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Inter Cell Interference
Coordination techniques inHETNETS
Almost Blank Sub-Frames approach
Elena Rodrguez Martnez
June 2013
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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& toðer ,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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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
toðer+ 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
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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.
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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
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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.
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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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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 #$: Soft-cell scheme #!
This technique has some important benefits in front of the resource partitionin&
technique. Since there is onl( one cell formed b( lo, po,er nodes and the macro site+ the
deplo(ment is easier because careful cell plannin& is not needed. i)en that lo, po,er nodes
can turn off their transmissions ,hen the( are not necessar(+ it is possible to sa( that the
technique is ener&eticall( efficient. The soft cell scheme also allo,s an efficient use of the
spectrum as there is no problem ,ith 3;S interference. In addition to these ad)anta&es+ due to
the fact that transmission nodes are transparent to ?E+ soft cells can pro)ide &reater mobilit(
robustness than deplo(ments ,ith separate cells. Traditional hando)er procedure is not
required ,hen mo)in& bet,een macro and lo, po,er nodes+ so the probabilit( of dropped
connections is lo,er.
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3)
SECT"O# 3*"#TERCE++ "#TER&ERE#CE MA#A'EME#T
No,ada(s+ the e)olution of the ph(sical la(er has reached a le)el+ ,here operation )er(
close to information theoretic bounds on achie)able spectral efficienc( for a &i)en si&nal to
interference and noise ratio 4SIN;5 is feasible #8. 7or this reason+ impro)in& the SIN; b(
reducin& or+ e)en better+ a)oidin& the inter-cell interference is the onl( ,a( to achie)e
si&nificant increases in spectral efficienc(. Therefore+ researchers are focusin& on inter-cell
interference coordination 4I3I35 ,hen definin& net &eneration mobile communication
standards+ such as FTE-Ad)anced 4FTE-A5.
3)1 BAC'RO(#$
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
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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 #=
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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 $%% ;elease 9 as an optional method to sol)e inter-cell intra-
frequenc( interference. The main idea of I3I3 is to di)ide each cell into t,o sections+ the cell
center and the cell ed&e+ and then allocate different subcarriers to users in different locations.
Theoreticall(+ it is impossible for subscribers at the ed&e of t,o nei&hborin& cells to use the
same frequenc(+ and+ in that ,a(+ the possibilit( of interference bet,een t,o nei&hborin&
cells is reduced.
I3I3 methods allo, achie)in& a sli&ht s(stem performance boost b( mana&in& the
s(stem band,idth and transmit po,er. Ho,e)er+ the frequenc(-domain I3I3 schemes
introduced in ;elease 9 and ,ere mainl( desi&ned for macro-onl( scenarios and onl( pro)ide
impro)ements for the ph(sical data channels+ ,hile failin& to offer protection for ph(sical
control channels carr(in& critical information for achie)in& &ood s(stem performance.
The basic idea on ,hich the I3I3 schemes rel( is to di)ide the ,hole a)ailable resources
into t,o &roups+ one for ser)e the cell-ed&e users and the other for the cell-center users.
Accordin& to the t(pe of resource schedulin&+ I3I3 techniques can be classified into hard
frequenc( reuse+ fractional frequenc( reuse and soft frequenc( reuse. The transmission po,er
of each portion of the spectrum is also controlled to pre)ent cell ed&es and nei&hborin& cells
from bein& affected.
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3)2)1/AR$ &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.
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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 #'
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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 $%% ma( not be effecti)e in the
case of HETNETS+ because+ as it ,as said before+ the( ,ere mainl( desi&ned for macro-onl(
scenarios. The basic &oal of I3I3 is the pro)ision of a more homo&eneous ser)ice to users
located in different re&ions of the net,ork+ i.e. mostl( to promote the cell-ed&e performance.
This aspect has &ained e)en more importance ,ith the introduction of multi-la(er
hetero&eneous net,orks #$.
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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
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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&
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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
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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 $: Illustration of ABSs used in different scenarios ##
7or back,ard compatibilit(+ certain si&nals must be transmitted in all DF sub-frames
e)en if the( should be muted. Therefore+ an ABS is characteri1ed b( minimum transmission+
but not completel( null0 that is ,h( these sub-frames are called almost blank. No control or
data si&nals are transmitted in ABSs+ onl( reference si&nals are sent+ namel(:
Q 3ommon reference si&nals 43;S5
Q %rimar( and secondar( s(nchroni1ation si&nals 4%SS and SSS5
Q
%h(sical broadcast channel 4%B3H5
Q
S(stem Information Block-# (SIB-#5 and pa&in& ,ith their associated ph(sical
do,nlink control channel 4%D33H5
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The main ar&ument for implementin& time domain partitionin& is to enable support for
ecessi)e ran&e epansion for those operators that do not ,ant to rel( on 3A.
ABS technique in the case of macro-femto scenarios ,ill be further eplained throu&h
the net section. The eplanation is focused on the scenario ,here macro stations and femto
nodes coeist in the same &eo&raphical area+ because as it ,as said pre)iousl(+ femto cells are
the sort of small cells most deplo(ed in HETNETS implementations.
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4)
SECT"O# 4*A+MOST B+A# S(B&RAMES .ROCE$(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 $%% standardi1ation+
t(pical )alues of tar&et SIN; for DF control channels transmission are -> dB or -8 dB #> so
that+ if the SIN; falls belo, this le)el+ the ?E cannot recei)e either control or data channels.
To implement the procedure of transmittin& ABS+ first of all+ the macro station needs to
kno, ,hich ?Es are sufferin& hi&h interferences from femto nodes. nce )ictim ?Es are
identified+ the macro station has to communicate the femto nodes that are causin&
interference that the( should launch ABS transmission mode. The SIN; le)el at the )ictim ?Es
should be track to determine ,hen the ?E is out of risk and+ then+ the macro station must
notifies the femto node to deacti)ate ABS transmissions. 3oordination bet,een both the
macro station and its ser)ed ?Es and the macro station and the femto nodes is indispensable
durin& all the process. The net scheme s(nthesi1es the procedure described:
2acro ?E
2acro ?E
7emto ?E3S femto node
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4)1 TRAC MAC
If a SIN; le)el belo, t
affected b( hi&h interferenc
The ABS transmission shoul
a,a( from the femto node+
le)el in the ?E rises abo)e
continuousl( all the ?Es it s
ualit( Indication 43I5 rep
important element of FTE t
indicate a suitable DF trans
)alue+ the channel qualit( is
?Es is essential to perform th
4)1)1 C/A##E+
In FTE+ 3I is a 8-bit i
based on the obser)ed SIN
&enerated0 the onl( require
feedback from ?E must be u
capabilit( such as the numbe
Track macro ?Es and
)ictims detection
techniques in HETNETS
7i&ure ': Scheme of ABS procedure
RO (ES A#$ -"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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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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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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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( $%% and are illustrated belo,:
' 2BS7 are sent to all the users in a cell 4broadcast5 or to a &i)en set of users in a cell 4multicast5
usin& a subset of the a)ailable radio resources. Information about the set of sub-frames that are
conO&ured as 2BS7 sub-frames in a cell is pro)ided as part of the s(stem information.
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7DD patterns:
4#L9+ #+ ABS5 #!!!!!!!+ K
%D33H> %DS3H
=
# frame 4#! ms5
4L9+ + ABS5 ##!!!!!!+ K
4$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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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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4)5 $EACT"-AT"O# O& ABSMO$E
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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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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 toðer. 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
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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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7i&ure : A&&re&ate throu&hput of the femto cells '
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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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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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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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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 $%&&
## Fope1-%ere1+ D.+ u)enc+ I.+ de la ;oche+ .+ Countouris+ 2.+ uek+ T..S. and "ie
[han& VEnhanced inter-cell interference coordination challen&es in hetero&eneous
net,orksW+ in )ireless Communica,ions# I''' Volume&1# Issue 0"#"une !##
#
8/10/2019 00000- Inter Cell Interference Coordination Techniques in HETNETS
52/53
Inter cell interference coordination techniques in HETNETSAlmost Blank Sub-frames approach
Date: ! "une !#$ %a&e ' of '$
#$ @olker %auli and Eiko