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When was the last time that you actually “printed” out directions before going on a trip? Do you remember searching through your car for a map that took 10 seconds to unfold and 10 minutes to refold? Mobile devices are changing the way we live our lives. Sooner than you may think, mobile applications will be able to monitor your day-to-day, week-to-week, month-to-month locations and activities and provide you with predictive concierge-like information and services based on your history. How is all of this possible? One key factor is the ability of mobile devices to quickly and accurately determine location.
The main method for determining location relies on satellites in earth orbit as part of the Global Navigation Satellite System. These space vehicles send specially-coded radio signals to earth that are decoded by GNSS receivers, thus enabling mobile devices to compute position, velocity and time.
Operated and maintained by the U.S. government, there are currently 24 active GPS satellites in orbit in six different orbital planes (each with four satellites). Orbiting at 20.2-kilometers above the earth, each satellite circles the planet once every 11 hours and 58 minutes, allowing five to eight GPS satellites to be in contact with any point on the earth at any given time. As of October 2011, the 24 satellites in the Russian GLONASS constellation are also in full operation, with three orbits each containing six satellites. These extra satellites, amongst other things, provide mobile devices with more signals to use for location determination. Additional GNSS constellations have been defined, including the European Union’s Galileo and China’s Compass systems, further increasing the number of visible satellite SVs.
Power levels of the satellite signal are relatively weak as compared to other commercial wireless technologies, thus GPS receivers must have a clear line of sight to satellites for an accurate location fix to be determined. This line of sight requirement can make it difficult to get a fix when inside a building or in urban built-up areas surrounded by buildings. For situations when GNSS signals cannot be received due to the lack of a clear line of sight to the SVs, there are other methods for determining location that do not rely on satellites for position determination.
One such system is observed time difference of arrival – the positioning solution of choice for LTE, which uses specially-coded signals (positioning reference signals) from terrestrial commercial wireless networks’ ENodeBs (base station towers on the ground). The OTDOA method uses neighbor cells (ENodeBs) to derive an observed time difference of arrival relative to the serving cell. This is achieved by performing calculations using the PRS, a highly detectable pilot signal even at low power levels, especially useful for indoor location determination.
Whether employing signals from GPS or GLONASS SVs, the location is calculated using trilateration. Essentially, a sphere with a radius equal to the distance between the receiver and the satellite is made around each of the measured satellites. These spheres intersect at the location of the mobile device. In order to determine location on the ground, a receiver has to determine the locations of at least three satellites; and four satellites are needed in order to compute a position in three dimensions (i.e., position and altitude).
The U.S. government mandates that the location of any mobile phone must be determined much faster than is possible using a GPS receiver alone. This is especially important in emergency situations, such as dialing 911. To solve this issue, mobile devices on commercial wireless networks request assistance data from the wireless network to locate the satellites much faster, speeding up the time-to-first-fix and improving SV detectability. This technology, known as A-GNSS, is used in GSM, WCDMA/HSPA, CDMA (1xRTT/1xEV-DO) and LTE devices today.
LTE mobile devices (user equipment or UEs) employ the OTDOA and A-GNSS technologies together in a “hybrid” operation. By using ENodeB signals and satellites from the GPS and GLONASS constellations, the location accuracy improves and indoor positioning can be realized.
With the global roll out of LTE by multiple wireless carriers and the increasing importance of location determination in our everyday lives, testing mobile device performance is critical. Test requirements also evolve along with the technology, so the test setup must be flexible – such as the R&S® TS-LBS solution shown in Figure 1:
Insert Fig 1. LTE A-GNSS (GPS/GLONASS) TEST SETUP
Such a test setup requires an LTE network emulator with the ability to simulate a real LTE network and the protocol required for A-GNSS and OTDOA. The GPS and GLONASS signals are simulated using a signal generator. The controller PC runs CONTEST software, enabling users to select, which tests to run, including the 37.571-1 3GPP A-GNSS minimum performance tests, and/or custom test plans mandated by major commercial wireless operators. Tests are fully automated, with logs and reports generated so that test results can be analyzed and issues resolved.
