Achieving Low Energy-per-fix with A-GPS Cellular Phones

Wayne Ballantyne, Greg Turetsky, Gary Slimak and John Shewfelt

Abstract:	Location Based Services are now achieving widespread deployment in GPS-equipped cellular phones. These phones typically utilize Assisted GPS (A-GPS) to improve sensitivity, TTFF, and Energy per Fix. A common operating requirement through many of these applications is the ability to monitor personnel or vehicle movements on a periodic basis, but without excessively shortening the device’s battery life. On older technology A-GPS wireless devices, a relatively high energy per fix forces the application developer to choose between suboptimal fix rate or degraded battery life For next generation A-GPS cellular phone architectures, an aggressive target of < 100 mJ/fix will be necessary to reduce GPS monitoring to a “background task” with no noticeable user impact on battery life. This paper will focus on, from a cellular phone design perspective, various methods that can be used to reduce power consumption. In order to achieve this target, everything from IC design methodologies, GPS algorithms and even smart middleware in the phone will be examined. The relationship of energy per fix and relative impact on cell phone battery life is discussed analytically and graphically. With present cell phone standby power consumption in the 7-12 mW range, it can be seen, for example, that the GPS monitoring power at 1 fix per minute, with 1.1 joules per fix, reduces the battery life by over 50%. Both handset-based (GPS fix computed in handset) and network-based (fix computed in infrastructure) are considered. The primary parameters affecting energy per fix in a handset-based GPS architecture are enumerated and analyzed. Improvements in energy per fix can be expected via bulk CMOS scaling, RF CMOS advances, and creative software approaches. A GPS “middleware” layer is shown to provide significant value in determining the need for and possibility of a low energy fix. In summary, a significantly lower energy per fix is realizable, if multiple hardware and software architectural changes are made in concert. This will enable location monitoring applications to operate truly as a “background task”, at acceptable fix rates, without perceptibly degrading the device battery life.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	2234 - 2242
Cite this article:	Ballantyne, Wayne, Turetsky, Greg, Slimak, Gary, Shewfelt, John, "Achieving Low Energy-per-fix with A-GPS Cellular Phones," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 2234-2242.
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