemes are used to impro)e the performance de&radation

caused due to these situations.

3)2

"#TERCE++ "#TER&ERE#CE COOR$"#AT"O#

I3I3 ,as introduced in $%% ;elease 9 as an optional method to sol)e inter-cell intra-

frequenc( interference. The main idea of I3I3 is to di)ide each cell into t,o sections+ the cell

center and the cell ed&e+ and then allocate different subcarriers to users in different locations.

Theoreticall(+ it is impossible for subscribers at the ed&e of t,o nei&hborin& cells to use the

same frequenc(+ and+ in that ,a(+ the possibilit( of interference bet,een t,o nei&hborin&

cells is reduced.

I3I3 methods allo, achie)in& a sli&ht s(stem performance boost b( mana&in& the

s(stem band,idth and transmit po,er. Ho,e)er+ the frequenc(-domain I3I3 schemes

introduced in ;elease 9 and ,ere mainl( desi&ned for macro-onl( scenarios and onl( pro)ide

impro)ements for the ph(sical data channels+ ,hile failin& to offer protection for ph(sical

control channels carr(in& critical information for achie)in& &ood s(stem performance.

The basic idea on ,hich the I3I3 schemes rel( is to di)ide the ,hole a)ailable resources

into t,o &roups+ one for ser)e the cell-ed&e users and the other for the cell-center users.

Accordin& to the t(pe of resource schedulin&+ I3I3 techniques can be classified into hard

frequenc( reuse+ fractional frequenc( reuse and soft frequenc( reuse. The transmission po,er

of each portion of the spectrum is also controlled to pre)ent cell ed&es and nei&hborin& cells

from bein& affected.


28/53


Date: ! "une !#$ %a&e 9 of '$

3)2)1/AR$ &RE(E#C, RE(SE

In this I3I3 scheme nei&hborin& nodes use different sets of resource blocks throu&hout

the cell at &i)en time0 that is ,h(+ t,o nei&hbor stations ,ill not use same resource

assi&nments for their ?Es. This is achie)ed di)idin& the sub-carriers into $+ 8 or = dis6oint sets+

dependin& on the number of nei&hborin& cells. These sets of subcarriers are assi&ned to an

indi)idual node+ in such a ,a( that nei&hborin& cells don/t use the same set of frequencies.

This method minimi1es si&nificantl( the interference at the cell ed&e+ impro)in& &reatl( cell

ed&e SIN;. The disad)anta&e is that the spectrum efficienc( drops b( a factor equal to the

reuse factor+ leadin& to reduced performance of the entire cell+ since full resources blocks are

not bein& utili1ed. This approach can be considered the opposite etreme to the con)entional

frequenc( plannin& ,ith frequenc( reuse of #.

7i&ure #=: Hard frequenc( reuse #'

3)2)2&RACT"O#A+ &RE(E#C, RE(SE

This I3I3 scheme is based on di)idin& the spectrum into t,o parts+ each of ,hich ,ill

ha)e different frequenc( reuse and transmission po,er. ne section is a common frequenc(

band used in all the cells0 in other ,ords+ this portion of the spectrum has a frequenc( reuse of

#. Nodes ,ould assi&n this full(-reused frequenc( portion+ ,hich has lo, po,er le)el+ onl( to

?Es located in the center of the cell. The other part of the spectrum+ ,hich has hi&h po,er

le)el+ is di)ided amon& the different nodes of the net,ork+ as in hard frequenc( reuse scheme.

This portion of spectrum is assi&ned to the cell-ed&e ?Es. In that ,a(+ t,o nei&hbor cells

cannot assi&n the same set of resource blocks to their cell-ed&e subscribers at &i)en time. This

scheme is particularl( useful for I3I3 in the uplink.


29/53



7i&ure #9: 7ractional frequenc( reuse #'

3)2)3SO&T &RE(E#C, RE(SE

The last I3I3 scheme is probabl( the preferred one. In this method all nodes in a net,ork

transmit in the ,hole s(stem band,idth+ but usin& different po,er schemes0 that is to sa(+

,ith a non-uniform po,er spectrum. The spectrum is di)ided into t,o re&ions+ one ,ith hi&h

po,er transmissions+ ,hich resources are allocated to the cell-center ?Es+ and the other one

,ith lo, po,er transmissions+ ,hich resources are assi&ned to cell-ed&e users. The specific

set of resources used in the ed&e of a cell cannot be used b( nei&hborin& cells+ but center users

ha)e a)ailabilit( of all resource blocks in all the cells. The spectrum dedicated for the cell ed&e

ma( also be used in the central re&ion if it is not bein& used at the cell ed&e.This I3I3 scheme

allo,s achie)in& impro)ed SIN; le)els for cell-ed&e ?Es ,hile keepin& suitable SIN; le)els for

cell-center users. This method is particularl( useful for I3I3 in the DF.

7i&ure #: Soft frequenc( reuse #'


30/53


Date: ! "une !#$ %a&e $! of '$

3)2)4RE+ATE$ ORS

Some simulations comparin& the different schemes of frequenc( reuse discussed before

can be found in #'. In this paper+ the( compare the three I3I3 techniques described ,ith the

case of no frequenc( plannin& amon& cells+ that is to sa(+ the case of frequenc( reuse of #.

As ,as alread( kne,+ their simulation confirm that reuse factor # deplo(ment

maimi1es the cell capacit(+ since all users can benefit from all the band,idth+ but fails to

&uarantee the cell ed&e throu&hput. The soft frequenc( reuse+ allocatin& more po,er to the

cell ed&e users+ and less to the cell center users+ has smaller capacit( than reuse factor #+ but it

enhances the cell ed&e throu&hput. Althou&h fractional frequenc( reuse and hard frequenc(

reuse schemes ha)e better transmission qualit( than soft frequenc( reuse+ the( sho, smaller

cell capacit(. So that+ the( conclude that the soft frequenc( reuse scheme is a &ood candidate

to enhance the cell ed&e throu&hput+ ,ithout sacriOcin& the a)era&e cell throu&hput.

3)3 E#/A#CE$ "#TERCE++ "#TER&ERE#CE COOR$"#AT"O#

I3I3 methods specified in release 9 and release of $%% ma( not be effecti)e in the

case of HETNETS+ because+ as it ,as said before+ the( ,ere mainl( desi&ned for macro-onl(

scenarios. The basic &oal of I3I3 is the pro)ision of a more homo&eneous ser)ice to users

located in different re&ions of the net,ork+ i.e. mostl( to promote the cell-ed&e performance.

This aspect has &ained e)en more importance ,ith the introduction of multi-la(er

hetero&eneous net,orks #$.


31/53


Date: ! "une !#$ %a&e $# of '$

7i&ure !: 2acro-pico scenario

The macro-femto scenario ,ith 3S: ?Es ser)ed b( the macro station ma(

eperience lar&e interference ,hen the( mo)e close to a femto node operatin& in

closed access mode+ this is also kno,n as macro cell co)era&e hole. This is because+

althou&h the femto node offers better si&nal qualit(+ ?Es are not allo,ed to connect

due to the fact that the femto node is operatin& in closed access mode.

7i&ure #: 2acro-femto scenario

To cope ,ith intra-frequenc( interference problems in HETNET+ eI3I3 techniques ha)e

been de)eloped in ;elease #! of $%% 4FTE-A5. These procedures can reduce inter-cell

interference not onl( on traffic channels+ as I3I3 schemes+ but also on control channels of the

DF. These techniques can be classified under t,o main cate&ories: frequenc( domain and time

domain solutions. Both of them ,ill be briefl( eplained belo,+ but throu&h the net section

the time domain technique ,ill be deepl( eamined.

2acro station

2acro ?E )isitor

4no access5

3S femto

node

2acro station%ico node

3enter ?E3ell ed&e ?E


32/53


Date: ! "une !#$ %a&e $ of '$

3)3)1&RE(E#C, $OMA"# TEC/#"(ES

The main frequenc( domain method for interference cancelation is based on carrier

a&&re&ation 43A5+ ,hich is one of the most important features of FTE-A 4$%% ;elease #!5. 3A

enables ?Es to be connected to se)eral carriers simultaneousl(. Besides that+ a terminal

supportin& 3A can be confi&ured b( hi&her la(er si&nalin& to enable cross-carrier schedulin& on

certain component carriers. This implies that a terminal recei)in& a DF assi&nment on one

component carrier 4335 ma( recei)e associated data on another 330 in other ,ords+ a node can

schedule its control information on a 33 and its data information on another 33. ne of the

main moti)ations for introducin& cross-carrier schedulin& ,as to enhance operations in

HETNETS in a multi-carrier deplo(ment. ##

The basic idea of this eI3I3 technique a)oid co-channel interference on DF is to create a

protected 33 for reliable reception of DF ph(sical si&nals+ s(stem information and control

channels at )ictim la(ers but ,here data can be recei)ed on an( confi&ured DF 33 )ia cross-

carrier schedulin&. So that+ the a)ailable spectrum is di)ided into t,o or more separate 33s.

There are t,o cate&ories of 33:

%rimar( component carrier 4%335: It is the main carrier in an( &roup. There ,ill

be a primar( DF carrier and an associated ?F primar( component carrier for

transmittin& control si&nalin&.

Secondar( component carrier 4S335: It is &enerall( used for data transmission.

There ma( be one or more S33.

B( assi&nin& the %%3 and the S33 to different net,ork la(ers0 that to sa(+ different cells+

at a &i)en time+ interference on control channels bet,een net,ork la(ers can be a)oided.

2acro cell

f#2acrocell data

Non-cell ed&e small

cell data

Small cell data+

includin& cell ed&e

3ontrol

f

Small cell

f#

f

7i&ure : 3ross-carrier schedulin&


33/53


Date: ! "une !#$ %a&e $$ of '$

In the case of the fi&ure abo)e+ the macro la(er schedules its control information on f#+

and can schedule its users on both f#and f. Interference on control and data is a)oided b(

schedulin& control and data information for different la(ers on different component carriers+

as it can be seen in fi&ure. Althou&h data information of ?Es ser)ed b( the lo, po,er node

located in the center of the cell can be scheduled on the same carrier as the ?Es ser)ed b( the

macro cell+ because the interference from the macro la(er on ?Es in the center of the small

cell can be tolerated+ ?Es in the ran&e epansion 1one must be scheduled in the other carrier

,here the macro ?Es are not scheduled.

This frequenc(-domain partitionin& technique is a natural choice to support

hetero&eneous deplo(ments for operators ,ho alread( rel( on 3A to eploit fra&mentedspectrum+ but it is onl( supported b( release #! terminals and on,ards+ so this feature cannot

be used b( release 9 and terminals.

3)3)2T"ME $OMA"# TEC/#"(E


34/53


Date: ! "une !#$ %a&e $8 of '$

lo, po,er users are cate&ori1ed into &roups: ?sers in the ran&e etension area+ ,ho suffer

from a hi&h le)el of interference and are onl( ser)ed durin& the ABS+ and users that are located

closer to the lo, po,er node+ so the( are not hea)il( affected b( the interference from the

macro+ and can be ser)ed b( an( sub-frame ,hether ABS or non-ABS.

In a macro-femto scenario+ the situation is )er( similar+ but+ in this case+ ABS are used at

lo, po,er nodes in order to miti&ate the interference in control channels of users ser)ed b(

the macro stations0 in other ,ords+ on this occasion+ the transmission acti)it( that is muted is

of the femto nodes. 2acro cells users in the )icinit( of a small cell can be scheduled ,ithin the

sub-frames o)erlappin& ,ith the ABSs of the femto nodes+ ,hich si&nificantl( miti&ate cross-

tier interference ##.

7i&ure $: Illustration of ABSs used in different scenarios ##

7or back,ard compatibilit(+ certain si&nals must be transmitted in all DF sub-frames

e)en if the( should be muted. Therefore+ an ABS is characteri1ed b( minimum transmission+

but not completel( null0 that is ,h( these sub-frames are called almost blank. No control or

data si&nals are transmitted in ABSs+ onl( reference si&nals are sent+ namel(:

Q 3ommon reference si&nals 43;S5

Q %rimar( and secondar( s(nchroni1ation si&nals 4%SS and SSS5

Q

%h(sical broadcast channel 4%B3H5

Q

S(stem Information Block-# (SIB-#5 and pa&in& ,ith their associated ph(sical

do,nlink control channel 4%D33H5


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Date: ! "une !#$ %a&e $' of '$

The main ar&ument for implementin& time domain partitionin& is to enable support for

ecessi)e ran&e epansion for those operators that do not ,ant to rel( on 3A.

ABS technique in the case of macro-femto scenarios ,ill be further eplained throu&h

the net section. The eplanation is focused on the scenario ,here macro stations and femto

nodes coeist in the same &eo&raphical area+ because as it ,as said pre)iousl(+ femto cells are

the sort of small cells most deplo(ed in HETNETS implementations.


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Date: ! "une !#$ %a&e $> of '$

4)

SECT"O# 4*A+MOST B+A# S(B&RAMES .ROCE$(RE

Throu&h this section it ,ill be further eplained ho, the time domain eI3I3 technique

ABS ,orks for miti&atin& interferences in macro-femto scenarios. As it ,as eplained

pre)iousl(+ ,hen an ?E ser)ed b( the macro station is in close proimit( to a femto cell

,orkin& in closed mode+ it suffers from stron& interference in the DF.

7i&ure 8: 2acro-femto interference scenario

Therefore+ the criterion to determine ,hen a femto node must transmit ABSs is based

on the )alue of the SIN; of the ?E. Based on technical documentation in $%% standardi1ation+

t(pical )alues of tar&et SIN; for DF control channels transmission are -> dB or -8 dB #> so

that+ if the SIN; falls belo, this le)el+ the ?E cannot recei)e either control or data channels.

To implement the procedure of transmittin& ABS+ first of all+ the macro station needs to

kno, ,hich ?Es are sufferin& hi&h interferences from femto nodes. nce )ictim ?Es are

identified+ the macro station has to communicate the femto nodes that are causin&

interference that the( should launch ABS transmission mode. The SIN; le)el at the )ictim ?Es

should be track to determine ,hen the ?E is out of risk and+ then+ the macro station must

notifies the femto node to deacti)ate ABS transmissions. 3oordination bet,een both the

macro station and its ser)ed ?Es and the macro station and the femto nodes is indispensable

durin& all the process. The net scheme s(nthesi1es the procedure described:

2acro ?E

2acro ?E

7emto ?E3S femto node


37/53

Inter cell interference coordinationAlmost Blank Sub-frames approach

Date: ! "une !#$

4)1 TRAC MAC

If a SIN; le)el belo, t

affected b( hi&h interferenc

The ABS transmission shoul

a,a( from the femto node+

le)el in the ?E rises abo)e

continuousl( all the ?Es it s

ualit( Indication 43I5 rep

important element of FTE t

indicate a suitable DF trans

)alue+ the channel qualit( is

?Es is essential to perform th

4)1)1 C/A##E+

In FTE+ 3I is a 8-bit i

based on the obser)ed SIN

&enerated0 the onl( require

feedback from ?E must be u

capabilit( such as the numbe

Track macro ?Es and

)ictims detection

techniques in HETNETS

7i&ure ': Scheme of ABS procedure

RO (ES A#$ -"CT"MS $ETECT"O#

he threshold is detected in a macro ?E0 that is

from a close femto node+ the ABS procedure

be stopped ,hen the ?E affected b( hi&h in

hich ,as causin& the interference0 in other ,or

the threshold. Therefore+ the macro base s

r)es. In order to do that+ the macro stations t

rt messa&e of all the ?Es it ser)es. This rep

at contains information sent from a ?E to hi

ission data rate. It is important to note that t

better. 3oordination bet,een the macro stati

e trackin& procedure.

(A+"T, "#$"CATOR &EE$BAC

te&er+ there are #' different )alues ran&in& fr

;. The $%% specification does not state ho

ent it must satisf( is that the block error rat

nder #!G. The 3I estimation process takes in

r of antennas and the t(pe of recei)er used for

Identif( a&&ressors

femto nodes

Acti)

Trac

d

Deacti)ation of ABS

mode

%a&e $= of '$

to sa(+ this ?E is

must be initiated.

terference mo)es

s+ ,hen the SIN;

ation must track

acks the 3hannel

rt messa&e is an

ser)in& node to

he lar&er the 3I

on and its ser)ed

m # to #'+ and is

3I should be

4BFE;5 ,ith 3I

o account the ?E

etection. The 3I

ation of ABS

mode

in& of SIN;

rin& ABS


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Date: ! "une !#$ %a&e $9 of '$

reported )alues are used b( the macro station for DF schedulin& and link adaptation$+ ,hich

are important features of FTE. Ho, often and ,hen the ?E feeds back 3I is controlled b( the

macro station.

The 3I can be periodic or aperiodic. The aperiodic 3I is carried b( %h(sical ?plink

Shared 3hannel 4%?S3H5 and it is onl( sent ,hen it is asked to b( the macro station. n the

other hand+ the periodic feedback is usuall( carried b( the %h(sical ?plink 3ontrol 3hannel

4%?33H5. Ho,e)er+ if the ?E is scheduled in the ?F+ the reported 3I mo)es to the %h(sical

?plink Shared 3hannel 4%?S3H5. This is because a ?E cannot transmit on both %?33H and

%?S3H simultaneousl( and+ unlike the %?S3H+ the %?33H is limited in capacit(. %eriodic 3I

feedbacks are sent periodicall( to the macro station0 the period bet,een consecuti)e 3Ireports is communicated b( the macro station to the ?E at the start of the 3I reportin&

process. If both of them are needed+ periodic and aperiodic reports+ onl( the aperiodic is sent.

The &ranularit( of 3I report can be di)ided into three le)els: ?E selected sub-band+

hi&her la(er confi&ured sub-band and ,ideband 3I report. In Sub-band le)el feedback+ the ?E

reports 3I for each sub-band. n the other hand+ in


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Date: ! "une !#$ %a&e $ of '$

Each )ictim ?E reports its measurements for the recei)ed si&nal po,er from ad6acent

femto nodes to its ser)in& macro station+ ,hich anal(1es these reports. In order to do that+ the

;S;% measurement report control messa&e is sent from a )ictim ?E to its ser)in& macro

station. The macro station defines tri&&erin& conditions under ,hich ?Es can send their

reports. The ;S;% measurement report messa&e contains a list of nei&hborin& cells of the ?E

specif(in& the measured ;S;% for each of them. The ;S;% from each one of the listed cells

represents the interference le)el seen b( the )ictim macro ?E from this femto node. The

measurement report includes the first 9 cells sorted in descendin& order dependin& on their

;S;% )alues0 that is to sa(+ the femto node ,ith the hi&hest interference le)el comes first. 7or

determinate the ;S;% measured from a specific femto node+ its cell specific reference si&nals

shall be used. If the ?E can reliabl( detect cell specific reference si&nals from other

nei&hborin& antennas+ it ma( use also these si&nals in addition ,ith the ones of the specific

femto node to determine the ;S;%8!. The ;S;% measurement report is the same as

re&ular scannin& reports used for hando)er.

Accordin& to this information+ the macro station identifies and selects the femto nodes

,ith hi&her le)el of interference and tri&&ers these femto nodes to acti)ate the ABS mode+

until the SIN; le)el+ that is to sa(+ the 3I )alue reported+ of the )ictim ?E raises to a certain

tar&et SIN;.

4)3 ACT"-AT"O# O& ABS MO$E

nce the macro station kno,s ,hich femto nodes are &eneratin& hi&h interferences on

the )ictims ?Es+ the ABS transmission mode in these femto nodes must be acti)ated. In this

process+ coordination bet,een the macro station and the femto nodes of the net,ork is

essential. The macro station not onl( has to notif( the femto nodes selected as a&&ressors that

the( must operate in ABS mode+ but also must specif( ,hich ABS pattern should be follo,ed.

8;eference si&nals are transmitted durin& the first and fifth 7D2 s(mbols of each slot ,hen the

short c(clic prefi is used and durin& the first and fourth 7D2 s(mbols ,hen the lon& c(clic prefi is

used #.


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Date: ! "une !#$ %a&e 8! of '$

The macro station decides ,hich ABS pattern ,ill be used takin& into account some

parameters such as the number of )ictim ?Es+ the le)el of interference at the )ictim ?Es+ their

locations in the cell and their requested ser)ices+ or in other ,ords+ their input load. It is also

important to consider the impact that the selected pattern ,ill ha)e on the performance of

the a&&ressors femto nodes+ since the throu&hput of the femto cells selected to operate in the

ABS mode ,ill be de&raded due to the blanked sub-frames.

4)3)1 ABS.ATTER#S

The ABS pattern specifies ,hich sub-frames in a &i)en frame ,ill be scheduled as ABS.

This information is echan&ed bet,een the macro station and the a&ressors femto nodes )iabitmap patterns. ?pon recei)in& the bitmap pattern+ either )ia customi1ed operation

administration and maintenance or )ia P interface+ the femto node starts ABS transmission.

So that+ the macro station can schedule data for its )ictim ?Es on sub-frames that o)erlap ,ith

the a&&ressor ABS transmissions. In order to enable this+ time-domain I3I3 requires time-

s(nchroni1ed transmissions bet,een base stations at least at the sub-frame boundaries.

ABS sub-frame patterns can be constructed b( confi&urin& so-called multicastLbroadcast

o)er sin&le-frequenc( sub-frames 42BS75' or L and b( not schedulin& unicast traffic 4or b(

reducin& transmit po,ers5 in certain sub-frames.

There is a set of a)ailable ABS patterns for 7DD and TDD deplo(ments. The blankin& rate

L9 4for 7DD5 and L#! 4for TDD5+ and the patterns shall appl( from sub-frame !. These

patterns are defined b( $%% and are illustrated belo,:

' 2BS7 are sent to all the users in a cell 4broadcast5 or to a &i)en set of users in a cell 4multicast5

usin& a subset of the a)ailable radio resources. Information about the set of sub-frames that are

conO&ured as 2BS7 sub-frames in a cell is pro)ided as part of the s(stem information.


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Date: ! "une !#$ %a&e 8# of '$

7DD patterns:

4#L9+ #+ ABS5 #!!!!!!!+ K

%D33H> %DS3H

=

# frame 4#! ms5

4L9+ + ABS5 ##!!!!!!+ K

4$L!+ #+ 2BS75 #!!!!#!!!! #!!!!!!!!!

TDD patterns:

4#L#!+#5 !!!!!!!!!#+ K

4L#!+5 !!!!!##!!! !!!!!##!!!

4L#!+#+2BS75 !!!!#!!!!# !!!!#!!!!#

>%h(sical channel transmitted in the DF. %D33H carries bet,een others the DF allocation

information+ ?F allocation &rants for the terminal.

=%h(sical channel transmitted in the DF. %DS3H is used for la(er # transport data transmission.

Supported modulation formats on the %DS3H are %SC+ #>A2 and >8A2.

# sub-frame 4# ms5

ABS cell-ed&e macro ?Es are scheduled in these

sub-frames 4no interference because the femto

node does not transmit control or data si&nals in

these sub-frames+ onl( reference si&nals5

non-ABS femto ?Es and cell-center macro ?Es

are scheduled in these sub-frames 4cell-ed&e

macro ?Es not allo,ed to be scheduled here

because the( are affected b( stron& interference5


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Date: ! "une !#$ %a&e 8 of '$

ther candidate patterns for consideration if the &roup is ok ,ith the ,ork load:

7DD patterns:

4$L9+#+ABS5 ###!!!!!+ K

The throu&hput of the femto cell operatin& in the ABS mode ,ill be de&raded since it

can onl( schedule its ?Es in the non-ABS. b)iousl(+ the hi&her the blankin& rate+ ma6or is the

de&radation. The macro station should &i)e to )ictims ?Es hi&her priorit( to be scheduled

durin& the ABS and prohibits their schedulin& durin& non-ABS. At the be&innin& of each sub-

frame+ the scheduler selects all flo,s that can be scheduled and decides ,hich schedulin&

al&orithm ,ill be assi&ned to each flo,. The scheduler is concerned mainl( ,ith throu&hput+

latenc( and fairness.There are different scheduler al&orithms pre-defined+ each of them &i)in&

preference toan( one of the mentioned concerns dependin& upon the user/s needs and

ob6ecti)es. The scheduler al&orithm selected 4i.e. the method b( ,hich data flo,s are &i)en

access to s(stem resources5 for the DF at the macro station is a )er( important decision. The

use of ABS patterns naturall( results in dramatic )ariations in the interference le)eleperienced b( macro ?Es+ and this represents a ma6or challen&e for the macro station to

conduct accurate link adaptation durin& the schedulin& process.

4)4 TRAC"#' O& S"#R$(R"#' ABS TRA#SM"SS"O#

In order to kno, ,hen the femto node can finish transmittin& ABS0 that is to sa(+ ,hen

the ?E is no lon&er affected b( hi&h interference+ the macro station has to anal(1e the ;S;%

reports as ,ell as the 3I feedback from its ser)ed ?Es durin& all the time that femto nodes

are ,orkin& in the ABS mode. The tar&et SIN; le)el for each )ictim macro ?E is based on its

location from the macro station ,here t(picall( the cell center ?Es tar&et SIN; is hi&her than

that of the cell ed&e macro ?Es. 7or decide if the ?E is no lon&er affected b( stron&

interference onl( non-ABS ha)e to be taken into account because durin& ABS the le)el of the

SIN; at the )ictim ?E ,ill considerable impro)e.


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Date: ! "une !#$ %a&e 8$ of '$

4)5 $EACT"-AT"O# O& ABSMO$E


44/53


Date: ! "une !#$ %a&e 88 of '$

parameter that it is set to !+. A macro ?E is identified as a )ictim if its filtered 3I after the

trackin& period becomes the 3I threshold or less: 3IfnJnstart R trackin& period 3Ithreshold.

The same procedure is acti)ated ,hen a )ictim ?E reports a 3I o)er the threshold

durin& a normal frame+ i.e. non-ABS. In this case+ if the filtered 3I at the end of the trackin&

period is hi&her than the threshold+ then the ?E is no lon&er considered a )ictim and the ABS

transmission is stopped.

4)6)2S"#REST"MAT"O#

@ictim macro ?Es eperience stron& interference in the sub-frames precedin& the ABS.

Ho,e)er+ due to the ABS acti)ation+ this interference le)el is hi&hl( miti&ated durin& the ABS+

and hence the reported 3I feedback 6ust before an ABS is misleadin&. Schedulin& decisions

are strictl( related to the channel qualit( eperienced b( each macro ?E+ and therefore+ ,ith

the 3I feedback. Since the reported 3I feedback 6ust before an ABS should not be used for

the calculation of the schedulin& metric durin& ABS7 periods+ in $ a strate&( to estimate the

SIN; le)el durin& an ABS is proposed+ based on the reported 3I feedbacks from pre)ious ABS

instants.

The measured SIN; le)el for the pre)ious ABS is considered as the epected SIN; le)el

for the net ABS. In order to consider the beha)ior of the process that &o)erns the chan&e of

SIN; le)el durin& ABS+ and the abrupt )ariations that ma( occur+ a Calman filter9 is used to

estimate the SIN; le)el durin& an ABS based on the reported SIN; le)el+ )ia 3I feedbacks+ for

the past ABS.

The discrete Calman filter con)er&es rapidl( to the eact measurements ,ith or $

iterations at most+ as it can be seen in the fi&ure belo,. Therefore+ it is a &ood procedure to

estimate the SIN; of a &i)en )ictim macro ?E durin& ABS from pre)ious 3I feedbacks for ABS.

9The Calman filter is a set of mathematical equations that pro)ides an efficient computational

4recursi)e5 means to estimate the state of a process+ in a ,a( that minimi1es the mean of the squared

error. The filter is )er( po,erful in se)eral aspects: it supports estimations of past+ present+ and e)en

future states+ and it can do so e)en ,hen the precise nature of the modeled s(stem is unkno,n 8.


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Date: ! "une !#$ %a&e 8' of '$

7i&ure >: 7ast response and accurate trackin& of the SIN; usin& Calman filter $

4)6)3

T/RO('/.(T $E'RA$AT"O# "# &EMTO CE++S

In order to reduce the throu&hput de&radation in femto cells due to the use of ABS+ a

no)el approach+ called ABS offsettin&+ is proposed in '. The aim of this is proposal is to

reduce the blankin& rate at the femto cells ,hile preser)in& the required optimal blankin& rate

at the macro cell. The main idea in this paper is that there is no need to force the entire

a&&ressor femto nodes in the macro cell area+ ma( be se)eral of them affectin& different ?Es+

to operate in the ABS mode ,ith the same pattern required b( the macro cell. Therefore+ each

a&&ressor femto node ,ill be confi&ured to operate in ABS mode ,ith a certain blankin& rate+

and it ,ill not start blankin& at sub-frame ! as defined in + but ,ill start blankin& at an

offset.

To achie)e this+ the s(stem resources are partitioned in t,o sta&es. 7irstl(+ all the macro

?Es are di)ided into t,o &roups+ normal ?Es and )ictim ?Es+ and the resources are di)ided

bet,een them+ that is+ the optimal and fair blankin& rate is specified. Secondl(+ )ictim macro

?Es are di)ided into &roups accordin& ,hich femto node is causin& stron& interference0 in

other ,ords+ all )ictim ?Es affected b( the same femto node are put to&ether. S(stem

resources are partitioned a&ain bet,een the different &roups of )ictim macro ?Es0 in other

,ords+ each coalition is assi&ned a part of the ABS amount+ achie)in& a reduction of the

blankin& rate associated to each &roup. It is important to note that there ma( be a resource

partition that is shared partiall( or completel( bet,een t,o coalitions. To &uarantee that the

effecti)e blankin& rate seen b( the macro cell is the optimal and fair blankin& rate decided in

the first sta&e+ each reduced blankin& pattern ,ill be associated ,ith an offset. The ABS


46/53


Date: ! "une !#$ %a&e 8> of '$

pattern selection problem in both the first and the second sta&e of resources partition is

sol)ed modelin& the s(stem as a Nash bar&ainin& problem.

7i&ure =: Scenario considerate

7emto cell #

7emto cell

7emto cell $

7emto cell 8

2acro cell

7i&ure 9: Associated reduced blankin& rates and offsets+ and effecti)e blankin& rate at the macro cell

4ABS pattern of $L95

The throu&hput of femto cells tri&&ered to operate in ABS mode decreases the hi&her

blankin& rate+ because it is relati)e to the amount of blanked sub-frames. Therefore+ usin&

reduced blankin& rates+ a&&ressor femto nodes throu&hput is not so de&raded as if it had to

follo, the optimal rate specified b( the macro.

In conclusion+ the ABS7 offsettin& hi&hl( compensate the decrease in the a&&ressor

femto nodes throu&hput+ and it &i)es hi&her throu&hput to the femto ?Es in ABS-mode

tri&&ered femto cells ,ithout affectin& the throu&hput of the macro cell ?Es+ both normal ?Es

and )ictim ?Es.

#

ABS L9

ABS #L9

$

8

ABS #L9

ABS #L9


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Date: ! "une !#$ %a&e 8= of '$

7i&ure : A&&re&ate throu&hput of the femto cells '


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Date: ! "une !#$ %a&e 89 of '$

CO#C+(S"O#S

In this pro6ect a deepl( research about hetero&eneous net,orks has been done ,ith a

special focus on the inter-frequenc( interference cancelation time-domain technique called

Almost Blank Sub-frames.

As a matter of fact+ mobile broadband traffic is increasin& and+ ,ith it+ also users*

epectations for hi&her data rates. Deplo(in& hetero&eneous net,orks is a con)enient means

to meet increasin& traffic demands and performance epectations+ because it enables for

substantial &ains in the capacit( and performance of ,ireless s(stems.

The ran&e epansion technique allo,s a better load balancin& o)er the different

net,ork la(ers+ increasin& the capacit( and robustness of the net,ork. Ho,e)er+ it is limited

b( inter-cell interference. 7emto nodes ,orkin& in closed access mode also produce hi&h

interferences to users that cannot connect to them. Therefore+ coordination bet,een

differents cells in the net,orks and interference mana&ement is essential to achie)e an

effecti)e hetero&eneous net,ork deplo(ment.

The )arious eI3I3 techniques eplained throu&h the pro6ect enable the impro)ement of

the performance de&radation caused due to the inter frequenc( interference. But+ despite the

&reat ad)ances that are bein& made in the field of eI3I3 techniques+ the( fail to completel(

cancel the interference in the control channels+ so that net,ork performance is al,a(s

de&raded.

7uture researches should be focused on implementin& the soft-cell scheme+ concept

described at the end of Section . This approach is based on the fact that the lo, po,er nodes

do not create a ne, cell different of the macro cell. Thereb(+ the problem that the

transmission cannot be carried out o,in& to the fact that reception of control channels is not

reliable is eliminated+ because reference si&nals are the same for lo, po,er nodes and for the

macro station.


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Date: ! "une !#$ %a&e 8 of '$

TERMS A#$ ABBRE-"AT"O#S

- second &eneration

$ - third &eneration

$%% - $rd eneration %artnership %ro6ect

8 - fourth &eneration

B3H - broadcast channel

BFE; - block error rate

BS - base station

3A - 3arrier A&&re&ation3A; - 3ompound Annual ro,th ;ate

3B< - 3ell Border


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Date: ! "une !#$ %a&e '! of '$

S3-7D2A - Sin&le-carrier frequenc(-di)ision multiple access

SIB-# - S(stem Information Block-#

SIN; - Si&nal to Interference and Noise ;atioSSS - secondar( s(nchroni1ation si&nal

TD-S3D2A - Time Di)ision S(nchronous 3ode Di)ision 2ultiple Access

TDD - Time-di)ision duplein&

?E - ?ser Equipment

?F - ?plink

?2TS - ?ni)ersal 2obile Telecommunications S(stem


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Date: ! "une !#$ %a&e '# of '$

RE&ERE#CES

# Claus I. %edersen+ Uuan(e .#8+ VE)ol)ed ?ni)ersal Terrestrial ;adio Access 4E-?T;A50 %h(sical la(er0

2easurementsW 09PP S,6:#2arch !##

#! Stefan %ark)all+ Erik Dahlman+ eor&e "Zn&ren+ Sara FandstrZm and Fars Findbom+

VHetero&eneous net,ork deplo(ments in FTEW+ 'ricsson revie3 $%&&

## Fope1-%ere1+ D.+ u)enc+ I.+ de la ;oche+ .+ Countouris+ 2.+ uek+ T..S. and "ie

[han& VEnhanced inter-cell interference coordination challen&es in hetero&eneous

net,orksW+ in )ireless Communica,ions# I''' Volume&1# Issue 0"#"une !##

#


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Date: ! "une !#$ %a&e ' of '$

#$ @olker %auli and Eiko

00000- inter cell interference coordination techniques in hetnets

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