Femtocells: Bringing Reliable Location and Timing Indoors
These tiny cellular base stations for homes and small business are all the buzz with wireless service providers today. Unlike cellular base stations (macrocells) that rely heavily on GPS timing and location, femtocells are typically deployed in indoor environments where GPS is less reliable. However, there’s a catch. . .
Femtocells — typically designed for use in residential or small business environments — are generating great interest among telecommunications service providers and consumers alike.
We can think of a femtocell as a cellular base station, shrunk down to the size of a WiFi router and connected to the broadband Internet connection (such as DSL or cable), which can handle several mobile handsets. Within the femtocell coverage area, voice and data calls approved to use the femtocell will be carried not through the macrocellular network, but through the femtocell itself.
These calls are connected via a licensed (GSM, CDMA, WiMAX) or an unlicensed (WiFi) interface. From the femtocell, the call is backhauled over the Internet and into the carrier network through which they are completed just like any other cell call.
Femtocells hold great promise for telecommunications service providers. They provide high-quality coverage across a small footprint, reduce customer churn, generate additional monthly service revenue, and reduce the traffic on the radio access network by offloading in-home voice and data traffic from the macro cell towers. For consumers, better quality of service and rates are among the enticements offered by femtocells.
Participating telecommunications standards associations including the 3rd Generation Partnership Project (3GPP) and the Femto Forum hope to establish globally applicable specifications for femtocell technology by the end of 2008. In the meantime, market research organizations predict a booming business within a very few years. In an ABI Research report released earlier this year, “Femtocell Market Data,” the company’s vice-president Stuart Carlaw predicted that shipments will likely total tens of millions by 2010.
The devil, of course, is in the details, and several different architectures are being employed for the femtocell carrier network interface under such names as Cellular Base Station, Collapsed Stack, UMA/GAN, and home-node B. But all of them require accurate position/location and timing to ensure robust operation and to avoid interference between cell towers and femtocells as well as to maintain capabilities and services that consumers depend on.
This article examines the location and timing needs for femtocells and explores the merits of various technologies in meeting these needs, with a focus on a hybrid system based on television transmissions and assisted GPS.
Femtocell Timing and Location Needs
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Signal Attenuation Inside Buildings
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Timing and Location Solutions
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Using Broadcast TV for Timing and Location
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Technical Advantages of TV Signals
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Frequency and Timing Accuracy
The excellent stability of the DTV pilots, showing a median variance (over both short 10-second and longer one-day intervals) of 6 ppb, is the source of the frequency stability provided by TV+GPS. By observing the pilot signals at the TV+GPS client and comparing the measurements with that of the Reference Monitors, TV+GPS can adjust the local clock to maintain the required 100 ppb frequency stability.
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TV+GPS Data Processing
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A number of solutions are being evaluated for femtocell timing and location, including using A-GPS, cell tower signals, and network timing. However none of these addresses both the timing and automatic location requirements adequately.
TV+GPS hybrid timing and location can meet the FCC’s E911 requirements needed to enable this important developing market.
The core of TV-positioning is the tremendous power advantage provided relative to GPS and superior building penetration. Hybridizing with A-GPS adds to that advantage.
In addition to the reciprocal technical advantages of their hybridization, TV and GPS are highly complementary in their geographical availability. In dense urban areas with large buildings and very challenging indoor settings, TV transmitter geometry is excellent. In remote areas where few TV towers exist, urban canyons and large multi-story steel-reinforced concrete buildings are rare; so, GPS provides the timing and location data.
At the signal processing level, precise timing derived from TV signals can be used to enhance AGPS performance.
For the complete story, including figures, graphs, and images, please download the PDF of the article, above.
ManufacturersFemtoSynch is the trade name for the TV+GPS product from Rosum Corporation, Mountain View, California, USA. In its FemtoSynch client, Rosum uses an AGPS receiver from ST-NXP Wireless, Plan-les-Ouates, Switzerland.
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