**1. Introduction**

56 Energy Efficiency – The Innovative Ways for Smart Energy, the Future Towards Modern Utilities

Comfort. In: Energy and Buildings, Munique, v. 34, n. 6, p. 661-665.

de uso Público e Coletivo. Brasília.

Performance of Tall Buildings. Earthscan, London.

[28] ABNT, Associação Brasileira de Normas Técnicas (1980). NBR 6401: instalações centrais de ar-condicionado para conforto: parâmetros básicos de projeto. Rio de Janeiro. [29] Agência Nacional de Vigilância Sanitária (2003). Orientação Técnica sobre Padrões Referenciais de Qualidade do Ar Interior, em Ambientes Climatizados Artificialmente

[30] Gonçalves, Joana Carla Soares; Umakoshi, Erica Metie (2010). The Environmental

[31] Hoppe, Peter (2002). Different Aspects of Assessing Indoor and Outdoor Thermal

The demand for higher data rates and improved energy-efficiency have motivated the deployment of short-range, low-cost, consumer-deployed cellular access points, referred to as femtocells [1]. Femtocells are consumer-deployed cellular access points, which interconnect standard user equipment (UE) to the mobile operator network via the end user's broadband access backhaul. Although femtocells typically support up to a few users, e.g., up to four users [2], they embody the functionality of a regular base station which operates in the mobile operator's licensed band. From the mobile operator perspective, the deployment of femtocells reduces the capital and operational costs, i.e., femtocells are deployed and managed by the end user, improves the licensed spectrum spatial reuse, and decongests nearby macrocell base stations. On the other hand, the end users perceive enhanced indoor coverage, improved Quality of Service (QoS), and significant User Equipment (UE) energy savings.

The deployment of femtocells is one of the most promising energy efficiency enablers for future networks [3-5, 23]. The study in [3] indicates that compared to a standard macrocell deployment, femtocell deployments may reduce the energy consumption on both the access network and the mobile terminals from four to eight orders of magnitude. Analogous results are derived in terms of system capacity per energy unit, although the performance degradation due to increased RF interference between the macro – femto and the femto – femto systems is not investigated. The latter effect is incorporated in [4], where it is shown that in-band macro – femto coexistence results in non-negligible performance degradation on the macrocell network layer. Nevertheless, improved QoS and significantly reduced energy consumption per bit are simultaneously achieved in the UE, with respect to the femtocell deployment density. To further reduce the energy consumption on the femtocell access point (FAP), the authors in [5]

© 2012 Xenakis et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

propose an idle mode procedure according to which the pilot transmissions are disabled in the absence of nearby cellular user activity. Compared to static pilot transmission, the proposed procedure is shown to significantly reduce the overall signaling overhead due to mobility.

Energy Efficient Mobility Management for the Macrocell – Femtocell LTE Network 59

HeNB HeNB

HeNB GW

S1

S1

E-UTRAN

The eNBs interconnect with each other through the X2 interface, while they connect to the EPC through the S1 interface [3]. The same implies for the connection between the HeNBs and the EPC, whereas different from the eNB case, the X2 interface between HeNBs is not supported.

S1

HeNB

In a cellular environment, MM typically consists of three phases [8] a) serving cell monitoring and evaluation, b) cell search and measurement reporting, and c) mobility decision/execution. The serving cell quality is monitored and evaluated on a periodic basis to sustain the service quality over an acceptable threshold. If the service quality falls below a policy-defined threshold, e.g. received signal strength or energy consumption, cell search and measurement reporting is triggered. The cell search and measuring procedure (which bands to sense, in what order, what measurement period and sampling rate to adopt, etc) can be either networkconfigured or user equipment (UE) based depending on the radio interface standard, the current UE state (e.g. idle or connected), the UE capabilities, and so on. In the former approach, the serving cell exploits its awareness on the surrounding cellular environment to configure the UE to derive and report back signal quality measurements on a predefined set of frequency bands or cells, e.g. provides the UE with a neighbor cell list (NCL) [8]. On the contrary, the UEbased approach is built on the UE capability to autonomously determine when and where to search for neighbor cells without any network intervention. In both cases, a handover (HO) decision entity incorporates the derived signal quality measurements to decide on whether the UE should move to another cell. This entity can reside either on the network (networkcontrolled approach) or the UE side (mobile-controlled approach) while the decision criteria can incorporate various performance measures such as a) signal quality measures, e.g. received signal strength and SINR, b) user mobility measures, e.g. speed, direction, and c) energy consumption at the UE side, e.g. Joule or Joule/bit. The mobility procedure where the user has no active connections (idle mode) is referred to as cell selection if the user is not camped on a cell or as cell reselection if the user is already camped on a cell. On the other hand, cell HO refers to the mobility procedure performed to seamlessly transfer ongoing user

Fig. 2 illustrates the overall LTE network architecture in the presence of HeNBs.

eNB

S1

S1

S1

S1

S1

X2

MME / S-GW MME / S-GW

eNB

**Figure 2.** Support of femtocells in the LTE network architecture

connections from the serving to the target cell (connected mode).

X2

eNB

X2

S1

S1

**Figure 1.** E-UTRAN HeNB Logical Architecture [6]

The Release 9 series of standards for the 3rd Generation Partnership Project (3GPP) the Long Term Evolution (LTE) system [6] is one of the first standards to provision the deployment of femtocells. In the context of LTE, a macrocell is referred to as evolved Node B (eNB), while a femtocell is referred to as Home eNB (HeNB). An LTE user is member of a Closed Subscriber Group (CSG) either if it is permitted to utilize a particular set of closed access femtocells or if it receives prioritized service on a particular set of hybrid access femtocells [7]. The standard describes the cell identification and access control procedures in the presence of LTE femtocells, along with the mobility management procedure for CSG femtocells. Fig. 1 depicts the logical architecture to support femtocells in the LTE system.

As shown in Fig. 2, two of the evolved packet core (EPC) network entities are directly involved in the support of HeNBs, i.e., the Mobility Management Entity (MME) and the Serving Gateway (S-GW). The MME implements the functions of core network (CN) signaling for MM support between 3GPP access networks, idle state mobility handling (e.g. paging), tracking area list management, roaming, bearer control, security, and authentication. On the other hand, the S-GW hosts the functions of lawful interception, charging, accounting, packet routing and forwarding, as well as mobility anchoring for intra and inter-3GPP MM. In the presence of femtocells, the evolved UMTS terrestrial radio access (E-UTRA) air interface architecture consists of eNBs, HeNBs, and HeNB gateways (HeNB GW). The eNBs provide user and control plane protocol terminations towards the UE, and support the functions of radio resource management, admission control, scheduling and transmission of paging messages and broadcast information, measurement configuration for mobility and scheduling, as well as routing of user plane data towards the S-GW. The functions supported by the HeNBs are the same as those supported by the eNBs, while the same implies for the procedures run between the HeNBs and the EPC. The HeNB GW acts as a concentrator for the control plane aiming to support of a large number of HeNBs in a scalable manner. The deployment of HeNB GW is optional; however, if present, it appears to the HeNBs as an MME and to the EPC as an eNB. The eNBs interconnect with each other through the X2 interface, while they connect to the EPC through the S1 interface [3]. The same implies for the connection between the HeNBs and the EPC, whereas different from the eNB case, the X2 interface between HeNBs is not supported. Fig. 2 illustrates the overall LTE network architecture in the presence of HeNBs.

**Figure 2.** Support of femtocells in the LTE network architecture

58 Energy Efficiency – The Innovative Ways for Smart Energy, the Future Towards Modern Utilities

**Figure 1.** E-UTRAN HeNB Logical Architecture [6]

propose an idle mode procedure according to which the pilot transmissions are disabled in the absence of nearby cellular user activity. Compared to static pilot transmission, the proposed procedure is shown to significantly reduce the overall signaling overhead due to mobility.

The Release 9 series of standards for the 3rd Generation Partnership Project (3GPP) the Long Term Evolution (LTE) system [6] is one of the first standards to provision the deployment of femtocells. In the context of LTE, a macrocell is referred to as evolved Node B (eNB), while a femtocell is referred to as Home eNB (HeNB). An LTE user is member of a Closed Subscriber Group (CSG) either if it is permitted to utilize a particular set of closed access femtocells or if it receives prioritized service on a particular set of hybrid access femtocells [7]. The standard describes the cell identification and access control procedures in the presence of LTE femtocells, along with the mobility management procedure for CSG femtocells. Fig. 1 depicts the logical architecture to support femtocells in the LTE system.

As shown in Fig. 2, two of the evolved packet core (EPC) network entities are directly involved in the support of HeNBs, i.e., the Mobility Management Entity (MME) and the Serving Gateway (S-GW). The MME implements the functions of core network (CN) signaling for MM support between 3GPP access networks, idle state mobility handling (e.g. paging), tracking area list management, roaming, bearer control, security, and authentication. On the other hand, the S-GW hosts the functions of lawful interception, charging, accounting, packet routing and forwarding, as well as mobility anchoring for intra and inter-3GPP MM. In the presence of femtocells, the evolved UMTS terrestrial radio access (E-UTRA) air interface architecture consists of eNBs, HeNBs, and HeNB gateways (HeNB GW). The eNBs provide user and control plane protocol terminations towards the UE, and support the functions of radio resource management, admission control, scheduling and transmission of paging messages and broadcast information, measurement configuration for mobility and scheduling, as well as routing of user plane data towards the S-GW. The functions supported by the HeNBs are the same as those supported by the eNBs, while the same implies for the procedures run between the HeNBs and the EPC. The HeNB GW acts as a concentrator for the control plane aiming to support of a large number of HeNBs in a scalable manner. The deployment of HeNB GW is optional; however, if present, it appears to the HeNBs as an MME and to the EPC as an eNB. In a cellular environment, MM typically consists of three phases [8] a) serving cell monitoring and evaluation, b) cell search and measurement reporting, and c) mobility decision/execution. The serving cell quality is monitored and evaluated on a periodic basis to sustain the service quality over an acceptable threshold. If the service quality falls below a policy-defined threshold, e.g. received signal strength or energy consumption, cell search and measurement reporting is triggered. The cell search and measuring procedure (which bands to sense, in what order, what measurement period and sampling rate to adopt, etc) can be either networkconfigured or user equipment (UE) based depending on the radio interface standard, the current UE state (e.g. idle or connected), the UE capabilities, and so on. In the former approach, the serving cell exploits its awareness on the surrounding cellular environment to configure the UE to derive and report back signal quality measurements on a predefined set of frequency bands or cells, e.g. provides the UE with a neighbor cell list (NCL) [8]. On the contrary, the UEbased approach is built on the UE capability to autonomously determine when and where to search for neighbor cells without any network intervention. In both cases, a handover (HO) decision entity incorporates the derived signal quality measurements to decide on whether the UE should move to another cell. This entity can reside either on the network (networkcontrolled approach) or the UE side (mobile-controlled approach) while the decision criteria can incorporate various performance measures such as a) signal quality measures, e.g. received signal strength and SINR, b) user mobility measures, e.g. speed, direction, and c) energy consumption at the UE side, e.g. Joule or Joule/bit. The mobility procedure where the user has no active connections (idle mode) is referred to as cell selection if the user is not camped on a cell or as cell reselection if the user is already camped on a cell. On the other hand, cell HO refers to the mobility procedure performed to seamlessly transfer ongoing user connections from the serving to the target cell (connected mode).

MM in the macrocell – femtocell network comprises many technical challenges in all three phases. Given the femtocell sensitiveness on user mobility and ambient radio frequency (RF) interference, serving cell monitoring and evaluation should be performed in a more frequent basis to sustain an acceptable service quality when connected to a femtocell. Considering the relatively small number of physical cell identifiers in prominent radio air-interfaces, more complicated yet backwards compatible cell identification procedures are required to facilitate cell searching and identification. Furthermore, maintaining and broadcasting a comprehensive Neighbor Cell List (NCL) to facilitate cell search and measurement reporting is not scalable in an integrated femtocell – macrocell network. To this end, novel UE-based cell search procedures are required to fully exploit the underlying femtocell infrastructure. The effectiveness of these procedures will have a great impact on the UE energy autonomicity and perceived QoS as explained in the following.

Energy Efficient Mobility Management for the Macrocell – Femtocell LTE Network 61

mobility criteria. Emphasis is given in reducing the number of the network-wide HO execution events, owing to the short femtocell radius and the ping-pong effect [9]. Nevertheless, the strongest cell HO decision policy [8] is considered for both macro-macro and femto-femto HO scenarios. According to it, the serving cell proceeds to a HO execution whenever the Reference Signal Received Power (RSRP) [6] of a neighbor cell exceeds over the respective RSRP status of the serving cell plus a policy-defined HHM, for a policydefined time period namely the Time To Trigger (TTT). The HHM is typically introduced to mitigate UE measurement inconsistencies, encompass frequency-related propagation divergences and minimize the ping-pong effect [9], i.e. consecutive HOs originating from the user movement across the cell boundaries. If comparable downlink Reference Signal (RS) power transmissions are assumed amongst the LTE cells, the strongest cell HO policy facilitates mobility towards a LTE cell with preferential propagation characteristics. However, this is not the case of the macrocell – femtocell LTE network where femtocells are expected to radiate comparably lower downlink RS power for interference mitigation on the macrocell layer [1]. Divergent RS power transmissions are expected even amongst the femtocell layer, in accordance with the adopted self-optimization procedure [5]. Apart from RS power transmission divergences, substantial RF interference divergences are also expected amongst the LTE cells. RF interference is an inevitable product of the unplanned femtocell deployment, both in terms of location and operating frequency, even if advanced interference cancellation and avoidance techniques are adopted [1-2, 14-16]. The RF interference divergences amongst the LTE cells may severely deteriorate the user-perceived QoS due to service outage and substantially increase the network signaling due to mobility,

if the interference-agnostic strongest cell HO decision policy is adopted.

an LTE backwards-compatible manner by suitably adapting the HHM.

In conclusion, apart from improved indoor coverage and enhanced user-perceived QoS, femtocells natively achieve significant energy savings at both the access network and the UE side. To this end, more sophisticated HO decision algorithms are required in the presence of LTE femtocells to fully exploit the native femtocell superiority both in terms of enhanced QoS and reduced energy consumption. The remainder of this chapter discusses an energyefficient HO decision policy for the macrocell - femtocell LTE network which aims at reducing transmit power at the mobile terminals [17]. The employment of the proposed policy is based on adapting the HO Hysteresis Margin (HHM) with respect to a mean SINR target and standard LTE measurements of the candidate cells' status. The incorporation of the SINR target guarantees QoS, while the utilization of standard LTE measurements provides an accurate estimation of the required UE transmit power per candidate cell. The benefit for employing the proposed HO decision policy is three-fold; improved energyefficiency at the LTE UEs, lower RF interference, and guaranteed QoS for the ongoing user links. Another important feature of the proposed HO decision policy is that even though it is fundamentally different from the predominant strongest cell HO policy, it is employed in

The remainder of this chapter is organized as follows. Section 2 models the macrocell – femtocell LTE in network under the viewpoint of MM and discusses the predominant

In the presence of ongoing user connections, cell quality measurements are usually performed during downlink (DL) and uplink (UL) idle periods provided either by Discontinuous Reception (DRX) or by packet scheduling (i.e. gap assisted measurements) [6]. However, the DRX periods are typically utilized for UE energy conservation while the measurement gaps can be utilized to extend the user service time. Taking this into account and considering that a) the short femtocell range results in more frequent cell search and measurement report triggering even under low to medium mobility scenarios, and b) the large number of neighboring cells will substantially increase the aggregated measurement time in dense femtocell deployments, it follows that cell search and measurement reporting may severely deteriorate the user-perceived QoS and deplete the UE battery lifetime. Moreover, searching for and deriving measurements on nearby yet non accessible femtocells should also be avoided, e.g. when a nearby femtocell belongs to a closed access group where the user is not subscribed. In prominent cellular standards, the mobility decision is typically based on signal quality, coverage or load balancing criteria [8, 9]. Given their preferential QoS and significantly reduced energy consumption on the UE side, femtocells are expected to be prioritized over macrocells during the mobility decision phase. However, the mobility decision and execution in an integrated femtocell environment is a non-trivial issue. Femtocell identification introduces non-negligible delay overhead while the limited femtocell capacity in terms of supported users may substantially increase the HO failure probability. The tagged user access status on the candidate femtocells should also be taken into account both to avoid unnecessary signaling overhead and minimize the HO failure probability due to HO rejection [9]. The femtocell sensitiveness on user mobility can substantially increase the number of mobility decision and execution events, increasing thus the network signaling overhead due to mobility management and compromising the UE service continuity when in connected mode.

HO decision affects various aspects of the overall network performance, which mainly include the Signal to Interference plus Noise Ratio (SINR) performance, the interference performance as well as the energy-efficiency at the access network nodes. Current literature includes various HO decision algorithms for the macrocell – femtocell network [10-12], which primarily focus on prioritizing femtocells over macrocells with respect to user mobility criteria. Emphasis is given in reducing the number of the network-wide HO execution events, owing to the short femtocell radius and the ping-pong effect [9]. Nevertheless, the strongest cell HO decision policy [8] is considered for both macro-macro and femto-femto HO scenarios. According to it, the serving cell proceeds to a HO execution whenever the Reference Signal Received Power (RSRP) [6] of a neighbor cell exceeds over the respective RSRP status of the serving cell plus a policy-defined HHM, for a policydefined time period namely the Time To Trigger (TTT). The HHM is typically introduced to mitigate UE measurement inconsistencies, encompass frequency-related propagation divergences and minimize the ping-pong effect [9], i.e. consecutive HOs originating from the user movement across the cell boundaries. If comparable downlink Reference Signal (RS) power transmissions are assumed amongst the LTE cells, the strongest cell HO policy facilitates mobility towards a LTE cell with preferential propagation characteristics. However, this is not the case of the macrocell – femtocell LTE network where femtocells are expected to radiate comparably lower downlink RS power for interference mitigation on the macrocell layer [1]. Divergent RS power transmissions are expected even amongst the femtocell layer, in accordance with the adopted self-optimization procedure [5]. Apart from RS power transmission divergences, substantial RF interference divergences are also expected amongst the LTE cells. RF interference is an inevitable product of the unplanned femtocell deployment, both in terms of location and operating frequency, even if advanced interference cancellation and avoidance techniques are adopted [1-2, 14-16]. The RF interference divergences amongst the LTE cells may severely deteriorate the user-perceived QoS due to service outage and substantially increase the network signaling due to mobility, if the interference-agnostic strongest cell HO decision policy is adopted.

60 Energy Efficiency – The Innovative Ways for Smart Energy, the Future Towards Modern Utilities

autonomicity and perceived QoS as explained in the following.

service continuity when in connected mode.

MM in the macrocell – femtocell network comprises many technical challenges in all three phases. Given the femtocell sensitiveness on user mobility and ambient radio frequency (RF) interference, serving cell monitoring and evaluation should be performed in a more frequent basis to sustain an acceptable service quality when connected to a femtocell. Considering the relatively small number of physical cell identifiers in prominent radio air-interfaces, more complicated yet backwards compatible cell identification procedures are required to facilitate cell searching and identification. Furthermore, maintaining and broadcasting a comprehensive Neighbor Cell List (NCL) to facilitate cell search and measurement reporting is not scalable in an integrated femtocell – macrocell network. To this end, novel UE-based cell search procedures are required to fully exploit the underlying femtocell infrastructure. The effectiveness of these procedures will have a great impact on the UE energy

In the presence of ongoing user connections, cell quality measurements are usually performed during downlink (DL) and uplink (UL) idle periods provided either by Discontinuous Reception (DRX) or by packet scheduling (i.e. gap assisted measurements) [6]. However, the DRX periods are typically utilized for UE energy conservation while the measurement gaps can be utilized to extend the user service time. Taking this into account and considering that a) the short femtocell range results in more frequent cell search and measurement report triggering even under low to medium mobility scenarios, and b) the large number of neighboring cells will substantially increase the aggregated measurement time in dense femtocell deployments, it follows that cell search and measurement reporting may severely deteriorate the user-perceived QoS and deplete the UE battery lifetime. Moreover, searching for and deriving measurements on nearby yet non accessible femtocells should also be avoided, e.g. when a nearby femtocell belongs to a closed access group where the user is not subscribed. In prominent cellular standards, the mobility decision is typically based on signal quality, coverage or load balancing criteria [8, 9]. Given their preferential QoS and significantly reduced energy consumption on the UE side, femtocells are expected to be prioritized over macrocells during the mobility decision phase. However, the mobility decision and execution in an integrated femtocell environment is a non-trivial issue. Femtocell identification introduces non-negligible delay overhead while the limited femtocell capacity in terms of supported users may substantially increase the HO failure probability. The tagged user access status on the candidate femtocells should also be taken into account both to avoid unnecessary signaling overhead and minimize the HO failure probability due to HO rejection [9]. The femtocell sensitiveness on user mobility can substantially increase the number of mobility decision and execution events, increasing thus the network signaling overhead due to mobility management and compromising the UE

HO decision affects various aspects of the overall network performance, which mainly include the Signal to Interference plus Noise Ratio (SINR) performance, the interference performance as well as the energy-efficiency at the access network nodes. Current literature includes various HO decision algorithms for the macrocell – femtocell network [10-12], which primarily focus on prioritizing femtocells over macrocells with respect to user In conclusion, apart from improved indoor coverage and enhanced user-perceived QoS, femtocells natively achieve significant energy savings at both the access network and the UE side. To this end, more sophisticated HO decision algorithms are required in the presence of LTE femtocells to fully exploit the native femtocell superiority both in terms of enhanced QoS and reduced energy consumption. The remainder of this chapter discusses an energyefficient HO decision policy for the macrocell - femtocell LTE network which aims at reducing transmit power at the mobile terminals [17]. The employment of the proposed policy is based on adapting the HO Hysteresis Margin (HHM) with respect to a mean SINR target and standard LTE measurements of the candidate cells' status. The incorporation of the SINR target guarantees QoS, while the utilization of standard LTE measurements provides an accurate estimation of the required UE transmit power per candidate cell. The benefit for employing the proposed HO decision policy is three-fold; improved energyefficiency at the LTE UEs, lower RF interference, and guaranteed QoS for the ongoing user links. Another important feature of the proposed HO decision policy is that even though it is fundamentally different from the predominant strongest cell HO policy, it is employed in an LTE backwards-compatible manner by suitably adapting the HHM.

The remainder of this chapter is organized as follows. Section 2 models the macrocell – femtocell LTE in network under the viewpoint of MM and discusses the predominant

$$\overline{\boldsymbol{\chi}}\_{\boldsymbol{u}\rightarrow\boldsymbol{s}}^{\boldsymbol{T}} = \frac{\overline{\boldsymbol{P}}\_{\boldsymbol{u}\cdot}^{\boldsymbol{T}} \overline{\boldsymbol{h}}\_{\boldsymbol{u}\leftrightarrow\boldsymbol{s}}^{\boldsymbol{T}}}{\sum\_{c\in\mathcal{C}\_{\text{n}}\hspace{1cm}\overline{\boldsymbol{\sigma}}^{\boldsymbol{T}}\boldsymbol{c}} \overline{\boldsymbol{h}}\_{c\rightsquigarrow\boldsymbol{s}}^{\boldsymbol{T}} + \boldsymbol{\Sigma}\_{\boldsymbol{u}'\nleftharpoons\boldsymbol{\mathcal{C}}\boldsymbol{u}\_{\operatorname{\bf R}} - \{\boldsymbol{u}\}} \overline{\boldsymbol{P}}\_{\boldsymbol{u}\boldsymbol{u}'}^{\boldsymbol{T}} \overline{\boldsymbol{h}}\_{\boldsymbol{u}\overset{\scriptstyle{\rm T}}{\rightsquigarrow}\boldsymbol{s}}^{\boldsymbol{T}} + \left(\overline{\boldsymbol{\sigma}}\_{\boldsymbol{s}}^{\boldsymbol{T}}\right)^{2}}\tag{1}$$

$$\overline{\boldsymbol{\chi}}\_{s \to \boldsymbol{u}}^{\boldsymbol{T}} = \frac{\overline{\boldsymbol{P}}\_{s \to \boldsymbol{u}}^{\boldsymbol{T}} \cdot \overline{\boldsymbol{h}}\_{s \to \boldsymbol{u}}^{\boldsymbol{T}}}{\sum\_{c \in \mathcal{C}\_{\text{fl}} = \{\boldsymbol{\xi}\}} \overline{\boldsymbol{P}}\_{c}^{\boldsymbol{T}} \cdot \overline{\boldsymbol{h}}\_{c \to \boldsymbol{u}}^{\boldsymbol{T}} + \sum\_{\boldsymbol{u}' \in \mathcal{U}\_{\text{fl}} = \{\boldsymbol{u}\}} \overline{\boldsymbol{P}}\_{\boldsymbol{u}'}^{\boldsymbol{T}} \cdot \overline{\boldsymbol{h}}\_{\boldsymbol{u}' \to \boldsymbol{u}}^{\boldsymbol{T}} + \left(\overline{\boldsymbol{\sigma}}\_{\boldsymbol{u}}^{\boldsymbol{T}}\right)^{2}}\tag{2}$$

$$\overline{P}\_{\textit{u}\rightarrow\mathcal{c}}^{\textit{T}} = \frac{\overline{\mathcal{V}}\_{target}^{\textit{u}} \Big( \sum\_{\mathcal{c}' \in \mathcal{C}\_{\mathtt{R}} = \{\mathcal{C}\}} \overline{\mathcal{P}}\_{\textit{c}'}^{\textit{T}} \cdot \overline{\mathcal{h}}\_{\textit{c}' \rightarrow \textit{c}}^{\textit{T}} + \Sigma\_{\textit{u}' \oplus \textit{U}\_{\mathtt{R}} = \{\mathcal{U}\}} \overline{\mathcal{P}}\_{\textit{u}\prime}^{\textit{T}} \cdot \overline{\mathcal{h}}\_{\textit{u}\prime' \rightarrow \textit{c}}^{\textit{T}} + \left( \overline{\sigma}\_{\textit{c}}^{\textit{T}} \right)^{\textit{T}} \tag{3}$$


$$\arg\max\_{\mathfrak{c}\in\mathcal{L}\_{\mathbf{u}}} RSRP\_{\mathfrak{c}\to\mathfrak{u}, \text{(dB)}}^{TTT} := \left\{ \mathfrak{c} \, \middle|\, RSRP\_{\mathfrak{c}\to\mathfrak{u}, \text{(dB)}}^{TTT} > RSRP\_{\mathfrak{s}\to\mathfrak{u}, \text{(dB)}}^{TTT} + HHM\_{\mathfrak{c}, \text{(dB)}} \right\} \tag{4}$$

$$RSRP\_{\mathbf{c}\to u}^{T} = P\_{\mathbf{c}, \mathbf{RS}}^{T} \cdot \overline{h}\_{\mathbf{c}\to u}^{T} \tag{5}$$

$$\overline{h}\_{u \to c}^{T} \cong \overline{h}\_{c \to u}^{T} = \frac{RSRP\_{c \to u}^{T}}{P\_{c, RS}^{T}} \tag{6}$$

$$\overline{\boldsymbol{I}}\_{c}^{T} = \left(\sum\_{c' \in \mathcal{C}\_{\text{n}} - \{c\}} \overline{\boldsymbol{P}}\_{c'}^{T} \cdot \overline{\boldsymbol{h}}\_{c' \to c}^{T} + \sum\_{u' \in \mathcal{U}\_{\text{n}}} \overline{\boldsymbol{P}}\_{u'}^{T} \cdot \overline{\boldsymbol{h}}\_{u' \to c}^{T} + \left(\overline{\boldsymbol{\sigma}}\_{c}^{T}\right)^{2}\right) \tag{7}$$

$$
\overline{P}\_u^T \triangleq \overline{P}\_{u \to s}^T = \frac{\overline{\nu}\_{target}^u \cdot P\_{s, RS}^T \overline{I}\_s^T}{RSR P\_{s \to u}^T} \tag{8}
$$

$$\overline{P}\_{u \to c}^{T} = \frac{\overline{\gamma}\_{target}^{u} \cdot P\_{c, RS}^{T} \left( \overline{l}\_{c}^{T} - \overline{P}\_{u}^{T} \overline{h}\_{u \to c}^{T} \right)}{RSRP\_{c \to u}^{T}} \tag{9}$$

$$
\overline{P}\_{u \to s}^{\overline{r}} > \overline{P}\_{u \to c}^{\overline{r}} \Rightarrow \tag{10}
$$

$$\frac{\overline{\boldsymbol{\gamma}}\_{target}^{u} \cdot \boldsymbol{p}\_{s,RS}^{T} \cdot \overline{\boldsymbol{I}}\_{s}^{T}}{^{RSRP}\_{s \rightarrow u}^{T}} > \frac{\overline{\boldsymbol{\gamma}}\_{target}^{u} \cdot \boldsymbol{p}\_{c,RS}^{T} \left(\overline{\boldsymbol{I}}\_{c}^{T} - \overline{\boldsymbol{P}}\_{u}^{T} \overline{\boldsymbol{h}}\_{u \rightarrow c}^{T}\right)}{^{RSRP}\_{c \rightarrow u}^{T}} \xrightarrow{} \tag{11}$$

$$RSRP\_{c\rightarrow u}^{T} > RSRP\_{s\rightarrow u}^{T} \cdot \frac{P\_{c, RS}^{T} \left(\overline{I\_{c}} - \overline{P}\_{u}^{T} \cdot \overline{h}\_{u \rightarrow c}^{T}\right)}{P\_{s, RS}^{T} \cdot \overline{I\_{s}}} \tag{12}$$

$$RSRP\_{c\rightarrow u, \{dB\}}^{TTT} > RSRP\_{s\rightarrow u, \{dB\}}^{TTT} + HHM\_{c, \{dB\}}^{UTPR} \tag{13}$$

$$HHM\_{c,(dB)}^{UTPR} = \begin{cases} 10\log\frac{P\_{c,BS}^{TTT}\left(\overline{l\_c^{TTT}} - \overline{P\_u}^{TTT}\cdot\overline{h\_{u\sim c}}\right)}{P\_{s,BS}^{TTT}\cdot\overline{l\_s}} & c, s \in \mathcal{C}\_n\\ & 10\log\frac{P\_{c,BS}^{TTT}\cdot\overline{l\_c}^{TTT}}{P\_{s,BS}^{TTT}\cdot\overline{l\_s}} & otherwise \end{cases} \tag{14}$$

$$\text{arg}\,\text{max}\_{\mathsf{c}\in\mathsf{L}\_{\mathsf{u}}}\mathsf{R}\mathsf{S}\mathsf{R}P\_{\mathsf{c}\to\mathsf{u},\{\mathsf{dB}\}}^{\mathsf{T}\mathsf{T}\mathsf{T}} := \left\{ \mathsf{c} \,|\, \mathsf{R}\mathsf{S}\mathsf{R}P\_{\mathsf{c}\to\mathsf{u},\{\mathsf{dB}\}}^{\mathsf{T}\mathsf{T}\mathsf{T}} > \mathsf{R}\mathsf{S}\mathsf{R}P\_{\mathsf{s}\to\mathsf{u},\{\mathsf{dB}\}}^{\mathsf{T}\mathsf{T}\mathsf{T}} + \,\mathsf{H}\mathsf{H}\mathsf{M}\_{\mathsf{c},\{\mathsf{dB}\}} + \,\mathsf{H}\mathsf{H}\mathsf{M}\_{\mathsf{c},\{\mathsf{dB}\}}^{\mathsf{U}\mathsf{T}\mathsf{R}\mathsf{R}} \right\} \tag{15}$$

Energy Efficient Mobility Management for the Macrocell – Femtocell LTE Network 67

consistent RSRP and RSRQ measurements. Notice that the measurement configuration and reporting phase in LTE is triggered on critical events [20], e.g. when the serving cell RSRP is below a network-configured threshold for a network-configured time period TTT. To facilitate subsequent parameter acquisition, each measurement report includes a measurement timestamp. The proximity configuration and indication signaling in Fig.3 and 4 is utilized for UE-based autonomous HeNB discovery, while the System Information (SI) acquisition and report signaling is required for HeNB identification and access control validation [6]. The serving eNB utilizes the reported UE measurements, sent on critical LTE events, for HO decision triggering (steps 8 in the reactive and 12 in the proactive approach)

**Figure 3.** Network signaling procedure for the reactive handover approach

Upon HO decision triggering, the serving eNB initiates a HO context request towards the MME including the corresponding measurement timestamp and target ECGI, i.e. steps 9 in Fig. 3 and 13 in Fig. 4. To minimize unnecessary network signaling, the MME verifies the

[21, 22].

Summarizing, the proposed UTPR policy is based on standard LTE measurements, while it is employed by introducing an adaptive HHM to the standard LTE HO procedure. The employment of the UTPR policy does not require any enhancements for the LTE UEs, however, an enhanced network signaling procedure is necessitated. Next section provides some insights on how the proposed policy could be employed in the context of the macrocell – femtocell LTE network.

## **4. Network signaling to employ the proposed handover decision policy**

To identify and ultimately utilize CSG femtocells within its proximity, each LTE UE maintains a CSG whitelist. The respective CSG whitelist per LTE user is also maintained in the Mobility Management Entity (MME), residing in the LTE Core Network (CN), in order to perform access control during the mobility execution phase. The closed and hybrid access LTE femtocells broadcast their CSG identity (CSG ID) along with a CSG indicator set to 'TRUE' or 'FALSE', respectively. Both these fields along with the E-UTRAN Cell Global Identifier (ECGI), used for global LTE cell identification, are signaled within the System Information Block Type 1 (SIB1) in the LTE downlink [6]. Although this information is not required during the LTE cell search and measurement phase, it is considered prerequisite during the LTE mobility decision and execution phase. To this end, a cell identification procedure is performed, where the UE is reconfigured to obtain the ECGI of the target LTE cell [6]. In the following, we identify and discuss two different LTE network signaling approaches to facilitate the employment of the proposed UTPR-based HO decision policy.

The employment of the proposed UTPR policy necessitates the incorporation of standardized LTE cell measurements on the tagged user's neighbor cell set, i.e. the downlink RS transmitted power ��� � and Received Interference Power ��� �� � ��. These measurements can be commuted through the S1 interface [6] to the serving LTE cell. The entire HO decision parameter set will be referred to as HO context in the following. Depending on whether the required HO context is reported and maintained in a LTE CN entity or not, e.g. the MME, two different network signaling approaches are identified i.e. the reactive and the proactive [24] In the reactive approach the HO context is obtained on request towards the target LTE cell, while in the proactive approach it is directly obtained on request to the MME. To employ the latter, the LTE cells are required to report their HO context status to the MME on a periodic basis. The reporting periodicity should be MMEconfigured and adapted according to the HO context request history, the LTE CN status and so on. Assuming that the serving eNB can be either a regular eNB or a HeNB, Fig. 3 and 4 illustrate the detailed network signaling [6] required in the reactive and the proactive HO context derivation approaches, respectively. Without loss of generality, it is considered that the serving and the target cell are connected to the same MME.

The cell search and measurement signaling steps for both approaches, i.e. steps 1-7 in the reactive and steps 5-11 in the proactive, are in accordance with [6]. During these steps, the serving eNB configures the UE to identify an appropriate neighbor cell set and derive consistent RSRP and RSRQ measurements. Notice that the measurement configuration and reporting phase in LTE is triggered on critical events [20], e.g. when the serving cell RSRP is below a network-configured threshold for a network-configured time period TTT. To facilitate subsequent parameter acquisition, each measurement report includes a measurement timestamp. The proximity configuration and indication signaling in Fig.3 and 4 is utilized for UE-based autonomous HeNB discovery, while the System Information (SI) acquisition and report signaling is required for HeNB identification and access control validation [6]. The serving eNB utilizes the reported UE measurements, sent on critical LTE events, for HO decision triggering (steps 8 in the reactive and 12 in the proactive approach) [21, 22].

66 Energy Efficiency – The Innovative Ways for Smart Energy, the Future Towards Modern Utilities

��� � ��������(��)

Summarizing, the proposed UTPR policy is based on standard LTE measurements, while it is employed by introducing an adaptive HHM to the standard LTE HO procedure. The employment of the UTPR policy does not require any enhancements for the LTE UEs, however, an enhanced network signaling procedure is necessitated. Next section provides some insights on how the proposed policy could be employed in the context of the

**4. Network signaling to employ the proposed handover decision policy** 

To identify and ultimately utilize CSG femtocells within its proximity, each LTE UE maintains a CSG whitelist. The respective CSG whitelist per LTE user is also maintained in the Mobility Management Entity (MME), residing in the LTE Core Network (CN), in order to perform access control during the mobility execution phase. The closed and hybrid access LTE femtocells broadcast their CSG identity (CSG ID) along with a CSG indicator set to 'TRUE' or 'FALSE', respectively. Both these fields along with the E-UTRAN Cell Global Identifier (ECGI), used for global LTE cell identification, are signaled within the System Information Block Type 1 (SIB1) in the LTE downlink [6]. Although this information is not required during the LTE cell search and measurement phase, it is considered prerequisite during the LTE mobility decision and execution phase. To this end, a cell identification procedure is performed, where the UE is reconfigured to obtain the ECGI of the target LTE cell [6]. In the following, we identify and discuss two different LTE network signaling approaches to facilitate the employment of the proposed UTPR-based HO decision policy.

The employment of the proposed UTPR policy necessitates the incorporation of standardized LTE cell measurements on the tagged user's neighbor cell set, i.e. the downlink

measurements can be commuted through the S1 interface [6] to the serving LTE cell. The entire HO decision parameter set will be referred to as HO context in the following. Depending on whether the required HO context is reported and maintained in a LTE CN entity or not, e.g. the MME, two different network signaling approaches are identified i.e. the reactive and the proactive [24] In the reactive approach the HO context is obtained on request towards the target LTE cell, while in the proactive approach it is directly obtained on request to the MME. To employ the latter, the LTE cells are required to report their HO context status to the MME on a periodic basis. The reporting periodicity should be MMEconfigured and adapted according to the HO context request history, the LTE CN status and so on. Assuming that the serving eNB can be either a regular eNB or a HeNB, Fig. 3 and 4 illustrate the detailed network signaling [6] required in the reactive and the proactive HO context derivation approaches, respectively. Without loss of generality, it is considered that

The cell search and measurement signaling steps for both approaches, i.e. steps 1-7 in the reactive and steps 5-11 in the proactive, are in accordance with [6]. During these steps, the serving eNB configures the UE to identify an appropriate neighbor cell set and derive

the serving and the target cell are connected to the same MME.

� and Received Interference Power ��� �� � ��. These

��� � �����(��) � �����(��)

���� � (15)

��� � ������������(��)

arg max���� ��������(��)

macrocell – femtocell LTE network.

RS transmitted power ���

**Figure 3.** Network signaling procedure for the reactive handover approach

Upon HO decision triggering, the serving eNB initiates a HO context request towards the MME including the corresponding measurement timestamp and target ECGI, i.e. steps 9 in Fig. 3 and 13 in Fig. 4. To minimize unnecessary network signaling, the MME verifies the

access status of the tagged UE on the target ECGI in steps 10 and 14, respectively. If the tagged user is not allowed to access the target eNB, the MME notifies the serving LTE cell accordingly.

Energy Efficient Mobility Management for the Macrocell – Femtocell LTE Network 69

The HO context requests and reports can be signaled in an aggregated manner in both the access (eNB, HeNB) and the core LTE network (MME, HeNB GW). For example, on multiple HO context requests towards a tagged eNB, the MME may send an aggregated HO context request including all the required measurement timestamps. A similar approach can be applied for the HO context report in the reverse direction. Although the reactive approach minimizes the required signaling between the MME and the target LTE cell, the overall network signaling will be highly correlated to the occurrence rate of HO triggering events. On the other hand, more frequent yet more deterministic signaling overhead is expected in the proactive approach, provided that the MME configures the HO context reporting periodicity on the eNBs. In addition to that, the proactive approach may significantly reduce the resulting HO decision delay compared to the reactive approach, provided that the HO context resides on the context-aware MME rather than the target LTE cell. However, certain operational enhancements are required in the MME to resourcefully support the proactive approach, in contrast with the reactive approach where no further LTE CN enhancements

This section includes selected numerical results to evaluate the performance of the proposed UTPR HO decision policy in the macrocell – femtocell LTE network. The simulation scenario is based on the evaluation methodology described in [22], while the proposed HO decision policy is compared against a strongest cell based policy, referred to as SCB policy in the

A conventional hexagonal LTE network is considered, including a main LTE cluster composed of 7 LTE cells, where each LTE cell consists of 3 hexagonal sectors. The wraparound technique is used to extend the LTE network, by copying the main LTE cluster symmetrically on each of the 6 sides. A set of blocks of apartments, referred to as femtoblocks, are uniformly dropped within the main LTE cluster according to the parameter ���, which indicates the femtoblock deployment density within the main LTE cluster, i.e., the percentage of the main LTE cluster area covered with femtoblocks. Each femtoblock is modeled according to the dual stripe model for dense urban environments in [22]. According to it, each femtoblock consists of two stripes of apartments separated by a 10 m wide street, while each stripe has two rows of �� = �� apartments of size 10 × 10 m. For a tagged femtoblock, femtocells are deployed with a femtocell deployment ratio parameter ���, which indicates the percentage of apartments with a femtocell [22]. Each femtocell initially serves one associated user, while in general, it can serve up to 4 users. Femtocells and femtocell users are uniformly dropped inside the apartments. Each LTE user is member of up to one CSG, where the CSG ID per user and femtocell is uniformly picked from the set {1, 2, 3}. Each LTE sector initially serves ten macrocell users, which are uniformly distributed

within it. Unless differently stated, it is assumed that �̅= 3 km/h and �� = 1 km/h.

The macrocell stations operate in a LTE band centered at 2000MHz, divided into � RBs of width 180 KHz and utilizing a 5MHz bandwidth. The macrocell inter-site distance is set to

are needed.

following.

**5. Numerical results** 

The key difference between the reactive and the proactive approaches is that in the former the MME forwards the HO context request towards the target eNB (steps 11-15), while in the latter the MME may directly provide the required HO context by utilizing the reports derived in steps 1-4 (Fig.4). It should be noted that the proactive context derivation signaling phase is indicatively located in steps 1-4, since it can be performed asynchronously with respect to the rest HO signaling procedure. In the absence of HO context close to the required measurement timestamp, the MME may decide to forward the HO context request towards the target eNB as in the reactive approach. Upon HO context acquisition, the HO decision algorithm in the serving eNB proceeds to a HO execution whenever necessary. In that case, a common HO execution signaling follows for both approaches (steps 17-24) [6].

**Figure 4.** Network signaling procedure for the proactive handover approach

The HO context requests and reports can be signaled in an aggregated manner in both the access (eNB, HeNB) and the core LTE network (MME, HeNB GW). For example, on multiple HO context requests towards a tagged eNB, the MME may send an aggregated HO context request including all the required measurement timestamps. A similar approach can be applied for the HO context report in the reverse direction. Although the reactive approach minimizes the required signaling between the MME and the target LTE cell, the overall network signaling will be highly correlated to the occurrence rate of HO triggering events. On the other hand, more frequent yet more deterministic signaling overhead is expected in the proactive approach, provided that the MME configures the HO context reporting periodicity on the eNBs. In addition to that, the proactive approach may significantly reduce the resulting HO decision delay compared to the reactive approach, provided that the HO context resides on the context-aware MME rather than the target LTE cell. However, certain operational enhancements are required in the MME to resourcefully support the proactive approach, in contrast with the reactive approach where no further LTE CN enhancements are needed.
