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Mm in hetnets

  1. 1. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® TOPICS IN RADIO COMMUNICATIONS Mobility Management Challenges in 3GPP Heterogeneous Networks David López-Pérez, Bell Laboratories Alcatel-Lucent . I smail Güvenc, Florida International University Xiaoli Chu, The University of Sheffield ABSTRACT since mobile UEs may trigger frequent han- dovers when they move across the small cover- In this article we provide a comprehensive age areas of LPNs. review of the handover process in heterogeneous In cellular networks, handovers allow a UE to networks (HetNets), and identify technical chal- transfer their active connections from its serving lenges in mobility management. In this line, we cell to a target cell in connected mode, while evaluate the mobility performance of HetNets maintaining quality of service [5]. In convention- with the 3rd Generation Partnership Project al homogeneous networks, UEs typically use the (3GPP) Release-10 range expansion and same set of handover parameters (e.g. hysteresis enhanced inter-cell interference coordination margin and time to trigger (TTT)) throughout (eICIC) features such as almost blank subframes the network. However, in HetNets (where (ABSFs). Simulation assumptions and parame- macrocells, picocells, femtocells, and relay nodes ters of a related study item in 3GPP are used to have different coverage area sizes), using the investigate the impact of various handover same set of handover parameters for all cells parameters on mobility performance. In addi- and/or for all UEs may degrade mobility perfor- tion, we propose a mobility-based inter-cell mance. For example, the use of range expan- interference coordination (MB-ICIC) scheme, in sion 1 in open access LPNs with different bias which picocells configure coordinated resources will affect when and where the handover process so that macrocells can schedule their high-mobil- is initiated in each cell. Therefore, in HetNets, ity UEs in these resources without co-channel there is a need for cell-specific handover param- interference from picocells. MB-ICIC also bene- eter optimization. Moreover, high-mobility fits low-mobility UEs, since handover parame- Macrocell UEs (MUEs) may run deep inside ters can now be more flexibly optimized. LPN coverage areas before the TTT optimized Simulations using the 3GPP simulation assump- for macrocells expires, thus incurring handover tions are performed to evaluate the performance failure due to degraded signal to interference of MB-ICIC under several scenarios. plus noise ratio (SINR). Handovers performed for high-mobility MUEs may also be unneces- INTRODUCTION sary (i.e. ping-pongs), when they quickly pass through the small coverage areas of LPNs. These In order to meet the upcoming exponential facts also impose the need for UE-specific han- growth of mobile data traffic [1], operators are dover parameter optimization. deploying more network infrastructures to make Due to its capital importance, mobility man- cellular networks closer to UEs, and thus agement challenges in HetNets have attracted increase spectrum efficiency and spatial reuse. In much interest from the wireless industry, this context, HetNets, which are comprised of research community, and standardization bodies coexisting macrocells and low power nodes [6–10]. Indeed, a new Study Item (SI) “HetNet (LPNs) such as picocells, femtocells, and relay mobility enhancements for LTE” has recently nodes, have been heralded as the most promis- been established in the 3GPP RAN 2 [6]. Results ing solution to provide a major performance in [6] indicate that mobility performance in Het- leap [2]. However, in order to realize the poten- Net deployments is not as good as in pure tial coverage and capacity benefits of HetNets, macrocell deployments, and that performance 1 With range expansion [2], operators are facing new technical challenges in, enhancements are needed for high-mobility a positive range expansion for example, mobility management, inter-cell UEs. As a result, the new 3GPP RAN 2 SI [6] bias is added to the down- interference coordination (ICIC) [3], and back- aims to develop strategies for improved small link RSS of picocell pilot haul provisioning [4]. Among these challenges, cell discovery/identification, automatic re-estab- signals at UEs to increase mobility management is of special importance. lishment procedures, enhanced mobility robust- picocells’ downlink cover- The deployment of a large number of LPNs may ness, and techniques for enhanced mobility state age footprints [3]. increase the complexity of mobility management, estimation in a HetNet environment. 70 0163-6804/12/$25.00 © 2012 IEEE IEEE Communications Magazine • December 2012C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  2. 2. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® In this article, we provide a comprehensive L1 filtering. For a typical setup (Fig. 1a), in review of the handover process in HetNets, and order to obtain an L1 filtered handover mea- In the proposed identify technical challenges in mobility manage- surement, downlink RSRP samples may be taken scheme, picocells ment, with a special focus on principal objectives every 40 ms, and then averaged over five succes- of the new 3GPP RAN 2 SI [6]. We also evalu- sive RSRP samples. configure coordinat- ate mobility performance in HetNets with 3GPP The L1 filtered handover measurements are ed resources so that Rel-10 enhanced ICIC (eICIC) features such as updated every handover measurement period almost blank subframe (ABSF), and propose a (e.g. 200 ms) at the UE, and averaged through a macrocells can novel mobility-based ICIC (MB-ICIC) scheme first-order infinite impulse response (IIR) filter, schedule their high- to further improve it. In the proposed scheme, as defined in Fig. 1a [11], to further mitigate the mobility UEs in these picocells configure coordinated resources so that effects of fading and estimation imperfections. macrocells can schedule their high-mobility UEs This moving averaging is performed in Layer 3, resources without in these resources without co-channel interfer- and is known as L3 filtering. Since successive co-channel interfer- ence from picocells. 3GPP simulation assump- log-normal shadowing samples are spatially cor- tions have been adopted in this study. related, the L3 filtering period is preferred to be ence from picocells. adaptive to the degree of shadowing correlation 3GPP simulation in the received signal. For high-mobility UEs, assumptions have OVERVIEW OF HANDOVER log-normal shadowing samples are not highly correlated, and thus it would be better to have a been adopted PROCESS IN 3GPP LTE shorter L3 filtering period than that for low- in this study. In this section we introduce the concepts of han- mobility UEs. A typical L3 filtering period is 200 dover, handover failure, ping-pong, picocell ms. range expansion, and eICIC based on ABSFs. A handover is then triggered if the L3 filtered handover measurement meets a handover event HANDOVER PROCESS entry condition. In LTE, there are eight types of In wireless communications networks, handovers handover event entry conditions (see [12], Sec- could be performed between different Radio tion 5.5.4): Access Technologies (RATs), carriers, or cells. • Event A1: Server becomes better than In this article, we discuss intra-RAT intra-carrier threshold. handovers. Specifically, we focus on 3GPP LTE • Event A2: Server becomes worse than hard handovers, in which UEs disconnect with threshold. the source cell before establishing a new connec- • Event A3: Neighbor becomes offset better tion with the target cell. than server. The 3GPP LTE handover process can typi- • Event A4: Neighbor becomes better than cally be divided into four phases: measurement, threshold. processing, preparation, and execution. Han- • Event A5: Server becomes worse than dover measurements and processing are per- threshold1 and neighbor becomes better formed by the UE. Handover measurements than threshold2. are usually based on downlink reference signal • Event A6: Neighbor becomes offset better received power (RSRP) estimations,2 while pro- than secondary server (this condition cessing takes place to filter out the effects of applies to carrier aggregation configura- fading and estimation imperfections in han- tions). dover measurements. After processing, if • Event B1: Inter RAT neighbor becomes according to the filtered measurements a cer- better than threshold. tain handover event entry condition is met, the • Event B2: Server becomes worse than UE alerts the serving cell and feeds back han- threshold1 and inter RAT neighbor dover measurements through a measurement becomes better than threshold2. report. Then, the preparation phase starts, in Intra-RAT intra-carrier handovers are trig- which the serving cell initiates the handover gered upon event A3. Once the event A3 condi- process, and prepares the handover execution tion is met, i.e. the L3 filtered RSRP of the together with the target cell. Finally, in the exe- target cell is larger than that of the serving cell cution phase, the serving and target cells per- plus a hysteresis margin (also referred to as form necessary network procedures with the event A3 offset), the UE starts the TTT timer assistance of the UE to transfer its connection (Fig. 1b). Only if the event A3 condition is satis- from the former to the latter. fied throughout the TTT, the UE alerts the serv- In LTE standards, the UE performs handover ing cell and feeds back this event A3 condition measurements and processing in Layer 1 (physi- through a measurement report, thus initiating cal) and Layer 3 (network), as shown in Fig. 1a the handover preparation process. Small values [11]. For handover measurements, the UE gen- of TTT may lead to too early handovers, increas- erally takes RSRP estimations over the cells ing ping-pongs, while large values of TTT may included in its neighboring cell list. In order to result in too late handovers, increasing handover 2 Handovers can be gov- remove the effects of fading from RSRP estima- failures. Therefore, the optimization of the TTT erned not only by signal tions, the UE obtains each RSRP sample as the according to the UE’s velocity carries capital strength but also by signal linear average over the power contributions of importance in mobility management [7], as it will quality. However, in this all resource elements that carry reference sym- be shown later. article, we have adopted bols within one subframe (i.e. 1 ms) and the con- Once the TTT successfully expires, as shown the simulation assump- sidered measurement bandwidth (e.g. six in Fig. 1b, the handover preparation phase starts. tions in [6], where the resource blocks), and thereafter further averag- The source cell issues a handover request mes- handover procedure is ing over several RSRP samples. This linear aver- sage to the target cell, which carries out admis- governed by RSRP mea- aging is performed in Layer 1, and is known as sion control procedures according to the quality surements. IEEE Communications Magazine • December 2012 71C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  3. 3. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® In order to address Handover decision Handover decision Handover decision problems caused by F(n-1) F(n) F(n+1) F(n+2) L3 L3 L3 the downlink trans- filtering filtering filtering mit power difference between eNodeBs (eNBs) and Picocell M(n) M(n+1) M(n+2) eNBs (PeNBs) in Het- L1 filtering L1 filtering L1 filtering Nets, cell selection methods allowing Time (t) UEs to associate with cells that do not pro- 40 ms (a) vide the strongest downlink RSRP are CQI<Qout(T310 started) Radio link failure necessary. Radio link monitor Radio problem Radio link failure timer (T310) T311 process detection Handover command Handover Handover Handover process Time-to-trigger (TTT) preparation time execution time Measurement report triggered Event entering condition Handover Handover (e.g. A3 condition) failure complete (b) 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 resource blocks) every 40 ms and recorded as RSRPL1(l). L1 filtering performs averaging over every 200 ms to pro- vide M(n) = 1/5 S4 RSRPL1(5n–k). Finally, L3 filtering performs averaging over every 200 ms to obtain k=0 F(n) = (1 – a)F(n – 1) + a10 log10{M(n)}, where a is the L3 filter coefficient; and b) Timers in radio link monitoring and handover processes. If a radio link failure is declared while the TTT is running, a handover failure happens. Alternatively (not shown in the figure), a handover failure may happen if the timer T310 has been triggered or is running when the handover command is received by the UE (indicat- ing control channel failure) [6]. of service requirement of the UE [13]. After HANDOVER FAILURES AND PING-PONGS admission, the target cell prepares the handover process, and sends a handover request acknowl- A UE is considered to be out of synchroniza- edge to the source cell. When the handover tion when its wideband SINR (also referred to request acknowledge is received at the source as channel quality indicator (CQI)) falls to cell, data forwarding from the source cell to the Q out (in dB), and to be back in synchroniza- target cell starts, and the source cell sends a tion when it reaches Q in (in dB). For tracking handover command (within a RRC message) to radio link failure (RLF) [14], a UE u uses two the UE. moving average windows, which have depths Finally, in the handover execution phase, the of 200 ms and 100 ms to compute its CQI val- UE synchronizes with the target cell and access- ues Q out,u and Q in,u , respectively. Both win- es it [13]. The UE sends a handover complete dows are updated once per frame, i.e. once message to the target cell when the handover every 10 ms. When Q out,u is lower than the procedure is finished. The target cell, which can threshold Q out , a synchronization problem then start transmitting data to the UE, sends a occurs, and the T310 timer (usually of 1 s path switch message to inform the network that duration) is triggered as shown in Fig. 1b. The the UE has changed its serving cell. Thereafter, T310 timer is stopped once Qin,u is larger than the network sends a UE update request message the threshold Q in , and the UE is considered to the serving gateway, which switches the down- back in synchronization. However, if the T310 link data path from the source cell to the target timer runs until it expires, the UE is consid- cell. The network also sends end marker packets ered out of synchronization, and an RLF is through the old path to the source cell, asking it declared [6]. Accordingly, a handover failure to release any resources previously allocated to happens if one of the following three condi- this UE. tions is met [6]: 72 IEEE Communications Magazine • December 2012C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  4. 4. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® 1 RLF happens during the time between sat- owing or fading is included, while in Fig. 2d, the isfying the event A3 condition and receiving effect of shadowing is considered. Comparing Optimizing handover a handover command (Fig. 1b). Fig. 2c with Fig. 2b, we can see that MUEs 2 T310 timer is triggered and still running invade the picocell expanded region without parameters to when a handover command is sent. handing over to the picocell due to the use of reduce handover fail- 3 The UE wideband SINR Qout,u is lower than TTT. Comparing Fig. 2d with Fig. 2c, we observe ures would increase Qout when a handover complete message is that due to shadowing effects, there is no clean- sent. cut boundary of the picocell coverage area with ping-pongs, and vice Note that if condition 2 or 3 occurs (which are or without range expansion, and UEs can con- versa [6]. This makes not shown in Fig. 1b for brevity), a packet data nect to the picocell in a much wider area. This control channel (PDCCH) failure is declared. demonstrates the importance of considering handover optimiza- The occurrence of a ping-pong is determined channel variations in mobility management. tion an intricate by the time duration that a UE stays connected In order to illustrate the concept of handover problem, which to a cell directly after a handover, namely time- failure, the locations where event A3 and RLF of-stay. The time-of-stay starts when the UE occur in a HetNet comprising a macrocell and a would be exacerbat- sends a handover complete message to the cell, picocell are plotted in Fig. 3, where the PeNB is ed by the large num- and ends when the UE sends a handover com- located at (1500, 1750) m, the eNB is located at plete message to another cell. If a UE has a (1500, 1500) m, and we adopt the same assump- ber of LPN overlaid time-of-stay less than the threshold Tp, e.g. 1 s, tions of Fig. 2c but with a larger TTT to facili- on macrocells in and the new target cell is the same cell as the tate the observation of RLF positions. In Fig. 3a, HetNet. source cell when handing over to the current MUEs were initially located outside the picocell seving cell, then the handover that terminates coverage area and moved toward the PeNB this time-of-stay is considered an unnecessary leading to macro-to-pico handovers, while in Fig. handover, i.e. a ping-pong [10]. 3b, PUEs were generated close to the PeNB location and moved toward the eNB, resulting in RANGE EXPANSION AND pico-to-macro handovers. In Fig. 3a, MUE RLF ALMOST BLANK SUBFRAMES locations are clustered, because MUEs were ini- tially served by different eNB sectors. From Fig. 3 In order to address problems caused by the we can infer that if MUEs or PUEs cross the downlink transmit power difference between RLF boundary before the TTT expires, when eNodeBs (eNBs) and Picocell eNBs (PeNBs) in moving in or out of the picocell coverage area, HetNets, cell selection methods allowing UEs to respectively, then RLF occurs and the UE expe- associate with cells that do not provide the riences handover failure. This figure also shows strongest downlink RSRP are necessary. A wide- that, with range expansion at the picocell, the ly considered approach is range expansion [3], in event A3 positions are pushed away from the which a positive range expansion bias is added to PeNB location, thus increasing the picocell cov- L3 handover measurements at UEs to increase erage area and potentially allowing for a better picocells’ downlink coverage footprints (Fig. 1a). spatial reuse. In Fig. 3a, since the gap between Although range expansion is able to mitigate the event A3 boundary and the RLF boundary is uplink inter-cell interference and provide load larger with picocell range expansion, it is more balancing in HetNets, it degrades the downlink likely that the TTT will expire before the UE signal quality of Picocell UEs (PUEs) in the SINR falls to Qout and the handover is complet- expanded region, since these PUEs are not con- ed successfully [10]. Picocell range expansion nected to the cell that provides the strongest thus facilitates the macro-to-pico handover. On RSRP (e.g. PUE-3 connected to PeNB-2 in Fig. the contrary, in Fig. 3b, range expansion chal- 2a). eICIC based on ABSFs can be used to miti- lenges the pico-to-macro handover, since the gate downlink inter-cell interference for range- space between the event A3 boundary and the expanded picocells [3]. ABSF are subframes in RLF boundary gets smaller, and it is more likely which no control or data signals but just refer- that the UE SINR falls to Qout before the TTT ence signals are transmitted. Specifically, macro- expires and a handover failure occurs [10]. In cells schedule ABSFs, and picocells schedule Fig. 3b, RLF may occur even earlier than event range-expanded PUEs in the subframes that A3, indicating the need for eICIC to support overlap with the macrocell ABSFs, so that their expanded region picocells. performance can be enhanced (e.g. subframes 2, 6, and 9 in Fig. 2a). HETNET MOBILITY PERFORMANCE ILLUSTRATIVE EXAMPLES In order to illustrate handover behaviors in the WITH 3GPP RELEASE-10 EICIC presence of picocell range expansion, the cover- A capital issue in mobility performance opti- age areas of a range-expanded picocell (with an mization is the tradeoff between handover fail- 8 dB bias) for three different handover scenarios ures and ping-pongs. Optimizing handover are depicted in Figs. 2b–2d, where the PeNB is parameters to reduce handover failures would located at (1500, 1650) m, the eNB is located at increase ping-pongs, and vice versa [6]. This (1500, 1500) m, UEs move at 3 km/h, eICIC is makes handover optimization an intricate prob- not implemented, and the handover preparation lem, which would be exacerbated by the large and execution times are neglected. In Fig. 2b, number of LPNs overlaid on macrocells in Het- handovers are performed based on geometry Nets. Moreover, UEs’ velocities are also an only, while in Figs. 2c–2d, handovers are per- important factor to consider. Tuning handover formed using a TTT of 160 ms and an event A3 parameters, e.g. TTT, based on mobility state offset of 2 dB. In Figs. 2b–2c, no effect of shad- information or measurement report processing IEEE Communications Magazine • December 2012 73C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  5. 5. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® Frame duration Interfering signal 1720 User connected to macrocell 1 2 3 4 5 6 7 8 9 10 Desired signal User connected to ER 1700 User connected to picocell eNB subframes Time PeNB location Subframe duration Expanded picocell Blanked subframe at eNB range 1680 Y-axis (meters) MUE-1 1660 PeNB-2 1640 eNB PUE-3 PUE-1 PUE-2 1620 PeNB-1 1 2 3 4 5 6 7 8 9 10 MUE-1 PeNB-2 subframes Time trajectory 1600 1 2 3 4 5 6 7 8 9 10 Blanked subframe at PeNB-2 PeNB-1 subframes Time 1580 1440 1460 1480 1500 1520 1540 1560 X-axis (meters) (a) (b) 1720 1720 User connected to macrocell User connected to macrocell User connected to ER User connected to ER 1700 User connected to picocell 1700 User connected to picocell PeNB location PeNB location 1680 1680 Y-axis (meters) Y-axis (meters) 1660 1660 1640 1640 1620 1620 1600 1600 1580 1580 1440 1460 1480 1500 1520 1540 1560 1440 1460 1480 1500 1520 1540 1560 X-axis (meters) X-axis (meters) (c) (d) Figure 2. 3GPP Release-10 eICIC scenarios and handover behaviors in the presence of picocell range expansion: a) range expansion at picocells. In 3GPP Rel-10, eICIC blanks subframes at the eNB side to improve performance of Pico UEs (PUEs) in the expanded region. 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 A3 offset, with shadowing. has been a widely used approach to mitigate • Set-1: TTT = 480 ms, A3 offset = 3 dB, handover failures. For example, high-mobility L3 Filter K = 4. UEs handover faster than low-mobility UEs [7]. • Set-2: TTT = 160 ms, A3 offset = 3 dB, UEs’ velocities become even more important in L3 Filter K = 4. HetNets, where handovers of high-mobility UEs • Set-3: TTT = 160 ms, A3 offset = 2 dB, to small cells should be prevented to avoid han- L3 Filter K = 1. dover failures or ping-pongs. • Set-4: TTT = 80 ms, A3 offset = 1 dB, In order to illustrate the tradeoff between L3 Filter K = 1. handover failures and ping-pongs and the • Set-5: TTT = 40 ms, A3 offset = –1 dB, impact of UEs’ velocities, we have performed L3 Filter K = 0. system-level simulations using the simulation The longest and shortest TTT durations are 480 scenarios and assumptions/parameters in [6], ms in simulation Set-1 and 40 ms in simulation which have been agreed by a large number of Set-5, respectively. For each simulation set, an vendors and operators. In these simulations, L1 and L3 filtering period of 200 ms was used, a hexagonal macrocell layout with 19 eNBs, along with full cell-loading. UEs were randomly 57 sectors, and an inter-eNB distance of 500 distributed over the entire simulation scenario, meters were used. Four PeNBs were random- and moved at a fixed speed randomly selected in ly distributed within each eNB sector cover- the set of {3, 30, 60, 120} km/h. UEs moved age area. Auto- and cross-correlated along straight lines toward randomly selected shadowing was included, as well as Rayleigh directions, and did not change directions until fading based on the typical urban model. We they hit the border of the simulation scenario. considered five different handover profiles When a UE hit the border of the simulation sce- based on [6]: nario, it bounced back and moved toward anoth- 74 IEEE Communications Magazine • December 2012C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  6. 6. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® 1820 1820 1800 1800 1780 1780 Y-axis (meters) Y-axis (meters) 1760 1760 1740 1740 1720 HO trigger locations (8 dB bias) 1720 HO trigger locations (8 dB bias) PeNB location PeNB location Range expansion Circle center Range expansion Circle center 1700 MUE RLF locations 1700 PUE RLF locations HO trigger locations (no bias) HO trigger locations (no bias) 1440 1460 1480 1500 1520 1540 1560 1440 1460 1480 1500 1520 1540 1560 X-axis (meters) X-axis (meters) (a) (b) 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. er randomly selected direction. Results are aver- ping-pongs mostly damage low-mobility UEs and aged over 100 random drops of picocells, and are alleviated with larger TTTs, e.g. in Set-1, each drop lasted for 200 s. Further details of the since extensive L1 and L3 filtering can be per- simulation and parameters are presented in formed to mitigate fading and estimation imper- Table 1 and [6]. fections [10]. Figure 4 presents the simulated handover These results confirm the tradeoff between failure and ping-pong rates for two different handover failures and ping-pongs, i.e. reducing cases: TTT mitigates handover failures, but increases • Picocells without range expansion or eICIC. ping-pongs, and vice versa. Among the five han- • Picocells using an 8 dB bias for range expan- dover profiles considered, Set-3 with an interme- sion and implementing eICIC with ABSF diate TTT of 160 ms yields the best configured at macrocells. handover-failure versus ping-pong tradeoff. We also observe that using range expansion with pic- HANDOVER FAILURES ocell eICIC decreases handover failures, but In terms of handover failure rates, the case of increases ping-pongs, with respect to the case of picocells with no range expansion or eICIC per- no range expansion or eICIC. For UE velocities forms worse than that of picocell range expan- up to 30 km/h, Release-10 eICIC is shown to sion with picocell eICIC (blanking subframes at offer reduced handover-failure and ping-pong the eNB side), as shown in Fig. 4a. This is rates, but it becomes harder to simultaneously because picocell coverage areas without range achieve both for higher UE velocities. Since we expansion are small, and thus MUEs may quick- envision dense and ad hoc future LPN deploy- ly run deep inside picocell coverage areas before ments, we anticipate the need for novel schemes the TTT expires, significantly degrading MUEs’ that are able to reduce both handover failures SINR before the handover process is completed. and ping-pongs. When using picocell range expansion with pico- cell eICIC, the number of handover failures is significantly reduced, because the event A3 MOBILITY-BASED INTER-CELL boundary is pushed away from the PeNB loca- tion by range expansion, and MUEs have more INTERFERENCE COORDINATION FOR time to handover before TTT expires. Pico-to- HETNETS macro handovers are not an issue due to eICIC. In both cases, handover failures mostly damage As previously shown, in co-channel deployments high-mobility UEs and are alleviated with short- of macrocells and picocells, high-mobility MUEs er TTTs [10], e.g. in Set-5. are likely to be victim UEs, because they may not be able to connect soon enough to a picocell PING-PONGS due to the TTT constraint, even when the pico- In terms of ping-pong rates, the case of picocells cell provides better link quality, and they may with no range expansion or eICIC performs bet- experience RLFs before completing the han- ter than that of picocell range expansion with dover process (Fig. 1b). picocell eICIC, as shown in Fig. 4b. When we In this section, we propose a mobility-based increase the picocell coverage area through ICIC (MB-ICIC) scheme that combines han- range expansion, cell selection oscillation caused dover parameter optimization with eICIC, so as by fading occurs in a larger area. As a result, the to reduce handover failure and ping-pong rates. number of ping-pongs increases. In both cases, On the one hand, in order to protect high-mobil- IEEE Communications Magazine • December 2012 75C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  7. 7. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® Parameter Macrocell dover failures), we propose the use of handover parameter optimization with large TTTs for low- Carrier frequency 2.0 GHz mobility UEs. In addition, macrocells may also leave certain resources blank (e.g. subframes 2, 6, System bandwidth 10MHz and 9 in Fig. 2a), so that picocells can schedule Number of eNB/sectors 19/57, with 500m ISD their range-expanded PUEs in these resources (i.e. traditional eICIC in Release 10) [3]. eNB antenna patterns (TR 36.814) 3D pattern HANDOVER FAILURES PeNB antenna patterns (TR 36.814) Omnidirectional pattern Handover failure rates under the proposed MB- eNB antenna tilt 15 degree ICIC are shown in Fig. 5a. With MB-ICIC, macrocells allocate high-speed UEs in coordinat- eNB antenna gain 15 dB ed subframes (i.e. ABSFs of picocells) so that PeNB antenna gain 5 dB their macro-to-pico handovers as well as strong pico-to-macro interference are avoided. In this UE antenna gain 0 dB way, the number of RLFs and thus handover failures is significantly reduced. However, such a Macrocell path loss model 128.1 + 37.6log10(R) dB performance improvement comes at the expense Picocell path loss model 140.7 + 36.7log10(R) dB of releasing some resources by the picocells. In order to minimize the throughput loss of pico- Shadowing standard deviation 8 dB (macrocell), 10 dB (picocell) cells, only high-velocity MUEs (e.g. > 60 km/h) are assigned to ABSFs of picocells, whereas low- Correlation distance of shadowing 25m velocity MUEs are handled through handover Macrocell shadowing correlation 0.5 (1) between cells (sectors) parameter optimization using large TTTs (e.g. Set-2) to suppress ping-pongs. It may also be Picocell shadowing correlation 0.5 between cells possible to semi-dynamically adjust the duty cycle of ABSFs at a picocell based on the per- Transmit power 46 dBm (eNB), 30 dBm (PeNB) centage of high-mobility UEs within a given time Penetration loss 20 dB window. Antenna configuration 1¥ 2 PING-PONGS Ping-pong rates under the proposed MB-ICIC are Picocell range expansion bias 8 dB (whenever applicable) also significantly reduced by MB-ICIC, as shown Cell loading 100% in Fig. 5b. This is because handovers for high- mobility UEs (e.g. > 60 km/h) are avoided UE speeds 3, 30, 60, 120 km/h through cooperative radio resource manage- UE noise figure 9 dB ment, while handovers for low-mobility UEs go through the standard handover procedure but Thermal noise density –174d Bm/Hz with long TTTs (e.g. Set-2), which reduce ping- pongs. Channel model Typical urban (6 rays) In order to allow a fair performance compari- Handover metric 1 Rx for RSRP measurement son, we quantify the gains of using the proposed MB-ICIC with respect to the case of picocell 2 Rx, Maximal ratio combining and range expansion and picocell eICIC for Set-3 SINR metric exponential effective SINR mapping (TTT of 160 ms), which has been shown to pro- vide the best handover-failure versus ping-pong RSRP measurement bandwidth 25 resource blocks tradeoff performance. When the UE’s velocity is L3 filter coefficient (a) 0.5 60 km/h, the handover failure and ping-pong rate reduction gains offered by the proposed Handover preparation (execution) delay 50 ms (40 ms) MB-ICIC are around 5.5 percent (by comparing Qout (Qin) Fig. 5a with Fig. 4a), and 10 percent (by compar- –8 dB (–6 dB) ing Fig. 5b with Fig. 4b), respectively. The gains T310 1s are larger at higher UE velocities. For example, at 120 km/h, the handover-failure and ping-pong Min. eNB-UE (PeNB-UE) distance 35m (10 m) rate reduction gains provided by MB-ICIC in comparison with the same reference case are Min. eNB-PeNB (PeNB-PeNB) distance 75m (40 m) around 13 percent and 12 percent, respectively. Table 1. Simulation parameters. MOBILITY STATE ESTIMATION Performance of the proposed MB-ICIC method ity UEs from handover failures (which have relies on the estimation of UE mobility state, 3 In a more general set- been shown to be a bigger issue for them than e.g. low-mobility, medium mobility, or high- ting, resources can be ping-pongs), we propose that picocells release mobility, which is also an objective of the ongo- coordinated in time (e.g., the use of certain resources 3 (e.g. subframes ing 3GPP RAN2 SI [6, 7]. In a homogeneous subframes), frequency 4 and 8 in Fig. 2a) so that macrocells can sched- network, the number of handovers within a given (e.g., component carriers), ule their high-mobility UEs in these resources time window can be compared with two different code (e.g., spreading without co-channel interference from picocells. thresholds to estimate whether a UE is at the codes in CDMA systems), On the other hand, in order to protect low- low, medium, or high mobility state. However, in and space (e.g., beam mobility UEs from ping-pongs (which have been a HetNet, using this approach no longer works directions) domains. shown to be a bigger issue for them than han- well due to varying cell sizes; higher densities of 76 IEEE Communications Magazine • December 2012C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®
  8. 8. C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND® 55 80 Set-1 (no RE, no eICIC) Set-1 (no RE, no eICIC) Set-2 (no RE, no eICIC) 50 Set-2 (no RE, no eICIC) Set-3 (no RE, no eICIC) Set-3 (no RE, no eICIC) 70 Set-4 (no RE, no eICIC) Set-4 (no RE, no eICIC) Set-5 (no RE, no eICIC) 45 Set-5 (no RE, no eICIC) Handover failure rate (percent) Set-1 (RE, pico eICIC) Set-1 (RE, pico eICIC) 60 40 Ping-pong rate (percent) Set-2 (RE, pico eICIC) Set-2 (RE, pico eICIC) Set-3 (RE, pico eICIC) Set-3 (RE, pico eICIC) Set-4 (RE, pico eICIC) 35 Set-4 (RE, pico eICIC) 50 Set-5 (RE, pico eICIC) Set-5 (RE, pico eICIC) 30 40 25 30 20 15 20 10 10 5 0 0 0 20 40 60 80 100 120 0 20 40 60 80 100 120 Velocity (km/hour) Velocity (km/hour) (a) (b) Figure 4. Simulated handover failure and ping-pong rates (with four randomly deployed picocells per sector, with or without an 8 dB range expansion (RE) bias and pico-side eICIC): a) Handover failure rates with or without range expansion (RE) and pico-side eICIC; and b) Ping-pong rates with or without range expansion (RE) and pico-side eICIC. 80 55 Set-1 (RE, MB-ICIC) Set-1 (RE, MB-ICIC) Set-2 (RE, MB-ICIC) 50 Set-2 (RE, MB-ICIC) Set-3 (RE, MB-ICIC) Set-3 (RE, MB-ICIC) 70 Set-4 (RE, MB-ICIC) 45 Set-4 (RE, MB-ICIC) Set-5 (RE, MB-ICIC) Handover failure rate (percent) Set-5 (RE, MB-ICIC) 60 Ping-pong rate (percent) 40 35 50 30 40 25 30 20 15 20 Mobility state threshold Mobility state threshold 10 10 5 0 0 0 20 40 60 80 100 120 0 20 40 60 80 100 120 Velocity (km/hour) Velocity (km/hour) (a) (b) 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. picocells yield mobility state estimates that are CONCLUSION biased toward medium and high mobility states [6]. In this article, we have investigated mobility One way to improve the mobility state esti- management challenges in HetNets, and have mation performance in HetNets is to scale each proposed a mobility enhancement scheme, handover count with a weight that is directly namely MB-ICIC, to mitigate handover failures proportional to the size of the cell involved in and ping-pongs. In MB-ICIC, picocells configure the handover [7]. Another approach is to use coordinated resources, which macrocells can use higher mobility state estimation thresholds for to schedule their high-mobility UEs. Handover higher picocell densities [9]. For a more accurate failure and ping-pong rates have been simulated estimation of a UE’s mobility state, Doppler fre- for a wide range of system and channel parame- quency measurements may also be utilized in ters, which are based on simulation assumptions combination with cell reselection count [15]. in a 3GPP RAN 2 SI. As compared with imple- Typically, a UE already measures Doppler fre- menting ABSFs only at macrocells, supporting quency for the purpose of channel estimation, ABSFs at both picocells and macrocells reduces and hence, measurement of the Doppler frequen- the handover failure and ping-pong rates for cy would not be a new function and may also be high-mobility UEs by around 13 percent and utilized for mobility state estimation purposes. 12 percent, respectively. IEEE Communications Magazine • December 2012 77C qM IEEE M ommunications q qM Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page MqM q Qmags THE WORLD’S NEWSSTAND®