Comments

"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

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"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 "