A-GNSS OTA Testing Helps You Find Your Way

Release v3.5.2 of CTIA’s Over-the-Air Test Plan incorporates GLONASS satellites and testing for all location-enabled wireless devices

If you’ve ever been in a hurry and sat in your car waiting for your navigation system to finish “searching for satellites” for what seems like an eternity, you’ll appreciate current mandates from CTIA, which include Over-the-Air (OTA) testing methodologies for wireless devices that use Assisted Global Navigation Satellite System (A-GNSS) positioning technology.

CTIA, a consortium representing the international wireless communications industry, mandated its latest version of the CTIA Test Plan for Wireless Device Over-the-Air Performance earlier this year. This document specifies test methodologies for comprehensive OTA testing for wireless devices, including positioning technologies. Related to device positioning, the changes included the expansion of A-GNSS OTA testing beyond the United States’ NAVSTAR Global Positioning System (GPS) to incorporate testing for the Russian Globalnaya Navigazionnaya Sputnikovaya Sistema (GLONASS) constellation. The revised Test Plan also incorporates A-GNSS OTA testing for location-enabled wireless devices beyond mobile phones such as tablets, laptops, and M2M devices.

How does GLONASS help GPS positioning?

GPS has a network of 31 satellites and GLONASS has a network of 28 satellites covering the earth. The 59-satellite combination of A-GPS + A-GLONASS gives A-GNSS-enabled devices:

  • Better location availability; i.e., a better chance of having a successful location determination due to the presence of more satellites overhead in any given area
  • Better location accuracy, due to the increased chance of more satellites being part of the position calculation

So what does this mean when you’re using location-based services?

To put it simply, increasing the number of visible satellites in the sky provides an improved geometry for better performance, especially in urban environments (see figure below). The increased coverage not only enhances the user experience in everyday location-based services (such as mapping and targeted retail applications), but provides critical positional accuracy when seconds count for wireless emergency services, such as North America’s Enhanced 911 (E911) or Europe’s E112.

Comparison of GPS only vs GPS + GLONASS satellite coverage
Comparison of GPS only vs GPS + GLONASS satellite coverage

In a benchmark study carried out by the Signals Research Group (SRG) in collaboration with Spirent Communications, a simulated predictive GNSS orbit file that included both GPS and GLONASS satellites revealed a definite benefit in combining the two constellations. In some cases, it was four times more likely that the mobile device would be able to accurately identify its location using combined A-GNSS versus the sole A-GPS solution.

Why perform A-GNSS OTA testing on wireless devices in addition to phones?

In the past, A-GNSS OTA testing was only required for wireless hand-held phones (as a simulation of calling E911 or utilizing LBS applications), and thus OTA test configurations included a head and/or hand phantom to determine any interference with antenna performance. As more wireless devices, such as tablets, laptops, and M2M devices, are becoming location-enabled, the latest CTIA Test Plan release extends the A-GNSS OTA testing requirements for these devices, which require a “free space” configuration to evaluate their positioning performance. The need for OTA testing becomes more critical as these wireless devices become VoLTE-capable: they must adhere to the positioning accuracy requirements for emergency services such as E911.

Increase in testing = increase in complexity

The addition of A-GLONASS not only increases location accuracy, but also the testing complexity associated with wireless devices that support this satellite positioning technology. At this time, only LTE devices are affected, as the industry has determined it is likely that only these devices will have GLONASS positioning capabilities. Key performance aspects such as radiated sensitivity and intermediate channel degradation  must be calculated specific to GLONASS in addition to previously-required GPS calculations. One interesting testing aspect involves the GNSS simulator configuration.

The simulator configuration for GPS OTA testing for all applicable network technologies (LTE, UMTS, GSM) is comprised of eight GPS satellites. However, the configuration for GLONASS test areas is quite different: it requires the simulation of only 6 satellites, but is comprised of 3 GPS and 3 GLONASS vehicles. This GLONASS test configuration uses both satellite systems because CTIA is driven by North American (NA) wireless carriers; they determined that real-world scenarios for wireless devices on NA carriers’ networks would likely include GPS satellites in the positioning calculation at all times.

Why perform A-GNSS OTA testing at all?

Validating the true performance of A-GNSS-enabled devices is challenging because accurate antenna performance characterization depends on the test environment. The nominal power from a satellite that reaches the ground is around -130dBm; the level that reaches a wireless device can be much lower due to its surroundings—several orders of magnitude lower than the weakest cell tower signals. Everyday use cases like the presence of a human head next to a phone or a human hand holding the device can cause significant shifts in the antenna pattern at these levels and degrade performance by affecting position location accuracy.

Conducted testing, though necessary, bypasses the mobile device’s antenna and associated circuitry; it cannot provide proper characterization of true device performance. OTA measurements are the best way to get the full picture, including obtaining the entire GNSS antenna pattern and determining any possible interference—whether due to human proximity or from the cellular radio or additional circuitry located in the device. To eliminate all outside interference, A-GNSS OTA testing requires the use of an anechoic chamber to obtain the GNSS antenna pattern accurately and to evaluate GNSS sensitivity in a realistic, isolated radiated environment.

Diagram showing A-GNSS testing
Configuration of test architecture for A-GNSS OTA Testing

Upcoming new mandates for A-GNSS OTA testing

CTIA’s OTA Test Plan v3.6 was released in June and should be mandated later this year. It incorporates two key changes for positioning tests:

  • Inclusion of wide-grip hand phantoms to accommodate devices from 72 to 92mm in width
    In the current release, devices wider than 72mm are not required to follow the CTIA mandate because the hand phantoms cannot accommodate them. As smartphones continued to increase in size, many of these devices were excluded from testing. V3.6 adds special considerations for those wider devices and specifies the hand phantoms required to perform the mandated tests. This may have a profound effect on the number of devices that will require testing; in our quick review of major NA carriers, just over 25% of new devices offered fit this category, such as the Apple iPhone 6+ or the Samsung Galaxy Note.
  • Inclusion of testing LPP control plane scenarios
    Prior to this v3.6, A-GNSS testing was limited to LPP user plane implementations. Since many wireless devices do not employ the user plane for positioning functions, those devices were excluded from the CTIA mandate. With this release, the requirement extends to LPP control plane. Control plane positioning is a required feature for all devices on major US carrier networks due to FCC regulations for positioning during emergency scenarios. This CTIA requirement change therefore increases the test load exponentially, as all wireless devices operating on US carrier networks will need to be tested.

For more information particular to the CTIA A-GNSS OTA testing standard and the changes mandated in February 2016, reference Spirent’s White Paper, A-GNSS Over-the-Air Performance Test Methodology: CTIA Test Plans and Their Implications.

comments powered by Disqus
× Spirent.com uses cookies to enhance and streamline your experience. By continuing to browse our site, you are agreeing to the use of cookies.