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Femtocells: Is There Room For Them On Your Network?

This method of boosting in-building cell coverage brings some nice benefits, but limited user capacity and competition from Wi-Fi mean you should evaluate carefully.

Sean Ginevan

July 20, 2007

5 Min Read

Beyond better coverage and faster data services, femtocells may also help lower cell phone costs by providing a "home zone" service, such as T-Mobile's T-Mobile@Home service. Generally, home zone services rely on either GAN (generic access network, also known as UMA) services or tying a user to a particular base station to create a home zone of up to several kilometers. Femtocells let operators create home zones limited to a building without users needing Wi-Fi access points--and without limiting users to expensive dual-mode handsets. With 25% of employees using their mobile phones at their desks, according to Strategy Analytics, the advantages of home zone services is clear.

However, femtocells aren't a slam dunk. From the carrier perspective, maintaining the integrity of the macro cellular network is the most important engineering goal, but the impact of femtocells is unclear. They'll have to obey conditions imposed by the macro cellular network. Otherwise, users walking or driving by a building that houses an open femtocell may roam onto it, then back onto the macro network. That's not good for carriers, since it could place strain on the back-end cellular infrastructure. In areas where there's a strong signal from the macro network, a femtocell may have to decrease its output power to the point where it provides only minimal coverage for the building it's in. Or a femtocell could be deployed on a separate frequency, which would consume some of the carrier's costly spectrum. Yet another option to address roaming complexities would be to restrict femtocells to specific phones so that outsiders don't accidentally roam onto them.

Base-station transmissions are also highly synchronized, meaning that a femtocell has to sync to an accurate time source. Unfortunately, synchronization can drive up the cost of femtocell use. Several options exist, including synchronizing to a network time protocol server via the IP backhaul connection; listening to timing signals from the macro network, which is viable only where the macro network exists; and using a GPS receiver for timing, though the GPS hardware needed for this approach increases cost and the femtocell or an antenna must have unobstructed GPS access.

Price-Conscious Path To The PSTN

(click image for larger view)

Beyond those drawbacks, an additional complication is that there is no standardized approach to integrating a femtocell into a cellular operator's core network; rather, four proposed architectures exist.

The first method is to create an IP tunnel, which encapsulates 3GPP (Third Generation Partnership Project) signaling to a radio network controller, sometimes referred to as a base station controller. The disadvantage for carriers is that, although lub sig- naling is standardized, there are generally vendor-specific features for each radio network controller, requiring operators to use femtocells and RNCs from the same vendor, creating lock-in. There are also scalability questions.


Femtocells aim to give users the ability to access cellular voice and data services in locations where signals are weak or spotty.

AirWalk, Ericsson, IPaccess, PicoChip Designs, NEC, Samsung, and Ubiquisys have offerings now. Others, including Motorola and Nokia, are hanging back, waiting for architectural decisions to shake out.

By offering mobile subscribers ubiquitous cellular-network voice and data access via femtocells, carriers won't miss out on those high-margin minutes. End users and customers enjoy better mobile voice and data performance as well as a stronger cellular signal and won't have to compete for resources from the macro cellular network. That said, femtocells' limited capacity--each can support just four to six phones--are a drawback. And there's competition: Both consumer- and enterprise-oriented mobile devices, for example, Apple's iPhone and various Nokia phones, are including support for Wi-Fi. As Wi-Fi becomes more ubiquitous, particularly in the consumer market, it may compete with cellular as the in-building wireless technology of choice, further limiting the potential market for femtocell technology.

A second method is to connect each femtocell to a separate and proprietary concentrator/RNC. The disadvantage of this approach is that operators would have to deploy additional network equipment solely to serve femtocells. A third option calls for connecting femtocells via IP to a UMA controller within the operator's core network. While UMA offers more flexibility in terms of supporting applications than a proprietary concentrator, it requires carriers to deploy additional equipment (in the form of UMA controllers) in their networks.

The final option is to have femtocells connect directly to the IP multimedia subsystem, or IMS, core of a cellular network with Session Initiation Protocol used for signaling (see diagram, above). The drawbacks are that IMS as a concept is relatively new, and those carriers on board with the idea are still in the process of deploying it throughout their networks, so femtocell projects would have to be delayed until upgrades are finished for each market.

Bottom line, femtocells represent an interesting way of tackling the problem of in-building coverage in small businesses and branch offices, where the extra cost for a voice-over-Wi-Fi-ready WLAN infrastructure and Wi-Fi-enabled smartphones may be prohibitive. However, Wi-Fi is clearly positioned as the in-building wireless networking technology of choice. Many enterprises are willing to invest in dual-mode phones to give employees access to mobile applications inside or outside of a building. And with Wi-Fi likely to be supported by many consumer phones in the next five years (witness Apple's iPhone), the long-term viability of femtocells in the enterprise is open to question. Still, for remote sites that need coverage, they're an option worth exploring.

Sean Ginevan is a technology analyst with the Center for Emerging Network Technologies at Syracuse University. Write to him at [email protected].

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