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The concept of femtocell is an integral part of the telecommunication industry's efforts to provide high throughput, high quality services into the users' home. In contrast to conventional cell types which are well-planned by the operators, femtocell base stations are supposed to be installed by customers themselves, similar to a WiFi access point. Unlike WiFi however, femtocells operate mainly in licensed bands, such that operators are in control of the radio interface. This brings new challenges as well as opportunities for femtocell design; these include sophisticated mobility and interference management, increased reliability, as well as deployment in a plug-and-play manner. Extensive progress in femtocell design has been made in Advanced WiMAX recently, which is associated with the IEEE 802.16m update in 2011. This report gives an overview and update on novel concepts and mechanisms for femtocell support in the air interface and network architecture which have been adopted into the IEEE 802.16m specification and the WiMAX Forum network specifications.
The massive increase in demand for wireless data traffic has created opportunities for new network architectures incorporating multi-tier base stations with diverse sizes. Support for small-sized low-power base stations (BS) such as femtocell BSs is gaining momentum in cellular systems, because of their potential advantages such as low cost deployments, traffic offloading from macrocells, and the capability to deliver services to mobile stations which require large amounts of data [1, 2, 3].
Femtocells will be supported by next generation cellular systems, such as Advanced WiMAX systems. Advanced WiMAX, which will provide up to 1Gbit/s peak throughput with the IEEE 802.16m  update in 2011, is one of the candidate technologies for the ongoing International Mobile Telecommunications (IMT)-Advanced programme for the 4th generation (4G) of mobile wireless broadband access. IEEE 802.16m defines the Wireless Metropolitan Area Networks (WirelessMAN)-Advanced Air Interface as an amendment to the ratified IEEE 802.16-2009 specification  with the purpose of enhancing performance such that IMT Advanced requirements are fulfilled. In Advanced WiMAX, femtocell support is one of the solutions to provide high performance services even in indoor scenarios.
In Advanced WiMAX, a femtocell BS, or WiMAX Femtocell Access Point (WFAP), is a low power BS that is intended to provide in house/SOHO coverage. With conventional macro or micro cells, indoor coverage is challenging and expensive to achieve due to the high penetration losses of most buildings. WFAPs are usually self-deployed by the customers inside their premises, and connected to the radio access network (RAN) of the service provider via available broadband connections like DSL or fiber-to-the-home (FTTH).
The self-deployment of WFAPs has consequences on the requirements for operation and management. The WFAP must be able to react, in a highly flexible manner, to different interference situations, since neither the location nor the radio propagation environment can be predicted in advance. Furthermore, the customers are in physical control of the WFAP meaning that they can switch it on or off any time. Other factors like unreliable backhaul connections must also be considered.
Considering these scenarios, some of the technical challenges and requirements can be identified as follows ,
- Tight integration into the existing WiMAX architecture for support of seamless mobility between
macrocells and WFAPs, low-complexity network synchronization and localization.
- Advanced interference mitigation techniques to guarantee quality of service and coverage in macrocells as well as femtocells.
- Access control for different groups of subscribers as well as energy efficient recognition of access rules by the mobile station (MS).
- Support for increased reliability and autonomous reaction on irregular network or WFAP conditions.
To design WFAPs capable of meeting these challenges and requirements, standardization efforts are being made in both IEEE 802.16 and WiMAX Forum. IEEE 802.16 Task Group "m" (TGm) defines the air interface support for femtocells. IEEE 802.16m is expected to be completed by the end of 2010, and scheduled for final approval as a technical standard in the first quarter of 2011. In parallel, the WiMAX Forum Network Working Group (NWG) is driving the development of specifications for femtocell solutions in the access and core network.
A high level description of some of the technologies to support WFAPs are in  and , but were not yet technically specified in the corresponding working groups at the time of writing. As IEEE 802.16m is close to completion, a significant number of technical details have been discussed, evaluated, and finally adopted into the specification. This report provides a detailed update on these recent developments in the standardization process for femtocell design in Advanced WiMAX.
In the next section we describe how WFAPs are integrated into the WiMAX network architecture. The Advanced Air Interface support for mobility in WFAPs, interference mitigation, WFAP reliability, and WFAP low duty mode, are introduced in the second section, which is mainly based on the most recent efforts in IEEE 802.16m. The report is concluded with some summarizing observations.
2. NETWORK ARCHITECTURE FOR WIMAX FEMTOCELLS
Figure 1 shows the general WiMAX network architecture with additional support for femtocells. In the following section, the main functional entities are described.
2.1. GENERAL WIMAX NETWORK ARCHITECTURE
The NSP (Network Service Provider) provides IP data services to WiMAX subscribers, while the NAP (Network Access Provider) provides WiMAX radio access services to one or more NSP. A WiMAX operator may own both NSP and NAP. A macro NAP contains one or more macro ASNs. A macro ASN is composed of an ASN Gateway and one or more BSs to provide mobile Internet services to subscribers. The ASN Gateway serves as the portal to an ASN by aggregating BS control plane and data plane traffic to be transferred to the CSN (Connectivity Service Network). An NSP contains the CSN which is composed of the AAA entity (Authentication, Authorization, and Accounting) and the HA (Home Agent) to provide a set of network functions (e.g. roaming, mobility, subscription profile, subscriber billing) that are needed to serve each WiMAX subscriber.
2.2. NETWORK ARCHITECTURE TO SUPPORT WIMAX FEMTOCELL
For the support of femtocells, a Femto NAP and a Femto NSP are introduced. Additionally, SON (Self-Organizing Networks) functions are added.
Femto NAP: A Femto NAP contains one or more Femto ASNs to provide short range radio access services to femtocell subscribers. When a WFAP is booted, it first communicates with the Bootstrap Server to download the initial configuration information, including the IP address of the Security GW. The WFAP and the Security GW authenticate each other and create a secure IPSec tunnel. The Femto Gateway acts as the portal to a Femto ASN that transfers both control and bearer data between MS and CSN, and control data between WFAP and Femto NSP.
Femto NSP: The Femto NSP manages and controls entities in the Femto ASN. The AAA function performs authentication and authorization of the WFAP. The management server implements management plane protocols and procedures to provide OAM&P (Operation, Administration, Maintenance & Provisioning) functions to entities in the Femto ASN. OAM&P enables the automation of network operations and business processes that are critical to WiMAX Femtocell deployment. WFAP management includes fault management, configuration management, performance management, and security management.
SON Functions to Support Femtocell: SON functions can be divided into self-configuration and selfoptimization. Due to the large number of WFAPs expected, self-configuration is primarily intended to enable auto-configuration and avoid truck rolls. However, since femtocell deployments are not planned by operators, it is very important that the configuration (e.g. radio parameters setting) should take its neighbors into account by not adding interference to the users. The wireless environment changes dynamically, as WFAPs can be powered on and off at any time. Self-optimization provides a mechanism to collect measurements from MS and fine tune system parameters periodically in order achieve optimal system capacity, coverage, and performance. Therefore, the SON server needs to interact with SON clients not only in the Femto ASN but also in the macro ASN. The information elements exchanged between the SON server and SON clients will be conducted on the management plane. Therefore, they will be transported using the same management plane protocols as defined in the femtocell management specification.
WFAP: For proper integration into the operators RAN, the WFAP enters the initialization state before becoming operational. In this state, it performs procedures such as attachment to the operators' network, configuration of radio interface parameters, time/frequency synchronization and network topology acquisition. After successfully completing initialization, the WFAP is integrated into the RAN and operates normally. In operational state, normal and low-duty operation modes are supported. In low-duty mode, the WFAP reduces radio interface activity in order to reduce energy consumption and interference to neighboring cells. The low duty mode will be discussed in more detail in a separate section.
Figure 1: WiMAX network architecture
2.3. AIR INTERFACE SUPPORT FOR WIMAX FEMTOCELLS
The IEEE 802.16m draft specification provides support for the operation of WFAPs and the integration of WFAPs in macrocell networks to provide functionality such as access control, network topology acquisition, mobility support, interference mitigation, reliability, and WFAP low duty mode. In the following section, these features are introduced in more technical detail.
2.4. FEMTOCELL ACCESS CONTROL
For the typical use case of WFAPs as a "private base station", access control schemes must be supported. A Closed Subscriber Group (CSG) containing a list of subscribers restricts access to WFAPs or certain service levels. IEEE 802.16m defines three modes for WFAP access; these are:
CSG-Closed WFAPs are accessible exclusively to members of the CSG, except for emergency services.
CSG-Open or hybrid WFAPs grant CSG members preferential access. However, subscribers which are not listed can still access the WFAP at a lower priority.
OSG (Open Subscriber Group) WFAPs are accessible by any MS much like a normal macro BS.
For efficient identification of subscriptions and accessibility of WFAPs, a femto-enabled MS can maintain a CSG whitelist, containing a set of WFAPs and corresponding attributes like geographical location or overlaying macrocell identifiers. To avoid large CSG whitelists, a CSG identifier (CSGID) is defined which describes a group of WFAPs within the same CSG. The CSGID can be derived directly without any additional information from the global unique BS identifier (BSID) of the WFAP.
3. NETWORK TOPOLOGY ACQUISITION
Knowledge of the network topology is critical for efficient interference mitigation and mobility management between macrocells and WFAPs and among WFAPs. Both macrocells and WFAPs have to be aware if a WFAP enters or leaves the environment, thus changing interference and mobility conditions. Furthermore, MSs can perform cell searching and handovers in a more efficient way if the type and the access policies of the WFAPs in connection range are known beforehand. IEEE 802.16m supports MS-assisted network topology acquisition, but the WFAPs can also scan the radio environment to find neighbor or overlay cells. Figure 2 shows some approaches for network topology acquisition.
3.1. MS acquisition of WFAP topology
IEEE 802.16m adopts an energy-efficient two-step scanning method for the MS to identify neighboring WFAPs, and further to efficiently identify whether an MS is allowed to access the WFAP. Identified WFAPs and their attributes can then be reported to overlaying macrocells and neighboring WFAPs. Base station types are differentiated by the frame preamble sequence, which is uniquely mapped to an IDcell identifier. The total number of preamble sequences is partitioned into subsets to differentiate between BS types. To make the scanning and possible network entry efficient, the set of IDcells is partitioned into sets for macro and non-macro cells, where the latter set is further partitioned into private (further partitioned into CSG-closed and CSG-open) and public cells (further partitioned into pico and relay).
The two-step scanning method works as follows. In the first step, an MS scans the frame preamble sequence to determine the BS type. However, the number of WFAPs within the coverage area of a macrocell may well exceed the number of available IDcell identifiers, such that the identity of a WFAP may not be resolved uniquely. To solve this WFAP ambiguity problem, the MS decodes in the second step the periodically broadcasted superframe header (SFH) to obtain the unique BSID identifier. Note that to save battery energy, the second step is only performed if necessarily. In the second step, the MS can also derive the CSGID of the WFAP, and compare with its local CSG whitelist to determine whether the detected WFAP is an accessible cell for the MS.
3.2. WFAP acquisition of neighboring cells
The WFAP can acquire the network topology from the backhaul, from the reporting MSs, or by active scanning. The WFAP can scan and measure its neighboring cells, such as overlaying macrocells, or other nearby WFAPs, for interference management and to assist the cell (re)selection of the MS. The WFAP in this way acts like an MS. However, in TDD (time division duplex) systems, the WFAP cannot transmit frame preambles and SFH during scanning. Hence, the WFAP broadcasts a SON-ADV (SON advertisement) message which includes the timing information of the scanning interval, in which the WFAP scans the other cells, while its own preambles and SFH may not be available for the MS in its coverage to scan. This message prevents the MS from scanning a WFAP which is not available for scanning.
Figure 2: Network topology acquisition
4. FEMTOCELL MOBILITY MANAGEMENT
Femtocell networks, especially in the case of dense deployments, are challenging for mobility and hand-over functions due to the large number of small cells with different access types. Special focus must be set on cell scanning functions to avoid high energy consumption on the MS side. Also, seamless hand-over must be supported to avoid QoS degradations. Figure 3 shows some optimized mobility management support in IEEE 802.16m.
4.1. Optimized MS scanning of WFAPs
Macrocell BSs and WFAPs can help MSs in the process of scanning for WFAPs by conveying information on the WFAP network topology. This is achieved by broadcast, unicast, and request-response message exchanges. Specifically, a macrocell BS can broadcast information on OSG WFAPs in their coverage area like carrier frequencies or IDcell partitions to reduce the scanning time for MSs. Furthermore, after successful association of an MS to the macrocell network, a macrocell BS can transmit a list of accessible neighboring WFAPs. An MS may also explicitly request a list of accessible WFAPs.
An MS may request additional scanning opportunities from a BS by sending a message including the detected IDcell index and carrier frequency information. Upon reception of the message, the BS can respond with list of accessible neighbor WFAPs.
Scanning of closed-subscriber group WFAPs should be avoided as far as possible as long as the subscriber is not authorized. Therefore, the MS may provide CSGIDs of CSG whitelists to the current serving base station to obtain instruction on how and when to scan, these instructions may include a list of WFAP BSIDs which are associated with the requested CSGIDs.
Furthermore, information on the location of WFAPs is also exploited. The CSG whitelist may include location information of CSG WFAPs, such as GPS info or overlay macrocell BSID. The network may also instruct (by sending a message, which may include a list of allowable WFAPs nearby the MS) the MS to scan WFAPs based on location information available at the network.
4.2 Handover support for WFAPs
Handovers between macrocells and WFAPs as well as inter-WFAP handovers should be on the one hand transparent and seamless for high QoS, on the other hand user preferences must be considered. For example, subscribers may prefer their home WFAPs even if the signal strength of the WFAPs is lower than that of adjacent macrocell BSs. To this end, the WirelessMAN-Advanced Air Interface defines handover and scanning trigger conditions, and target BS priorities for femtocells based on the BS type. Trigger conditions can be defined such that an unwanted handover from a home WFAP to the macrocell network is avoided, or vice versa handovers to WFAPs are preferred.
In addition, the network can instruct the MS on how to prioritize the cell (re)selection. For example, the network or the serving BS can send the MS a message that includes a prioritized list of the candidate target base stations.
Figure 3: Optimized handover scheme
5. INTERFERENCE MANAGEMENT
Since WFAPs are overlaid by macrocells, interference occurs not only in the tier of WFAPs, but also across tiers, i.e. between WFAPs and macrocells. Advanced interference management is therefore crucial for viable operation of femtocell networks, especially in dense-deployment scenarios. Several factors need to be considered, such as whether the WFAP and overlay macrocell use the same frequency, whether multiple frequency carriers are available, whether interference management is applied to control channel or data channel or both, and whether the interfered MS is in connected or in idle mode.
In order to provide seamless connectivity and high QoS to mobile stations, the WirelessMAN-Advanced Air Interface supports advanced interference management methods with a set of technologies targeting different scenarios. The purpose is to achieve efficient inter-cell interference mitigation with acceptable complexity in an optimized manner. Advanced interference management crosses multiple layers, such as physical layer (e.g., power control, carrier change), MAC layer (e.g., signaling, messages, resource management such as resource reservation), network layer (e.g., security, SON server coordination), and other higher layers (e.g. mobile station QoS requirement and provisioning). Some of these technologies are described below:
Resource Reservation and Blocking - A CSG WFAP may become a strong source of interference for non-member MSs which are associated to a nearby macrocell. In this case, the WFAP blocks a radio resource region (i.e. a time/frequency partition of the radio frame) exclusively for non-member MSs for communication with the macrocell BS.
Power control - A CSG WFAP adjusts the transmit power to reduce interference at non-member MSs. For example, the transmit power may be reduced to satisfy the minimum QoS requirements of its member MSs if the WFAP is strongly interfering non-member MS(s). The power level may be restored again to provide better QoS to its member MSs as soon as the non-member MS left the coverage area of the WFAP.
Coordinated Fractional Frequency Reuse (FFR) - Both WFAP and macrocells coordinate their frequency partitions and the associated power levels over the frequency partitions.
Frequency carrier change -The WFAP can change to another frequency carrier with less interference if there are more than one frequency carriers available.
Spatial coordinated beamforming - If beamforming is supported by the WFAP, the WFAP and/or the macrocell can coordinate their antenna precoding weights to avoid or mitigate interference.
Femtocell-macrocell coordinated hand off scheme - A CSG WFAP can hand off some of its member MSs to a nearby macrocell so that the WFAP can adjust radio resources (for example by means of power control or radio resource reservation) to reduce interference to non-member MSs served by the macrocell. The timing of the resource adjustment can be adaptively set to accommodate the QoS requirements from both the WFAP member MSs and the non-member MSs.
Femtocell type change under service agreement - If required, a CSG WFAP can temporarily change its subscriber type (e.g. from CSG-Closed to CSG-Open) if it strongly interferes with a non-member MS, such that the MS can hand-over to the now CSG-Open WFAP. The subscriber type is restored as soon as the non-member MS leaves the coverage area of the WFAP.
Figure 4: Two-step interference management in case a CSG-closed WFAP generates high interference at a non-member MS.
One of the biggest problems for the operation of heterogeneous macrocell/WFAP deployments is the creation of coverage holes for macrocell users by CSG-closed WFAPs. If a mobile station is not a member of the subscriber group of a CSG-closed WFAP, the received signal power is experienced as interference. This may lead to service degradation and in the worst case to connection loss - i.e. a coverage hole is created. To solve this problem, IEEE 802.16m defines a two-step solution as shown in Figure 4:
Step 1: After scanning, an MS detects that the only BS with acceptable signal quality is a CSG-closed WFAP where the MS is not listed as member. Normally, a non-member MS should not try to access the CSG-closed WFAP . However in the exceptional case of a coverage hole generated by the CSGclosed WFAP, the non-member MS can signal the coverage hole situation to the WFAP by means of a reserved CDMA ranging code.
Step 2: The WFAP can notify the macrocell and a network entity such as a SON server to request
coordinated interference mitigation. It has to be noted that the coordinated interference management not only means the non-member MS served by the macrocell BS will get desired QoS, but also the WFAP tries to guarantee its member MSs desired QoS. Depending on the scenario, interference mitigation approaches such as resource reservation, power control, FFR, or beam-forming can be applied.
The two-step approach can be used in a general case when MS is connected with macrocell, where in Step 1 the MS can report the interference to the macro BS, while in Step 2 the macro BS can coordinate with the interfering WFAP via the backhaul network and then the interference mitigation approaches can be applied.
5.1 WFAP SERVICE RELIABILITY
Since WFAP BSs are under physical control of the customers, normal operation may be interrupted for various reasons. Typical examples are loss of power support or backhaul connectivity problems. Also, operators may schedule maintenance times and network topology reacquisition or interference mitigation procedures. However, service continuity should be maintained as much as possible in these cases. IEEE 802.16m introduces features for increased reliability and continuous service to the MS (see Figure 5) should such scenarios arise.
Figure 5: Illustration of WFAP reliability design
As a basic rule, the WFAP will try to inform the network and MSs in case of any service disruptions. On the air interface, this is done by means of a periodically broadcasted message. The message encodes the reason for the unavailable time, relevant system parameters like transmit power reduction and frequency allocation index, and eventually the duration of the air interface absence, if known beforehand. Additionally, a list of recommended BSs for the MS to handover can be included. The message is broadcasted until the WFAP disables the air interface. This allows the MS to initiate a hand-over to a BS based on the recommended list or to any previously cached neighbor BS list of its preference. Alternatively, the WFAP can instruct the MS to handover to other
BSs before a scheduled unavailable time is due. For optimized network re-entry to a WFAP which becomes available again, the WFAP may store MAC context information of the served MSs (e.g. basic capabilities, security capabilities, etc.).
If the WFAP recovers from failure of backhaul, or power down, or reconfiguration or it regains some resources from interference coordination, it may inform the network or notify the current serving BS of the MS through the backhaul network interface. Based on the cell types of the current serving BS and the WFAP and the associated mobility management policy, the current serving BS may then initiate a handover back to the WFAP, where the recovered WFAP is prioritized.
5.2. WFAP OPERATION IN LOW DUTY MODE
A novel optional operational mode, denoted as Low Duty Mode (LDM) was introduced into IEEE 802.16m in order to reduce interference and energy consumption in femtocell deployments. The principle of the LDM is to reduce air interface activity as much as possible by transmitting on the air interface only if it is required. To this end, default LDM patterns consisting of available intervals (AI) and unavailable intervals (UAI) are defined which enables a pattern of activity and inactivity for the WFAP. The UAI allows disabling or switching off of certain parts of WFAP hardware components such as the transmitter chain. Another possibility is to use UAIs for scanning and measurement of the radio environment in order to improve interference mitigation or for synchronization to the macrocell network. During an AI, the WFAP is available for any kind of transmission just as in normal operation state besides being guaranteed to be available for scanning by the AMSs.
The LDM is designed with two basic paradigms: First, a WFAP may enter LDM only in case there is no active MS attached. This rule is established in order to avoid complex signaling and possible QoS degradation at the user side. Second, the impact on the operation of the MS should be minimized in order to keep implementation costs low.
The Default LDM pattern is either pre-provisioned, unicasted during network entry or may be broadcasted, such that MSs have the necessary information on when the WFAP is available, for example for requesting bandwidth. The WFAP switches back to normal mode on explicit request from the backhaul network or implicitly by receiving any triggering of data activity from the MSs.
An AI will be scheduled by the WFAP whenever there is an operational need for it. Therefore, the resulting AI pattern at the WFAP is the superposition of the Default LDM pattern and any additional AIs necessary for normal MS operation. This is illustrated in Figure 6. Note that for interference mitigation, it is desirable to reduce the transmitting time of the LDM as much as possible, for example by aligning paging and LDM Default Patterns.
Figure 6: WFAP operation in low duty mode
The femtocell concept is supported in Advanced WiMAX for low cost deployment, high throughput and high quality of service in indoor scenarios. Recent standardization activities in IEEE 802.16m and the WiMAX Forum, the technical details for the next generation WiMAX femtocell design have been defined. Advanced WiMAX introduces innovative solutions for femtocell support into the WiMAX network architecture and the WirelessMAN-Advanced Air Interface. This report highlighted the challenges and design principles of Advanced WiMAX Femtocells. The features and mechanisms which solve the unique problems of deploying and operating femtocell networks have been illustrated. These include network topology acquisition, enhanced mobility management, coordinated interference management, increased service reliability, and operation of Low Duty Mode.