Femtocells: Bringing Reliable Location and Timing Indoors

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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
Femtocells have similar requirements for location and timing as macro base stations on a hilltop, and for the same reasons. Accurate real-time location helps service providers meet regulatory mandates and operators’ business needs for such things as emergency caller location identification, verification of licensed spectrum operations, and monitoring customer usage.

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Femtocell Challenges
Two factors complicate efforts to meet the timing and positioning needs of femtocells: 1) they are generally deployed inside buildings where signal reception is poor, and 2) the service provider has little control over the placement of the femtocell within the customer’s home.

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Signal Attenuation Inside Buildings
Fading effects, building attenuation caused by building materials, vary greatly by the frequency of the RF signal . . . More detailed building attenuation models have been developed such as the one described in the Working Papers column in the July/August 2008 issue of Inside GNSS. These models are helpful in understanding attenuation effects with greater precision.

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Timing and Location Solutions
A variety of methods have been explored to provide timing and location for femtocells.
As GPS is already widely used in cellular base stations, many companies are promoting the use of GPS and assisted-GPS (AGPS) as a femtocell timing and location solution. AGPS (or multiple constellation versions of assisted GNSS) replaces missing satellite broadcast data when access is intermittent, difficult, or impossible due to signal obstruction.

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Using Broadcast TV for Timing and Location
All global standard TV signals include timing information that can be used for precise frequency, synchronization, and location. Rosum’s first-generation system, for instance, exploited Advanced Television Systems Committee (ATSC) digital and National Television System Committee (NTSC) analog TV signals used throughout North America and in South Korea.

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Technical Advantages of TV Signals
Broadcast TV signals have numerous characteristics that make them desirable for use in location and timing.
TV signals are strong, with broadcast power levels of hundreds of kilowatts or megawatts, providing a positioning link margin 50 dB greater than GPS. (Positioning link margin is the ratio of the lowest usable power level and the outdoor power level and represents the amount of building attenuation that can be suffered and still have usable signals.)

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Frequency and Timing Accuracy
TV+GPS can provide highly accurate frequency stability of better than 100 ppb, commonly 10 ppb. Data collected from TV reference monitors show the inherent stability of broadcast TV signals.

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
The TV+GPS client uses the aiding data to detect and measure the timing of TV and GPS signals, which are then used to compute frequency, timing, and location for the femtocell. The solution uses detailed knowledge of the pilot frequency offset (PFO – the deviation from the FCC nominal broadcast frequency) and the frame rate offset (FCRO – deviation from the standard-specified 24.2-millisecond frame duration) to find and lock onto the signals and to measure their precise frequency and timing.

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TV+GPS Performance
TV-based positioning has GPS-level (single-digit meter) accuracy in environments with clear line-of-site visibility to regional TV towers. The greatest value of using TV for positioning and timing, however, manifests itself in obstructed or indoor environments. Most wireless users spend the bulk of their day indoors, and over half of wireless emergency calls are made from inside buildings.

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Conclusion
Femtocells represent a tremendous market opportunity for telecommunications service providers, and a great boost in cellular network performance for consumers. Customers, however, expect that femtocells will work where they put them in their homes and that their 911 calls will receive proper treatment.

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.

Additional Resources
[1] 3rd Generation Partnership Project (3GPP), “3G Home NodeB Study Item Technical Report,” September 23, 2008. Report available at <http://www.3gpp.org/ftp/Specs/html-info/25820.htm>
[2] Alliance for Telecommunications Industry Solutions, “Location Technology Performance Data - Define Topologies & Data Collection, ATIS-0500011,” February 2007
[3] National Institute of Standards and Technology, “NIST Construction Automation Program Report No. 3, Electromagnetic Signal Attenuation in Construction Materials,” October 1997
[4] http://www.ctia.org/advocacy/research/index.cfm/AID/10323
[5] http://www.fcc.gov/pshs/services/911-services/voip/Welcome.html
[6] Network Reliability and Interoperability Committee (NRIC) VII, February 15, 2005, Focus Group 1A. “Near Term Issues for Emergency/E9-1-1 Services”
[7] Discussion of the FCC’s E911 rule can be found at <http://www.fcc.gov/pshs/services/911services/enhanced911/Welcome.html>.
[8] More details about the FCC’s E911 Phase 2 rules at <http://www.fcc.gov/Bureaus/
Engineering_Technology/Documents/bulletins/oet71>
[9] For detailed treatment of the femtocell/carrier interface architectures, please see the FemtoForum (www.femtoforum.org), UMA (www.umatoday.com), and 3GPP (www.3gpp.org) websites.

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