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Monday, 23 November 2009

WiMAX Femtocell System Architecture


So what does it take to build a WiMAX Femtocell solution?

WiMAX Femtocell can be visualized as a scaled down version of WiMAX macro-cell solution. In addition to the capabilities of a WiMAX macro-cell, other required features of a WiMAX Femtocell are the following:

Spectrum: WFAP operates over licensed spectrum using standard WiMAX wireless air interface and protocol.

Form factor: WFAP can be standalone (similar to WiFi access points) or integrated with DSL or cable modems.

Transport: WFAP uses transport network of subscribers’ DSL, FTTH or cable-based broadband connection.

User Capacity: Since WFAP is deployed inside a building; a WFAP needs to support at least 5-6 subscribers.

Power Output: With a range of roughly 10 meters, power output should be kept very low, no more than a 2.4 GHz WiFi product.

Deployment Support: Operating in a licensed spectrum a WFAP may face interference from neighboring base stations (femto or macro). Therefore, a WFAP should have the capabilities to automatically adjust to minimize the interference.

Local Breakout: A WFAP should optionally support the capability to route incoming or outgoing traffic directly to the destination through the Internet Service Provider (ISP) network. This approach will bypass the WiMAX service provider network, thus offloading WiMAX service provider network and reducing the cost of service to the subscriber.

Performance: A Femtocell solution should fit as per the WiMAX network architecture defined by the WiMAX forum. The deployment should not limit the number of WFAPs that are able to connect with a designated ASN Gateway unless operator specified. A network deployment should allow different ISPs to connect WFAP with ASN Gateway in the core network.

Hand-over: A Femtocell solution should allow handovers between WFAP and WiMAX macro cells or with other adjacent WFAPs.

Security: A Femtocell solution should use a secure channel of communication (for both control plane and data plane) with ASN Gateways in the core network. The core network must authenticate and authorize a WFAP before it starts offering services to MS/SS in its coverage area. A WFAP may authenticate the ASN Gateway with which it gets connected. A WFAP should keep its air interface disabled unless it is authenticated and authorized to start communication with the ASN Gateway in the core network. A Femtocell may support close subscriber group (CSG) database i.e. a list of subscribers allowed to access the WFAP, and its management.

Accounting: For providing different rate plans to subscribers accessing services through WFAP, a WFAP needs to make sure that it is recognized by the core network.

Location Information: A WFAP should support location identification procedures with the core network. Location information can then be used for emergency services or location based services.

Air Interface: A WFAP should provide at least 10 meters of coverage area in a residential set up without any exclusion zone around it.

Network Synchronization: A WFAP should support mechanism to synchronize with external network to provide services that require strict air interface co-ordination. Some of the services are soft-handovers, support for idle mode paging, and multicast-broadcast (MCBCS) services.

Quality of Service: A WFAP should support marking of incoming/outgoing packets with appropriate DSCP code, as configured by a service provider. This would allow support for defined service level agreements (SLAs) when the service is delivered through a WFAP.

Manageability: A WFAP should implement DSL forum’s defined TR069 protocol to allow an operator to remotely manage a WAFP. It must allow an operator to remotely disable/enable the air interface service.


The WiMAX network architecture for femtocell systems is based on the WiMAX basic network reference model that differentiates the functional and business domains of NAPs from those of the network service providers (NSPs). The NAP is a business entity that provides and manages WiMAX radio access infrastructure, while the NSP is the business entity that manages user subscriptions, and provides IP connectivity and WiMAX services to subscribers according to negotiated service level agreements (SLAs) with one or more NAPs. A NAP is deployed as one or more access service networks (ASNs), which are composed of ASN gateways and BSs, while the NSP includes a home agent, authentication, authorization, and accounting (AAA), and other relevant servers and databases.

In a WiMAX network supporting a femtocell, a new business entity called the femto-NSP is introduced, which is responsible for the operation, authentication, and management of WFAPs. The femto-NSP is logically separated from the conventional WiMAX NSPs responsible for MSs’ subscriptions, and it includes femto-AAA and femtocell management/self-organizing network (SON) subsystems.

The femtocell management system is an entity to support operation and maintenance (O&M) features of the WFAP based on TR-069 or DOCSIS standards. Because potentially many femto BSs will be deployed in overlay coverage of macrocell BSs and have to support handover to/from macrocell BSs or neighbor femto BSs, the operating parameters of femto BSs have to be well organized and optimized. Femto BS parameter configuration and network performance, coverage, and capacity optimization can be done in an autonomous fashion by using SON functions. A SON server provides SON functions to measure/analyze performance data, and to fine-tune network attributes in order to achieve optimal performance.

A femto-NAP implements its infrastructure using one or more femto-ASNs; an ASN is defined as a complete set of network functions needed to provide radio access to a WiMAX femtocell subscriber. The reference model for a the femto-ASN is defined based on some changes to the conventional ASN to address specific needs of WFAPs, which typically reside at customer premises, and are operated and managed remotely by a femtocell operator over third party IP broadband connection. The femto-ASN reference model includes a WFAP connected to a femto-GW serving as the ASN-GW, through a new entity called a security gateway (SeGW). The SeGW provides IP Security (IPsec) tunnels for WFAPs, and is responsible for authentication and authorization of the WFAPs. The WFAP is connected to a femto-ASN gateway (femto-ASN GW) and other functional entities in the network through this IPsec tunnel. The management system is connected to WFAP through Rm for remote configuration, and it will also include the SON server function, to be defined in the next releases of the femto architecture.

The femto-ASN GW is an entity that controls WFAPs, and performs bearer plane routing to the CSN and Internet as well as control plane functions similar to ASN-GW providing the link to the connectivity service network (CSN) and other ASNs with mobility and security support in the control plane and IP forwarding. In addition to common functionalities of the ASN-GW, the femto-ASN GW supports femto-specific functionalities such as closed subscriber group (CSG) subscriber admission control, femtocell handover control, WFAP low-duty mode management, and femtocell interference management.


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