Hybrid spectrum arrangement and interference mitigation for coexistence between LTE macrocellular and femtocell networks

Transcription

1 Bai and Chen EURASIP Journal on Wireless Communications and Networking 2013, 2013:56 RESEARCH Open Access Hybrid spectrum arrangement and intererence mitigation or coexistence between LTE macrocellular and emtocell networks Yong Bai 1* and Lan Chen 2 Abstract When deploying two-tier LTE cellular networks with coexistence o macrocells and underlaid emtocells, the strategies o spectrum arrangement need to be investigated to eiciently utilize the scarce spectrum resource. Co-channel spectrum usage and dedicated-channel spectrum usage are two traditional strategies o spectrum arrangement. Nevertheless, there are pros and cons o them rom the perspective o achievable network capacity in two-tier LTE network as they result in dierent amounts o available spectrum at each tier and disparate cross-tier intererences. To improve overall spectrum utilization, we propose a novel approach o spectrum arrangement, which is called hybrid spectrum arrangement, to take advantage o their merits. In our proposal, an underlaid emtocell can select its spectrum usage mode according to a criterion aiming to beneit both macrocell and emtocell in terms o achieved capacities. Consequently, the emtocells embedded in macrocell are sel-organized as inner and outer emtocells, which operate in dedicated-channel spectrum usage and co-channel spectrum usage, respectively. Then, we examine distinct characteristics o cross-tier intererences in the context o hybrid spectrum arrangement and present corresponding schemes to mitigate the residual signiicant intererences. Analysis and system level simulation are given to validate the eectiveness o our proposed methods or two-tier LTE cellular network. 1 Introduction A cellular network is a radio network made up o a numberoradiocells.eachcellisservedbyabasestation(bs) to provide radio coverage over a limited area. A macrocell has the widest range o cell size and is served by high-power BS with antennas mounted above rootops. A emtocell has the coverage o m and is served by an indoor BS to support stationary or low-mobility users at homes or in small oices. The BS o emtocell is usually user-installed with a connection to the cellular operator network through a wired broadband backhaul such as digital subscriber line (DSL). Furthermore, the cells o dierent sizes can be deployed in a hierarchical cell structure (HCS) to provide multi-tier network *Correspondence: bai@hainu.edu.cn 1 College o Inormation Science & Technology, Hainan University, 58 Renmin Ave., Haikou, Hainan , China Full list o author inormation is available at the end o the article connectivity [1,2]. Macrocells are deployed as one radio tier to cover wide areas, and emtocells are embedded inside macrocells as another radio tier to supply sporadic overage. This overlaying approach has beneits on capacity gain, better coverage, and reduced battery consumption o handsets [3]. In this article, we consider a two-tier 3GPP long term evolution (LTE) network made up o macrocells and underlaid emtocells. Within the overlapped coverage areas in such a two-tier system, a user equipment (UE) can access either macrocell or emtocell and can switch its access tier by perorming vertical hando. Speciically, when a emtocell is conigured in the closed subscriber group (CSG) manner (also called closed access ), only the users included in the emtocell s access control list are allowed to use the emtocell resources. On the other hand, a emtocell can also be conigured in the open access manner, in which any user is allowed access to the emtocell. For simplicity in this article, a UE is called an MUE when it 2013 Bai and Chen; licensee Springer. This is an Open Access article distributed under the terms o the Creative Commons Attribution License ( which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

2 Bai and Chen EURASIP Journal on Wireless Communications and Networking 2013, 2013:56 Page 2 o 15 is associated with a macrocell BS (namely enodeb or enb in LTE network), and it is called a HUE when it is associated with a emtocell BS (namely Home enodeb or HeNB in LTE network). Ater acquiring licensed spectrum, wireless cellular operator aces the question o how to utilize this scarce resource eiciently, especially when a two-tier cell structure is employed [4]. The irst strategy is letting two tiers share the licensed spectrum such that macrocell and emtocell operate in co-channel requency reuse, which is reerred to as co-channel spectrum usage in this article. The other strategy is that the licensed spectrum is partitioned to separate portions and each tier operates in a dedicated spectrum portion. The latter strategy is reerred to as dedicated-channel spectrum usage in this article. There are both pros and cons o them rom the perspective o achievable network capacity. With co-channel spectrum usage, each radio tier is granted largest amount o available spectrum, whereas the cochannel requency reuse o two tiers imposes severe cross-tier intererences and consequently is considered the highest risk deployment. On the other hand, the crosstier intererence is lower with dedicated-channel spectrum usage, whereas the amount o available spectrum is reduced at each tier. According to Shannon ormula, both the higher cross-tier intererence (with co-channel spectrum usage) and the reduced amount o spectrum (with dedicated-channel usage) can become the capacitylimiting actor. To tackle the drawbacks o two traditional strategies o spectrum arrangement, we propose a novel strategy o spectrum arrangement, called hybrid spectrum arrangement, to take advantage o their merits. In our proposal, each HeNB can select a preerable spectrum usage mode by sel-coniguration. It is supposed that proper selection o spectrum usage mode can avoid whichever o the available bandwidth and the cross-tier intererence becoming the capacity limiting actor. Thereore, the selection o spectrum usage mode should properly trade o the amount o available spectrum and the crosstier intererence. In our proposal, a emtocell selects its spectrum usage mode according to a criterion such that both macrocell and emtocell can be beneited in terms o higher achieved capacity. Since the cross-tier intererence is largely dependent on the relative locations between HeNBs and enbs, the criterion or the decision making o a HeNB can be urther translated to its relevant spatial condition, i.e., the relative location o a HeNB to its embedding enb, where the embedding enb is the closest enb rom the HeNB. According to a spatial threshold, the emtocells embedded in a macrocell are dierentiated to inner and outer emtocells, which operate in the mode o dedicated-channel spectrum usage and the mode o co-channel spectrum usage, respectively. It turns out that the spectrum usage modes o emtocells are mixed within a macrocell. This is why we call the proposed strategy hybrid spectrum arrangement. When the strategy o hybrid spectrum arrangement is employed, the cross-tier intererences have their distinct characteristics comparing with that with two traditional strategies. For successul deployment o such a strategy in the two-tier LTE cellular network, we examine the severeness o our cross-tier intererence scenarios under hybrid spectrum arrangement and propose the intererence mitigation methods to alleviate the residual signiicant intererences among them. Finally, this article stresses that the strategy o hybrid spectrum arrangement along with the related intererence mitigation methods ormulate as an overall solution to improve the spectrum utilization or two-tier LTE cellular network. The main contributions o the present article can be summarized as ollows. First, we point out that two spectrum usage modes (i.e., co-channel spectrum usage and dedicatedchannel spectrum usage) have pros and cons rom the perspective o achievable network capacity ater analyzing their disparate eects on the amount o available spectrum and the cross-tier intererences. Then, we argue that the capability o emtocells to select their spectrum usage modes lexibly helps to improve overall spectrum utilization. Furthermore, we propose a practical criterion o spectrum usage selection or emtocells aiming to beneit both macrocell and emtocell rom the perspective o achieved capacities, and we discuss that the decision making o a emtocell on its spectrum usage mode can be translated to the relevant spatial condition the relative location o a HeNB to its embedding enb. Next, we present the novel strategy o spectrum arrangement, hybrid spectrum arrangement, as a natural consequence o above proposed criterion and the enabled selection capability o emtocells. In act, the emtocells embedded in macrocell can be sel-organized to inner and outer emtocells according to their spatial conditions, and they operate in dedicated-channel spectrum usage and co-channel spectrum usage, respectively. We give detailed analysis o two-tier LTE network to decide the spatial thresholds on downlink and uplink or the decision making o emtocells. Moreover, we discuss the distinct characteristics o cross-tier intererences under hybrid spectrum arrangement and propose mitigation methods to address two residual severe intererences the downlink intererence rom HeNB to MUE and the uplink intererence rom HUE to enb. Eventually, we ormulate an overall solution to improve spectrum utilization or two-tier LTE cellular network by

3 Bai and Chen EURASIP Journal on Wireless Communications and Networking 2013, 2013:56 Page 3 o 15 combining the proposed strategy o hybrid spectrum arrangement and the related intererence mitigation methods. This article extends our previous study [5,6]. In this article we clariy the criterion or selecting the spectrum usage mode and derive the spatial thresholds on downlink and uplink in more details, and we examine the impacts o hybrid spectrum arrangement on all our cross-tier intererence scenarios rather than two among them in [6]. Moreover, we ormulate an overall solution to improve spectrum utilization or two-tier LTE cellular network by combining the strategy o hybrid spectrum arrangement presented in [5] and the related intererence mitigation methods proposed in [6]. For perormance evaluation, we conduct more simulation experiments by using the system parameters that conorm with the latest 3GPP LTE speciications. The rest o this article is organized as ollows. Section 3 discusses the related study on spectrum arrangement and intererence mitigation or two-tier cellular network, and underlines the dierences o our study with the related studies. Section 3 describes how spectrum usage selection can help to improve spectrum utilization in two-tier LTE cellular network. Section 4 presents the strategy o hybrid spectrum arrangement and its implementation methods in two-tier LTE system. This section also gives detailed analysis to derive the spatial thresholds on downlink and uplink or the decision making o emtocells. Section 5 examines the severeness o our cross-tier intererence scenarios under hybrid spectrum arrangement and presents methods to mitigate two residual signiicant intererences. In Section 6, we give perormance evaluation o proposed methods via system-level simulation o two-tier LTE system. Section 7 concludes this article. 2 Related study In the literature, there has been some related study o spectrum management in the context o two-tier cellular network. Claussen [7] studied the easibility o co-channel operation between user-deployed emtocells and an existing macrocell network, and discussed key requirements or such operation such as auto-coniguration and public access. Chandrasekhar and Andrews [8] proposed an optimum decentralized spectrum allocation policy or twotier FDMA (including OFDMA) network. Oh et al. [9] proposed a requency planning or emtocells in cellular networks using ractional requency reuse (FFR). Partial co-channel spectrum arrangement between macro- and emto- cells is investigated in [10]. With the coniguration o partial co-channel spectrum arrangement as well, Lima et al. [11] investigated coordination mechanisms to opportunistically reuse resources without compromising ongoing transmissions on overlay macrocells. Güvenç et al. [12] presented a hybrid requency assignment or emtocells mainly aiming to maintain the emtocell s coverage on the user s premises. In order to cope with the cross-tier intererence in twotier cellular networks, intererence mitigation schemes by employing dynamic radio resource management (including resource partitioning, allocating, and scheduling), power control, handover, cognitive radio have been reported. Bharucha et al. [13] ocused on mitigating downlink emto-cell to macro-cell intererence through dynamic resource partitioning, in the way that HeNBs are denied access to downlink resources that are assigned to macro UEs in their vicinity. To avoid the strong intererence between a emtocell and close-by macrocellassociated mobile stations, Sahin et al. [14] proposes a method that jointly utilizes the spectrum sensing results as well as scheduling inormation obtained rom the macrocell BS. The authors in [15] proposed intererence mitigation strategies that adjusts the maximum transmit power o emtocell users by open-loop or closed-loop power control to suppress the cross-tier intererence at a macrocell BS. Shi et al. [16] analyzed the mechanism or generating the uplink intererence scenarios and provided guidelines or intererence mitigation in two-tier macro and emto co-existing UMTS networks. In [17], downlink emto-to-macro control channel intererence is analyzed and intererence reducing methods are proposed with co-channel emtocell deployment. A distributed emtocell power allocation scheme is proposed in [18] by exploiting limited channel inormation o neighboring macro MSs to eectively reduces cross-tier intererence on them. Ndong et al. [19] addressed the uplink cross-tier intererence problem and proposes a resilient solution to the near-ar eect issue by utilizing the interering macrocell inormation eedback through inrastructure network at the HeNB or interered signal recovery o the symbols sent by the HUE. A sel-organizing emtocell ramework which is composed o three complementary control loops or co-channel deployment is presented in [20]. López- Pérez et al. [21] proposed an intererence avoidance technique combining intracell handovers and power control in OFDMA two-tier macrocell-emtocell networks. Torregoza et al. [22] proposed a cognitive emtocell network architecture that incorporates cognitive radio and emtocells, and propose a joint power control, base station assignment, and channel assignment scheme or cognitive emtocell networks. Other relevant articles include [23-25]. Zhang et al and Yu et al [23,24] suggest intelligent spectrum allocation using cognitive radio or home networks and smart grid, and [25] studied a parallel spectrum sensing or balancing sensing perormance and eiciency in cognitive radio networks.

4 Bai and Chen EURASIP Journal on Wireless Communications and Networking 2013, 2013:56 Page 4 o 15 Here we underline the dierences o the contributions in this article with the related works. Regarding the spectrum arrangement in two-tier cellular network, there are some similar ideas in the related study [12] as our proposed hybrid spectrum arrangement. However, the strategy o hybrid spectrum arrangement is not ully explored in two-tier LTE system and there is a lack o complete solution with such an arrangement. In [12], the threshold distance that separate the inner and outer regions was investigated in the context o 3G wireless system and determined based on intererencelimited coverage area (ILCA) o a emtocell. In this article, the criterion to determine the inner and outer regions is based on the achievable throughputs o two LTE tiers instead o the coverage o emtocells. Furthermore, the intererence mitigation methods in the related works are mainly studied in the context o co-channel spectrum usage. For instance, the intererence mitigation methods presented in [16,19] were targeted or the uplink intererence rom a visiting MUE to near-by HeNB, which is not a signiicant intererence scenario with hybrid spectrum arrangement. Thus, instead o investigating all the possible cross-tier intererences, we examine the cross-tier intererence in the context o hybrid spectrum arrangement, identiy two signiicant intererence scenarios including the intererence rom HeNB to nearby MUEs and the intererence rom HUEs to the enb. Then, we propose the intererence mitigation methods targeted or the above two severe cross-tier intererences. As ar as we know, the proposed intererence mitigation methods or these two scenarios in this article have not been reported in the literature beore. Finally, this article gives a more complete solution or spectrum utilization in two-tier LTE cellular network by combining the strategy o hybrid spectrum arrangement and necessary intererence mitigation methods under such an arrangement. 3 Spectrum usage selection to improve spectrum utilization in two-tier LTE cellular network 3.1 Comparison o spectrum arrangement strategies The main purpose o studying the strategies o spectrum arrangement or two-tier LTE cellular network is to increase the overall spectrum utilization, which can be measured by area spectral eiciency (ASE). The ASE o a cellular system is deined as the achievable throughput per unit area or the bandwidth available. The ASE o combined macrocells and emtocells can be measured in terms o bit/s/hz/cell-site, where the cell-cite is the area o a macrocell site. The joint capacity o two-tier wireless network is given by C m + n i=1 C i where C m is the throughput achievable by a macrocell, C i is the throughput achievable by ith emtocell, and n is the total number o emtocells in a macrocell. Then the ASE o two-tier wireless network can be represented as η = C m + n i=1 C i πr 2 (1) w t where R is the radius o macrocell, and w t denotes the total amount o available spectrum or the two-tier wireless network. Furthermore, the achievable throughput o macrocell (emtocell) can be estimated by the Shannon ormula C = w log 2 (1 + γ) (2) where C is achievable throughput, w is the amount o spectrum available, and γ is the received signal-tointererence-plus-noise ratio (SINR). Note that (2) regards the intererence as white Gaussian noise and gives the worst-case estimate o achievable capacity. Assume that the total amount o licensed spectrum is w t Hz. With co-channel spectrum usage, each tier can use all w t Hz, i.e., w = w t ; With dedicated-channel spectrum usage, w t Hz spectrum is partitioned to two spectrum potions o w m Hz and w Hz (i.e., w m + w = w t ), where w m and w are the dedicated amounts o spectrum or macrocell and emtocell, respectively. Comparing two traditional strategies o spectrum arrangement, there are pros and cons o them rom the perspective o achievable network capacity. With co-channel spectrum usage, each radio tier is granted largest amount o available spectrum,w t, but the cochannel requency reuse in two tiers imposes severe cross-tier intererences and consequently is considered the highest risk deployment. In this case, the SINR value o γ is lower than that with dedicated-channel spectrum usage, and it can be deduced rom (2) that higher crosstier intererence tends to become the capacity-limiting actor in this case. On the other hand, the cross-tier intererence is lower with dedicated-channel spectrum usage, and the SINR is higher than that with co-channel spectrum usage. Whereas the amount o available spectrum is reduced at each tier as w m < w t and w < w t.itcanbe deduced rom (2) that the reduced amount o spectrum tends to become the capacity-limiting actor in this case. 3.2 Spectrum usage selection to improve spectrum utilization As we discussed, the cross-tier intererence impacts the overall network capacity. On generating the cross-tier intererence, macrocell and emtocell can play the role o either the interering party (aggressor) or the interered party (victim). As listed in Table 1, the cross-tier intererences can be classiied accordingly to our intererence scenarios: the intererence scenario I the uplink intererence rom HUE (aggressor) to enb (victim), the intererence scenario II the downlink intererence rom

5 Bai and Chen EURASIP Journal on Wireless Communications and Networking 2013, 2013:56 Page 5 o 15 Table 1 Cross-tier intererence scenarios in two-tier LTE network Number Aggressor Victim I HUE enb uplink II HeNB enb downlink III MUE HeNB uplink IV enb HeNB downlink HeNB (aggressor) to MUE (victim), the intererence scenario III the uplink intererence rom MUE (aggressor) to HeNB (victim), and the intererence scenario IV the downlink intererence rom enb (aggressor) to HUE (victim) [2]. Moreover, the cross-tier intererence experienced by a speciic emtocell is dierent rom one another. It is apparent that the severeness o cross-tier intererence is relevant to the spatial condition o emtocell the relative location between a HeNB and an enb. For instance, the relative location aects the intererence scenarios IV and III. The closer a HeNB locates to its embedding enb, the higher the cross-tier intererence in the intererence scenario IV and III. Since the emtocells located at dierent locations experience dierent cross-tier intererence, the achieved capacities o them are dierent as well. For the emtocells close to the embedding enb, the cross-tier intererence tends to be the capacity-limiting actor; whereas or the emtocells at the cell edge o a macrocell, the spectrum available tends to be the capacity-limiting actor. To increase overall spectrum utilization, it would be better o i each HeNB can select its operation mode rom two traditional spectrum usage modes to avoid whichever o the available bandwidth and the cross-tier intererence becoming the capacity-limiting actor. In other words, a HeNB should be able to operate in either the mode o co-channel spectrum usage or the mode o dedicatedchannel spectrum usage. The selection should be based on a proper criterion, which is presented in the ollowing section. 3.3 Criterion o selecting spectrum usage mode by emtocells To improve overall spectrum utilization, the major objective o usage mode selection is to maximize the ASE as represented in (1), which is equivalent to maximize joint two-tier capacity C m + n i=1 C i. In addition to maximizing the ASE, another aspect to determine the spectrum arrangement is the achievable throughput at each tier. The joint two-tier network capacity can be higher with a selected usage mode, whereas the achievable throughput o one tier (C m or C i ) may be too low to support enough services or mobile users. Thereore, each tier should provide a minimum prescribed throughput. From the above discussion, the criterion o spectrum arrangement or two-tier LTE cellular network needs to take into account both the joint two-tier capacity and the required capacity at each radio tier. Hence, instead o adopting the criterion to solely maximize the joint two-tier capacity, we adopt a criterion o selecting the spectrum usage mode as ollows: The co-channel spectrum usage is chosen only when both macrocell and emtocell preer it in terms o higher capacity; otherwise, dedicatedchannel spectrum usage is chosen. This criterion is a suboptimal with respect to the joint capacity o macrocell and emtocell in all circumstances, but it is practical one as it excludes the involvement o collecting relevant inormation or calculating joint two-tier capacity. Furthermore, higher achieved capacities at both macrocell tier and emtocell tier imply that the cross-tier intererence is not the dominating actor that limits the network capacity. 4 Hybrid spectrum arrangement or two-tier LTE cellular network 4.1 Hybrid spectrum arrangement and its implementation The two traditional spectrum usages result in disparate amounts o spectrum and cross-tier intererences. For selecting a proper spectrum usage mode, a emtocell needs to trade o the amount o available spectrum and the experienced cross-tier intererence caused by two spectrum usage modes. Assuming the value o w t with co-channel spectrum usage and the values o w m and w with dedicated-channel spectrum usage are known, we now examine the eects o cross-tier intererence on the emtocell s decision making. Since the cross-tier intererence is largely dependent on the spatial condition o a emtocell, the criterion o spectrum usage selection can be translated to its relevant spatial condition the location o HeNB to enb. According to their spatial conditions, the emtocells are distinguished to be inner and outer emtocells. The HeNBs o inner and outer emtocells operate in dedicated-channel spectrum usage and in co-channel spectrum usage, respectively. With above discussion, hybrid spectrum arrangement is a natural consequence when multiple emtocells within a macrocell select their spectrum usage mode by sel-coniguration. Denote the distance between a HeNB and its embedding enb as d, and denote the distance threshold to dierentiate inner and outer emtocells as d th. As shown in Figure 1, a emtocell is viewed as an inner emtocell when a HeNB is located at the position where d d th ;otherwise,itis viewed as an outer emtocell. Since inner and outer emtocells adopt dierent spectrum usage modes, the spectrum usage modes o emtocells within a macrocell become mixed. Hence, the proposed method or spectrum arrangement is called hybrid spectrum arrangement in this article. As shown

6 Bai and Chen EURASIP Journal on Wireless Communications and Networking 2013, 2013:56 Page 6 o 15 Figure 1 Dierentiation o inner and outer emtocells according to a spatial threshold in a LTE macrocell. in Figure 2, with hybrid spectrum arrangement, w t Hz spectrum is partitioned to two spectrum potions, w m Hz and w Hz; w m is assigned to macrocell, w is assigned to inner emtocells, and w t can be used by outer emtocells. In practical implementation, the parameter o d can be estimated at HeNB corresponding to the received pilot power o enb, i.e., reerence signal received power (RSRP) in LTE network. The RSRP can be estimated by the added unctionality at HeNB or can be estimated by HUE and sent to HeNB as a measurement report via an added radio resource control (RRC) signalling message. In this way d th corresponds to a pilot power threshold, p th. Denote the estimated value o RSRP as p r. A emtocell is viewed as an inner emtocell i p r p th ;otherwise,itisviewedasan outer emtocell. Using p th or decision making is regarded as a better approach rather than using d th because p r relects the actual link quality with pathloss and shadowing between a HeNB and its embedding enb. Assume the spectrum usage mode is allowed to be determined separately on LTE downlink and uplink, the values o d th or downlink and uplink can be set to dierent values. The procedure to determine the spectrum usage mode by a HeNB is shown in Figure 3. Firstly, a HeNB detects Figure 3 Procedure to determine the spectrum usage mode by HeNB. the received RSRPs o neighboring enbs. The highest detected value o RSRP o an enb (p max r )isthoughtto be received rom the embedding enb with respect to the HeNB. Then, the HeNB compares p max r with p th,and determines the adopted spectrum usage mode based on the comparison result. 4.2 Downlink and uplink analysis to determine the spatial thresholds Next, we clariy how to determine the values o spatial threshold d th on downlink and uplink or selecting the spectrum usage mode. In the analysis, we consider the case o n = 1, i.e., only one emtocell exists within a macrocell. For the case o n > 1, i.e., multiple emtocells exist within a macrocell, we evaluate the perormance by system-level simulation Downlink analysis First, we evaluate the downlink throughput o macrocell. According to Shannon ormula, the downlink throughput achieved by macrocell depends on the bandwidth and SINR. The downlink throughput o macrocell with co-channel spectrum usage, Cm dl,co-channel, and that with dedicated-channel spectrum usage, Cm dl,dedicated,canbe estimated by and C dl,co-channel m = w t log 2 (1 + γ dl,co-channel m ) (3) Figure 2 Hybrid spectrum arrangement or LTE macrocell and emtocells. C dl,dedicated m = w m log 2 (1 + γ dl,dedicated m ) (4) where γm dl,co-channel and γm dl,dedicated denote the downlink SINRs o MUEs with co-channel spectrum usage and dedicated-channel spectrum usage, respectively. Note that the downlink throughput o macrocell should be estimated as the sum o achievable throughput o assigned

7 Bai and Chen EURASIP Journal on Wireless Communications and Networking 2013, 2013:56 Page 7 o 15 resource blocks (RBs) o MUEs. The sum computation is omitted here and hereater or terminology simplicity. ThedownlinkSINRoMUE,γm dl, can be evaluated by where s m Imm dl Im dl p t,m γm dl = s m Imm+I dl m dl +N p = t,m x α 1 (5) 0 nm 1 i=1 p t,m x α 1 i +p t, l α β ρ +wn 0 received downlink signal power at MUE downlink macrocell-to-macrocell intererence downlink emtocell-to-macrocell intererence N background noise power transmit power o enb x 0 distance between severing enb and MUE α 1 path loss exponent at macrocell n m number o enbs x i distance between MUE and ith enb p t, transmit power o HeNB l distance between MUE and HeNB α 2 path loss exponent at emtocell β penetration loss in db rom outdoor to indoor ρ adjacent channel intererence ratio (ACIR) in db N 0 power spectral density o background noise w w = w t with co-channel spectrum usage; w = w m with dedicated-channel spectrum usage. Here the downlink intererence imposed on MUEs includes both intra-tier intererence rom neighboring macrocells, Imm dl, and cross-tier intererence rom emtocells, Im dl. The cross-tier intererence with the dedicatedchannel spectrum usage is assumed to be adjacent channel intererence, which is reduced by adjacent channel intererence ratio (ACIR, typically over 40 db), compared to the co-channel intererence with co-channel spectrum usage. Since macrocell downlink is an intererence-limited system, we have Im dl N. Due to the act that the transmit power o HeNB is much less than that o enb (p t, p t,m ) and additional wall penetration loss β (typically db) on p t,, Im dl Idl mm.thus,γ m sd m,and Im d γm dl,co-channel γm dl,dedicated.withw t > w m,wehave Cm dl,co-channel > Cm dl,dedicated by comparing (3) and (4). The interpretation o the analysis result is as ollows. Due to the act that the transmit power o HeNB is much less than that o enb and the penetration loss o walls in emtocell, the co-tier intererence rom other macrocells is the dominant downlink intererence or outdoor MUEs rather than the cross-tier intererence rom emtocells. Thus, larger bandwidth with the co-channel spectrum usage helps to increase the capacity o macrocells rather than the dedicated-channel spectrum usage. Hence, macrocell preers the co-channel spectrum usage on downlink to maximize its capacity or the MUEs on average. Thereore, to satisy the our proposed criterion or usage selection, the decision o selection on downlink depends on comparing the achieved throughputs o emtocell under two spectrum usage modes. Next, we evaluate the downlink throughput o emtocell. The downlink throughput o emtocell with co-channel spectrum usage, C dl,co-channel, and that with dedicated-, can be estimated by channel spectrum usage, C dl,dedicated and C dl,co-channel C dl,dedicated = w t log 2 (1 + γ dl,co-channel ) (6) = w log 2 (1 + γ dl,dedicated ) (7) where γ dl,co-channel and γ dl,dedicated denote the downlink SINRs o HUEs with co-channel spectrum usage and dedicated-channel spectrum usage, respectively. ThedownlinkSINRoHUE,γ dl, can be evaluated by where s Im dl γ = s I dl m +N = p t, r α 2 (8) (p t,m d α 1 + nm 1 i=1 p t,m y α 1 i )10 10 β ρ +wn 0 received signal power at HUE downlink macrocell-to-emtocell intererence r distance between HUE and HeNB d distance between HeNB and enb y i distance between HUE and ith enb w w = w t with co-channel spectrum usage; w = w with dedicated-channel spectrum usage. Here only cross-tier intererence imposed on HUEs (Im dl ) is considered since no co-tier intererence between emtocells exists when n = 1. It can be observed rom (8) that γ dl depends on d, which is the distance between a HeNB and its embedding enb. The closer a emtocell is located rom enb, the higher the downlink macrocell intererence the HUEs suer. When a emtocell is located very close to enb, the downlink intererence dominates rather than noise, i.e., Im dl N. DuetolargeACIR,Im dl with dedicatedchannel spectrum usage is much less than that with co-channel spectrum usage. Hence, the downlink throughput o a emtocell with dedicated-channel spectrum usage is larger than that with co-channel spectrum usage (i.e., C dl,dedicated > C dl,co-channel )whenhenbis

8 Bai and Chen EURASIP Journal on Wireless Communications and Networking 2013, 2013:56 Page 8 o 15 located very close to enb. Thus, the emtocell preers dedicated-channel spectrum usage at a close distance to enb. With the increase o d, the intererence rom macrocell (Im dl ) reduces with co-channel spectrum usage due to increased pathloss, and hence the achieved capacity o emtocell (C dl,co-channel ) increases. When the emtocell is located at a certain distance away rom enb such that the intererence rom macrocell (Im dl )reducessigniicantly, bandwidth w becomes the dominant actor on determining the achievable throughput instead o SINR (γ ). Consequently, C dl,co-channel >C dl,dedicated when a HeNB locates at a certain distance away rom enb. Thus, a emtocell preers co-channel spectrum usage when it locates at a certain distance away rom enb. Thedistancethreshold,dth dl, can be determined under the condition that the achieved downlink throughput o emtocell with co-channel spectrum usage is the same as that with dedicated-channel spectrum usage, i.e., d = dth dl i C dl,co-channel (d) = C dl,dedicated (d). For the case o n > 1, our simulation result shows that the macrocell stills preers co-channel spectrum usage with reasonable density o emtocells deployed in the macrocell. On the other hand, the emtocells can still keep the decision as n = 1 and resort to intererence mitigation method to alleviate the residual intererences Uplink analysis On uplink, we irst evaluate the achievable uplink throughput o emtocell. The uplink SINR o HeNB, γ ul, can be evaluated by γ ul = s ul Im ul ˆp t, ˆp t,m n m u i s ul I ul m + N = ˆp t, r α2 nm i=1 ˆp t,m u α 1 i 10 β ρ 10 + wn 0 (9) received signal power at HeNB uplink macrocell-to-emtocell intererence transmit power o HUE transmit power o MUE number o MUEs in macrocells distance between HeNB and ith MUE. I no active MUE is near the HeNB, the emtocell uplink is largely a noise-limited system. Hence, Im ul 0, and γ sul N. Considering the throughput o emtocell on average, C ul,co-channel > C ul,dedicated. Thus, the emtocell preers co-channel spectrum usage to maximize its capacity on uplink. Thereore, to satisy the criterion or usage selection, the spectrum usage mode or uplink depends on the choice o macrocell. Next, we evaluate the achievable uplink throughput o macrocell. The uplink SINR o macrocell, γm ul,canbe evaluated by γ ul m = s ul m I ul mm + Iul m + N ˆp t,m ˆx α 1 i = nmo 1 i=1 ˆp t,m v α 1 i + n h 1 i=1 ˆp t, d α 1 i 10 β ρ 10 + wn 0 (10) where n mo v i n h d i number o MUEs outside o the interested enb distance between ith MUE and the interested enb number o HUEs in emtocell distance between ith HUE and the interested enb. As γm ul is a unction o d, the closer the emtocell is located to the enb, the higher uplink intererence the macrocell suers. The achievable uplink throughput o macrocell with co-channel spectrum usage is less than that with dedicated-channel spectrum usage (i.e., Cm ul,co-channel < Cm ul,dedicated ) when a HeNB is located close to the embedding enb. However, the uplink throughput in co-channel spectrum usage becomes larger than that in dedicated spectrum usage (i.e., Cm ul,co-channel > Cm ul,dedicated )when the HeNB is located at certain distance away rom the embedding enb. The speciic distance that the uplink throughputs are equal with two spectrum usage modes can be set to be the uplink distance threshold, dth ul, i.e., d = dth ul i Cul,co-channel m (d) = Cm ul,dedicated (d) to determine the uplink spectrum usage mode. When the HeNB is located arther than dth ul,theem- tocell is viewed as an outer emtocell and operates in co-channel spectrum usage; otherwise, it operates in dedicated-channel spectrum usage. For the case o n > 1, the aggregate intererence on the macrocell uplink rom both MUEs in other macrocells and HUEs needs to be considered. The intererence level can be measured by intererence over thermal (IoT), which is deined as the ratio o the intererence plus thermal noise to thermal noise. The uplink aggregate IoT o macrocell, IoT ul m,isevaluatedby ( I ul IoT ul m = 10 log mm + Iul m + N ) 10 db. (11) N The maximal allowed aggregate intererence in macrocell uplink can be set as the total IoT threshold, IoT th tot.i IoT th tot is per-deined, the allowed IoT introduced by emtocells, IoT th, can be calculated ater taking out the current aggregate IoT at macrocell, IoT cur m,romiotth tot.thatis ( ) IoT th = 10 log IoTth tot IoTcur m + 1 db. (12) 10

9 Bai and Chen EURASIP Journal on Wireless Communications and Networking 2013, 2013:56 Page 9 o 15 IoT th can be used or deciding whether additional emtocell (when n > 1) can be allowed to operate in the co-channel spectrum usage. In LTE network, a UE may employ uplink ractional power control depending on its pathloss to the connected BS such that it does not always transmit with its ull power. Hence, ˆp t, varies according to a HUE s distance to HeNB, and ˆp t,m varies according to a MUE s distance to enb. In the next section, we discuss that the uplink cross-tier intererence rom HUEs o emtocells to the enbs (i.e., the intererence scenario I) can be alleviated with proper system parameter tuning under ractional power control. In the above analysis and the ollowing simulation to determine the spatial thresholds, we assume the macrocells are ully loaded, i.e., all their RBs are allocated to support the traics o MUEs. This assumption leads to a conservative decision on the value o spatial threshold, which hence gives the priority to guarantee the service provisioning o macrocells. When considering the temporal varying nature o the resource usage at macrocells, the thresholds or the spectrum usage mode selection can be made to be time variant accordingly. Hence, the emtocells can select its spectrum usage mode with respect to both temporal and spatial conditions. To implement such an approach in LTE networks, the HeNB requires more advanced hardware to detect timely resource usage o macrocell or needs message exchange between enb and HeNB. Due to the highly dynamic nature o the resource usage o macrocell, it is hard or the HeNB to obtain timely and precise inormation o resource usage at macrocell. Frequent changing operation mode o emtocells also interrupts the ongoing services provided to the HUEs. A compromised approach might be more easible: the selection o usage mode by emtocells is updated based on longer time observation o the resource usage at macrocell so that it does not change very requently; or instance, the change o selection mode occurs only at some time between daytime and nighttime. 5 Intererence mitigation with hybrid spectrum arrangement With our proposed hybrid spectrum arrangement, the our intererence scenarios have their distinct characteristics when comparing with pure co-channel spectrum usage and pure dedicated-channel spectrum usage. Next, we examine the severeness o our intererence scenarios, identiy the residual signiicant intererence scenarios, and propose the intererence mitigation methods or them. 5.1 Discussion on intererence scenario IV First, we give a discussion on the intererence scenario IV the downlink intererence rom enb to HUEs. With our proposed hybrid spectrum arrangement, a HeNB operates in the dedicated-channel spectrum usage when it is located less than dth dl ; otherwise, it operates in the co-channel spectrum usage. Hence, the HUEs in the inner emtocells do not experience much downlink intererence rom enb. Though the HUEs in the outer emtocells suer downlink intererence rom enbs, the gain o larger bandwidth in outer emtocells surpasses the drawback o intererence such that outer emtocells can still achieve higher downlink capacity with co-channel spectrum usage. Thus, the intererence scenario IV has been mitigated with hybrid spectrum arrangement. 5.2 Discussion on intererence scenario III With hybrid spectrum arrangement, an enb operates in dedicated-channel spectrum usage with w m Hz when an inner emtocell exists. The spectrum used by the outer emtocells (w t ) can be distinguished to two portions, MUE-absent portion (w ) and MUE-possible portion (w m ). The MUE-absent portion can only be assigned to HUEs by HeNB; it can not be assigned to MUEs by enb. On the other hand, the MUE-possible portion used by outer emtocells is overlapped with macrocell and the resource in this portion can be assigned to MUEs by enb. Thereore, a HeNB always have MUE-absent portion (w ) as clean resource to be assigned to its HUEs. When an MUE associated with an enb is near a CSG HeNB, the HeNBcanperormRRMtoreallocatetheresourceused by HUEs to MUE-absent portion. In this way the intererence scenario III (the uplink intererence rom MUE to HeNB) can be easily mitigated with hybrid spectrum arrangement. 5.3 Intererence mitigation method or intererence scenario II The intererence scenario II reers to the intererence rom downlink HeNB to MUE. With hybrid spectrum arrangement, dierent portions o spectrum are dedicated to macrocells and inner emtocells. Thus, an MUE does not suer much downlink intererence rom a HeNB when it lies within the coverage an inner emtocell, regardless o whether the emtocell operates in the closed-access or open-access manner. Whereas when a MUE lies within the coverage o a closed-access outer emtocell and its ID is not in the authorized list o the CSG emtocell, the MUE suers high downlink intererence rom HeNB because the spectrum used by the outer emtocell is overlapped with that used by MUEs. The problem is severer when an MUE lies within the indoor coverage o outer emtocell due to the near-ar eects. In this case, an indoor MUE at the macro cell edge receives highly attenuated signal rom the enb with wall penetration loss, but receives excessive intererence signal originating rom a HeNB directly without such penetration loss. Consequently, dead zone exists or an MUE when it lies too close to an indoor HeNB: a connection can not be established with enb or

10 Bai and Chen EURASIP Journal on Wireless Communications and Networking 2013, 2013:56 Page 10 o 15 an ongoing connection between an MUE and its serving enb can be interrupted. To give priority o QoS assurance or MUEs, the severe intererence scenario II should be nulliied. To ulil this task, both the serving enb and the interering HeNB can perorm radio resource management (RRM) operations (e.g., smart resource scheduling) or the interered MUE. The serving enb can resort to channel-dependent scheduling or the interered MUE. However, i all the RBs (Resource Blocks) in the MUE-possible portion (w m )are used by the serving enb, no clean RBs are available at the serving enb to be reallocated or the interered MUE. On the other hand, the interering HeNB is not aware o the existence o the near-by interered MUEs and it can not perorm RRM operations even i not all the available RBs are used. In this article, it is proposed that a HeNB can be notiied that it is interering a MUE and then reallocates its resource to mitigate the downlink intererence imposed on the MUE. In the ollowing, we describe two schemes to ulil the task o notiication o near-by MUE to the interering HeNB. The irst scheme is implemented over wired networks, and the other scheme is implemented over the air Notiication o interered MUE to HeNB over wired networks The irst method to notiy a HeNB that it intereres a near-by MUE is implemented over wired networks. The procedure is shown in Figure 4. When the downlink perormance o MUE degrades by detecting the increase o packet loss rate and packet delay, the MUE checks its experienced downlink IoT. I the change o downlink IoT is larger than a threshold, the MUE attributes the observed intererence to the existence o near-by HeNB ater double checks: comparing the estimated RSRP o HeNB with a pre-deined threshold and conirming that HeNB s physical cell identiier (PCI) is within csg-physcellidrange. I multiple HeNBs are detected by the interered MUE, the HeNB with the highest estimated RSRP is viewed as the interering HeNB. Ater detecting the interering HeNB, the MUE sends the serving enb a measurement report containing the HeNB s PCI inormation. When PCI conusion occurs or the interering HeNB, the serving enb asks the MUE to urther read the E-UTRAN cell global identiier (ECGI) o the interering HeNB. Ater that, the serving enb sends the interering HeNB a message, named as MUE ARRIVAL, to indicate that an MUE is near-by rom it and being interered. The message o MUE ARRIVAL is routed via mobility management entity (MME) and sent to the interering HeNB over S1 Interace. When a HeNB receives a message o MUE ARRIVAL, it is aware that a near-by MUE is being interered and then it perorms RRM operations to release resource or the interered MUE. The RRM operations can be conducted in the ollowing steps. First, it can reallocate some o the RBs used by HUEs to the MUE-absent portion; second, it can reduce the RBs that are in the MUE-possible portion used or non-real-time traics by HUEs; third, it can reduce the RBs that are in the MUE-possible portion used or real-time traics by HUEs i the QoS o HUEs is over qualiied or can be downgraded. When the MUE changes to IDLE mode rom ACTIVE mode, or the estimated RSRP o the interering HeNB is less than a pre-deined threshold, the MUE sends another measurement report to its serving enb, and then the serving enb sends the interering HeNB a message, named as MUE DEPARTURE, to indicate that the interered MUE is not suering the intererence rom it anymore. When a HeNB receives a message o MUE DEPARTURE, it is aware that an interered MUE moves away and it can use all the available spectrum or its HUEs. The approach o notiying interered MUE to HeNB over wired networks does not require additional hardware o HeNBs, but has a larger delay than the other approach, which we present next Notiication o interered MUE to HeNB over the air The other scheme to notiy a HeNB that it intereres a near-by MUE is implemented over the air. The procedure is shown in Figure 5. In this scheme, some subcarriers o a RB in the MUE-absent spectrum portion (w )isreserved or notiying HeNB that an MUE is suering its downlink intererence. The HeNBs o outer emtocells do not allocate this RB to HUEs or the data transmission; the HeNBs o inner emtocells can still use the reserved RB. When a MUE is interered by a HeNB, the MUE sends a signal, as MUE ARRIVAL, on a prescribed subcarrier o the dedicated RB. When the MUE changes to IDLE mode rom ACTIVE mode, or the estimated RSRP o the interering HeNB is less than a pre-deined threshold, the MUE sends another signal, as MUE DEPARTURE, on another prescribed subcarrier o the dedicated RB. To detect the notiication signals, the HeNB perorms energy detection on the reserved RB. Instead o using message exchange between MUE and HeNB, this scheme can be easily implemented without the requirement o strict synchronization between the MUE and the interering HeNB. The delay o notiication over the air is the air propagation delay rom an MUE to HeNB which is negligibly small, but this approach has resource overhead since it needs to reserve a RB or accomplishing the notiication task. 5.4 Intererence mitigation method or intererence scenario I The intererence scenario I reers to the intererence rom uplink HUEs to enb. When there are multiple HeNBs, the

11 Bai and Chen EURASIP Journal on Wireless Communications and Networking 2013, 2013:56 Page 11 o 15 Figure 4 Notiication o interered MUE to HeNB over wired networks. impact o the cross-tier intererence rom HUEs on enb uplink perormance can not be neglected with ractional uplink power control. Uplink ractional power control lets a UE transmit at a power level depending on its pathloss to BS. The ormula or ractional uplink power control is P = min{p max, P log 10 M + αpl} where P max is UE s maximum transmit power, P 0 is cell/ue speciic value, M isthenumberoassignedrbs,pl is the pathloss rom UE to enb/henb, and α is pathloss compensation actor [27]. In our simulation evaluated in Section 6, P 0 is irst set to 60 dbm, and α is set to 0.6. It is seen that the uplink throughput o enb reduces with the increase o number o HeNBs. However, i P 0 is set to 75 dbm or the HUEs, the imposed intererence on enb uplink is reduced signiicantly while the HUEs can still maintain their perormance. The simulation result shows that the achieved throughput o enb does not vary much with the increase o number o HeNBs when P 0 is set to 75 dbm or HUEs. Thus, dierent values o P 0 or macrocell and emtocell are recommended to be used in the practical system as a means to alleviate the cross-tier intererence scenario I. 6 Perormance evaluation by system-level simulation The simulation parameters are given in Table 2, which are in line with 3GPP LTE speciications [2,28,29]. In our simulation, the overlaying LTE network consists o macrocells and emtocells. The simulated network layout assumes a hexagonal grid with 19 macrocell BSs (enbs) and 3 sectors per enb with a center-excited structure. The central macrocell is a reerence cell-site when collecting perormance results; other 18 macrocells act two rings o intererers to the central macrocell. The cell radius o macrocells is 500/ m, and the emtocells are assumed to have a circular coverage with radius 10 m. The horizontal antenna pattern o macrocell is set to [ 12 ( ) 2 be A(ϕ) = min ϕ ϕ 3dB, Am ],whereϕ 3dB = 70 degrees, A m = 25 db; the antenna pattern o emtocell is set to be omnidirectional, i.e., A(ϕ) = 0dB. For the case o n = 1, there is only one HeNB that is located in the central macrocell with distance, d, to the central enb. For the case o n > 1, there are n HeNBs that are located randomly in each macrocell o all 19 macrocells. The number o active MUEs is 10 per sector in macrocell and the number o active HUEs is 4 per emtocell. The active UEs, whose number is decided in the initialization phase and kept constant or the whole simulation time, are uniormly distributed over the network area. A single transmit antenna at the UE and two receive antennas at the BS are used with maximal ratio combining (MRC). With co-channel spectrum usage, the available bandwidth or macrocell and emtocell are both 20 MHz. With dedicated-channel spectrum usage, the available bandwidth or macrocell and emtocell is