Establishing the location fix of an E911 call from a CDMA mobile phone is typically accomplished using a built-in GPS device. Handset-based GPS receivers to date have mostly worked using a time-multiplexed scheme (TM-GPS), where the GPS signal and the telephone call signal sequentially toggles back and forth. This method is attractive because only one RF radio is working at a time and is easy to implement.
Simultaneous GPS comes to the fore
Simultaneous GPS (S-GPS), where both GPS and telephone call operate simultaneously, is becoming increasing common because it provides ~4-dB improved sensitivity and enables location-based services (LBS) for service providers. S-GPS-capable chipsets from manufacturers such as Qualcomm have been commercially available since 2004 and are being widely adopted in today's CDMA handsets.
A single antenna is preferred whenever possible because of lower costs and smaller space use. Figure 1 illustrates a generic block diagram for the GPS receive chain in a single antenna tri-band phone.
Fig. 1. In a single-antenna tri-band phone, the GPS receive chain often looks like this.
The GPS receive chain consists of a GPS filter, a low-noise amplifier (LNA), an inter-stage filter, and down converter with each component contributing to the overall system sensitivity. The term GPS sensitivity is defined as the lowest signal power level at the antenna port where the handset should be able to locate the GPS satellite 98% of the time.
For both TM and S-GPS operation, important variables contributing to system sensitivity are the first GPS filter's IL and LNA NF, while the second GPS filter's IL and down-converter NF have minimal affect. Figure 2 illustrates how the total system noise is impacted by each of the discrete components.
Fig. 2. A plot of system noise versus each component's insertion loss, or noise figure, shows the importance of first GPS filter IL and LNA NF.
Since the first filter's insertion loss (IL) and LNA NF directly affect the GPS system noise, those values should be minimized during component selection. Filter IL is governed by the quality factor, Q, of the filter, with the higher the Q the better. Of the two filter technologies used in cell phones today, surface-acoustic wave (SAW) and film bulk acoustic resonator (FBAR), FBARs have, in general, demonstrated higher Q and lower IL.
