The Impact of Small Cells on

Transcription

1 The Impact of Small Cells on MME Signaling METHODS TO REDUCE AND OPTIMIZE MME CORE SIGNALING CAUSED BY SMALL CELLS APPLICATION NOTE

2 ABSTRACT As more subscribers use mobile broadband services from their smartphones and tablets, the demand for bandwidth and coverage is increasing. As a result, small cells are becoming a key component of a mobile service provider s Long Term Evolution (LTE) network deployment strategy. However, adding small cells to an LTE network can significantly increase the signaling load on the core network and, in particular, on the Mobility Management Entity (MME). This application note explains how the smart signaling techniques of the Alcatel-Lucent 9471 Wireless Mobility Manager (WMM) can help mobile service providers (MSPs) minimize the signaling impact of small cells on the core network. Based on Alcatel- Lucent experience in large-scale LTE networks, we also provide practical advice on how to effectively manage core network signaling when adding small cells to the LTE network.

3 Table of contents Executive summary / 1 The rise of LTE core network signaling / 1 Small cells overview / 2 Key small cell effects on the MME / 4 Number of S1-MME links / 4 S1-MME link recovery times / 4 Message broadcasts / 5 Paging and tracking area management / 5 Paging strategies for small cells / 6 Traditional paging methods are not effective / 6 CSG paging optimization / 7 Alcatel-Lucent 9471 WMM CSG paging optimization / 7 Alcatel-Lucent 9471 WMM smart paging / 7 Combining paging optimization methods for small cells / 8 Tracking area overview / 9 Dynamic tracking area management / 11 Tracking area deployment strategies for small cells / 12 Conclusion / 12 References / 13 Acronyms / 14

4 Executive summary Packet core network signaling is increasing and, as a result, is becoming a concern to mobile MSPs. The increase in signaling is primarily due to widespread user adoption of mobile broadband services through smartphones and tablets. To meet this demand for high-speed mobile broadband, MSPs are updating their networks to LTE networks and adding small cells as part of a heterogeneous network (HetNet) architecture. Small cells are an economical and efficient solution to augment network capacity and coverage as well as to provide the quality of experience (QoE) needed for real-time, mobile broadband services. There are two small cell network designs. In the first, the small cell is directly connected to the Evolved Packet Core (EPC). In the second, a small cell gateway is added to aggregate traffic from the small cells to the EPC. There are pros and cons for either option but if an MSP chooses the first option, the volume of signaling on the EPC increases significantly, particularly on the MME, the dedicated control plane element in the EPC. To address the increase in core signaling caused by small cell deployments, Alcatel- Lucent has used its field experience gained from large-scale LTE network deployments and expertise in mobility management to develop idle mobility smart signaling capabilities on the Alcatel-Lucent 9471 Wireless Mobility Manager (WMM), which acts as the MME/SGSN in the Wireless Packet Core solution. These 3GPP-compliant smart signaling capabilities can reduce signaling caused by paging and tracking area (TA) management in LTE networks by up to 80 percent as well as extend mobile device battery life. The 9471 WMM also supports Closed Subscriber Group (CSG) paging optimization, which further reduces network signaling. Both of these paging optimization methods can be combined in small cell deployments. The 9471 WMM is built to meet the new, more demanding signaling requirements of LTE and small cell networks. The rise of LTE core network signaling To better understand the network signaling impact of LTE small cells, it s instructive to review current large-lte macro network deployments and subscriber behavior. Signaling traffic in early LTE network deployments, and particularly in the new EPC, is significantly higher than in existing 2G/3G packet data core networks. One reason for this is the change in the 3rd Generation Partnership Program (3GPP) network architecture. With LTE and its flatter, all-ip architecture, there is now direct connectivity between the macro cell (and potentially small cells) and the MME, the dedicated control plane element in the new EPC. The MME is responsible for the subscriber and session management functions in the LTE signaling network. As a result, potentially hundreds of thousands of cells and millions of subscribers will connect to MMEs and generate far more control plane signaling than on existing 2G/3G Serving Gateway Support Nodes (SGSNs). 1

5 Alcatel-Lucent analysis of field data from several large LTE network deployments found that an MME can experience a sustained signaling load from 500 to 800 messages per user equipment (UE) during the normal peak busy hours and up to 1500 messages per user per hour under adverse conditions (for example, a malfunctioning device, poor coverage or network engineering mistakes). As LTE networks become more widely deployed, subscriber mobile usage behavior is changing. More subscribers are accessing mobile broadband through their smartphones and tablets as they become accustomed to better mobile broadband experience of LTE. In large US metropolitan markets where LTE is available, Alcatel-Lucent LTE Traffic Engineering has found that peak network usage can be as high as 45 service requests per UE per hour during peak busy hours. These smart devices also employ shorter dormancy timers to conserve battery power but, in so doing, they increase signaling. Together, these factors increase the potential for network congestion and signaling storms in the EPC if not properly managed. In 3G networks the SGSN is buffered to a large degree from these radio access spikes by the Radio Network Controller (RNC), but in LTE networks the MME bears the full force of these signaling storms. As a result, MSPs who are migrating to LTE and adding small cells must take steps to ensure that their core control plane network can support the expected increase in signaling volume. Specifically, MSPs need to deploy a carrier-grade, next-generation MME/SGSN platform that has the scalability and CPU performance to support a converged 2G/3G/LTE heterogeneous network and also has the capability to intelligently manage this traffic to reduce overall core signaling. Core signaling can be reduced by optimizing traffic and suppressing surges in traffic when abnormal network behavior due to specific events causes spikes in traffic between network nodes or between UEs and the network. Through field experience in largescale LTE networks, Alcatel-Lucent has seen spikes of over two orders of magnitude in instantaneous load due to mass operations (such as large-scale Attach requests) for all UEs associated with a single node. Small cells overview Small cells are wireless infrastructure equipment that operates in licensed bands within MSP cellular networks. Small cells are often deployed to increase bandwidth capacity, improve coverage and provide a better QoE for real-time mobile broadband services. They are a critical component of an MSP s HetNet strategy. HetNets combine a broad range of wireless technologies with flexible radio access options, enabling MSPs to cost-effectively provide ubiquitous coverage along with the high-bandwidth capacity required to deliver a superior QoE in all environments. Because of their size and low power requirements, small cells are simpler and can be deployed more quickly than macro cells through remote integration capabilities. Small cells also support multi-vendor environments through advanced interference mitigation techniques and intelligent traffic management. Today, small cells are being deployed in both 3G and LTE networks with the integration of Wi-Fi to provide an alternative access option and additional broadband capacity. 2

6 In LTE networks there are three types of small cells with varying capacity, coverage, power requirements and mounting options: metrocells, enterprise cells and, in the future, residential cells. A metrocell is a smaller version of a macrocell that is typically deployed by MSPs in either indoor or outdoor public spaces to provide increased capacity in urban hotspots or coverage in a rural location. Metrocells connect directly to the EPC over the S1 interface. Another metrocell deployment option is to install metro radios in large public venues or on the sides of high-rise buildings. The metro radios can be daisy-chained together using an industry-standard Common Public Radio Interface (CPRI) that connects back to a common enodeb baseband unit (BBU), which connects to the EPC. Enterprise and residential cells, referred to in the 3GPP standards as Home enhanced NodeBs (HeNBs), have smaller form factors with less subscriber capacity and coverage than metrocells. They are typically installed indoors and have three network deployment options [1, 2]. The first option is to deploy a small cell gateway (also called an HeNB gateway) behind the small cells and have the gateway aggregate the traffic and forward it appropriately to the EPC. In this option, the HeNB gateway provides both the control (S1-MME) interface to the MME and the user (S1-U) interface to the serving gateway (SGW). In the second option, the HeNB-gateway is again used but only for aggregation of control plane traffic (S1-MME only). The HeNB user plane is directly connected to the SGW. The third option is for the HeNB to be directly connected to the EPC, just like a macro or metro cell. Figure 1 shows the Alcatel-Lucent small cell architecture and the different deployment options. Figure 1. Alcatel-Lucent LTE small cell architecture CSG List Srv HSS S6d 9471 WMM (MME) LTE EPC S11 Macro Cell X SR Mobile GW (SGW/PGW) S1-MME S1-U 9766 LTE SC GW (optional) enodeb (d2u) CPRI 976x Metro Cells 9762 Enterprise Cell (HeNB) 9768 Metro Radio UE (CSG ID) 3

7 Small cells have three different access modes [1, 2]: Open access: The HeNB operates as a normal cell, allowing any user of a Public LAN Mobile Network (PLMN) to access it with proper subscription authorization. Closed access: The HeNB restricts access, allowing only members of a Closed Subscriber Group (CSG) permission to access the cell. A CSG cell that is part of a PLMN broadcasts its CSG identity (CSG ID) only to User Equipment (UE) members that contain that cell identity in their CSG list. Each cell of an HeNB can belong to only one CSG, but an HeNB can belong to different CSGs with different CSG IDs. Hybrid access: A variant of open access where any subscriber has access to the cell but the cell also operates as a CSG cell, which gives priority of services to members of a CSG when congestion occurs. Key small cell effects on the MME Small cell effects on the MME depend on the number and type of small cells added to the network and whether the small cells are directly connected to the MME or are connected through the intermediary small cell gateway. The most significant impact on the MME and core network signaling due to small cell deployments are increases in: Number of S1-MME links S1-MME link recovery times during network failures Message broadcasts to enodebs and TAs Paging volumes Tracking area management The Alcatel-Lucent 9471 Wireless Mobility Manager (WMM) is the combined MME/SGSN in the converged 2G/3G/LTE Alcatel-Lucent Wireless Packet Core (WPC) solution. It is LTE design optimized using the latest-generation, industry-standard computing hardware and field-proven mobility software to provide the scaling, capacity and performance needed for large-scale LTE macro and small cell networks. Number of S1-MME links If small cells are to be directly connected to the EPC, the MME must be able to terminate and manage a very large number of S1-MME links because each small cell (metrocell or HeNB) requires one Stream Control Transmission Protocol (SCTP) association. If the MME is instead connected to the HeNB gateway, only one S1-MME link (and SCTP association) is required per HeNB network. The Alcatel-Lucent 9471 WMM provides the S1-MME link and SCTP scaling needed for either deployment option, with support for up to 50,000 S1-MME links (SCTP associations) now and hundreds of thousands of links with the platform s evolution to Network Functions Virtualization (NFV). S1-MME link recovery times If there is a network signaling issue and the S1-MME link is dropped, it is imperative that the MME quickly re-establish link connectivity to prevent service interruption. When SCTP connections are established on the 9471 WMM hub card, the association data is also stored on the standby instance on the alternate hub card. If a failover occurs, only resynchronization of the link data is required; and it can be processed much faster than when the link must be completely re-established. 4

8 Message broadcasts The MME must be able to broadcast emergency Commercial Mobile Alert System (CMAS) alerts, evolved multimedia broadcast/multi-cast service (embms) control messaging, and overload control Start/Stop messages to a large volume of enodebs and small cells quickly and without a sudden surge in messaging volume. The 9471 WMM supports broadcast message pacing and overload control throttling capabilities to ensure reliable delivery. Paging and tracking area management An area that makes a significant contribution to the overall control plane traffic load, and allows room for innovation by the MME, is paging and TA management. Figure 2 shows the signaling load on an MME for different procedures taken from field data of a large North American LTE service provider in a large metropolitan market. Paging messaging is 29 percent of the total signaling load at the MME. Tracking area updates (TAUs) can also generate a moderate signaling load on the MME, in this case 5 percent of the total messaging volume. Figure 2. MME signaling distribution in US market Inter-radio access transmission 2% Tracking area updates 5% Attach/detach 1% Paging 29% Session management 62% Active mobility 1% These two idle mobility procedures must be carefully managed by the MSP, especially when small cells are introduced. If the same paging and TA management methods employed in 2G/3G networks are extended to 4G/LTE networks, it is highly likely that paging broadcasts will increase significantly and consume a high percentage of control plane capacity, leading to congestion and, in a worst-case scenario, enodeb or small cell paging overload. 5

9 While little can be done to change the user behaviors and device actions (such as frequent Active/Idle transitions) that lead to a particularly high volume of service requests seen in busy LTE networks, there is much that the MME can do to locate idle users with the highest efficiency to minimize network load due to paging and TA management. Paging strategies for small cells Traditional paging methods are not effective Traditional 2G/3G paging methods do not work when deploying LTE networks with small cells, so MSPs must employ a different paging strategy. Based on LTE traffic modeling done by Alcatel-Lucent [3], the optimal paging volumes an enodeb and persubscriber UE can support using traditional 2G/3G paging methods (for example, Best Effort [BE] escalation and priority routing area/location area [RA/LA] paging) is when the TA size is from 30 to 40 enodebs. Figure 3 shows the volume of paging messaging as the TA size is increased when both traditional (non-optimized) paging strategies and optimized paging (9471 WMM smart paging) are used for three different first-attempt paging success rates: 80 percent, 90 percent and 93 percent. The graph shows that if traditional, non-optimized paging methods are used, the volume of paging messaging rises rapidly as the TA size increases beyond 200 enodebs; paging volumes begin to overload the signaling network. With 9471 WMM smart paging, the paging volumes are significantly lower, even with large TAs. Figure 3. MME paging load by TA size (fixed subscriber level) 2000 UE Page Messages/Hour Page attempt success rate WMM opt 80% WMM opt 90% WMM opt 93% non-opt 80% non-opt 90% non-opt 93% Tracking Area Size (Number Cells per TA) 6

10 When MSPs add small cells into existing LTE networks, it will likely be a mix of cell types and deployment options to address specific public, enterprise and residential applications. As a result, paging on the network will increase as existing LTE TAs increase in size, with some exceeding 200 cells per TA. Also, new TAs will be added for HeNB gateways for residential and enterprise networks. Paging optimization of small cells and MMEs will be critical to prevent the cells from becoming overloaded and being dominated by a high paging load. CSG paging optimization The 3GPP standards for small cells support paging optimization through the use of CSGs [2]. As was described in the Small Cells Overview section, a small cell (or HeNB) can be deployed in any of three access modes: closed, open or hybrid access. An HeNB in closed access mode contains one or more CSG cells that broadcast a specific CSG ID and restrict access to only the subscriber members that possess that CSG ID in their UE s CSG list. CSG paging optimization limits the paging broadcast to only the HeNBs whose CSG cells match the CSG ID in the UE s CSG list. This will significantly reduce the paging volumes when LTE HeNBs are deployed in residential and enterprise applications. Alcatel-Lucent 9471 WMM CSG paging optimization The 9471 WMM supports access restrictions based on CSG membership. When a CSG is associated with an LTE HeNB, the cell can operate in either of two modes: Closed access: Only UEs belonging to the CSG can access the cell. For emergency calls any UE can access the cell. Hybrid access: Any UE can access the cell but UEs belonging to the CSG have preferential access and evolved-utran Radio Access Bearer (erab) QoS during congestion conditions. The 9471 WMM supports two global configuration options to use or not use CSG paging optimization: MME-activated CSG paging and HeNB-activated CSG paging. If paging optimization is activated in the MME, the 9471 WMM sends paging messages to only the CSG cells that are in UE s subscriber data list. If the CSG paging optimization in HeNB is activated, the MME includes a list of CSG IDs from the UE CSG subscription data in the paging message for all the macro enodebs and the HeNB-gateway. Alcatel-Lucent 9471 WMM smart paging To effectively manage and reduce MME signaling volume, paging methods between the last seen enodeb and TA are needed. The 9471 WMM supports smart paging methods such as paging last seen n enodebs (n = 1, 2, 3, 4, 5). The 9471 WMM can store in memory up to five prior visited enodebs per UE, including prior visited enodebs that are part of a different TA than the last seen enodeb. These smart paging methods are fully compliant with 3GPP standards and can be used in networks with any vendor s enodebs. Analysis of LTE traffic patterns in large scale-networks [3] determined that paging last seen n enodebs is particularly effective in reducing the broadcast paging traffic for data services while still maintaining a highly successful first page attempt success rate. The 9471 WMM paging is highly flexible, supporting the configuration of different paging policies for each type of service. This helps MSPs lower the volume of broadcast paging while still meeting end-user latency requirements for each type of service. 7

11 A 9471 WMM paging policy typically includes the following parameters: Paging by service type, including voice, SMS, data, or QoS Class Identifier [QCI] level Number of paging attempts Timing between paging attempts Method of paging used in each attempt As shown in Figure 4, the 9471 WMM s smart paging features can reduce signaling messaging over basic TA and TA list paging methods by as much as 80 percent depending on the TA size. Figure WMM smart signaling paging benefits Number pages/ue/hr Standard TA/TA List Paging 9741 WMM Smart Paging TA size 10 TA size 30 TA size 100 Combining paging optimization methods for small cells When small cells are deployed in an LTE network, paging increases and it is imperative for MSPs to find methods to reduce this load. The 9471 WMM supports CSG paging optimization and smart paging methods for both LTE macro networks and small cell networks. By combining these methods, MSPs can reduce the overall paging load in any deployment scenario, whether small cells are directly connected to the 9471 WMM or indirectly connected through the HeNB gateway. For example, when residential or enterprise (HeNB) small cells are deployed there are three paging scenarios to consider: All of the HeNBs are directly connected to the MME All the HeNBs are connected to an HeNB-gateway Some HeNBs are directly connected to the MME while others are connected through an HeNB gateway In all three cases a mix of open, hybrid or closed access cells can be included. When all the HeNBs are directly connected to the MME, HeNBs that are open or hybrid access mode can use 9471 WMM smart paging methods such as last seen n HeNBs on the first or any subsequent paging attempt. For closed mode HeNBs, the 9471 WMM 8

12 (MME) initiates CSG paging optimization by sending a paging procedure to only the HeNBs that have both the TAI in the TAI list and the CSG ID in the allowed CSG list of the UE. If the UE is not located in n paging attempts within the closed mode HeNB, the paging policy can be expanded to include hybrid or open cells in subsequent attempts and 9471 WMM smart paging methods can be used. In the second scenario, when all the HeNBs are connected to an HeNB-gateway, the 9471 WMM does not know the access mode of the HeNB cells. In this case, the 9471 WMM initiates the paging procedure; it sends the UE s allowed CSG list in the message to the HeNB-gateway that has the UE s TAI list. The HeNB gateway applies CSG paging optimization by forwarding the page to only the closed mode HeNBs that are part of the UE s allowed CSG list. However, this does not preclude the HeNB network from using 9471 WMM smart paging methods. The 9471 WMM can still forward smart paging methods to the HeNB gateway to the open and hybrid mode HeNBs and to closed mode HeNBs on subsequent paging attempts depending on how the MSP constructs the paging policy. In the final scenario, some HeNBs are directly connected to the MME while others are connected through an HeNB gateway. Either MME-activated or HeNB-activated CSG paging optimization for closed mode HeNBs can be applied first depending on the HeNB connectivity to the MME or the HeNB gateway. As in the other scenarios, 9471 WMM smart paging methods can still be applied to open or hybrid mode HeNBs initially and to closed mode HeNBs in subsequent paging attempts depending on how the paging policy is designed. Tracking area overview In LTE networks, a TAU procedure is initiated when the UE moves between macro or small cells that are in different tracking areas (TAs). A TA represents a group of cells within a defined geography of the evolved Universal Terrestrial Radio Access Network (eutran). Like a location area (LA) or routing area in GSM/UMTS networks, the TA helps the MME locate the UE in the LTE network by paging only those cells in the TA where the UE was last registered. If the UE is not located in the TA where it was last seen, the paging broadcast can be expanded to a TA list. Figure 5 shows the concept of TA lists of a UE that is registered in multiple TAs. A TA list is a group of adjacent TAs that is managed by the MME and periodically sent to the UE. The MME performs broadcast paging to the enodebs or small cells that are in this TA list if in the first paging attempt the UE was not located in the last seen TA or if the incoming service request warrants a larger broadcast. Figure 5. Tracking area and Tracking area list TA1 TA3 TA2 UE TA List = TA1 + TA2 + TA3 TRACKING AREA DEPLOYMENT STRATEGIES FOR SMALL CELLS Upon receiving a TA list from the MME, the UE is registered in multiple TAs. This 9

13 reduces the number of required TAU procedures and the resultant signaling load as long as the UE remains in the TAs of its TA list. A TA TAU procedure is initiated only when the UE crosses the TA boundary into another TA that is not in its TA list or when the periodic TAU timer expires. In both macro and small cell networks, the MME manages the UE s TA list. With the addition of small cells in LTE networks, both the number of TAs and the size (number of cells per TA) are expected to grow. For example, if metrocells are added to an existing macro network, the existing TA size will increase unless a new TA is created. Large TAs mean a greater signaling load when paging at the TA level. As HeNBs and HeNB gateways are introduced, the number of TAs will increase because each HeNB gateway must have its own unique TA. Tracking area updates can occur more frequently if a UE is moving along the border between enodebs or between HeNBs that are in different TAs and one of the TAs is not in the UE s TA list. This is known as the toggling location registration effect (see Figure 6), which generates multiple TAU procedures as the UE moves in and out of a TA boundary. Mobile service providers must be careful when designing TAs and TA lists so that the TAs are neither too large, which increases the amount of broadcast paging, nor too small, which leads to frequent TAU procedures. Figure 6. UE TAU Toggling Across TA Boarder TAU toggling TA1 TA2 In large-scale LTE networks, service provider mistakes are made and poor TA deployment strategies are implemented because of a lack of understanding of the signaling impacts that result. Figure 7 shows a large-scale metropolitan LTE network with multiple TAs that are split geographically and enodeb cells that are isolated from the rest of the enodebs in the TA. Despite the best efforts of MSPs, tight management of TA borders can be lost. Without correction, these events can lead to dramatic increases in TAUs at borders, and up to 3x the paging load as the TAs become oversized or UEs toggle across TA borders. This spike in signaling due to poor TA design is shown in Figure 8. Figure 7. LTE enodeb TA Assignments Figure 8. Ave. UE signaling load/hr by MME market 10

14 Dynamic tracking area management Alcatel-Lucent addresses the TAU toggling problem with a dynamic TAU feature on the 9471 WMM. This configuration parameter controls the automatic addition of tracking area identities (TAIs) to the UE s registered TA list based on the UE s mobility history. This configuration parameter also dynamically updates the UE with the most optimal TAI list, adding and removing TAs accordingly. By constantly optimizing the UE TAI list, fewer TAU procedures are required (TAU signaling is suppressed) and the potential for toggling effects at TA boundaries is reduced. Figure 9 shows two different examples of this dynamic TA management feature as the UE moves in a circular pattern between two TAs; TA (A) and TA (B), or between three different tracking areas, TA (1), TA (2) and TA (3). In the first case, the UE is currently registered in TA (A) but has previously been in TA (B). In the second case, the UE is currently located in TA (1) but the 9471 WMM has observed it moving in a circular pattern between TA (1), TA (2) and TA (3). Upon detecting this circular pattern, the 9471 WMM automatically sends a TAI list to the UE with all of the TAIs in that circular pattern. Figure 9. Dynamic TA management Dynamic UE TA List = TA(A)+TA(B) Dynamic UE TA List = TA(1)+TA(2)+TA(3) 9471 WMM TA(B) TA(3) TA(A) TA(1) TA(2) Dynamic TA list updating can be set automatically without operator intervention. Tracking area updates are signaling intensive and consume a lot of UE battery power, about 10 mw per TAU procedure for an LTE smartphone. This power estimate includes the TAU messaging; the UE scanning effort and enodeb attachment. A typical approach to suppress the frequency of TAUs is to adjust the MME s periodic TAU timer to a longer duration. However, by configuring the 9471 WMM to use dynamic TA management and its TAU suppression capabilities, fewer TAU procedures will occur, so less UE battery power will be consumed. Table 1 compares the power consumed by an LTE smartphone of an active mobile subscriber using the 9471 WMM dynamic TA management features with an MME with basic TAU management capability and various TA sizes: 800 mw/hour for transmit 500 mw/hour for receive 5.5 mw/hour while in Idle mode 11

15 The table shows that the 9471 WMM s dynamic TA list management TAU suppression techniques generate fewer TAU messages and consume less UE battery power, resulting in extended smartphone battery life of up to several hours depending on the TA size and the subscriber s activity level. Longer smartphone battery life results in happier subscribers. Table 1. UE Battery Savings with Dynamic Tracking Area Management BASIC MANAGEMENT DYNAMIC TA MANAGEMENT HIGH MOBILITY MODEL TA SIZE (enodebs) # TAUs/ UE/h UE POWER USED FOR TAUs # TAUs/ UE/h UE POWER USED FOR TAUs UE POWER SAVINGS UE BATTERY LIFE SAVED PER DAY (ACTIVE TIME) mw mw mw 92 minutes mw mw 61.1 mw 49 minutes mw mw 30 mw 24 minutes Tracking area deployment strategies for small cells When deploying small cells into existing LTE networks, MSPs should consider the following: If the MME pool areas remain the same size, the total number of cells per TA will increase significantly. The addition of small cells into already large TAs could force MSPs to split some TAs. New TAs may be needed when large concentrations of small cells (for example, in high-rise buildings) are added. If TAs are split or new TAs are added, the physical boundaries of the current TA borders should be maintained and optimized as much as possible to preserve existing TA/LA mapping. Every HeNB gateway requires a new TA, which then leads to large TA counts per MME pool. A large increase in total TA border area due to the splitting of TAs and the addition of new TAs brings a greater chance that a UE will toggle between adjacent cells in a TA border region, resulting in increased TAUs. TAU suppression will become increasingly important to reduce signaling. A TA size of over 200 enodebs or HeNBs will significantly increase overall paging volumes unless the additional cells are primarily closed access. A balanced network design must be achieved between contructing large TAs, which increases the volume of paging, and constructing small TAs,which increases the volume of TAUs. Conclusion Adding small cells to an existing LTE network can have a significant impact on EPC network signaling. How significant the impact is depends on several factors: the number of and type of small cells, the concentration of small cells in a specific area, whether the small cells are directly connected to the EPC or are connected through a small cells gateway, and their access mode (open, closed or hybrid). As the dedicated control plane element in the EPC, the MME is impacted by the addition of small cells in several key areas: S1-MME link scalability and the ability to provide rapid recovery from failed links Increased volume of broadcast messaging Paging methods and strategies Tracking area management and deployment strategy 12

16 The Alcatel-Lucent 9471 WMM has the capacity, performance and unique software capabilities to address each of these areas. This is evident in the innovative paging and tracking area management capabilities. The 9471 WMM can significantly reduce paging by as much as 80 percent through smart paging techniques that support a new paging method and paging policies that consider the service type or QCI level of the bearer. The 9471 WMM also supports CSG paging optimization to limit the amount of broadcast paging in small cell networks. These paging optimization techniques can be combined in an LTE macro/small cell network. By reducing paging, MSPs can support more subscribers per MME and defer additional MME capital investment. Tracking area management signaling can also become significant, particularly when small cells are introduced. Existing TAs may become too large and may need to be split, and new TAs may be required to support enterprise and residential applications. The 9471 WMM has innovative TA management capabilities that reduce the signaling associated with TAUs by detecting cyclic movement of a UE between TAs and providing dynamic TA list management that delivers an always-optimized UE TA list. By reducing paging and TA updates, the 9471 WMM can extend UE battery life. With all of these capabilities, the 9471 WMM is built to meet the new, more demanding signaling requirements of LTE and small cell networks. Find out more about the Alcatel-Lucent 9741 WMM at products/9471-wireless-mobility-manager References 1. 3GPP TS , 2013, GPP TR , 2009, 3. Alcatel-Lucent, LTE End-to-End Traffic Modeling Analysis,

17 Acronyms 2G/3G/4G 3GPP BBU CMAS CPRI CPU CSG embms erab EPC eutran HeNB HetNet LA LTE MME MSP NFV PLMN QCI QoE QoS RNC SCTP SGSN SGW SMS TA TAI TAU UE Wi-Fi second-generation/third generation/fourth generation 3rd Generation Partnership Program baseband unit Commercial Mobile Alert System Common Public Radio Interface central processing unit Closed Subscriber Group evolved multi-media broadcast/multi-cast service evolved UTRAN Radio Access Bearer Evolved Packet Core evolved Universal Terrestrial Radio Access Network Home enhanced Node B heterogeneous network location area Long Term Evolution Mobility Management Entity mobile service provider Network Functions Virtualization public land mobile network QoS Class Identifier quality of experience Quality of Service radio access network Stream Control Transmission Protocol Serving GPRS Support Node serving gateway Short Message Service tracking area tracking area identity tracking area update user equipment Wireless Fidelity Alcatel, Lucent, Alcatel-Lucent and the Alcatel-Lucent logo are trademarks of Alcatel-Lucent. All other trademarks are the property of their respective owners. The information presented is subject to change without notice. Alcatel-Lucent assumes no responsibility for inaccuracies contained herein. Copyright 2013 Alcatel-Lucent. All rights reserved. NP EN (October)