New approaches to an old problem

Dec 1, 2007 12:00 PM, By Dave Plank

Wireless users, especially in the enterprise and public-safety sectors, have been waiting for affordable, reliable in-building coverage since the first police officer hit the “xmit” key on his two-way radio. Recent technological developments — as well as deployments of nascent fixed/mobile convergence, or FMC, solutions in some areas — now have operators wondering if the wait is finally over.

In September, Sprint Nextel rolled out a consumer-friendly femtocell base station designed to work with the company's phones and improve subscribers' in-home performance. The Airave, manufactured by Samsung and about the size of a conventional 802.11 base station, is available in parts of Denver and Indianapolis, and the company says wider availability is on the way. Semiconductor companies, including Analog Devices of Norwood, Mass., also have announced chip-level products for such applications.

Femtocells are small, stand-alone units designed for deployment in buildings with an eye toward increasing network coverage. Such devices initially used Bluetooth signals to communicate with handsets, but most vendors now are moving toward Wi-Fi as the protocol of choice.

The Sprint Nextel system allows customers to use their mobile phone in their homes, routing the conversation through their broadband Internet connection, says Ajit Bhatia, director of product management for the carrier. The company sells the Airave for $50, with monthly service costing $15 for a single handset or $30 for a family plan.

Whether such products offered by Sprint Nextel and other carriers — T-Mobile rolled out a similar, Wi-Fi-based offering earlier this year — will be successful could be determined by the answer to this question: Will individual consumers, who already pay for service that promises to follow them almost anywhere, pay extra for supposedly better coverage only inside their own homes? From a technical standpoint, other important questions loom: What are the implications of large numbers of femtocells operating in a small area, e.g., a high-rise apartment building or condominium? And specifically, what is the potential for interference with other cellular or Wi-Fi traffic?

This last question is particularly vexing for enterprise users anxious to harness the potential benefits of femtocell deployments, including ease of use (one number to reach a salesperson, for instance, rather than separate office and mobile numbers) and cost reduction. Even in many traditional offices, cellular devices are becoming the primary means of communication as employees move around to meetings in different rooms or even buildings.

Most buildings rely on outside cell towers for coverage, but concrete, steel and other building materials play havoc with cellular signals. Even when such signals can be received inside, they may be weak or intermittent. Also, conflicting signals among multiple outside cellular towers can cause devices to “hunt,” constantly switching from one source to another. And users who rely on outside signals still face capacity limitations due to other users on the network.

Femtocells (and their wider coverage-area cousins, picocells) address these problems by providing strong signals throughout a building. In this respect, they have the same aim as distributed antenna systems (DAS), the traditional solution to in-building coverage.

Femtocells and DAS both typically require an 8-10 dB advantage over signals from outside cells for mobile devices to reliably find and stay on their frequencies for voice communications, said Stefan Scheinert, chief technology officer for LGC Wireless of San Jose, Calif. That requirement may increase to 20 dB for data. Such strong signals often require multiple femtocells, but because each cell uses the same frequency, multiple cells often must overlap. This causes devices to hunt for the strongest signal, which in turn degrades data-rate performance. DAS users address this dilemma through careful antenna-location planning and signal-meter work — precisely the labor- and expertise-intensive efforts that femtocells are supposed to eliminate.