Do you know where your femtocell is?

Femto Hub Blog (Oct 6 2009) Americas , Operators

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  1. Do you know where your femtocell is?

    By Paul Callahan

    The issue of femtocell location recently hit the press as a potential speed bump in the AT&T 3G Microcell rollout.  But AT&T was quick to clarify that the issue was reported incorrectly.

    "UPDATE: Seth Bloom, spokesman for AT&T told Fierce Wireless that the company is not seeking clarification from the FCC regarding the 3G MicroCell deployment in general nor is it seeking clarification regarding e911 issues as it relates to femtocells. The 3G Microcell complies with all FCC e911 requirements and the trials in Charlotte and Raleigh are going smoothly."

    What’s going on here?  It turns out that GPS is fast becoming a requirement for femtocells.  Here are a few of the reasons:

    • CDMA needs GPS for accurate network timing.  GPS provides very accurate synchronization to ensure the proper channel separation required for CDMA networks.  As a result, all CDMA macro-cellular base stations have GPS antennas and receivers, as do CDMA femtocells.  The GPS chips used for network timing purposes are more sophisticated – and a bit more expensive – than those used in your average Garmin Nuvi.  Obviously, these chips provide location data for CDMA femtocells as well.

    • UMTS wants GPS for location.   In the UMTS world, network timing is more flexible and can be derived by other means.  UMTS femtocells can derive timing over the air from macro-cellular signals, or from Network Timing Protocol (NTP) servers on the Internet.  The primary desire for GPS comes from the U.S. FCC requirements for Enhanced 911 (E911).  In E911 Phase 2, the wireless operator must provide the location of the call within 50-300 meters so emergency services can find the mobile caller.  To date, there is no U.S. law requiring operators to use GPS receivers in femtocells, although the idea has been considered.  Similarly, the UK’s Ofcom is currently studying the issue.

    • Some operators want GPS to control unauthorized re-location.  Why?  If you want to take your femtocell to your beach house on the Jersey shore, that might be okay.  It will depend on the operator.  But if you would use your U.S.-purchased femtocell from a hotel room in Hong Kong, that would violate the local spectrum licenses.  In either case, the mobile operator will want to know where the femtocell is located when it phones home.

    • Other operators want GPS to avoid using spectrum in the wrong places, even within a country in which they operate.  It turns out that spectrum plans are not always contiguous over geographic areas.  For example, certain operators may only have a license to use 850 MHz in a specific region, but have the right to use both 850 MHz and 1900 MHz in another.  As a result, the femtocell needs to know where it is before it lights up its radio.

    The notion of an operator keeping tabs on where its product is being used is naturally cause for a lot of speculation, but in the case of GPS and femtocells, it is for straightforward operational purposes. Femtocells are part of the operators’ networks and for network planning purposes the operators need to know exactly where each femtocell is.

    Paul Callahan is vice president of business development for Airvana.

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Reply Permalink
On 10/6/09 tyoung said:
"Thanks, Paul for your article. This is a very important topic for this forum.

Let me add a few points from the prospective of a Location and Timing vendor, Rosum.

The challenge with using GPS for femtocells is, of course, that GPS is designed for outdoor usage, and indoor performance is sketchy. To get a little technical, GPS is designed for outdoor signal levels of -130dBm, and the best assisted GPS chipsets on the planet today can use that signal down to -160dBm or so. (I'm being generous here to use the tracking sensitivity, not acquisition sensitivity). This means that if the signal is blocked 30dB by the environment - trees, building materials in the walls, roofs, etc. - more than 30dB, then you have no location or timing. If you are using GPS for location (not timing and frequency) then you need 4 satellites, so the 4th strongest satellite must see no more than 30dB of blockage. One concrete wall knocks the signal down 26dB. (By the way, that same wall is even worse on WiMAX, thus the strong need for femtocells to support WiMAX deployments.)

So this is the challenge of using GPS for femtocells, and why femtocells have long GPS antenna cables and consumers are encouraged to push them to the window. The problem is that 1) stringing an antenna across the room is not always desirable or possible, 2) needing to teach consumers about GPS antenna placement is not good for the femtocell market ("24" and "The Da Vinci Code" tell us that GPS works everywhere - it does not, or the FCC would require indoor E911 testing), 3) Femtocells don't belong by the window in the first place - the goal here is to provide perfect coverage throughout the residence, so the femtocell belongs in the center of the residence, where you can use the building attenuation to mitigate interference with other femtocells and the macro cell network 4) using GPS adds unnecessary cost to the femtocell and pressure on cost is very high - GPS satellites are in motion, of course, and performance is periodic especially in marginal environments (think metro DC, Boston, NYC, SF, etc.), so sometimes it works and sometimes it doesn't - for CDMA and WiMAX which require network synch, this means that you have to design in a rather expensive clock (TCXO of some sort) for the sole purpose of keeping the device alive through the GPS outage - this is known as "GPS holdover".

My company, Rosum, was founded by original architects of the GPS system to provide timing and location in indoor and urban areas where GPS fails. We have a multi-sensor approach - we look at all available GPS, broadcast TV signals (TV has an 80 dB power margin, which is a tremendous advantage where femtocells live and it is best in and around metro areas where GPS is worst), network timing if available, macro cell sniffing if available, and we fuse all this information together to provide timing and location for femtocells. Further, because we eliminate the GPS holdover problem, we can save money in the femtocell design as well. Learn more at www.rosum.com, or contact me at tyoung@rosum.com

Todd Young
VP Business Development
Rosum Corporation

"
Reply Permalink
On 10/12/09 Ubiq said:
"Femtocell location is important to operators and consumers alike, not just in the US. In my femtocellpioneer blog I have considered the use of GPS and other location methods from a global perspective.

Will Franks.

http://femtocellpioneer.blogspot.com/2009/10/location-gps-and-femtocells-global.html "

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