PowerDown: Achieving Low Energy-Per-Fix in Cell Phones



Accelerating deployment of location-based services requires reducing the energy-per-fix (EPF) to a minimum, as continuous tracking shortens cell-phone battery life. This article focuses on realizing a low-current RF front end, minimizing RF active time, and developing an intelligent GPS middleware layer to determine whether a low EPF is likely or necessary.


Location-based services will soon achieve widespread deployment in GPS-equipped cellular phones. These phones typically use assisted GPS (AGPS) to improve sensitivity, time-to-first-fix (TTFF), and energy-per-fix (EPF). A common operating requirement of many of these applications is the ability to monitor personnel or vehicle movements on a periodic basis, without excessively shortening the device's battery life.

On older-technology AGPS wireless devices, a relatively high EPF forces the application developer to choose between sub-optimal fix rate or degraded battery life. Next-generation AGPS cellular phone architectures require an aggressive target of <100 milliJoules (mJ) per fix to reduce GPS monitoring to a background task with no noticeable user impact on battery life. We focus here on methods to reduce power consumption from a cellular phone design perspective, examining integrated chip (IC) design methodologies, GPS algorithms, and smart middleware in the phone.

With present cell phone standby power consumption in the 7–12 milliwatt (mW) range, the GPS monitoring rate of one fix per minute at 1.1 joules per fix reduces battery life by more than 50 percent. Bulk complementary metal-oxide semiconductor (CMOS) scaling, RF CMOS advances, and creative software approaches — particularly a GPS middleware layer to determine the need for and possibility of a low-energy fix — can improve EPF and enable location monitoring applications to operate truly as a background task, at acceptable fix rates, without perceptibly degrading the device battery life.

In user location-monitoring via a cell phone (subsequently referred to as the mobile subscriber, or MS), the user's position is sampled typically at intervals of 1 to 10 minutes. The captured fixes are usually buffered, then uploaded to the web-based tracking application server every 20–30 minutes. With this approach, the energy expended in uploading the fixes is relatively small, and most of the incremental current drain impact is due to the GPS receiver's energy used in acquiring a single fix, or EPF. This energy is referenced to the cellular phone battery itself, typically a 3.6V lithium-ion type with a capacity in the 500–1500 mA-Hr range.

The additional power consumed by a handset with an EPF of X mJ is given by

where P batt is the power in mW from the battery for a GPS fix interval of Fix_int minutes.

The resulting current drain from the battery due to GPS location sampling is