mm in hetnets

IEEE Communications Magazine • December 201270 0163-6804/12/$25.00 © 2012 IEEE

1 With range expansion [2],a positive range expansionbias is added to the down-link RSS of picocell pilotsignals at UEs to increasepicocells’ downlink cover-age footprints [3].

INTRODUCTION

In order to meet the upcoming exponentialgrowth of mobile data traffic [1], operators aredeploying more network infrastructures to makecellular networks closer to UEs, and thusincrease spectrum efficiency and spatial reuse. Inthis context, HetNets, which are comprised ofcoexisting macrocells and low power nodes(LPNs) such as picocells, femtocells, and relaynodes, have been heralded as the most promis-ing solution to provide a major performanceleap [2]. However, in order to realize the poten-tial coverage and capacity benefits of HetNets,operators are facing new technical challenges in,for example, mobility management, inter-cellinterference coordination (ICIC) [3], and back-haul provisioning [4]. Among these challenges,mobility management is of special importance.The deployment of a large number of LPNs mayincrease the complexity of mobility management,

since mobile UEs may trigger frequent han-dovers when they move across the small cover-age areas of LPNs.

In cellular networks, handovers allow a UE totransfer their active connections from its servingcell to a target cell in connected mode, whilemaintaining quality of service [5]. In convention-al homogeneous networks, UEs typically use thesame set of handover parameters (e.g. hysteresismargin and time to trigger (TTT)) throughoutthe network. However, in HetNets (wheremacrocells, picocells, femtocells, and relay nodeshave different coverage area sizes), using thesame set of handover parameters for all cellsand/or for all UEs may degrade mobility perfor-mance. For example, the use of range expan-sion1 in open access LPNs with different biaswill affect when and where the handover processis initiated in each cell. Therefore, in HetNets,there is a need for cell-specific handover param-eter optimization. Moreover, high-mobilityMacrocell UEs (MUEs) may run deep insideLPN coverage areas before the TTT optimizedfor macrocells expires, thus incurring handoverfailure due to degraded signal to interferenceplus noise ratio (SINR). Handovers performedfor high-mobility MUEs may also be unneces-sary (i.e. ping-pongs), when they quickly passthrough the small coverage areas of LPNs. Thesefacts also impose the need for UE-specific han-dover parameter optimization.

Due to its capital importance, mobility man-agement challenges in HetNets have attractedmuch interest from the wireless industry,research community, and standardization bodies[6–10]. Indeed, a new Study Item (SI) “HetNetmobility enhancements for LTE” has recentlybeen established in the 3GPP RAN 2 [6]. Resultsin [6] indicate that mobility performance in Het-Net deployments is not as good as in puremacrocell deployments, and that performanceenhancements are needed for high-mobilityUEs. As a result, the new 3GPP RAN 2 SI [6]aims to develop strategies for improved smallcell discovery/identification, automatic re-estab-lishment procedures, enhanced mobility robust-ness, and techniques for enhanced mobility stateestimation in a HetNet environment.

ABSTRACT

In this article we provide a comprehensivereview of the handover process in heterogeneousnetworks (HetNets), and identify technical chal-lenges in mobility management. In this line, weevaluate the mobility performance of HetNetswith the 3rd Generation Partnership Project(3GPP) Release-10 range expansion andenhanced inter-cell interference coordination(eICIC) features such as almost blank subframes(ABSFs). Simulation assumptions and parame-ters of a related study item in 3GPP are used toinvestigate the impact of various handoverparameters on mobility performance. In addi-tion, we propose a mobility-based inter-cellinterference coordination (MB-ICIC) scheme, inwhich picocells configure coordinated resourcesso that macrocells can schedule their high-mobil-ity UEs in these resources without co-channelinterference from picocells. MB-ICIC also bene-fits low-mobility UEs, since handover parame-ters can now be more flexibly optimized.Simulations using the 3GPP simulation assump-tions are performed to evaluate the performanceof MB-ICIC under several scenarios.

TOPICS IN RADIO COMMUNICATIONS

David López-Pérez, Bell Laboratories Alcatel-Lucent

I.smail Güvenc, Florida International University

Xiaoli Chu, The University of Sheffield

Mobility Management Challenges in3GPP Heterogeneous Networks

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IEEE Communications Magazine • December 2012 71

In this article, we provide a comprehensivereview of the handover process in HetNets, andidentify technical challenges in mobility manage-ment, with a special focus on principal objectivesof the new 3GPP RAN 2 SI [6]. We also evalu-ate mobility performance in HetNets with 3GPPRel-10 enhanced ICIC (eICIC) features such asalmost blank subframe (ABSF), and propose anovel mobility-based ICIC (MB-ICIC) schemeto further improve it. In the proposed scheme,picocells configure coordinated resources so thatmacrocells can schedule their high-mobility UEsin these resources without co-channel interfer-ence from picocells. 3GPP simulation assump-tions have been adopted in this study.

OVERVIEW OF HANDOVERPROCESS IN 3GPP LTE

In this section we introduce the concepts of han-dover, handover failure, ping-pong, picocellrange expansion, and eICIC based on ABSFs.

HANDOVER PROCESSIn wireless communications networks, handoverscould be performed between different RadioAccess Technologies (RATs), carriers, or cells.In this article, we discuss intra-RAT intra-carrierhandovers. Specifically, we focus on 3GPP LTEhard handovers, in which UEs disconnect withthe source cell before establishing a new connec-tion with the target cell.

The 3GPP LTE handover process can typi-cally be divided into four phases: measurement,processing, preparation, and execution. Han-dover measurements and processing are per-formed by the UE. Handover measurementsare usually based on downlink reference signalreceived power (RSRP) estimations,2 while pro-cessing takes place to filter out the effects offading and estimation imperfections in han-dover measurements. After processing, ifaccording to the filtered measurements a cer-tain handover event entry condition is met, theUE alerts the serving cell and feeds back han-dover measurements through a measurementreport. Then, the preparation phase starts, inwhich the serving cell initiates the handoverprocess, and prepares the handover executiontogether with the target cell. Finally, in the exe-cution phase, the serving and target cells per-form necessary network procedures with theassistance of the UE to transfer its connectionfrom the former to the latter.

In LTE standards, the UE performs handovermeasurements and processing in Layer 1 (physi-cal) and Layer 3 (network), as shown in Fig. 1a[11]. For handover measurements, the UE gen-erally takes RSRP estimations over the cellsincluded in its neighboring cell list. In order toremove the effects of fading from RSRP estima-tions, the UE obtains each RSRP sample as thelinear average over the power contributions ofall resource elements that carry reference sym-bols within one subframe (i.e. 1 ms) and the con-sidered measurement bandwidth (e.g. sixresource blocks), and thereafter further averag-ing over several RSRP samples. This linear aver-aging is performed in Layer 1, and is known as

L1 filtering. For a typical setup (Fig. 1a), inorder to obtain an L1 filtered handover mea-surement, downlink RSRP samples may be takenevery 40 ms, and then averaged over five succes-sive RSRP samples.

The L1 filtered handover measurements areupdated every handover measurement period(e.g. 200 ms) at the UE, and averaged through afirst-order infinite impulse response (IIR) filter,as defined in Fig. 1a [11], to further mitigate theeffects of fading and estimation imperfections.This moving averaging is performed in Layer 3,and is known as L3 filtering. Since successivelog-normal shadowing samples are spatially cor-related, the L3 filtering period is preferred to beadaptive to the degree of shadowing correlationin the received signal. For high-mobility UEs,log-normal shadowing samples are not highlycorrelated, and thus it would be better to have ashorter L3 filtering period than that for low-mobility UEs. A typical L3 filtering period is 200ms.

A handover is then triggered if the L3 filteredhandover measurement meets a handover evententry condition. In LTE, there are eight types ofhandover event entry conditions (see [12], Sec-tion 5.5.4): • Event A1: Server becomes better than

threshold.• Event A2: Server becomes worse than

threshold.• Event A3: Neighbor becomes offset better

than server.• Event A4: Neighbor becomes better than

threshold.• Event A5: Server becomes worse than

threshold1 and neighbor becomes betterthan threshold2.

• Event A6: Neighbor becomes offset betterthan secondary server (this conditionapplies to carrier aggregation configura-tions).

• Event B1: Inter RAT neighbor becomesbetter than threshold.

• Event B2: Server becomes worse thanthreshold1 and inter RAT neighborbecomes better than threshold2.Intra-RAT intra-carrier handovers are trig-

gered upon event A3. Once the event A3 condi-tion is met, i.e. the L3 filtered RSRP of thetarget cell is larger than that of the serving cellplus a hysteresis margin (also referred to asevent A3 offset), the UE starts the TTT timer(Fig. 1b). Only if the event A3 condition is satis-fied throughout the TTT, the UE alerts the serv-ing cell and feeds back this event A3 conditionthrough a measurement report, thus initiatingthe handover preparation process. Small valuesof TTT may lead to too early handovers, increas-ing ping-pongs, while large values of TTT mayresult in too late handovers, increasing handoverfailures. Therefore, the optimization of the TTTaccording to the UE’s velocity carries capitalimportance in mobility management [7], as it willbe shown later.

Once the TTT successfully expires, as shownin Fig. 1b, the handover preparation phase starts.The source cell issues a handover request mes-sage to the target cell, which carries out admis-sion control procedures according to the quality

2 Handovers can be gov-erned not only by signalstrength but also by signalquality. However, in thisarticle, we have adoptedthe simulation assump-tions in [6], where thehandover procedure isgoverned by RSRP mea-surements.

In the proposed

scheme, picocells

configure coordinat-

ed resources so that

macrocells can

schedule their high-

mobility UEs in these

resources without

co-channel interfer-

ence from picocells.

3GPP simulation

assumptions have

been adopted

in this study.


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IEEE Communications Magazine • December 201272

of service requirement of the UE [13]. Afteradmission, the target cell prepares the handoverprocess, and sends a handover request acknowl-edge to the source cell. When the handoverrequest acknowledge is received at the sourcecell, data forwarding from the source cell to thetarget cell starts, and the source cell sends ahandover command (within a RRC message) tothe UE.

Finally, in the handover execution phase, theUE synchronizes with the target cell and access-es it [13]. The UE sends a handover completemessage to the target cell when the handoverprocedure is finished. The target cell, which canthen start transmitting data to the UE, sends apath switch message to inform the network thatthe UE has changed its serving cell. Thereafter,the network sends a UE update request messageto the serving gateway, which switches the down-link data path from the source cell to the targetcell. The network also sends end marker packetsthrough the old path to the source cell, asking itto release any resources previously allocated tothis UE.

HANDOVER FAILURES AND PING-PONGS

A UE is considered to be out of synchroniza-tion when its wideband SINR (also referred toas channel quality indicator (CQI)) falls toQout (in dB), and to be back in synchroniza-tion when it reaches Qin (in dB). For trackingradio link failure (RLF) [14], a UE u uses twomoving average windows, which have depthsof 200 ms and 100 ms to compute its CQI val-ues Qout,u and Q in,u, respectively. Both win-dows are updated once per frame, i.e. onceevery 10 ms. When Qout,u is lower than thethreshold Qout, a synchronization problemoccurs, and the T310 t imer (usually of 1 sduration) is triggered as shown in Fig. 1b. TheT310 timer is stopped once Qin,u is larger thanthe threshold Qin, and the UE is consideredback in synchronization. However, if the T310timer runs until it expires, the UE is consid-ered out of synchronization, and an RLF isdeclared [6]. Accordingly, a handover failurehappens if one of the following three condi-tions is met [6]:

Figure 1. L1 and L3 filtering procedures, radio link monitor process, and handover process in 3GPP LTE:a) L1 and L3 filtering procedures. RSRP is measured over one subframe (1 ms and, e.g., 6 resourceblocks) every 40 ms and recorded as RSRPL1(l). L1 filtering performs averaging over every 200 ms to pro-vide M(n) = 1/5 S4

k=0 RSRPL1(5n–k). Finally, L3 filtering performs averaging over every 200 ms to obtainF(n) = (1 – a)F(n – 1) + a10 log10{M(n)}, where a is the L3 filter coefficient; and b) Timers in radiolink monitoring and handover processes. If a radio link failure is declared while the TTT is running, ahandover failure happens. Alternatively (not shown in the figure), a handover failure may happen if thetimer T310 has been triggered or is running when the handover command is received by the UE (indicat-ing control channel failure) [6].

L3filtering

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In order to address

problems caused by

the downlink trans-

mit power difference

between eNodeBs

(eNBs) and Picocell

eNBs (PeNBs) in Het-

Nets, cell selection

methods allowing

UEs to associate with

cells that do not pro-

vide the strongest

downlink RSRP are

necessary.


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1 RLF happens during the time between sat-isfying the event A3 condition and receivinga handover command (Fig. 1b).

2 T310 timer is triggered and still runningwhen a handover command is sent.

3 The UE wideband SINR Qout,u is lower thanQout when a handover complete message issent.

Note that if condition 2 or 3 occurs (which arenot shown in Fig. 1b for brevity), a packet datacontrol channel (PDCCH) failure is declared.

The occurrence of a ping-pong is determinedby the time duration that a UE stays connectedto a cell directly after a handover, namely time-of-stay. The time-of-stay starts when the UEsends a handover complete message to the cell,and ends when the UE sends a handover com-plete message to another cell. If a UE has atime-of-stay less than the threshold Tp, e.g. 1 s,and the new target cell is the same cell as thesource cell when handing over to the currentseving cell, then the handover that terminatesthis time-of-stay is considered an unnecessaryhandover, i.e. a ping-pong [10].

RANGE EXPANSION ANDALMOST BLANK SUBFRAMES

In order to address problems caused by thedownlink transmit power difference betweeneNodeBs (eNBs) and Picocell eNBs (PeNBs) inHetNets, cell selection methods allowing UEs toassociate with cells that do not provide thestrongest downlink RSRP are necessary. A wide-ly considered approach is range expansion [3], inwhich a positive range expansion bias is added toL3 handover measurements at UEs to increasepicocells’ downlink coverage footprints (Fig. 1a).Although range expansion is able to mitigateuplink inter-cell interference and provide loadbalancing in HetNets, it degrades the downlinksignal quality of Picocell UEs (PUEs) in theexpanded region, since these PUEs are not con-nected to the cell that provides the strongestRSRP (e.g. PUE-3 connected to PeNB-2 in Fig.2a). eICIC based on ABSFs can be used to miti-gate downlink inter-cell interference for range-expanded picocells [3]. ABSF are subframes inwhich no control or data signals but just refer-ence signals are transmitted. Specifically, macro-cells schedule ABSFs, and picocells schedulerange-expanded PUEs in the subframes thatoverlap with the macrocell ABSFs, so that theirperformance can be enhanced (e.g. subframes 2,6, and 9 in Fig. 2a).

ILLUSTRATIVE EXAMPLESIn order to illustrate handover behaviors in thepresence of picocell range expansion, the cover-age areas of a range-expanded picocell (with an8 dB bias) for three different handover scenariosare depicted in Figs. 2b–2d, where the PeNB islocated at (1500, 1650) m, the eNB is located at(1500, 1500) m, UEs move at 3 km/h, eICIC isnot implemented, and the handover preparationand execution times are neglected. In Fig. 2b,handovers are performed based on geometryonly, while in Figs. 2c–2d, handovers are per-formed using a TTT of 160 ms and an event A3offset of 2 dB. In Figs. 2b–2c, no effect of shad-

owing or fading is included, while in Fig. 2d, theeffect of shadowing is considered. ComparingFig. 2c with Fig. 2b, we can see that MUEsinvade the picocell expanded region withouthanding over to the picocell due to the use ofTTT. Comparing Fig. 2d with Fig. 2c, we observethat due to shadowing effects, there is no clean-cut boundary of the picocell coverage area withor without range expansion, and UEs can con-nect to the picocell in a much wider area. Thisdemonstrates the importance of consideringchannel variations in mobility management.

In order to illustrate the concept of handoverfailure, the locations where event A3 and RLFoccur in a HetNet comprising a macrocell and apicocell are plotted in Fig. 3, where the PeNB islocated at (1500, 1750) m, the eNB is located at(1500, 1500) m, and we adopt the same assump-tions of Fig. 2c but with a larger TTT to facili-tate the observation of RLF positions. In Fig. 3a,MUEs were initially located outside the picocellcoverage area and moved toward the PeNBleading to macro-to-pico handovers, while in Fig.3b, PUEs were generated close to the PeNBlocation and moved toward the eNB, resulting inpico-to-macro handovers. In Fig. 3a, MUE RLFlocations are clustered, because MUEs were ini-tially served by different eNB sectors. From Fig. 3we can infer that if MUEs or PUEs cross theRLF boundary before the TTT expires, whenmoving in or out of the picocell coverage area,respectively, then RLF occurs and the UE expe-riences handover failure. This figure also showsthat, with range expansion at the picocell, theevent A3 positions are pushed away from thePeNB location, thus increasing the picocell cov-erage area and potentially allowing for a betterspatial reuse. In Fig. 3a, since the gap betweenthe event A3 boundary and the RLF boundary islarger with picocell range expansion, it is morelikely that the TTT will expire before the UESINR falls to Qout and the handover is complet-ed successfully [10]. Picocell range expansionthus facilitates the macro-to-pico handover. Onthe contrary, in Fig. 3b, range expansion chal-lenges the pico-to-macro handover, since thespace between the event A3 boundary and theRLF boundary gets smaller, and it is more likelythat the UE SINR falls to Qout before the TTTexpires and a handover failure occurs [10]. InFig. 3b, RLF may occur even earlier than eventA3, indicating the need for eICIC to supportexpanded region picocells.

HETNET MOBILITY PERFORMANCEWITH 3GPP RELEASE-10 EICIC

A capital issue in mobility performance opti-mization is the tradeoff between handover fail-ures and ping-pongs. Optimizing handoverparameters to reduce handover failures wouldincrease ping-pongs, and vice versa [6]. Thismakes handover optimization an intricate prob-lem, which would be exacerbated by the largenumber of LPNs overlaid on macrocells in Het-Nets. Moreover, UEs’ velocities are also animportant factor to consider. Tuning handoverparameters, e.g. TTT, based on mobility stateinformation or measurement report processing

Optimizing handover

parameters to

reduce handover fail-

ures would increase

ping-pongs, and vice

versa [6]. This makes

handover optimiza-

tion an intricate

problem, which

would be exacerbat-

ed by the large num-

ber of LPN overlaid

on macrocells in

HetNet.


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has been a widely used approach to mitigatehandover failures. For example, high-mobilityUEs handover faster than low-mobility UEs [7].UEs’ velocities become even more important inHetNets, where handovers of high-mobility UEsto small cells should be prevented to avoid han-dover failures or ping-pongs.

In order to illustrate the tradeoff betweenhandover failures and ping-pongs and theimpact of UEs’ velocities, we have performedsystem-level simulations using the simulationscenarios and assumptions/parameters in [6],which have been agreed by a large number ofvendors and operators. In these simulations,a hexagonal macrocell layout with 19 eNBs,57 sectors, and an inter-eNB distance of 500meters were used. Four PeNBs were random-ly distributed within each eNB sector cover-age area. Auto- and cross-correlatedshadowing was included, as well as Rayleighfading based on the typical urban model. Weconsidered five different handover profilesbased on [6]:

• Set-1: TTT = 480 ms, A3 offset = 3 dB,L3 Filter K = 4.

• Set-2: TTT = 160 ms, A3 offset = 3 dB, L3 Filter K = 4.



• Set-5: TTT = 40 ms, A3 offset = –1 dB, L3 Filter K = 0.

The longest and shortest TTT durations are 480ms in simulation Set-1 and 40 ms in simulationSet-5, respectively. For each simulation set, anL1 and L3 filtering period of 200 ms was used,along with full cell-loading. UEs were randomlydistributed over the entire simulation scenario,and moved at a fixed speed randomly selected inthe set of {3, 30, 60, 120} km/h. UEs movedalong straight lines toward randomly selecteddirections, and did not change directions untilthey hit the border of the simulation scenario.When a UE hit the border of the simulation sce-nario, it bounced back and moved toward anoth-

Figure 2. 3GPP Release-10 eICIC scenarios and handover behaviors in the presence of picocell range expansion: a) range expansion atpicocells. In 3GPP Rel-10, eICIC blanks subframes at the eNB side to improve performance of Pico UEs (PUEs) in the expandedregion. We further consider blanking subframes at the PeNB side to improve mobility performance; b) geometry-based handover; c)handover using 160 ms TTT and 2dB A3 offset, but without shadowing or fast fading; and d) handover using 160 ms TTT and 2dB A3offset, with shadowing.

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er randomly selected direction. Results are aver-aged over 100 random drops of picocells, andeach drop lasted for 200 s. Further details of thesimulation and parameters are presented inTable 1 and [6].

Figure 4 presents the simulated handoverfailure and ping-pong rates for two differentcases:• Picocells without range expansion or eICIC.• Picocells using an 8 dB bias for range expan-

sion and implementing eICIC with ABSFconfigured at macrocells.

HANDOVER FAILURESIn terms of handover failure rates, the case ofpicocells with no range expansion or eICIC per-forms worse than that of picocell range expan-sion with picocell eICIC (blanking subframes atthe eNB side), as shown in Fig. 4a. This isbecause picocell coverage areas without rangeexpansion are small, and thus MUEs may quick-ly run deep inside picocell coverage areas beforethe TTT expires, significantly degrading MUEs’SINR before the handover process is completed.When using picocell range expansion with pico-cell eICIC, the number of handover failures issignificantly reduced, because the event A3boundary is pushed away from the PeNB loca-tion by range expansion, and MUEs have moretime to handover before TTT expires. Pico-to-macro handovers are not an issue due to eICIC.In both cases, handover failures mostly damagehigh-mobility UEs and are alleviated with short-er TTTs [10], e.g. in Set-5.

PING-PONGSIn terms of ping-pong rates, the case of picocellswith no range expansion or eICIC performs bet-ter than that of picocell range expansion withpicocell eICIC, as shown in Fig. 4b. When weincrease the picocell coverage area throughrange expansion, cell selection oscillation causedby fading occurs in a larger area. As a result, thenumber of ping-pongs increases. In both cases,

ping-pongs mostly damage low-mobility UEs andare alleviated with larger TTTs, e.g. in Set-1,since extensive L1 and L3 filtering can be per-formed to mitigate fading and estimation imper-fections [10].

These results confirm the tradeoff betweenhandover failures and ping-pongs, i.e. reducingTTT mitigates handover failures, but increasesping-pongs, and vice versa. Among the five han-dover profiles considered, Set-3 with an interme-diate TTT of 160 ms yields the besthandover-failure versus ping-pong tradeoff. Wealso observe that using range expansion with pic-ocell eICIC decreases handover failures, butincreases ping-pongs, with respect to the case ofno range expansion or eICIC. For UE velocitiesup to 30 km/h, Release-10 eICIC is shown tooffer reduced handover-failure and ping-pongrates, but it becomes harder to simultaneouslyachieve both for higher UE velocities. Since weenvision dense and ad hoc future LPN deploy-ments, we anticipate the need for novel schemesthat are able to reduce both handover failuresand ping-pongs.

MOBILITY-BASED INTER-CELLINTERFERENCE COORDINATION FOR

HETNETS

As previously shown, in co-channel deploymentsof macrocells and picocells, high-mobility MUEsare likely to be victim UEs, because they maynot be able to connect soon enough to a picocelldue to the TTT constraint, even when the pico-cell provides better link quality, and they mayexperience RLFs before completing the han-dover process (Fig. 1b).

In this section, we propose a mobility-basedICIC (MB-ICIC) scheme that combines han-dover parameter optimization with eICIC, so asto reduce handover failure and ping-pong rates.On the one hand, in order to protect high-mobil-

Figure 3. Coverage areas of macrocell/picocell with and without range expansion, and RLF locations from macrocell and picocell per-spectives (Qout = –8 dB): a) Macrocell perspective; and b) picocell perspective.

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ity UEs from handover failures (which havebeen shown to be a bigger issue for them thanping-pongs), we propose that picocells releasethe use of certain resources3 (e.g. subframes 4 and 8 in Fig. 2a) so that macrocells can sched-ule their high-mobility UEs in these resourceswithout co-channel interference from picocells.On the other hand, in order to protect low-mobility UEs from ping-pongs (which have beenshown to be a bigger issue for them than han-

dover failures), we propose the use of handoverparameter optimization with large TTTs for low-mobility UEs. In addition, macrocells may alsoleave certain resources blank (e.g. subframes 2, 6,and 9 in Fig. 2a), so that picocells can scheduletheir range-expanded PUEs in these resources(i.e. traditional eICIC in Release 10) [3].

HANDOVER FAILURESHandover failure rates under the proposed MB-ICIC are shown in Fig. 5a. With MB-ICIC,macrocells allocate high-speed UEs in coordinat-ed subframes (i.e. ABSFs of picocells) so thattheir macro-to-pico handovers as well as strongpico-to-macro interference are avoided. In thisway, the number of RLFs and thus handoverfailures is significantly reduced. However, such aperformance improvement comes at the expenseof releasing some resources by the picocells. Inorder to minimize the throughput loss of pico-cells, only high-velocity MUEs (e.g. > 60 km/h)are assigned to ABSFs of picocells, whereas low-velocity MUEs are handled through handoverparameter optimization using large TTTs (e.g.Set-2) to suppress ping-pongs. It may also bepossible to semi-dynamically adjust the dutycycle of ABSFs at a picocell based on the per-centage of high-mobility UEs within a given timewindow.

PING-PONGSPing-pong rates under the proposed MB-ICIC arealso significantly reduced by MB-ICIC, as shownin Fig. 5b. This is because handovers for high-mobility UEs (e.g. > 60 km/h) are avoidedthrough cooperative radio resource manage-ment, while handovers for low-mobility UEs gothrough the standard handover procedure butwith long TTTs (e.g. Set-2), which reduce ping-pongs.

In order to allow a fair performance compari-son, we quantify the gains of using the proposedMB-ICIC with respect to the case of picocellrange expansion and picocell eICIC for Set-3(TTT of 160 ms), which has been shown to pro-vide the best handover-failure versus ping-pongtradeoff performance. When the UE’s velocity is60 km/h, the handover failure and ping-pongrate reduction gains offered by the proposedMB-ICIC are around 5.5 percent (by comparingFig. 5a with Fig. 4a), and 10 percent (by compar-ing Fig. 5b with Fig. 4b), respectively. The gainsare larger at higher UE velocities. For example,at 120 km/h, the handover-failure and ping-pongrate reduction gains provided by MB-ICIC incomparison with the same reference case arearound 13 percent and 12 percent, respectively.

MOBILITY STATE ESTIMATIONPerformance of the proposed MB-ICIC methodrelies on the estimation of UE mobility state,e.g. low-mobility, medium mobility, or high-mobility, which is also an objective of the ongo-ing 3GPP RAN2 SI [6, 7]. In a homogeneousnetwork, the number of handovers within a giventime window can be compared with two differentthresholds to estimate whether a UE is at thelow, medium, or high mobility state. However, ina HetNet, using this approach no longer workswell due to varying cell sizes; higher densities of

Table 1. Simulation parameters.

Parameter Macrocell

Carrier frequency 2.0 GHz

System bandwidth 10MHz

Number of eNB/sectors 19/57, with 500m ISD

eNB antenna patterns (TR 36.814) 3D pattern

PeNB antenna patterns (TR 36.814) Omnidirectional pattern

eNB antenna tilt 15 degree

eNB antenna gain 15 dB

PeNB antenna gain 5 dB

UE antenna gain 0 dB

Macrocell path loss model 128.1 + 37.6log10(R) dB

Picocell path loss model 140.7 + 36.7log10(R) dB

Shadowing standard deviation 8 dB (macrocell), 10 dB (picocell)

Correlation distance of shadowing 25m

Macrocell shadowing correlation 0.5 (1) between cells (sectors)

Picocell shadowing correlation 0.5 between cells

Transmit power 46 dBm (eNB), 30 dBm (PeNB)

Penetration loss 20 dB

Antenna configuration 1¥ 2

Picocell range expansion bias 8 dB (whenever applicable)

Cell loading 100%

UE speeds 3, 30, 60, 120 km/h

UE noise figure 9 dB

Thermal noise density –174d Bm/Hz

Channel model Typical urban (6 rays)

Handover metric 1 Rx for RSRP measurement

SINR metric 2 Rx, Maximal ratio combining andexponential effective SINR mapping

RSRP measurement bandwidth 25 resource blocks

L3 filter coefficient (a) 0.5

Handover preparation (execution) delay 50 ms (40 ms)

Qout (Qin) –8 dB (–6 dB)

T310 1 s

Min. eNB-UE (PeNB-UE) distance 35m (10 m)

Min. eNB-PeNB (PeNB-PeNB) distance 75m (40 m)

3 In a more general set-ting, resources can becoordinated in time (e.g.,subframes), frequency(e.g., component carriers),code (e.g., spreadingcodes in CDMA systems),and space (e.g., beamdirections) domains.


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picocells yield mobility state estimates that arebiased toward medium and high mobility states[6].

One way to improve the mobility state esti-mation performance in HetNets is to scale eachhandover count with a weight that is directlyproportional to the size of the cell involved inthe handover [7]. Another approach is to usehigher mobility state estimation thresholds forhigher picocell densities [9]. For a more accurateestimation of a UE’s mobility state, Doppler fre-quency measurements may also be utilized incombination with cell reselection count [15].Typically, a UE already measures Doppler fre-quency for the purpose of channel estimation,and hence, measurement of the Doppler frequen-cy would not be a new function and may also beutilized for mobility state estimation purposes.

CONCLUSION

In this article, we have investigated mobilitymanagement challenges in HetNets, and haveproposed a mobility enhancement scheme,namely MB-ICIC, to mitigate handover failuresand ping-pongs. In MB-ICIC, picocells configurecoordinated resources, which macrocells can useto schedule their high-mobility UEs. Handoverfailure and ping-pong rates have been simulatedfor a wide range of system and channel parame-ters, which are based on simulation assumptionsin a 3GPP RAN 2 SI. As compared with imple-menting ABSFs only at macrocells, supportingABSFs at both picocells and macrocells reducesthe handover failure and ping-pong rates forhigh-mobility UEs by around 13 percent and 12 percent, respectively.

Figure 5. Simulated handover failure and ping-pong rates (with four randomly deployed picocells per sector, 8 dB range expansion bias,and MB-ICIC): a) Handover failure rates under MB-ICIC; and b) ping-pong rates under MB-ICIC.

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Mobility state threshold Mobility state threshold

Figure 4. Simulated handover failure and ping-pong rates (with four randomly deployed picocells per sector, with or without an 8 dBrange expansion (RE) bias and pico-side eICIC): a) Handover failure rates with or without range expansion (RE) and pico-sideeICIC; and b) Ping-pong rates with or without range expansion (RE) and pico-side eICIC.

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Set-1 (no RE, no eICIC)Set-2 (no RE, no eICIC)Set-3 (no RE, no eICIC)Set-4 (no RE, no eICIC)Set-5 (no RE, no eICIC)Set-1 (RE, pico eICIC)Set-2 (RE, pico eICIC)Set-3 (RE, pico eICIC)Set-4 (RE, pico eICIC)Set-5 (RE, pico eICIC)


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REFERENCES[1] Cisco, “Global Mobile Data Traffic Forecast Update,

2011-2016,” Feb. 2012, White Paper, pp. 1–29. [2] A. Damnjanovic et al., “A Survey on 3GPP Heteroge-

neous Networks,” IEEE Wireless Commun. Mag., vol.18, no. 3, June 2011, pp. 10–21.

[3] D. López-Pérez et al., “Enhanced Inter-Cell InterferenceCoordination Challenges in Heterogeneous Networks,”IEEE Wireless Commun. Mag., vol. 18, no. 3, June2011, pp. 22–31.

[4] D. Aziz and R. Sigle, “Improvement of LTE HandoverPerformance Through Interference Coordination,” Proc.IEEE Vehic. Tech. Conf. (VTC), Barcelona, Spain, Apr.2009, pp. 1–5.

[5] TS 23.009, “Handover Procedures,” 3GPP TechnicalReport, v.10.0.0, Apr. 2011.

[6] TR 36.839, “Mobility Enhancements in HeterogeneousNetworks,” 3GPP Technical Report, v.2.0.0, Aug. 2012.

[7] R2-122474, “Summary of Email Discussion [77bis#32]LTE/HetNet Mobility: MSE,” 3GPP Agenda Item 7.10.4,May 2012.

[8] Samsung, “Mobility Support to Pico Cells in the Co-Channel HetNet Deployment,” Stockholm, Sweden,Mar. 2010, 3GPP Standard Contribution (R2-104017).

[9] D. López-Pérez, I. Guvenc, and X. Chu, “MobilityEnhancements for Heterogeneous Wireless NetworksThrough Interference Coordination,” Proc. IEEE Int’l.Wksp. Broadband Femtocell Technologies (colocatedwith IEEE WCNC), Paris, France, Apr. 2012, pp. 69–74.

[10] —, “Theoretical Analysis of Handover Failure and Ping-Pong Rates for Heterogeneous Networks,” Proc. IEEEInt’l. Wksp. Small Cell Wireless Networks (co-locatedwith IEEE ICC), Ottawa, Canada, June 2012.

[11] M. Anas et al., “Performance Analysis of HandoverMeasurements and Layer 3 Filtering for UTRAN LTE,”Proc. IEEE Int’l. Symp. Personal, Indoor, Mobile RadioCommun. (PIMRC), Athens, Greece, Sep. 2007, pp. 1–5.

[12] TS 36.331, “Radio Resource Control; Protocol Specifi-cation,” 3GPP Technical Report, v.10.4.0, Dec. 2011.

[13] D. Pacifico et al., “Improving TCP Performance Duringthe Intra LTE Handover,” Proc. IEEE Global Telecom-mun. Conf. (GLOBECOM), Dec. 2009, pp. 1–8.

[14] TS 36.300, “Evolved Universal Terrestrial Radio Access(E-UTRA) and Evolved Universal Terrestrial Radio AccessNetwork (E-UTRAN),” 3GPP Technical Report, v.10.5.0,Oct. 2011.

[15] NTT DOCOMO, Inc., “Enhanced Mobility State Estima-tion by Doppler Frequency Measurements,” Qingdao,China, Aug. 2012, 3GPP Standard Contribution (R2-124007).

BIOGRAPHIESDAVID LÓPEZ-PÉREZ [S’08, M’12] ([email protected]) is a Research Engineer at Bell Labs, Alcatel-Lucent, Dublin, Ireland. Prior to this, he received hisBachelor (B.Sc.) and Master (M.Sc.) degrees in Telecommu-nication from Miguel Hernandez University, Spain, in Sept.2003 and Sept. 2006, respectively, and his Doctor in Phi-losophy (Ph.D.) title from University of Bedfordshire, UK, in

April. 2011. From Aug. 2010 until Dec. 2011, he wasResearch Associate, carrying post-doctoral studies, at theCentre for Telecommunications Research (CTR) at King’sCollege London (KCL), London UK. From Feb. 2005 untilFeb. 2006, he was with VODAFONE Spain, working at theRadio Frequency Department in the area of network plan-ning and optimization. He has been invited researcher atDOCOMO USA labs, Palo Alto, CA in 2011, and CITI INSA,Lyon, France in 2009. In May 2007, he was awarded with aPh.D. Marie-Curie fellowship. With 30 years of age, he haspublished more than 50 book chapters, journal and confer-ence papers in recognized venues, and has been awardedas Exemplary Reviewer for IEEE Communications Letters. Heis or has been guest editor of IEEE Comm. Mag., ACMSpringer MONE and EURASIP JCNC, and editor and/orauthor of several cellular HetNet related books, e.g. “Het-erogeneous Cellular Networks: Theory, Simulation andDeployment” Cambridge University Press, 2012. Moreover,he is or has also been co-chair of several HetNet relatedworkshops, e.g., the 1st IEEE WCNC Workshop on Broad-band Femtocells, the 2nd IEEE 2011 GLOBECOM Workshopon Femtocell Networks (FEMnet).

ISMAIL GÜVENC [S’01, M’06, SM’10] ([email protected])received his Ph.D. degree in electrical engineering fromUniversity of South Florida, Tampa, FL, in 2006 (with out-standing dissertation award). He was with Mitsubishi Elec-tric Research Labs in Cambridge, MA, in 2005, and withDOCOMO Innovations, Inc., Palo Alto, CA, between 2006-2012. Since Aug. 2012, he has been an assistant professorin the Department of Electrical and Computer Engineering,Florida International University. His recent research inter-ests include heterogeneous wireless networks and futureradio access beyond 4G wireless systems. He has publishedmore than 60 conference and journal papers, and severalstandardization contributions. He co-authored/co-editedthree books for Cambridge University Press, is an editor forIEEE Communications Letters and IEEE Wireless Communi-cations Letters, and was a guest editor for two specialissue journals on heterogeneous networks. He holds 13patents, and another 13 pending.

XIAOLI CHU [S’03, M’06] ([email protected]) is a lecturerin the Department of Electronic and Electrical Engineeringat the University of Sheffield, UK. She received the Ph.D.degree in Electrical and Electronic Engineering from theHong Kong University of Science and Technology in 2005.From September 2005 to April 2012, she was with theCentre for Telecommunications Research at King’s CollegeLondon. Her current research interests include heteroge-neous networks, interference management, cooperativecommunications, cognitive communications, and greenradios. She has published more than 40 journal and con-ference papers and book chapters. She is a guest editor ofthe Special Issue on Cooperative Femtocell Networks forACM/Springer Journal of Mobile Networks & Applications.She has been TPC co-chair of several international work-shops. She received the UK EPSRC Cooperative Awards inScience and Engineering for New Academics in 2008, theUK EPSRC First Grant in 2009, and the RCUK UK-China Sci-ence Bridges Fellowship in 2011.


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