A-GPS Antenna Performance: Over-the-Air Test Method

September 1, 2009 By: Brock Butler, Hakan Alparslan, Michael D. Foegelle, Ronald Borsato GPS World

With the rise in location-based services (LBS) applications and the need to meet E9-1-1 positioning requirements, the number of mobile cellular devices supporting Assisted GPS (A-GPS) is steadily growing. As a key enabling LBS technology, A-GPS offers customers higher position accuracy, quicker location fixes, and improved service coverage in difficult locations, such as urban and in-building environments. As a result, mobile operators and device manufactures are looking for testing choices that quantify and benchmark real-world A-GPS device performance.

Until recently, all industry-defined A-GPS test methodologies focused on testing the performance of a device over a cabled RF connection, bypassing the GPS antenna and associated circuitry. This approach does not give a complete picture of real-world device performance and its affect on end-user experience of LBS applications. To achieve this, testing must assess the performance of GPS antennas and other device factors that can only be determined with an over-the-air (OTA) test methodology (FIGURE 1).

FIGURE 1. Over-the-air test methodology

The need for OTA performance testing of cellular and Wi-Fi wireless devices has long been a key requirement in the overall testing process. Over the years, standard OTA performance test plans have been created by organizations such as CTIA-The Wireless Association, 3GPP, and Wi-Fi Alliance. OTA testing is performed in a controlled radiated environment, called an anechoic chamber, using specialized equipment to provide a known signal to the device under test. A key aspect of this testing is that all signals are transmitted and received wirelessly, as they are in the real world. This ensures that all interaction factors between the radio and the rest of the wireless platform, including radiation pattern and platform interference, are taken into account when determining overall wireless performance.

More recently, industry organizations, including CTIA, have recognized the need to create standardized test procedures for A-GPS OTA testing to objectively specify and validate acceptable performance. A CTIA subgroup has completed a section on A-GPS OTA testing, incorporated in version 3.0 of the CTIA Test Plan for Mobile Station Over-the-Air Performance (hereafter referred to as the Test Plan). This article explains the general test methodology defined in this specification; it applies to Universal Mobile Telecommunications System (UMTS), Global System for Mobile Communications (GSM), and Code Division Multiple Access (CDMA) devices.

OTA Test Method

The goal of OTA testing is to obtain a snapshot of performance of the device under test (DUT) in all directions around it. For example, consider a requirement to compare the amount of light emitted from a lightbulb around the room in all directions. It is necessary to look at the lightbulb from all directions to measure and compare the results.

The radiated energy from or to the DUT is measured by placing a measurement antenna (MA) a fixed distance away from the device. Because the DUT can be randomly oriented with respect to the MA, a dual-polarized measurement antenna is used to measure two orthogonal polarizations, recording the total radiated energy irrespective of the relative orientation.

To cover all points on the surface of a sphere surrounding the device, it is necessary to be able to move the MA relative to the DUT in two orthogonal axes. This requires some form of spherical positioning system to move the MA and/or the DUT in spherical coordinates around theta (u) and phi (f) axes to achieve full spherical coverage.

FIGURE 2 illustrates a typical test system in which the DUT is rotated in two axes and which is capable of performing OTA testing for A-GPS.