A ETHOD AND DEVICE IN A COMMUNICANON NETWORK
Ehds4EdC-et&e.!rclse!
The invention relates to communication networks, and in particular to a method in a
first device for estimating the quality of a sEnal lransmined lrom a secood device to a
third device.
Backoround io the lnvsntion
Femtocell base stations in a Long Term Evolution (LTE) communication network
(otherwise known as Home evolved Node Bs - HeNBs - or Enterprise evolved Node
gs - EeNBs) are small, lor.r-power, indoor cellular bas€ stations for residenlial or
business use. They provide better netwoft coverage and capacity than that available
in such environments from the overtying macrocelluhr LTE. network. ln gddition,
femlocell base stations us,e a broadband connection to rec€ive data from and send
daia back to the operato/s network (known as'backhaul")
As ferllg:ell base stations can make use of the same frequencies as macrocell base
stations in the maqocellular networ*, and as lhey are loc€ted within the @verage area
ot one or more macrocell base stations in the macrocellular net!,t,ort, it is necessary to
ensure that dowrlink transmissions from lhe femtoc€ll base stalon to mobilE devrces
(otherwise known as User Equipments - UEs) using lhe lemioc€ll base station do not
inlerrere subslanlialty wfi downlink transmissions from macrocell base slations lo
mobile devices using the macrocell base stations.
Typically, lhis interlerence is mitigaled by placing a cap on the po!{rr that the femtocelt
base_station c€n use lo transmit signals to mobile devices The cap on the power can
be set such that, al a specified pathloss from lhe femloce base station (for example 80
dB), a signal received by a mobile device from a macrocell base station woutd meet a
specfied quality level (for example a target signal to inierference plus noise ratio -
SINR). The determination of the cap is subjecl to a minimum and maximum power
rcstdclon on lhe lransmission power of the fer oce base station, for exampte O OOI
W and 0.1 W respectively
However, this approach has limitalions in that lhe transmission po!,ver of lhe femtocell
base statpn will be
lhe femlocell base
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that need protecting. This cap- c4n lead to the data throughM ror mobile devices
communicating with the femlocell base station being unnecesiarily reskicted
ln providing an approach ror sening the maximum permined fansmission po\,t er for
downlink transmissions ,rom femtocell base stations, it is necessary for the femtocell
base station to determine if there are neaby mobile devices that need protecting.
Therefore, there is a need for a melhod in which the iemtocell base station can
detemine lhe quality of signals being iransmitted from a mobile devbe lo another base
station
Summarv of the lnvention
Therefore, acaording to a rlrst aspect of the invenlion, there is provided a method of
estimating a quality of a signal, the method in a lirst device comprising measuring a
signal transmitted from a second device lo a third device; determining a value of a
metric from an auloconelalion tunclion of the measured signal; and determining an
estimate of the quality ofthe signalfrom the determined metric
Preferably, the step of measuring conprises measuring the signal in the time domain,
and the step of determining a value of a metric comprises determining the
autocorrelation function of the time domain signaland noise.
Preferably, the step of detemining a value of a metric comprises determining the
autoconelation function comprises normalising the measured signal to give a sequence
r; taking the fast Fourier transfom of lhis sequence to give t; determining the squared
magnitude of each sample in f; and taking the inverse fasl Fourier transfom of lhe
sequence resulting frorn the step of determining the Squared magnitude to give an
autoconelation sequence a.
ln a prefened embodiment, the step of determining a value oI a metric ftom an
autoconelation function of the measured signal comprises c€lculating the magnitude or
squared magnitude of lhe autoconelation function.
Preferably, the step of determining a value of a metric Iurther comprises adjusting or
zeroing the centraltap in lhe output ofthe step ofcalculating.
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ln a furlher embodiment, the step of detemining a value of a metric fudher comprises
adjusting or zeroing the tap adjacent the central tap in the outpul ot the step of
calculatrngln
one embodiment, the step of determinrng a value of a metric comprises identifying
the tap with the largest magnitude or squared magnitude in the taps remaining in lhe
output of the step of c€lculaling: and setting the metric to lhe value of said magnitude
or said squared magnitude ofthe identified tap.
ln one erhbodiment, the step of determining a value of a metric further comprises
adjusting the value of the mel.ic based on the distance of the iient ied tap from lhe
cenlraltap
ln another embodimenl the step o, determining a value of a metric furlher comprises
adjusting the value of the metric based on a function of a peak to average power ratio
of thg measured signal.
ln this embodiment, the step of adjusling the value ofthe metric based on a funclion of
a peak lo average power ratio o[ the measured signal preferably cornpnses, in the
event that the peak to average power rato of lhe measured signal is below a threshold
value, adjusting the value oflhe metric to a minimum value
ln one embodiment, the step of determining an eslimate of the quality of the signal from
the determined metric comprises comparing the determined melric to a look-up lable.
ln an altemative embodiment, the step of determining an eslimate of the quality of the
signal from-- lhe determined metric comprises using a curv+lltting technique to match
the determined metric to a predelermrned relalionship between values tor the metric
and the quality ofthe signal.
Prefe.ably, the step of measuring comprises measu.ing a Zadoff-Chu reference signal
transmitled from the second device to the third device, and the quality of the signal is a
signal to noise ratio.
Preferably, ihe step of measuring a Zadotf-Chu relerence signal comprises estimating
the posiiion of the Zadoff-Chu reference signal in time.
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Preferably, the step of measuring comprises measuring a portion of the zadoff-Chu
reference signal.
5 ln one embodiment, the method further comprises the step of using a scheduler to
ensure that no signals will be iransmitted io the first device from other devices
associated therewith that might interfere with the execution of the step of measuring.
According to a second aspect of the invention, there is provided a network elemenl for
10 use in a communication network, lhe network element being configured to perform the
method described above-
Brief Descriotion of the Drawinqs
The invention will now be described in detail, by way of example onv' with reference to
15 the following drawings, in which:
Figure '1 shows an exemplary communication network;
Figure 2 is a flow chart illustrating a method of setting a maximum permitted
20 transmission power for a femtocell base station;
Figure 3 is a flow chart illustrating the method of Figure 2 in more detail,
Figures 4(a) and 4(b) are graphs illustrating the autoconelation function for time
25 domain reference signals with low and high signal to noise ratios respeqtively;
Figure 5 is a graph illustrating a plot of autoconelation function peaks against signal lo
noise ratiol
30 Figure 6 is a graph illustrating a plot of peak to average power ratios against signal to
noise ratio;
Figure 7 is a graph illustrating a plot of autocorrelation funclion peaks against signallo
noise ratio in which the scatter has been reduced;
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Figure I is a flow chart illustrating a method ofestimating a signal quality of a reference
signal in an uplink in accordance w h an exemplary embodimenl of the invention;
Figure I is a graph illustrating the change in throughput on a macrocell downlink
against femtocell base station density in a macrocell sector;
Figure 10 is a graph illustrating the change in throughput on a macrocell dolvnlink
against femtocell base slatlon densty tor a user equipment at the edge of the
macroc€lli
Figure 11 is a graph illustrating the change in throughput on a femtocell downlink
against ,emtocell base slation density; and
Figure 12 is a graph illustrating the change in throughput on a femtocell downlink
against femtocell base station density for a user equipment at lhe edge of the
femtocell.
Detailed Description ot the Prelened Embodiments
Ahhough the invention will be described below with reference to an LTE communication
network and femtocell base stations or HeNBs, it will be appreciated that the invention
is applicable to other types of thid or subsequent generation netwoft in which
femtocell base statpns (whether for home or business use), or their equivalents in
those networks, can be deployed Moreover, although ln the embodiments below the
femtocell base stations and macrocelt base stations use the same air interface (LTE), it
will be appreciated that lhe invention c€n be used in a situaljon in which lhe macrocell
and femtocell base slations use the same or corresponding frequencies but different air
interface schemes (for example the macrocell base stations could use WCDMA whrle
the femtocell base sktions use LTE).
Figure 1 shows part of an exemplary communication network 2 in which the inventon
can be implemenled. The communication network 2 includes a plurality of macrocell
base stations 4 (only one of which is shown in Figure 1) thal each define a respective
cove.age area - indicated by macrocell 6. ln an LTE communicalion network, the
macrocell base stations 4 are refered to as evolved Node Bs (eNBs).
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One o「more Femtoce‖ base slauOns 8(Home eNBs‐ HoNBs)can be locatedぃ出hin the
∞ verage area 6 or the macrcce‖ base sla“on 4(a“hOugh onv one femtOcel base
stallon 8 is shown in Figure l),w lh each femloco‖ base stalion 3 dern ng a respective
∞verage area― indicated by feml∝ el 10
n wll be appreciated that FIgure l has nol been drawn to scale,and thatin most reaiworld
implementatbns the∞ verage area 10 of the l¨ tocel base stalon 3面‖be
signincantw smallerthan he∞ ve嘲e area 6 ofthe macroce‖ base station 4
A number of moblo devlces(UEs)12 are also ocated in the communicatlon nemrk 2
w“hin the coverage area 6 ofthe macroce‖ base statOn 4
Four mobile dev ces 12o, 12b. 12c and 12d are each assocレned wllh the macr-ll
base sta“ on 4,meaning that they transmn andrOr r_lve conlrol slgna‖ ing and/or data
using the macroca‖ base station 4 11面‖be noled lhat akhough the mobile d● vk"12d
ヽalso wnhin the cOverage area lo oithe fmlocel base stat on 8,n is assOcialod wnh
the macroce‖ base staton 4(lhis COuld be due to the signal strength of he macrclce‖
base statbn 4 being signincantv better for moble device 12d than the signai strength
of the remtoce‖ base stalon 8 or the tmto崚‖base stalion 8∞ uld be reshcted:o
specinc subsoibers that don't include mclbile device 12d. elc) Mobiie dev● s 12a.
12b.12c and 12d are refered to collecllvely herein as・macro uEs・,as they are the
mob‖e devlces7user equipmenls(uEs)aulCiated tth the macr― ‖base station 4
Twclfutter mob‖e devices.12e and 121.are l● cated tthin lhe coverage area 10 ofthe
femtoce‖ base stalon e and are curentv assOCiated wth the remtOce‖ base sta“ on 8,
meaning that they transmn and/Or recelve controi signalling and/or data using the
remtoce‖ base sta“ on 8 Moble devices 12e and 12,are reFered to colect～ev herein
as iemlo uEs・asthey are he mob‖ e deviceyuser equipments(UEs)assOCiated wnh
the femtoce‖ base sta“on 3
As desc●bed above.l is necessary to ensure that the downlink transmisslons from the
femtoce‖ base station e tO the femto uEs 12e and 12f do nol prevent nearby mac“ ン
UEs(such as macroUE 12d)from being able to successfu‖ y rece～ e down"nk
transrnissions rrOm the macroce‖ base staton 4 A similar requrement exisls for a
mobile device that is associated wnh anOther tttoce‖ base station, in that the
dclwnhnk transmissions from the Femtocel base station 8 1o the femloUEs 12e and 12,
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should not prevent those mobile devices from successfu‖ y recerving the down‖ nk
transmissions from theirfemtoce‖ base statbn
As desc■bed above.lhS prclЫ emおaddressed h∞ nvenlbnd nemrks」野J四塁
"曰cap to the transmission p"呻r used by femtoce‖ base statbns 8 to transml slgnals to
remto uEs This cap is set to a va!ue that prevents these downlnk signals from
causing an undesirable level of interfere― to rr ob“ e devices that are not assoclated
wnh lhe femtoce‖ base station 8 that are in or near the coverage area 10 of the
femtoce‖ base stalon 3(such aS moble devce 12d in Figure l) ThiS Cap is appled to
the transmission power re9ardless of whether there are any moble devices in o「noar
the∞ verage area 10 of the femtocel base s● いon 3(so r wOuu be appiied,for
example,even r mob“ e devi∝ 12d was not presenl)
However.as i‖ ustrated in F19ure 2.l is deternined whether here afe any mobile
devices that are not ass● ●lated mh the femtocel base stal on 3 ulal require proteclon
from interference caused by down‖ nk translnissions or the feml_‖ base statlon e
(Step 101),and the transmission power cap for the femlocel base statわn8is set
accordingly(step 103)
A more delaled method of operaung a femtoce‖ base slahon e is llustはed in Figure 3
in FIgure 3,steps lll.113.117 and l19∞ respond io lhe step of detemining(step
101)in FIgure 2
!n the roilowing, anhough ule methOd ぃ
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be desclbed w th reference to protecting
moble device 12d“ e a maCro uE)that iS associated u“h mac● cel base slalbn 4
from dclwnlnk transmissiOns frorn the rem10ce‖ base staton 3. 1ぃ
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be appreciated
that a similar method can be used to protecl a mobile device that is associaled w“ h
anotherfemtoce‖ base slat on
;n step 111,the femlα ,l base slation 3 attmpts to dent"r there are any macr● UEs
12 that are r― Iving down“ nk lransrnissions from a macr∝ e‖ base slaton 4
in LTE,macroUEs 12 transm“ infomatbn to the macroce‖ base statbn 4 before.
dunng Or anerthe receipt of a downlnk transmissbn from the macroce‖ base stalion 4.
for example an acknomedgement(AC● NACK)signa,a channel qualty indicator
(CQり,SOunding signals,dala signals,etc Therebre,the t輛t●cel base staton e can
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monitor uplink channel(s) us€d by the macro-UEs for these transmissions to detetmine
if there are any mobile devbes neaby that might need protecling trom its dolrnlink
transmissions.
ln step 113, it is determined whether any signals detecied in step '111 originate from
mobile devices that are not being se.ved by (or associated with) the temtocell base
station 8.
ll the lemtocell bass station 8 does nol deled any signals from rnacreuEs 12, lhen the
ferntocell base slation I can assume that there are no maoo-UEs nea.by that need
proleding fiom its dor nlink transrnissions. ln this cass, in stsp 115, the maximum
permiled transrnissign poryer for lhe femtocell base station 8 can be set to a high or
relatively high value, tor example an upper limit for the transmission po$Er (such as 0.1
W in LTE). The method then retums to step 1 1 1 and repoats periodically
ll lhe fen ocell bas€ station I does detecl signals from macro-UEs '12, then lhe method
moves to step 117 in whbh the femtocell base station 8 estinEtes a quality gf a
detecled signal. This quality can be a signal to noise ralio (SNR), a signal to noise plus
interference ratio (SNIR), a signal strength. or any other measure oI the quality of a
transmitted signal ln some implementalions, depending on the way in which the
lemtoc€ll base station I delecls signals in the uplink, the terntocell base staton I may
be able to distinguish signals fro.n multiple rnacro-UEs 12 and c€n estimate the quality
of each of the signals Ho rever, in altemative implernentatjons, the femtocell bas€
station I may not be able to distinguish the signals and therefore perlorms the
estimation on lhe signal wilh the highest quality.
ln a preferred embodiment of the invention, lhe femtocell base station 8 idenlifies
characlerislics of the Zadoff-Chu reference signal and estimales the signal to noise
ratio (SNR) of this signal This embodiment b descriH in more detail below wilh
rcferen@ to Figure 4 ll will be noted that in this embodiment the femlocell base
station I does not distinguish bettr€en signals f.om multiple macrg-UEs 12 and
therefore estimates the SNR for the signal with the highest quality.
ln an ahemative implementation, the femtocell base station 8 deteds and decldes the
data in the uplink and determrnes a quality of the data stlnals. lt will be appreciated by
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those sk‖ ed h the an hat anemal～ e techn● ueS Can be used by the remtoce‖ base
station e to detemine a qua1ly ofthe signals in the uplink
The femtOce‖ base stall●n31hen∞mpares lhe esumated qualty(or the highest
esumaed qual的r the remlα
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base sta“ on 3 can es“ male the qual対for muniple
SIOnalS)Vnh a threshou value(step l19)In a preferred im● efnentabon where the
qua‖ty ls a signalto nolse ralb,the threshold can be a vaiue in the range of lo dB to
30 dB
lt面‖be noted that a nlacr● uE 12面‖need most proted on from the downlnk
transmissions of the remtOcel base stalon e when
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is near to lhe edge of the
∞verage area 6 ofthe macroce‖ base stalion 4,as the downlnk slgnals rece～ ed at
●e macro uE 12 froln the rracroce‖ base saion 4面‖be relatvely weak in tht
sluation,the macrouE 12面‖need to be transmming ns up‖ nk signals at a relal"ev
high pOwer(due to ns distance from the macr● ●e“ base sta“on 4) By estimaung a
qualty of the uplink slgnal(whiCh輌‖be affected by the transmlssbn power of the
macro uE 12d and ns prOxim● lo lhe tmtocel base sta輛on 3).the fem10cel base
station 8 can deterrnine whether`and/or the extent to which,the macκ卜UE 12d needs
proteding from the downlnk transmissions of he femloc● ‖base statbn 3
Therefore.lFthe estimated qual● exceeds the threshold value then the femtoce‖ base
slalion 8 assumes that the macro uE 12d that onginated the sunai needs siOnncanl
protttion from the dOwnlnk transmissions of lhe femlocel base slal on 3, and the
maximum pernhed transmlssion pOwer for the remtrce‖ base sta“ on 8 shouu be set
at a low or rela“ veり1●w value(Step 121) For example,the maximum pemted
lransmission pOwer can be set to a lcwer hmn for the transm ssion pOwer(suCh as
0 001 W in LTE)
in one implementalon,the remtOce‖ base slabon 3 sets the maximum pemmed
transrnission pOwer such that,al a spMled Palh oss from the femtocel base sla“ on 8
(fOr eXampL 80 dB),a Signal received by the macro uE 12d froln the macrocF‖ base
statlon 4 meels oris estimated to meet a specned qual"level(fOr example a target
signal(o interference plus noise ratio― SINR)asin a∞ nvenibnal ne¨rk
35 The method then retums to step lll and repeats penodically
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lf the eslimated quality does not exc€ed the lhreshotd value then the femtocell base
station I sets the ma.ximum permitted transmission pov{er to an intermediate value that
lies between an upper and lower limil lor the transmission pourer (step 123). Thus, the
femtocell base station I provides 6orne protection tor the macro-UE 12d, while allowing
downlink transmissions from the ferntocell base station 8 lo be transmitted at a higher
pouer than conventional techniques permit. ln this way, the data throughput tor femtc
UEs 't 2e and 1 2f can be impfoved over the conventional technique.
ln a prefened impl€mentation, the intermediate value ror lhe rnaximum pemitted
tansmission pov{er is sel€cted based on the difference between the estimaled quality
of the signal and the threshold value. ln particular, lhe value for the maximurn
permiued transmissign po$rer c€n increase in propodion io the difference between the
estimated quality of the signal and lhe threshold value (up to an upper limit. if
applic€ble). ln a prefened embodment where the quality is a signal to noise ratio, if
lhe estimated SNR is 5 dB below lhe threshold value, then the ma.ximum permined
lransmit power c€n be set to be 5 dB above the lov, or relatively low value, subject to
the upper limil gn the maximum pemitted transmil po{rer.
Again, the method retums to step 111 and repeats p€rbdic€lly.
ln one implementation of the invention, steps 113 and 1'17 can be combined, in that the
len{ocell base station 8 estimates a qualily (such as lhe SNR) ol a signal in the uplink
and if the estin€ted quality is above a panhuhr threshold, then a detection of a macro
UE 12 is assumed to have been made. This threshold could be the same or different
lo the lhreshold used in step 1 1 9.
It will be appreciated that a macrGUE 12d may move into the vicinity of the femtocell
base station 8 (r.e rnlo or near lo lhe coverage area 10 of the lemtocell base station 8)
without needing to transmit anythhg to its associated maqoc€ll bas€ statioo 4 (for
example if lhe macro-UE 12d is not receiving any do.rnlink transmissions from lhe
macrocall base station 4), which means thal the fen{ocell base station I will nol be
able to delec{ the macro-UE 12d in step 1 l1
However, as lhe maqo-UE 12d may need io monitor downlink control channels from
the macrocell base station 4 (for exarnple a broadcast channel - BCH, ot a physical
downlink control channel - PDCCH), it is nec€ssary to make sure that the macro-UE
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12d is able to receive these down‖ nk transmissbns A lhough these channels are
des[gned to be relat,ev rObust against intererence.the femtocel base staOon 3 may
stm inlerfere mh these channe:s r the tttnsmissbn p_ris sufrcbntv high
ThererOrer in One im口ementa“ on the remlocel base station e penOdicaily or
internmenuy sels the maximum pemmed transmission power io the lower"mm.in
order to provide the maximum protα■ion for any macn● uEs 12d inおvにin ty.
ires膵慧
"e of whether the remtoce‖
base station 8 detx■ s any scnals in steps lll
and l13 For example,the remtOcel base station 8 can set the maximum perrnmed
transmもsiOn pOwer io lhe ower lmm for loo mimseccnds every l second Thls輛‖
provde oppoltunnbs fOr any mac“ 卜UEs 12d that are nct tansmming anソ uplnk
slgna:s to listen for downlink transmissions from the macroce‖ base station 4
ln an anematve implerrlenta」on,the femtoce‖ base slation e can set the maxhum
pern tted transrnission pOwerto the ower“ mn wheneverthe remtOce‖ base station 3 is
transmming signaにal the same ume thal the macrocel base station 4 is transmtting
∞ntrol channel signals ln Particular,the bmtoce‖ base sta“ on e wi‖ 、,ca!V be
synchronised M“ h the macrocel base slatlon 4 and the∞ ntrol channel stnals輌‖be
sent at predeterrnined umes and On predelorlnined resource bl● cks(RBs),sO the
femtoce‖ base slation輌‖know when the macroce‖ base statlon 4面‖be tfansmmng
ule cOnlrOl channel signa:s For eXample, in LTE, scrne contrc1l channe:signals are
transmned once every lms(eg PFICH,PDCCH).W lh the nrst rOur Of fOurteen
柳bOiS transmmed per l ms cawing contrOl channel signais Olher control channels
le g PBCH,PSCH)are Sentless frequently and use approximatev seven symbols oul
d every 140 sッmbols and a subset oflhe available resource blocks
EЫ ima10n ofthe ouJ卜of an udmk reference● onJ
As descrlbed above, in a prererred embodiment of the invenlon, the femtoce‖ base
強tion 3 dentlFes charactenst cs oFthe ZadofFChu reference signal and eslmales the
st9na1lo noise ra“ o(SNR)o,thiS SignaL
un‖ ke vvcDMA nemorks,in LTE the charadenstics Of uplink reference sona,s are
slgnncantけdifferent to the characte商slics of both data transm ssions and therrrlal
noise Ths melhod eゃlons d fFerences in the autocclFebtiOn functon bemeen a
pOnion Ofthe“ me domain rererence stnal and(lhered)Gaussian noise
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For an uplink referenc€ signal occupying a small number o, frequency dom€in resgurce
blocls, it uoutd be expecled that the autoconetalion tunction with high SNR wouu
deviate from that due to (filtered) Gausshn noise However, even with a wi,leband
speslrally flat reference signal, such as SO resourc€ blocks (the maximum for a .lO MHz
syslem), the adocorrelatbn function of a portion of the time domain reference signal
deviates fro.n the fltered Gaussian ngise cas€.
This is lrue for a[ the Zadoff-Chu basis sequences, attiough the nature of the
autocorelation funclion does depend on the panicular Zadofi-Chu basis sequence. An
example of the autoconelation tunciion fof lotv and high SNR cases wfi 50 resourc€
blocks is shown in Figures 4(a) and 4(b) resp€ctivety
It can be seen in Figure 4 that the low SNR case is dominated by the autoconelation
function of the filtered Gaussian noise. ! dile lhe high SNR case is dominated by lhe
autoconelation function of the reference signat.
Frgure 5 shows the results of a simulatbn in which the autoconelation peaks from a
single reference signat, excluding the centrat lap, is plofled against the SNR This ptot
nas obtained over a r€nge of different reference signal pafttmete.E. numbeE of
resource blocks, numbe.s of macro_UEs, SNRS frorn each macro.UE and fiequencl
rcsource assignmenls The simulation also included fading effeqts.
Thus. it c.n be seen from Figu.e 5 that this rnetric, based on the autoconetatlon
ftrnction, c€n be used to estimale or predict Ihe SNR in many cases. Ho$rever. there
arc a number of points in the plot where although the SNR E high, the rnetric remains
lor'r. This sc€ter to the right hand side of the plot is potentialty p,oblematic, srnce in
these cases nearby macro-UEs might nol be protected by the femtocell base station 8.
This scaner c€n be due to fading as well as diflerences between the autoconelaton
functions of lhe different Zadoff-Chu basis sequences
An altemative ctass of metric for the estmation of fhe SNR can be based on the
slatistics of the time domarn wavefgm. One simpte mekic is the peak to average
po(er ratio (PAPR). High SNR reference srgnals should have low pApR, Mereas
Gaussian noise has a relativety high pApR.
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Resuns fOrthis mem (in“near unls)are shown in Figure 6 and“
“
n be seen thal
there ts an even lamer s“ ter apparentin the PAPR moいc than the am● cOreiallon
meblc,and as such the PAPR met面c(and olher mel面cs based on slatisllcs of the
power)are less atlract～e ror esumaling the SNR ofthe up“ nk reference slgnaL
However,"has been Observed thal lhe“attelng bemeen the autocorelat on and
PAPR metncs is independent,「e for lhe probiematt points tth high sNR but
abnorrnalりl●W aul― 同albn melnc,the PAPR tends to rernain low(as eXpected for
high SNR signaに) Fo「such pOints, the amocOrela“ on metnc∞n be adlusted
(upWards) ThiS approach can be used to reduce the scatter in the autocOrrelation
memc,andぃ。refore improve the eslimation oflhe sNR Fo「examp膊,rthe PAPR ρ
(in‖ near unls)is less than 3.then a minimum value can be appiod to lhe n∝
颯に,th`
minimum vaiue being glven by 400+(3っ
)・50
Two add“ ional approaches for futter reducing the scatter in the aul… rela"on metnc
have been idenl rled
Firsiv.as the amocO∝ lauOn peaks OF the reference slgnals tend to redu∝
in
magn“ ude mh distanco frOm the main cenlral peak, then son申
e shaping o, the
autocorrelalon function can be applled To avoid an increase in the・
僣ise detαxion・
はe.l is importanl that this is onv dOne fOr sampbs in the auα っrreb“ On function
which are already signiたantv aboVe the noise levei― and sO a threshold ls app“ ed
pnorto appりing this sha● ng For exampb,r the metnc is greater than 120 and the
tel rrcm the centre tapも
"lhen the melnc can be increased by 0 6n
Secondり,lhe scatter can be reduced by obtaining resuns over mukiple measuremenls,
rOr examp e by taking the maximum metnc Oblained from a sel or fOur or eight
measurements
Bソusing al o,these technlques,the scatterin the aulocorreiabon melnc is slgnmcantv
reduced Figure 7 illustrates the resu llng rela10nship bemeen the amOcOrelation
metnc and the sNR
The remiOce‖ base statbn 3 can make use Or the rebtionship bemeen the
autocOrela“ on function and the sNR lo deternine the sNR of an up“ nk sonal A
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method of estimating the SNR of the Zadoff-Chu rgference signal in accordance with
an embodiment of the invention ls shown in more detail in Figure g
Firslly, the femtocell base station 8 obtiains a .rough' synchronization to the macroce
(via a network monitor mode, or, if the slandards allotv, via macrocell timing
measuremenl reporls included in mobile device measuremenls, or via the )(2
interlace).
This rough synchronization altows the femtocell base station 8 to estimate roughly
where in time the uplink reference signals from macro-UEs are likely to be. ln nearly all
cases, this is the cenlre symbol in the O.sms uplink suFframe.
It will be appreciated that this estimaton will be subject to some error due to
propagation delay from the macrocell base station 4 and lhe timing advance used by
macro-uEs 12 ln the c€se ol over-the-air synchronization, which is assumed
hereafter, the enor will be up to one macrocell roundlrip propagation delay, which for a
cell of skrn is 33us Mich is roughly half the duration of an orlhogonal ftequency
division multiplexing (OFDM) symbol. The enor means that signals received from
rnacro-UEs 12 may arrive earlier than expected at the femtocell base station g.
Therefore, ln step 201 of Figure 8, the femtocell base station g measures or captures a
portion of the uplink reference symbol to give a time domain reference signal. For
erample, the femtocell base station 8 obtains the time domain reference signal from
the first 512 samples of lhe referenc€ symbol (assuming a 10 MHz bandwidlh with
1024 samples plus a cyclic prefix per OFDM symbol) Despite the timing uncertainty
for over-th+air synchronization, this captured portion of the reference symbol should
only contain reference stgnal samples from macro-UEs 12 lhat ate near to the
femtocell base station 8 (i.e. there shouldn't be any samptes of data symbols).
ln this slep, a scheduler in the femtocell base slation I may be used to ensure that
lhere will be no uplink transmissions from femto-UEs 12 lo the femtocell base station 8
that might interfere with this measurement
ln step 203, lhe femtocell base station 8 determines lhe autoconelation function for the
35 time domain reference signal and (filtered) Gaussian noise.
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ln one implementation, the femtocell base station I does this by normalizing the
captured time domain signal lo give unit power, with the resuhing sequence being
denoted r, taking the fasl Fourier transform (FFT) of this sequence to give f, calculaling
the squared magnitude (1'2+O2) for each sample of t and laking the inverse FFT of the
resuhing sequence to give the autoconelation sequence a.
As the autoconelation sequence a determined in step 203 is symmetrical (see Figure
4), only half of lhe samples in a need to be retained by the femtocell base station g for
further processing.
ln step 205, the femtoc€lt base station 8 takes the magnitude (or, in atemative
implementalions, the squared magnitude) of sequenc€ a and then, in step 207, adjusts
or zeros the central tap (corresponding to zero time lag in the autocorrelation function)
It may also be nec€ssary to adjust or zero the tap adjacent to the centrat tap if this tap
is significantly influenced by Ultering in the rec€ive path. Such liltering has a fixed
characlenstic so the decision as to adjust or zero this tap is a design decision
Then, in step 209, the femlocell base station I Unds the tap with lhe largest magnitude
(or squared magnitude) in the remaining taps, and sets the value of a metric m to this
magnitude (or squared magnitude)
The femtocell base station I can then determine the signal to noise ratio of the uplink
reference signal using this metric (step 211). The value ofthe SNR for lhe determined
netdc m can be determined from the relationship shown in Figure 5 or Figure 7, for
example using a curve-fitting technique or a look,up table
As described above, the accuracy of the SNR estimation c€n be improved by
considering the PAPR ol the signal, shaping the autoconehtion function based on the
distance of the peak used to determine the metric from the central tap and/or the metric
may be eslmated from signals received in muftiple time slots.
Therefore, the melric /n may be adjusted as a function of distance from the central tap
lor example by applying a simple linear function to the metric m determined in step
35 209. This linear function can be as described above
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Additionally or altematively, the metdc m may be adjusted as I lunclion ol the peak to
average po,ver ratio ol the captured po.tion of lhs uplink referenc€ symbol. Specitical,
if the PAPR is below a threshold (ror example 3 in linear units) then a minimum value
can be imposed on the metic (again lhis can be a simple linear tunc{bn ol PAPR).
Again, this linear tunciion can be as described above.
Again, additionally or altematively, the metdc m or SNR may be estimated horn uptink
rsference signals c€ptured in muhide time slots and, for example, the highest vatue of
the SNR obtain€d ftorn lhese measurements can be used by the femtocell base station
I to adjust its maximum permitted transmission p@rer.
Figuros I to 12 illustrate the p€rtormance benefG of the approach described above.
Figure 9 illustrates how lhe datia throughput on a downlink frorn a mac.oc€ll base
station is atrecled by an increasing number of aciive femtocell base stations within the
coveEue area of lhe macrocell base station for both a conventional lixed porver c€p
and the scheme descdbed above. ln padicular, it c€n b€ seen that there is a negligible
dirferenc€ in the dala throughput betv€en the conventionat scheme and the scheme
desc.ibed above
FEure 10 illustrates how the data lhroughput on a dovrnlink lrom a macrocell base
siation to cell edge (5 pe,c€ntile) nracrGuEs is afieded by an increasing number of
active femtocell base stations within ihe coverage area of the macrocell base station
for a convenlional scherrle and a scheme as dEscribed above. Agah, lhere is almost a
negligible difierence betn een the tu,o schemes.
Figure 11 plots lhe data throughput on a downlink from a femtocell base station against
lhe number of aclive lemtocell base stations within the coverage area of the macrocelt
base station ,or bolh a conventional fixed po|er c€p and the scheme according to the
invention. lt can be seen that the scheme described above provides an approximate
inqease in data throughpul of 5 Mb/s regardless ol the number of active remtocell base
strations, which is roughly equivalent lo an improvement of 25% in the data throughput
Figure '12 plots the data throughput on a downlink ftom a femtocell bas€ station to cell
edge (5 percentile) femtquEs against the number of aclive femtocell base statons
within lhe coverage area of the mac.ocell base station for a conventional scheme and a
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scheme as described above. lt c€n be seen that for ce edge (5 percentile) lemlo-UEs
the scheme describ€d above provides an app{oximate inqease in data throughpul of
190 kb/s regardless of lhe number of ac{rye femtocell bas€ statbns, Mich translates lo
an eighGfold inqease in the daia throughput.
Therefore, ihese graphs iMic€te that lhe adaptation ol the maximum p€rmitted
transmission pq$rer according to the nvention provides performance beneltts for femto-
UEs over the conventional fxed marimum permitted transmission pou/er schenre, while
offering the same protection to the macrocell base station donnlink
Although the invention has been described in terms of a method of estimating a signal
quality, ii will be appreciated that the invention can be embodied in Lemtocell base
station ihat comprises a processor and lransceiver circuitry configured lo perform the
described method.
Furthermore, while ihe invention has been presented as a meihod n a femtocell base
stalion of estimating a quality of a signal trans.nitted ftom a macro-UE to a macr@ell
base station (or from a femteuE lo another femtoce base stiation) that allows the
femlocsll base station to controt iis maximum permined lransmission pow€r, it will be
appreciated lhat the signal quality estimated using the rnethod according lo the
inventbn can be used for other purposes, and can be perlomed by elemenls in a
communication netwo.k olh€r than femtocell base stations, such as macroc€ll base
statons (eNBs) or mobile devices.
V\rtlile lhe invention has been illustrated and described in detait in the drawings and
foregoing description, such illuslration and description are to be considered illustrative
or exemplary and not restrictive, the invention is not limited to the disclosed
em bodiments.
Variations to the disctosed embodiments can be undeEtood and ef,ected by those
skilled in the art in p.actcing lhe claimed invention, ftom a sludy of the drawings, the
disclosure, and lhe appended claims. ln the claims, the word 'comprising" does not
exclude other elements or steps, and the indefinite article .a'or "an, does not exclude
a plurality. A single processor or other unit may fufit the funclons of severat items
reciled in the claims The mere lact that cenain measures are recited in mutually
diflerent dependent claims does not indicate lhat a combinatbn of these measures
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cannot be used to advantage A computer progftm may be stored/distributed on a
suitable medium, such as an optic€lstorage medium or a solid-state medium supplied
together with or as part of other hardware, but rnay atso be distributed in other forms,
such as via the lntemel or other wired or wireless telecommunication systems. Any
5 reference signs in the claims shoutd not be construed as limiting lhe scope

1 A femtoceli base station, comprising at least one processor to
determine if a macro-UE. which transmits a signal to a macrocell base station, needs
protection from downlink transmissions of the femtoceli base station based on monitoring for
the presence of an uplink channel of the macro-UE;
measure a quality of a signal corresponding to a delected uplink channel of the
macro-UE and
perform power control of a transmit power of Ihe femtoceli base station based upon a
companson ot the measured signal quality with a threshold vafue wherein when the
measured sjortai quality exceeds the threshold value the transmit power of the femtoceli
base station is set to a lower limit
2 Femtoceli base station as clawed in efaim 1. wherein the at (east one processor ts
configured to rrveasure ihe signal quality by measuring at least one of a reference signal and
a sounding signal af 'he uplm* channel or me macro-UE
Femtoceli case station as claimed In cla*m 1 wherein the at least one processor ts
configured to measure the signal quality of the detected uphnk channel by detecting
deviations ot a values o' an aulocarre&lion function for the uplink channel signal From
Gaussian noise
A Fermoceil base station as claimed m claim 1, wherein the at least one processor is
configured to determine it the macro-Ur is in a coverage area of the femtoceli base station
aepending upon the measured signal o^ai ty
5 Femtoceli base station as claimed in claim 1, wherein the processor is configured to
measure the quality of the uplink signal based on a value of an autocorrelation of the
delected uplink channel
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6 A method of detecting a mobile device associated with a macrocell base station and
performing interference protection at a small cell base station, the method comprising:
monitoring for the presence of an uplink channel of the macro mobile device to
determine if the macro mobile device, which transmits a signal to a macrocell base station,
needs protection from downlink transmissions of the small cell base station,
measuring a quality of a reference signal corresponding to a detected uplink channel
of the macro mobile device based on a value of an autocorrelation of the detected uplink
channel, and
performing adjustment of the femtocell base station cell coverage area based on a
comparison of the measured signal quality with a threshold value
7 The method of claim 6, wherein when the measured signal quality exceeds the threshold
value, the transmit power of the femtocell base station is set to a lower limit