Femtocells: Is There Room For Them On Your Network?

Businesses are mobilizing an increasing number of applications, including e-mail and CRM. In a recent reader poll, 70% of respondents said they use mobile/wireless applications to access organizational data; communications mechanisms were fairly evenly split among Wi-Fi and cellular networks. Pervasive mobile coverage is clearly important, and femtocells are one way to ensure good inside cellular connectivity for customers and employees, even in locations where that's a challenge.

Much of the value and ROI of application mobility projects depends on pervasive connectivity. And without good cellular coverage, mobile applications are used less, which translates into a lower average revenue per user for carriers. In-building wireless systems, like newly conceived femtocells, can fill in coverage gaps, driving increased ROI for enterprises and average revenue per user for carriers.

There are many ways of boosting mobile coverage, including the use of passive repeaters, distributed antenna systems, and fixed- mobile convergence. Femtocells promise to let users roam onto an in-building network without a pricey dual-mode phone. A number of niche vendors have stepped forward with these products, and some big manufacturers are surveying the field. At an average cost of $150 to $200, a single femtocell could support four to eight users within a range of roughly 300 feet, depending on the power output of the femtocell and the physical characteristics of the site. Traffic can be backhauled over standard IP connections, like DSL or cable modems, yet let cellular phones obtain cellular access for both voice and 3G data.

The big disadvantage of current femtocells is their low capacity; picocells, which are more expensive but allow for greater per-cell call capacity, are useful both for providing indoor coverage for buildings with high user density as well as for boosting outdoor call capacity in a macro cellular network. And there's competition from Wi-Fi vendors. The progress being made toward fixed-mobile convergence within the enterprise means femtocells may have a tough time making inroads in this market.

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Femtocells deliver numerous benefits for end users, including better coverage within a building. As users move beyond the coverage of the macro cellular network provided by base stations run by the carrier, they'll be able to roam seamlessly onto the in-building network provided by the femtocell, using existing handsets.

Femtocells should also provide 3G services that otherwise wouldn't be available from the macro network because of low signal strength or because the carrier hasn't rolled out faster radio technologies such as EV-DO. And users may be able to get better network speeds from the femtocell because they won't have to compete with others for either radio or backhaul resources on the macro network. Of course, they may still have to compete with other users for femtocell access.SHOW ME THE ROI

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.

(click image for larger view)CORE CONNECTION
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.

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].