interference mitigation methods for lte-advanced

8/13/2019 Interference Mitigation Methods for LTE-Advanced

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Interference Mitigation Methods for LTE-Advanced

Networks with Macro and HeNB Deployments

Agnieszka Szufarska, Krystian Safjan,

Stanisaw StrzyNokia Siemens Networks - Research

Wrocaw, Poland

{agnieszka.szufarska, krystian.safjan,

stanislaw.strzyz}@nsn.com

Klaus I. Pedersen, Frank Frederiksen

Nokia Siemens Networks - ResearchAalborg, Denmark

{klaus.pedersen, frank.frederiksen}@nsn.com

AbstractThis paper is focused on interference mitigation for

LTE-Advanced multi-layer networks with macro and HeNBs.

The autonomous HeNB power setting in the downlink is

studied for different HeNB access methods. Our studies show

that using the so-called escape carrier configuration and

autonomous HeNB power setting is a promising strategy for

enabling gradual introduction of user-deployed HeNBs in

existing macro-layer networks.

Keywords: escape carrier, HeNB power setting, HeNB access

I. INTRODUCTION

Performance optimization of Long Term EvolutionAdvanced (LTE-A) networks is studied in this paper. In

particular, we focus on the benefits coming with deploymentof home base stations (HeNBs) for offering improvedhotspot coverage and user performance, as well as trafficoffload from the macro-layer network. It is assumed that theHeNBs are installed indoor [1], having a fairly lowmaximum transmit power of 20 dBm, and a backhaulconnection realized over the users private Internet

connection.

The deployment of HeNBs offers several performancebenefits for operators and end-users, however, it can alsocause challenges in terms of interference management andguaranteeing a certain service quality in the network. Anexample of a typical deployment of macro-eNB and HeNB isshown in Figure 1, where HeNB #1 is located close to themacro cell centre (experiencing strong signal from overlayMacro-eNB), while HeNB #2 is located at the macro celledge region (experiencing weak signal from the eNB).

Path loss for users located indoor to their serving Macro-eNB is typically significantly larger than path loss between a

user and any HeNB located inside the same building as agiven user (except the case when e.g. there are several wallsbetween a UE and a HeNB).

Figure 1. Illustration of macro-HeNB interference problem.

Clearly, the main reason for this high Macro-eNB pathloss is a combination of high propagation distance between aUser Equipment (UE) and a Macro-eNB and the penetrationloss when the radio signals enter a building. Due to thiseffect and despite the relatively low HeNB maximumtransmit power (20 dBm), most scenarios lead to betterindoor coverage and service quality from the indoor HeNB

than could possibly be obtained from the Macro-eNB,whenever an indoor user is examined. This hot spot effectis foreseen as a source of gains for HeNB owners. Theindoor users who could otherwise only receive low qualitysignals from the nearest Macro-eNB are now given a realchance to of accessing high data rate services via HeNBs.

A dominant access mode for residential HeNBs isexpected to be the closed subscriber group (CSG) access. Inthis mode of operation, only the users listed in the HeNBAccess Control List stored in the core network are allowed to

be served by the HeNB. Hence, co-channel macro-users thatare in close proximity of HeNB and are not members of theCSG will experience decrease in service quality which, at

some point, may lead to coverage holes for such users.

Furthermore, if Macro-eNB and HeNB are deployed onthe same frequency (co-channel case) following thedeployment case as illustrated in Figure 1, yet another

problem can be observed. Looking at the macro coveragearea, the HeNB #1 located in the cell center may havecoverage problems due to high Downlink Reference SignalReceived Power (DL RSRP) from a nearby Macro-eNB. Atthe cell edge, however, it is the HeNB #2 which is a sourceof excessive downlink interference towards users connectedto the Macro-eNB. These challenges need to be properlyaddressed in order to make HeNB usage and deploymentwidely accepted by operators.

In this paper, we study deployment strategies of HeNBsin an existing macro-layer network with the objective ofmaximizing the overall network performance without

jeopardizing the wide area macro-layer coverage andcapacity. We focus on downlink transmission, as it is themost challenging case from an interference management

point of view while considering deployment of HeNBs(HeNB #2 in Figure 1). Performance of both data and controlchannels is analyzed. We start our analysis by studying thecase with plain co-channel deployment of Macro-eNB andHeNBs. We demonstrate that the use of simple autonomous

Macro-eNB HeNB #2HeNB #1

Wanted signalInterference

978-1-4244-8327-3/11/$26.00 2011 IEEE


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HeNB power setting methods is effective method to mitigatethe effects of excessive HeNBs to macro-users interference.The effects of using different access constraints for theHeNBs are also studied, ranging from strict CSG throughhybrid subscriber group (HSG) to fully open subscribergroup (OSG). As expected, the best overall performance isachieved for OSG [2], while the cases with deployment ofCSG HeNBs are the most challenging since macro users are

excluded from accessing a near-by HeNB may experiencecoverage holes. Following this finding, a number of possibleresource partitioning schemes between Macro-eNB andHeNB are studied. Our focus is on studying resource

partitioning schemes in the frequency domain on a carrierresolution. Both static and dynamic resource partitioningschemes are considered.

The paper is organized as follows; Section II inspectsrelated work and highlights the new study aspects brought bythis paper. In Section III a short summary of studiedinterference management schemes for macro+HeNBscenarios is provided and followed by main assumptions andmethodology used for system-level semi-static simulations

presented in Section IV. Detailed discussion of the results isprovided in Section V, while Section VI concludes the paperby summarizing major findings.

II. RELATED WORK

Current work on interference in co-channel macro+HeNBdeployments with open subscriber group case (as e.g. in [2])

presents the starting point for the investigations presented

here. However, since networks with HeNBs in open access

mode do not suffer from coverage holes, coexistence studies

of macro- and HeNB-cells in closed subscriber group mode

are more related to the scope of this paper. Many authors,e.g. in [3] or [4], propose different frequency reuse schemes

as a solution for the interference avoidance in macro+HeNBscenarios. The other approach proposed e.g. in [5] is

interference avoidance using multiple antennas and beam

forming in HeNB-cell nodes.

The aspects of LTE-Advanced downlink control channel

(CCH) performance for heterogeneous networks have not

been widely considered in the literature of the topic yet. It

has been however, discussed within the 3GPP RAN1 Group

[6].

This paper is complementing the current state of the art withstudies on performance of autonomous HeNB with power

setting in various classical CSG modes and investigates

relaxed HeNB co-channel access methods, while

investigating e.g. hybrid mode, i.e. when considering CSGwith visitors. Moreover, usage of additional carriers

available in LTE-Advanced is studied in form of the escapecarrier (EC) solution.

III. INTERFERENCE MANAGEMENT

The focus of this paper is the analysis of solutions thatallow management of interference affecting macro users inclose proximity of CSG HeNB cells. For the reference case

presented in Figure 1, we consider co-channel Macro-Home-eNB deployment, with HeNBs using the CSG access

mode. The primary case considered here is the cell edgesituation (HeNB #2 in Figure 1). If a HeNB is placed in alocation where signal from the nearest Macro NB is weak,then the area in which the signal from HeNB is stronger may

be relatively large, and the interference created by the HeNBto the macro UE becomes significant. This effect, combinedwith the restricted CSG access to the HeNBs, is seen as themost challenging scenario and a real problem that has to be

solved before mass deployment of residential HeNBs mayoccur.

A.Autonomous HeNB power settingOne of the techniques to optimize the performance of co-

channel deployment of HeNB is to autonomously set/adjustHeNB transmission power in order to reduce the probabilityof coverage holes for Macro eNB users. The assumed HeNBPower Control (PC) algorithm is described in [7]. Thisregulation restricts the transmit power of HeNB located closeto macro cell edge while allowing HeNB located nearby theMacro-eNB to transmit with higher power values. It isassumed that a HeNB has a simple UE receiver capabilityand may operate in Network Listen Mode (NLM). The NLM

is a measurement mode where a HeNB stops transmitting itsdata and senses the Macro-eNB signals by the built-in UEfunctionality. The autonomous HeNB power setting in DL is

based on sensing the received signal power from co-channelmacro stations at irregular time instants. In [7] it is proposedthat the NLM mode is enabled at least when a HeNB is

physically installed but it may also be triggered uponadditional events (power on/off, etc.). The HeNB transmit

power is adjusted according to the following formula

Ptx = max( min( PM + , Pmax ), Pmin ) [dBm], (1)

where parameters Pmin, Pmax are the minimum and maximumHeNB transmit power settings (i.e. defined by a standard),

while PMis the measured received power from the strongestco-channel Macro NB. The parameter is a scalar used toalter the slope of PC mapping curve and, as such, can be

adjusted e.g. to reflect different sizes of macro cells. ,expressed in dBm, is the parameter used for altering theeffective dynamic range of PM. These parameters areconsidered to be configured statically for each HeNB (e.g.configured by the operator).

B.Frequency domain configurationsAnother technique that is considered to improve DL

performance of macro users located in close proximity ofCSG HeNB is the separation of frequency resources betweenmacro and HeNB network layers. Since it is sub-optimal to

divide the resources into eNB and HeNB only-pools, it isproposed to restrict HeNB operation and assure that onlysome of the available resources are free from HeNBinterference towards the macro layer. The separation ofresources can either be done in frequency domain (multiplecarriers as supported with scalable bandwidth for LTE-Advanced) or in time domain (some of the time frames areexcluded from HeNB operation) [9][11]. In this paper wefocus on frequency-domain resource separation. Figure 2summarizes different frequency configurations considered inthis study. Cases with two carriers (in Figure 2 denoted as f1


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and f2) are considered in order to provide isolation betweenthe macro and HeNB-layers. A simple option for such ascheme is to assign both f1 and f2 to the macro cell layers,while allowing HeNBs to operate only on f2, thus leaving f1free from any direct HeNB interference. The f1 carrier in thiscase serves as the escape carrier for Macro-eNB users (seescheme [B] in Figure 2). The macro UEs located in the closevicinity of HeNBs should then be served on f1, while other

Macro NB UEs, outside interference region of HeNBs, canthen be served on f2. If the signal quality in the vicinity ofCSG HeNB is low then the macro UE will automaticallyrequest an inter-frequency handover to f1. For cases with thesame bandwidth for f1 and f2, we assume that equal numberof macro connected UEs are assigned to f1 and f2,respectively.

Figure 2. Selected spectrum arrangement possibilities for macro+HeNB

deployments. The yellow parts of the band symbolically represent control

channels.

C.Relaxed HeNB access constraintsThe third technique that can be considered in order to

improve performance of macro users heavily interfered by a

HeNB is based on relaxing the access criteria at the HeNB.

In the majority of previous macro+HeNB studies, very strictCSG access constraints have been assumed for the HeNBs.

However, from an interference management point of view,

one option for the HeNB owner is to include visitors in the

HeNB access list of allowed users. Such scenario can be

called CSG with visitors. In this case all users located in

the same indoor location as the HeNB are allowed to access

the node.

Another scenario with even more relaxed access

constraints is a hybrid mode in which the HeNBs are open

for all users (also the ones located outside direct HeNB

coverage area). In this case only part of the resources is

available for non-CSG members. This approach is aligned

with the current understanding of hybrid mode in 3GPP[11]. In our study for this case we assume that 25% of the

available channel resources (physical resource blocks) at

each HeNB are available to be shared by the non-CSGmembers that connect to the node.

IV. SIMULATION METHODOLOGY

A. Scenario and simulation modelThe simulation methodology used to generate the results

presented in this paper is aligned with the 3GPP RAN4recommended approach for HeNB evaluation [1]. In this

paper, a dense urban scenario is considered with one dual-stripe building with four floors placed in each macro cell.The simulations are done in snapshot mode, with buildings,HeNBs and users dropped randomly in each snapshot.Whenever a HeNB is placed indoor, one corresponding userwith access rights to the HeNB is placed in the sameapartment. In addition, 10 macro-users are randomly placedin each macro-cell area so that 80% of those users are placed

inside the buildings with HeNBs. The most essentialparameters of the model are summarized in TABLE I. Allthroughput values are generated using physical layerabstraction which takes relevant MIMO configuration intoaccount and is adjusted to approximately follow the LTE-Advanced link performance. The serving cell selection

procedure is based on Reference Signals Received Power(RSRP) measurements performed by each UE, i.e. a userconnects to the cell that offers the highest RSRP value at agiven location.

TABLE I. HIGHLIGHTS OF SIMULATION PARAMETERS

Parameter Setting

System

configuration

LTE-Advanced, 10 MHz bandwidth (50 PRBs)

2x5 MHz for frequency-domain coordination

Macrocelldeployment

Hexagonal and regular, 500 m inter-site distance, 21sectors simulated, statistics gathered over 3 central

sectors to avoid edge effects

HeNBdeployment

1 dual-stripe block per sector (4 floors)

{4, 8, 16, 32} HeNB per dual-stripe floor, all active.

Transmit Power Macro eNB: 46dBm

Home eNB: controlled in range 0..20dBm

User deployment 10 UEs per sector with access to Macro-eNBs only(80% of macro users indoor)

1 additional UE for every deployed HeNB (with

access rights to the HeNB)

Wall penetrationloss

External wall: 10dB, internal wall: 5dB

Scheduler Round robin

Trafic model Full buffer

Shadowing As in[1]: lognormal, correlated. For links between aHeNB and a UE served by this BS correlated = 4dB,

for all other links = 8dB.

Fast fading Not simulated

AntennaConfiguration

eNB: 2Rx, 2Tx, 3 sectors, 2D pattern, 14dBi gain

HeNB: 2Rx, 2Tx, omnidirectional, 5dBi gain

UE: 2Rx, 1 Tx, omnidirectional, 0dBi gain

10 m

10 m

10 m

10 m

10 m

f1

f1 f2

eN

HeN

eN

HeN

[A]

Co- channel

[B]

Escape carrier

static division


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For HeNBs it is assumed that restricted access (CSG) isapplied. For the users with HeNB access rights RSRP is stillused as the main cell selection criterion such that the UEscan still connect to Macro-eNBs in case the macro cell RSRPis the strongest.

V. PERFORMANCE EVALUATION RESULTS

In this section, the simulation results for the threedifferent interference management schemes described inSection III are presented. The plots are shown for a baselineHeNB density of 4 HeNB per dual-stripe floor. Only Figure5 is a visualization of results for various HeNB densities.

A. Co-channel methodsIn Figure 3 the throughput cumulative distribution

function (cdf) for the users connected to Macro-eNBs is

shown. The figure shows that under ideal CCH performance

the co-channel deployment without power control will

introduce a coverage hole for ~7 % of the Macro connected

users. When applying power control for the HeNB, the

coverage can be improved such that only ~2 % of the macroconnected users are potentially experiencing connection

loss. The best performance is observed for the hybrid accesswithout PC, where it is seen that the 5

thpercentile as well as

the median throughput level are increased by 350% and

43%, respectively, when compared to the reference case

(CSG with PC).

Figure 3. Performance for Co-channel methods, PC settings: =1,

=55[dBm].

It should be noted that applying PC for the hybrid access

mode will lower performance for macro UEs. This is caused

by the smaller coverage area of the HeNBs what limits

capability of providing offload through the hybrid share.

B.Escape carrierThe second method of protecting the macro connected

UEs is using the escape carrier spectrum arrangement

scheme as illustrated in Figure 2 [B]. The results for this

scenario are shown in Figure 4. By introduction of the

escape carrier, the coverage hole problem is mitigated,

offering 5th

percentile average throughput of 520 and 611

kbps (without and with PC respectively) for the macroconnected UE. This is achievable despite the fact that all

UEs in the escape carrier operate at a reduced carrier

bandwidth. It is observed that the reduction of the peak data

rate through limiting the available system bandwidth for theusers is not impacting the results. Again, it is seen that

applying PC along with the escape carrier configuration

reduces the macro connected UE experienced throughput,

although this effect is not as clear as for the hybrid CSG

HeNB case.

C. Control Channel impactWhen evaluating the network performance, it is crucial that

all data as well as control channels meet certain signal

quality conditions. For instance, macro connected UEs in

close proximity of CSG HeNB could potentially experience

problems receiving Control Channels (CCH) from a HeNB.

For CCHs Block Error Rate (BLER) level below 1 % isassumed to illustrate the requirements [6]. The approximate

minimum required SINRs (obtained by means of link-level

simulations) for different control channels are listed in the

header of TABLE II. [6].

When evaluating whether there are potential CCH problems,

these SINR thresholds are compared to the observed SINRdistributions for different macro+HeNB cases to determine

the percentage of users that are potentially experiencing

CCH problems. The results from this evaluation are shown

in TABLE II. , where it is seen that the co-channel case

without PC will potentially experience severe controlchannel problems, e.g. with the Physical HARQ Indication

Channel (PHICH) being the channel with the highest

outage. On a general level, this reference case will lead to

problems for all CCHs (14% to 23% of users affected).

Further, it is seen that the escape carrier and hybrid access

methods provide best protection of CCH not more than2% of users experience problems on the two most sensitive

Control Channels: Physical Downlink Control Channel(PDCCH) and Physical Hybrid Automatic Repeat Request

Indicator Channel (PHICH). For these two CCHs, it is

possible to introduce limited power boosting to compensate

for poor performance and thereby relax the SINRrequirements.

TABLE II. Percentage of users suffering from CCH problems (minimumSINRs required to achieve 1% BLER are given in table header for eachcontrol channel)

Scenario

PBCH

-8.5dB

PCFICH

7dB

Dynamic

BCH

5dB

PDCCH

3.8dB

PHICH

3.2dB

Co-channel, PC off 14 16 19.5 22 23

CSG with visitors, PC off 9 11 15 18 20

Co-channel, PC on 5 5.5 7 9 10

CSG with visitors, PC on 2 2.7 4 6 7.5

EC, PC off 0 0 0 0 2

Hybrid, PC off 0 0 0 1 2

Hybrid, PC on 0 0 0 1 1.5

EC, PC on 0 0 0 0 1

0 2 4 6 8 100

0.2

0.4

0.6

0.8

1Macro user throughput

Throughput [Mbps]

cdf

0 0.5 1

0

0.05

0.1

0.15

CSG, PC off

CSG, PC on

CSG with Visitors, PC off

CSG with Visitors, PC on

Hybrid Access, PC off

Hybrid Access, PC on


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Figure 4. Performance for Escape carrier and CSG mode.

D.Data channel performance overviewThe results presented earlier considered the baseline HeNB

density, while the results in Figure 5 shows the 5th

percentile

of macro UE throughput and the 50thpercentile HeNB userthroughput for different densities of HeNB deployments.

The baseline configuration is 4 HeNBs per floor, while the

performance at the 5th

percentile outage level is shown for

increased HeNB densities. From these results it is generally

observed that the macro user throughput tends to decrease

with higher number of CSG HeNBs, as the additional HeNB

increase interference level. For high CSG HeNB densities, it

is clear that both resource partitioning and HeNB PC isbeneficial. Although such techniques reduce the

performance of HeNB users, it is still observed that they

experience much higher throughput than macro users. For

the cases with HSG HeNB the best overall performance is

achieved by letting the HeNBs operate at their maximumpower level to attract as many users as possible, i.e.increased offload of the macro layer. Thus, for the HSG

HeNB cases, we observe increased macro-user outage

performance with increasing number of HeNBs, since more

users are offloaded from macro.

Figure 5. Data channel performance for various number HeNBs per floor.

VI. SUMMARY AND CONCLUSIONS

In this study we have shown that a simple co-channeldeployment of macro-eNBs and CSG HeNBs are likely toresult in macro-layer coverage holes if not interferencemanagement is applied. In such coverage holes, the macroconnected UEs are not able to correctly decode controlchannels from their serving macro cells. Using Network

Listen Mode at the CSG HeNBs and implementing simpleautonomous power setting can improve performance ofmacro users and reduce probability of experiencing themacro-layer coverage holes. However, in order to fully avoidgeneration of the coverage holes, we find that either relaxedHeNB access constraints (e.g. HSG or OSG) or partialresource partitioning e.g. in frequency domain (escape carriermode) have to be used. Using such techniques helpimproving both the data and control channel performance formacro-UEs in HeNB vicinity, while still providing attractive

performance for HeNB-UEs. For scenarios with CSGHeNBs, the escape carrier configuration combined withsimple autonomous HeNB power setting is demonstrated to

be a promising configuration. The latter is a simple andpractical feasible interference management scheme. Topicsof future research include analysis of time-domain resource

partitioning schemes as currently under discussion for LTERel-10, as well as more detailed system performanceassessment including mobility mechanisms.

REFERENCES

[1] 3GPP, R4-092042, Simulation assumptions and parameters forFDD HeNB RF requirements, May 2009

[2] H. Claussen, "Performance of Macro- and Co-Channel Femtocellsin a Hierarchical Cell Structure," PIMRC 2007. IEEE 18th

International Symposium on , vol., no., pp.1-5, 3-7 Sept. 2007

[3] T. Lee, J. Yoon, S. Lee, J. Shin, "Interference management in

OFDMA Femtocell systems using Fractional Frequency Reuse",ICCCAS 2010, pp.176-180, 28-30 July 2010

[4] R. Juang, P. Ting, H. Lin, D. Lin, Interference Management ofFemtocell in Macrocellular Networks, Wireless

Telecommunications Symposium (WTS), 2010 , vol., no., pp.1-4,

21-23 April 2010

[5] S. Park, W. Seo, Y. Kim, S. Lim, D. Hong, Beam SubsetSelection Strategy for Interference Reduction in Two-Tier

Femtocell Networks, Wireless communications, vol. 9, no. 11,pp.3440-3449 November 2010.

[6] 3GPP, R1-101451, Downlink CCH performance aspects for co-channel deployed macro and HeNBs, February 2010.

[7] 3GPP, R4-093644, HeNB Interference management for LTE Rel-9 via power control, October 2009.

[8] 3GPP, TR 36.921, Evolved Universal Terrestrial Radio Access(E-UTRA); FDD Home eNode B (HeNB) Radio Frequency (RF)

requirements analysis, April 2010

[9] 3GPP, R1-093340, Blank Subframes for LTE, Alcatel-Lucent,August 2009.

[10] 3GPP, R1-105708, TDM muting patterns for het-netcoordination, Texas Instruments, October 2010

[11] 3GPP, TS 36.300, Evolved Universal Terrestrial Radio Accessand Evolved Universal Terrestrial Radio Access Network

0 2 4 6 8 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Macro user throughput

Throughput [Mbps]

cdf

CSG, co-channel, PC off

CSG, co-channel, PC on

escape carrier, PC off

escape carrier, PC on

0 0.5 1

0.05

0.1

0.15

0.2

0.25

0.3

5 10 15 20 25 30 35 40 45

0

0.2

0.4

0.6

0.8

1

1.2

81632

81632

8

1616

8

3232

816

32

8

1632

4

8

1632

8

16

32

HeNB user throughput @50% CDF [Mbps]

Macrouserthroughput@

5%C

DF[Mbps]

HeNB user performance (50%) vs, Macro user performance (5%)

4

4

4

4

44

4

CSG, Co-Channel, PC off

CSG, Co-Channel, PC on

Escape Carrier, PC off

Escape Carrier, PC on

CSG with Visitors, PC off

CSG with Visitors, PC on

Hybrid Access, PC off

Hybrid Access, PC on

interference mitigation methods for lte-advanced

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