Electronic Design

Myriad Infrastructure Options Bombard Femtocell Design

Wireless communications continue to grow exponentially, but so do the architecture variations and related signaling protocol alternatives.

Cell phones are everywhere, and femtocells may follow. Designed for homes and small businesses, these desktop cell-phone basestations connect to the cell-phone network via an existing high-speed Internet connection using a DSL or cable TV modem.

The femtocell is a home version of the micro and pico cells used in buildings and other densely populated environments. They enable the network to handle more subscribers, and they improve indoor handset performance. And not only will they greatly improve home cell-phone connectivity, they’ll also help unburden the carriers’ already heavily loaded backhaul channels.

Femtocells promise unparalleled in-building coverage (i.e., five bars guaranteed in your home or office), dedicated highspeed 3G data services, and aggressive bundled tariff packages that allow flat-fee rates for unlimited voice and data calls while using your femtocell, for example. Beyond these three major benefits, femtocells are seen as a platform for service innovation, primarily by exploiting the fact that the device will be deployed in the customer’s residence or office.

Multiple architecture options are already on the market and under consideration for standardization (see the table). These include the Universal Mobile Telecommunications System (UMTS), Iub-over-IP, Iu-over-IP, Unlicensed Mobile Access/ Generic Access Network (UMA/GAN), and the IP Multimedia Subsystem (IMS).

Proponents of the UMTS-based architecture cite its ability to leverage existing mobile core network equipment, standards, and capabilities to deliver femtocell service faster with minimal added equipment investment. UMTS-based approaches provide an accelerated path to macro-network equivalent services, so operators can trial the femtocell value proposition with customers earlier.

The first UMTS-based approach is the Iub-over-IP architecture (Fig. 1). The femtocell access point (FAP), also known as the Home NodeB (HNB), takes on the role of the Node B (i.e., 3G basestation). The femtocell gateway (FGW), also known as the Home NodeB Gateway (HNB-GW), lies between the FAP and the radio network controller (RNC).

This approach specifically suits small-office/home-office (SOHO) or single-home environments where only a few subscribers would access services through the FAP. Depending on the number of FAPs connected to the FGW, the RNC and FGW could even be collapsed into one device.

The FAP communicates with the FGW using a 16-bit cell ID that uniquely identifies a femtocell. The FGW multiplexes the traffic coming from different FAPs and forwards it to the RNC using the Framing Protocol (FP). The FGW doesn’t modify any of the FP packets, especially the FAP identifier. At startup, the FAP establishes a security association with the FGW to avoid compromising subscriber information over the public IP network. The FAP could use TR-069 or some other similar mechanism to discover and obtain IP addresses from an auto-configuration server (ACS).

The RNC would handle the resource management (bearer and control) functionality. The RNC and FGW would take care of delay jitter for the bearer traffic and control signaling (specifically forced/hard handover mobility management).

To communicate with the pre-R4 Core Network (CN), which doesn’t support IP transport, the RNC will have to accomplish IP-to-ATM and vice-versa transport conversion. In this architecture, the FAP looks like a normal basestation to the CN. Also, all of the mobility management (MM) and call control (CC), including handover, remains in the CN equipment domain.

The second UMTS-based approach is Iu-over-IP (Fig. 2). The FAP takes on the role of both the NodeB and the RNC. A collapsed architecture makes it possible to offload existing RNCs, freeing up macro-network capacity while still leveraging existing protocols and cellular MM and CC procedures.

The configuration suits small and medium business (SMB) and multiple dwelling unit (MDU) environments, where multiple customers can access services through the FAP. The Iu-over- IP configuration creates a greater demand on the FAP to handle frequent MM and resource management (RM) procedures and places an even greater premium on advanced quality-of-service (QoS) capability in the FAP itself. The FAP communicates with the FGW using a 12-bit RNC-ID. The FGW tunnels Radio Access Network Application Part (RANAP) signaling from the FAP to the CN. It may also convert the IP transport from/to ATM transport using SIGTRAN functionality if the CN doesn’t support IP transport.

At startup, the FAP establishes a security association with the FGW to avoid compromising subscriber information over the public IP network. The FAP could use TR-069, or some other similar mechanism could be employed to discover and obtain IP addresses from an ACS for the FGW. In addition, the FAP uses the ACS to obtain RM parameters and algorithms to be exercised in the femtocell environment.

In this approach, the FAP handles most of the RM (i.e., bearer and control) functionality within the femtocell environment. It defers to the CN for these procedures during femtocell-tomacrocell handover.

The UMA/GAN-based architecture takes advantage of a proven and standards-based approach for providing cellular-grade services over the best-effort Internet (Fig. 3). UMA is an approach to fixed-mobile convergence that bridges cellular radio and unlicensed 802.11 Wi-Fi technologies.

The standard UMA architecture requires a dual-mode phone with specialized client software for managing handover and call control when moving from cellular to Wi-Fi networks and vice versa. The UMA/GAN-based femtocell architecture moves the specialized client, or UMA client, from the handset to the FAP, eliminating the requirement for a dual-mode phone while leveraging UMA’s signaling and QoS innovations.

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UMA/GAN resembles the femtocell approach, though femtocell extends the licensed 2G/3G wireless spectrum to the customer premises instead of bridging licensed and unlicensed wireless technologies. So, the underlying UMA/GAN architecture can likewise be extended to support the femtocell approach by overloading the GAN controller (GANC) with Iu FGW functionality.

This architecture best suits service providers who have an existing GAN infrastructure, but want to offer innovative high-value, high-bandwidth services using High Speed Packet Access (HSPA) capability. Such capability requires higher bandwidth than what’s offered by current 802.11 deployments.

The FAP presents the Up interface to the FGW, which also acts as the GANC. The FGW communicates with the CN using the Iu interface. At startup, the FAP establishes a security association with the FGW to avoid compromising subscriber information over the public IP network. The FAP could use TR-069 or some other similar mechanism to discover and obtain IP addresses from the ACS for the FGW. The FGW treats the femtocell as an IP-based device, and these nodes communicate using IP address and port numbers.

The FAP converts voice packets to RTP packets and forwards them to the FGW, which may have to convert the RTP packets back to voice, based on the CN transport network. Transcoding multiple times due to different transport networks may lead to loss of voice quality, which can be fixed by implementing strict QoS at the Up interface.

Proponents of the IMS-based architecture are looking to take advantage of the mobile operator’s move to an all-IP based core network (Fig. 4). The evolved IMS CN provides an excellent platform for service innovation by exploiting easily extensible technologies like the Session Initiation Protocol (SIP).

The IMS-based architecture suffers from the lack of a proven MM and CC model and limited standards around handover. However, the business case for femtocells is intertwined with the ability to deliver new services in IMSbased femtocells.

In this approach, the FAP interworks the UMTS signaling plane with the SIP signaling protocol over the public IP network. On the IMS core side, the FAP may interface directly with softswitches providing call session control function (CSCF) functionality using SIP and interface directly with the home subscriber server (HSS) using the Diameter protocol for authentication, authorization, and accounting (AAA) functionality.

Alternatively, the FAP may choose to interface with these devices through an aggregating packet data gateway.

On the bearer plane, the FAP forwards voice traffic toward the IMS core as Real-Time Protocol (RTP) packets. QoS depends on the public IP network’s capablities, including reliability and minimization of packet delays and loss. TR-069 could again be used for zerotouch initial system configuration and service provisioning of the FAP.

Handovers in the IMS-based approach are inter-CN in nature, i.e., between the mobile switching center (MSC) and the serving GPRS support node (SGSN). The FAP will handle most RM (bearer and control) functionality within the femtocell environment and would defer to the CN during femtocellto- macrocell handover. The latter is a key issue to resolve from a standardization perspective if this model is to reach widespread operator acceptance.

Femtocells provide a potent weapon for mobile operators as they compete for additional minutes used within the home. They let operators improve coverage, increase capacity, reduce customer churn, and provide innovative high-quality services, driving increased average revenue per user (ARPU).

Yet challenges remain. How much interference will occur between closely located femtos in apartment buildings, and what can be done about it? How much of a carrier’s backhaul can be relieved of the massive increases in 3G data traffic and services like video?

Success will depend on further innovation and architecture standardization. The good news is that significant strides toward a common standard have been made within the 3rd Generation Partnership Program (3GPP) and 3GPP2. Specifically, 3GPP has announced that an agreement has been reached on a standard architecture that leverages existing UMTS capabilities as well as innovations from the UMA approach.

The new interface between the FAP and FGW, known as Iu-h, still needs to be specified further, but this represents a significant step forward toward harmonization around a single architecture. There is more work yet to be done on the details of the new Iu-h interface, as well as open actions to drive a next-generation standard based around SIP/IMS. The rapid move toward an initial standard, though, represents the industry’s strong desire for successful femtocell rollout.

Until the standard is ironed out, however, femtocell device manufacturers must be able to support multiple architecture approaches, since operators want to perform trials now, and each carrier has divergent requirements for the architecture it wants to utilize. This choice often depends more on an operator’s existing network assets and evolution plans than on the independent merits of the various femtocell architectures.

As a result, femtocell device manufacturers need to develop significant breadth of support for different protocols inhouse, which is a complex and time-consuming endeavor, or partner with key telecom technology experts who can support all of the different approaches immediately.

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