GPS World, June 2014

consumer oem Wireless LBS FIgure 3 BeiDou coverage over Europe The different colors show the percent of time that four or more BeiDou satellites are visible above a 10 mask angle must have the ability to search a very large number of code frequency bins at once Host Based As much as possible we want to make use of the host application processor AP and memory This allows for tight integration with assistance data which is coming from the host other sensors and other wireless data such as Wi Fi and Bluetooth for indoor locations A hostbased architecture also keeps size and cost as low as possible With Host Offload A significant trend in location applications is the need for always on low power location The host AP cannot be used for continuous position updates since it draws too much power So while we want hostbased location when the host AP is active such as when navigating with turn by turn directions and a map we also want a host offload capability so that the GNSS chip can compute positions internally while the host is asleep Interchangeability The ultimate requirement for multi system GNSS is the ability to use any combinations of satellites as if they were all in the same constellation This is summarized as any four satellites will do Front End Architecture From a cell phone tablet perspective the signals in space are all in the L1 band with frequencies as shown in FIgure 4 The key architecture feature of the GNSS front end is that it should have three separate RF chains for the three separate frequencies of interest see FIgure 5 Baseband Architecture The preferred architecture of a chip as shown in FIgure 6 is host based to take advantage of the large host CPU when it is active When the host CPU is asleep a small low power on chip CPU is leveraged for background always on location This enables applications such as geofencing to run without significantly reducing battery life When the host is active such as when you are actively using the phone for turn by turn navigation the host AP is on and we want to make as much use as possible of the host AP and memory This allows for tight integration with assistance data coming from the host other sensors and other wireless data such as Wi Fi and Bluetooth for indoor locations A host based architecture also keeps size and cost as low as possible even with host ofÀoad capability which adds very little to the size of the chip Receiver Intersystem RF Biases With the three different bands of frequencies we will get RF group delays in the receiver front end These must be calibrated out by the receivers designer as part of the chips system design If the group delay between BeiDou and GPS is not calibrated it will lead to approximately three meters of bias between the two systems FIgure 7 Once it is calibrated there is essentially no bias Satellite Intersystem Biases Different GNSS constellations run off their own master clocks referenced to different realizations of UTC GPS is referenced to UTC USNO QZSS is referenced to UTC NICT GLONASS 1561098 157542 1602 A D A D A D Band Separation Filters Gain Control and Filters Gain Control and Filters Gain Control and Filters FIgure 5 Front end architecture showing three RF chains To Baseband Processing FIgure 4 Frequencies of interest for GNSS in cell phones GPS World June 2014 www gpsworld com 48