A-GPS Over-The-Air Test Method: Business and Technology Implications

Transcription

1 A-GPS Over-The-Air Test Method: Business and Technology Implications White Paper Ron Borsato, Spirent Communications Dr. Michael D. Foegelle, ETS-Lindgren

2 Introduction With the rise in Location Based Services (LBS) applications and the need to meet E911 requirements, the number of mobile cellular devices supporting Assisted GPS (A-GPS) is steadily growing. As one of the enabling LBS technologies, A-GPS offers customers higher position accuracy, quicker location fixes, and improved coverage of service 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 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, as shown in Figure 1. This approach does not give the complete picture of real-world device performance and its impact on the end-user experience of LBS applications. To achieve this, GPS performance testing needs to include all relevant components. An Over-The-Air (OTA) test methodology, shown in Figure 2, is the best solution to address this need. This white paper presents an overview of the business and technology drivers for OTA A-GPS testing, which describes a method to satisfy the radiated testing requirements of all involved parties. While the white paper is intended for Department Managers, Lab and R&D Managers, and Engineers already working with A-GPS or OTA, the introductory content in the following sections is also beneficial for those unfamiliar with these concepts. Figure 1. Conducted RF Signal Figure 2. Over-The-Air RF Signal 2

3 Conventional Standalone GPS vs. Assisted GPS Mobile devices can make use of several different approaches to determine their current location. Some of these Cell Identification, Uplink Time Difference of Arrival (U-TDOA), Advanced Forward Link Trilateration (AFLT), and Enhanced Observed Time Difference (E-OTD) rely on the cellular network. Another popular method, used by devices that support Wireless LAN (WLAN), is mapping known WLAN access points then using this information to approximate a device s current location. However, since the majority of these approaches have limitations, including accuracy and availability, the most common method employed in mobile devices today is the Global Positioning System (GPS). GPS is a Global Navigation Satellite System that has been fully operational since Devices with embedded GPS receiving capability are able to accurately compute their current position almost anywhere on earth where satellite signals can be received. The reliability, accuracy, and availability of this technology have driven widespread adoption. Mobile devices with GPS have two options when determining their current location: 1) Conventional standalone GPS capabilities and 2) Assisted GPS (A-GPS) Conventional Standalone GPS With conventional standalone GPS, the GPS receiver in the mobile device is solely responsible for receiving satellite signals and computing its location. This method requires the device to track at least four satellites to compute its location. The same method is used by nearly all Personal Navigation Devices (PNDs). Anyone who has used these devices has experienced the long time delay associated with getting a position fix when the device is first powered on. PNDs also have limitations in obtaining a position fix in challenging environments, such as indoors, in urban canyons, and under dense foliage. Assisted GPS Using wireless connectivity, mobile devices can also support assisted GPS (A-GPS). A-GPS improves the location determination performance by obtaining assistance data from the network over the wireless communication channel, enabling: Faster initial acquisition of satellites. An effective increase in GPS sensitivity, which can result in position fixes in more challenging environments. Some position calculations to be offloaded to a remote server, freeing the device s processor to service more critical functions. These advantages are the reason nearly all mobile devices with GPS chipsets support A-GPS. Figure 3. GPS User in Urban Environment 3

4 Over-The-Air Testing The need for OTA performance testing of cellular and WiFi 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 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 subgroup of the CTIA organization has completed work to include for the first time, a section on A-GPS OTA testing. This is incorporated in the version 3.0 release of the CTIA Test Plan for Mobile Station Over-The-Air Performance (hereafter referred to as the CTIA OTA Test Plan). The general test methodology defined in this specification is explained in the following sections of this white paper. 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. The certification organization for Global System for Mobile Communications (GSM) and Universal Mobile Telecommunications System (UMTS) devices sold into North America, the PCS Type Certification Review Board (PTCRB), are Until recently, all A-GPS testing to industry standards was performed likely adopters of the new version of the CTIA OTA Test Plan. It is also likely that other industry using a cabled RF connection. As a bodies will adopt similar methods in About CTIA - The Wireless Association consequence, devices that pass all the future. tests in the existing conformance CTIA-The Wireless Association is an Industry organizations are not standards may perform poorly in industry consortium representing the the only ones interested in mandating the real world. This results in an inferior end-user experience of LBS wireless communications industry in A-GPS OTA requirements. Many network operators also believe this applications and has led some the United States. Founded in 1984, testing is very important and some industry leaders to create their own this organization represents network already have A-GPS OTA test programs A-GPS OTA solutions. operators, device manufacturers, wireless in place. Now that version 3.0 of data/networking companies, and other contributors to the wireless sector. In the CTIA OTA Test Plan is finalized, many of these network operators are expected to adopt the methodology addition to lobbying the U.S. Congress and in this specification to help ensure FCC on behalf of the wireless industry and the performance of the A-GPS-capable operating one of the industry s largest trade devices they offer their customers. shows, CTIA maintains a wireless device certification program intended to ensure a high standard of quality performance for consumers. 4

5 A-GPS Over-The-Air Test Method This section gives an overview of the A-GPS OTA test method specified in the version 3.0 release of the CTIA OTA Test Plan. A-GPS Over-The-Air testing requires specialized equipment beyond that required for conducted testing over an RF cable. The test method described in this section applies to UMTS, GSM, and Code Division Multiple Access (CDMA) devices. Required Equipment and Setup movement of the MA relative to the DUT requires some form of spherical positioning system. The goal of OTA testing is to obtain a snapshot of the performance of the device-under-test (DUT) in all directions around the device. For example, consider a requirement to compare the amount of light emitted from a light bulb around the room in all directions. It is necessary to look at the light bulb from all directions to measure and compare the results. There are two common ways to achieve this. The first is to mount two orthogonal positioners, one on top of the other, to rotate the DUT in two axes. In this combined axis scenario, the MA remains fixed, while the DUT rotates in two axes. The second involves placing the DUT on a turntable and using a separate positioner to move the MA up and down around it. The DUT is configured for typical use cases. For a mobile device, this includes use of a phantom head and hand to simulate the effects of a device held against the human head. In either case, from the viewpoint of the DUT, the MA moves north/south (theta (q) axis) and east/west (phi (f) axis) around it, resulting in full spherical coverage. For hand-held applications, such as personal navigation using A-GPS, a phantom hand is used to hold the device in the same way a user typically would. Thus, the RF shadows and near field effects caused by the proximity to these phantoms can be taken into account when determining the device performance. To avoid unwanted interference from outside signal sources, and prevent interference with other communication systems, the DUT and MA must be shielded from the outside world. This is done by placing them inside an RF shielded room. However, while the shield reflects external energy 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. away from the DUT, it also reflects energy radiated from the DUT back towards the MA and vice-versa. This can result in the energy being measured more than once. This duplication occurs because the energy can be measured directly from the DUT, as well as after it reflects off the walls of the room. To prevent this from happening, the room must be lined with RF absorbing material to reduce unwanted reflections. The result In all likelihood, the device will be operating in a highly is a fully anechoic chamber where all of the walls, the floor, scattered environment when operating near the limit of and the ceiling are lined with RF absorber. its sensitivity. In this case, the device does not favor any particular polarization. The test methodology for A-GPS OTA testing utilizes an MA with linear polarization, as opposed to circular polarization to remain compatible with the existing CTIA OTA Test Plan. Outside the chamber, the measurement antenna must be connected to test instrumentation to measure the power radiated from the DUT, or to transmit signals at a known level to the DUT to determine its receiver sensitivity. The path loss associated with cabling, measurement antenna gain, and To cover all points on the surface of a sphere surrounding range path loss must be applied to correct the test equipment the device, it is necessary to be able to move the MA relative to reading to correspond to what is occurring at the DUT. To the DUT in two orthogonal axes. Imagine looking at a globe of the Earth and wanting to ensure that you have observed every part of its surface equally. You would have to move north and south, as well as east and west, to cover the entire globe. This 5 determine radiated power from the DUT, a signal analyzer or power meter is typically used. To determine the receiver sensitivity of the DUT, a Network Emulator (NE), or satellite simulator in the case of A-GPS testing, provides the known downlink signal.

6 A-GPS Over-the-Air Test Method (cont d) Satellite Simulator Base Network Station Emulator Dynamic Range Signal Conditioning RF Switch Matrix PC Running Test Automation Software Broadband Signal Analyzer RF Absorber Material Dual Polarized Measurement Antenna Communication Signal Path Measurement Signal Path DUT Multi-Axis Positioner Communication Antenna Positioning Controller Fiber Optic Control Lines Fully Anechoic Chamber Figure 4. Typical OTA Equipment Setup (Diagram used with permission of ETS-Lindgren) Depending on what test instrument must be connected to the MA, it is often not practical to maintain the communication link to the DUT through the MA. Thus, a separate communication antenna is typically used to provide a dedicated communication path between the NE and DUT. This can provide a low loss uplink path when the MA is used for downlink-only tests. It can also provide bi-directional communication signaling when the MA is connected to a signal analyzer for power measurement. Because most communication test equipment is designed for conducted testing, additional signal conditioning components are usually required to adapt the Over-The-Air signals to the available dynamic range of the instrumentation. An RF switch matrix is used to provide all of the necessary routing between the component parts of the system. Test Procedure and Interpretation of Results Because the GPS radio is receive-only, the main interest is in evaluating receiver sensitivity from various directions around the device. The resulting Effective Isotropic Sensitivity (EIS) pattern then determines the average radiated receiver sensitivity across the entire sphere around the device, referred to as Total Isotropic Sensitivity (TIS), or across a portion of the sphere. In addition to determining the baseline radiated sensitivity of the GPS receiver, the effect of cellular communication on the GPS receiver is evaluated to ensure that the GPS receiver performance is not degraded due to interference from the mobile phone transmitter. Finally, a PC running test automation software is used to control the positioning system and capture the desired measurements from all orientations around the DUT. Figure 4 illustrates a typical test system in which the DUT is rotated in two axes and which is capable of performing OTA testing for a number of technologies. 6 Traditionally, a TIS measurement (the measurement of an EIS pattern) is determined by performing a sensitivity search at each point around the device. The signal level transmitted to the device is lowered until a target error rate is reported by the device. That defines the limit of the device s receiver sensitivity for that direction. The result is a contoured radiation pattern where the peaks represent nulls in the antenna pattern where more power was required to get the signal through, and the valleys correspond to the peaks in the antenna pattern where the device is the most sensitive.

7 Determination of the TIS for an A-GPS device is complicated by the time involved in determining a good vs. bad result. A single A-GPS fix can take over 20 seconds, and repeated fixes are required as the power is lowered. It is also required to obtain a level of statistical confidence that the appropriate sensitivity level has been determined. To do this from all directions around a device could easily require days of testing. As an alternative, a method has been developed to take measurements from the A-GPS device itself to help determine the radiation pattern of the device. The resultant pattern is then normalized to a single EIS sensitivity search to determine an estimate for the entire EIS pattern. The test procedure consists of five steps: 1. Antenna pattern 2. Linearization 3. Radiated sensitivity 4. TIS, UHIS, and PIGS calculation 5. Intermediate channel degradation In addition to understanding the test method for A-GPS OTA, it is important to understand the significance of each measurement and how it is used to quantify the A-GPS performance of devices. This allows device manufacturers to create better-performing devices and helps network operators ensure that devices launched on their network perform well. Antenna Pattern Figure 5. Typical GPS Antenna Pattern Antenna pattern measurement is important in quantifying the true performance of GPS antennas in mobile devices. As devices become smaller, more powerful, and priced lower, the trade-offs between size, cost, and performance become more difficult. This is also true for the GPS antennas now embedded in nearly all high-end mobile devices, and an increasing number of mid- and low-end devices. For these devices to deliver a good user experience for location-based applications, the GPS antenna pattern should be compromised as little as possible. The antenna pattern of a device can be impacted by a number of factors including, but not limited to: GPS antenna design Device form factor Location of the GPS antenna in the device Presence of a human head or hand near the device The first part of the A-GPS OTA Test Plan calls for measurement of the GPS antenna pattern. An antenna pattern can be represented visually to identify the wireless device s ability to effectively receive signals from different directions. Imagine the antenna at the center of the shape in Figure 5; the areas with large peaks signify the directions from which the antenna receives signals most effectively. 7

8 A-GPS Over-the-Air Test Method (cont d) interactions between the radio, antenna, and device platform Figure 6 illustrates the impact of a human head on a GPS antenna pattern. Note the large valley at the location of the are not the same as the performance of the individual system head. components. For A-GPS OTA testing, the antenna pattern is established by radiating a known GPS signal power level and obtaining full spherical coverage around the device. By keeping track of the GPS power levels that the DUT measures, it is possible to plot how well the device receives GPS signals at different angles of arrival. For A-GPS, the metric used to characterize the antenna pattern is the carrier-to-noise ratio (C/N0 ) of the GPS signal. For the CTIA-defined tests, 60 discrete positions are required for full spherical coverage. Measurements are made in two axes (five angles in the theta (q) axis and 12 in the phi (f) axis). Figure 6. Impact of Human Head on Antenna Pattern Figure 7 stresses the importance of GPS antenna location. Additionally, two orthogonal antenna polarizations (e.g. In this case, the antenna is at the bottom of the device (notice parallel to the theta (q) and phi (f) directions of motion) must the peaks facing downwards) when the device is held upright. be measured to determine the total power received at each Since the device clearly fails to effectively receive GPS signals point, for a total of 120 measurements. See Figure 8 for an from directly overhead, it is likely to be a relatively poor illustration. performer. Figure 7. Poorly Performing Antenna Pattern Figure 8. Dual Axis Rotation with Two Antenna Polarization While it s not uncommon to perform passive tests to evaluate an antenna radiation pattern by feeding it with an Eight GPS satellites are simulated during the antenna RF cable, this is avoided for OTA testing for several reasons. pattern measurement. The C/N0 ratio is measured by the The RF cable itself can drastically change the radiation device under test for each individual satellite, and the average pattern of a device, especially for electrically small devices C/N0 is used as the metric for each discrete antenna pattern like a typical mobile phone. In addition, the results from measurement. 8

9 Linearization The antenna pattern produced in this manner relies on the DUT to perform measurements on the received GPS signals. However, the DUT is not a measurement device with a traceable calibration. In order to provide that traceability, the pattern measured by the DUT needs to be corrected to eliminate any non-linearities introduced by the DUT. By mapping the average or median C/N 0 report from the DUT back to a range of signal levels generated by the calibrated signal source (e.g. GPS satellite simulator), a set of corrections for the pattern data can be obtained, essentially transferring the calibration traceability of the signal generator to the DUT. This linearization process results in much more accurate antenna pattern data once this correction is applied. The exact linearization procedure can be carried out in multiple ways and is not covered in this white paper. Section and Appendix E.4 in version 3.0 of the CTIA Test Plan for Mobile Station Over-The-Air Performance describes this procedure in more detail. Please refer to the CTIA Certification Web site at for further information. Radiated sensitivity is measured by lowering the GPS signals until the DUT is unable to meet the specified performance requirements. The test is performed at the device orientation and MA polarization that resulted in the highest C/N 0 measurement in the upper hemisphere. The satellite scenario and performance metrics used for the test are in accordance with the industry standards for the respective wireless standard in use (3GPP TS for UMTS, 3GPP TS for GSM, or TIA -916 for CDMA); with the exception that the actual sensitivity level is found, as opposed to determining pass/fail at a particular signal level. Once the EIS has been determined at this one point, the linearized pattern is normalized to the corresponding device orientation and polarization and then subtracted from the EIS (in db) to produce an EIS pattern. Thus, the remaining EIS points are estimated from the one measured EIS value and the measured pattern shape, rather than measuring each EIS point individually. Radiated Sensitivity Another important test step is to measure the radiated sensitivity, or Effective Isotropic Sensitivity (EIS), of the device. Average GPS signal levels in clear sky conditions are very low, typically -130 dbm, which are much lower than cellular signal levels. It is important for a GPS-enabled mobile device to be able to receive in a low-signal environment. A device s GPS sensitivity reflects, to a great extent, the ability of its antenna to receive low-powered signals. Figure 9. Average GPS Signal -130 dbm at Surface of Earth The GPS performance of mobile devices is closely correlated with the user experience of location-based applications. When using devices indoors, or in areas where the sky is obstructed by trees or other obstacles, the already-low GPS signal levels are further attenuated. As a result, devices with good GPS sensitivity work in many situations where others with poorer sensitivity do not. Some devices on the market today can use GPS signal levels below -150 dbm. 9

10 A-GPS Over-the-Air Test Method (cont d) TIS, UHIS, PIGS Calculation Once the complete EIS pattern is determined, the TIS, UHIS, and PIGS all of which are isotropic sensitivity measurements can be calculated. As discussed previously, TIS is a metric that represents the average sensitivity of a device in a radiated environment. It represents the lowest signal level that the device would be able to operate with if it was radiated with equal power level from all directions. TIS is convenient because it is a single metric that represents the overall radiated sensitivity performance of the device, making it easy to benchmark devices against each other. For TIS, the entire spherical antenna pattern is used (see Figure 10). Upper Hemisphere Isotropic Sensitivity (UHIS) is similar in concept to TIS but it represents the average radiated sensitivity performance of a device above the device s horizon (see Figure 11). UHIS is calculated by integrating/averaging the EIS pattern over the upper hemisphere from theta (q) = 0 to 90 degrees. Similarly, Partial Isotropic GPS Sensitivity (PIGS) is calculated using antenna pattern data from the upper hemisphere as well as 30 degrees below the horizon (see Figure 12). Figure 10. TIS Figure 11. UHIS Figure 12. PIGS Figure 13 and 14 illustrate the importance of UHIS. The devices in both images show the same antenna pattern, but the one in Figure 14 is inverted. Despite identical TIS values, the device in Figure 13 yields better UHIS, with better performance in an environment with partial clearance where only the overhead sky is unobstructed. PIGS is an important metric because devices often receive signals reflected off the ground, for example, while standing indoors next to a window, as shown in Figure 15. Another advantage of using PIGS is the fact that the device will not be held in a completely vertical orientation with respect to the ground, so it can be considered to account for some range of variation around the vertical orientation. Figure 13. Good UHIS Performance Figure 14. Poor UHIS Performance 10 Figure 15. Indoors with Reflections from Floor

11 Intermediate Channel Degradation (ICD) In addition to measuring the EIS pattern to determine TIS and other related metrics, an Intermediate Channel Degradation (ICD) test is performed for each band supported by the mobile device. A-GPS performance may be affected by the device s active cellular connection due to the cellular subsystems interfering with the GPS receiver. As a result, GPS performance can degrade due to self-jamming when different cellular channels are used. These effects can only be measured effectively using OTA testing, since the interfering cellular signal does not reach the GPS receiver in a conducted test. The ICD procedure tests the A-GPS performance across a variety of wireless operating channels (hereafter referred to as intermediate channels). To test this, a C/N 0 measurement is performed at the mid channel frequency in a particular wireless operating band at the same pattern peak used for the GPS sensitivity measurement. The same C/N 0 measurements are repeated at various intermediate channels for that particular operating band. The final ICD measurement is defined as the difference between the C/N 0 measurement at the mid channel and the lowest C/N 0 at any intermediate channel (including the mid-channel). Therefore, the GPS intermediate channel degradation is always zero or greater. The ICD measurement is performed for each operating band that the device supports. ICD is an important measurement, because user experience can be severely impacted when GPS performance degrades due to the use of cellular frequencies that may be specific to a given network. Even if a device is targeted for one network operator market and its associated frequencies, users may roam to other networks while traveling. Figure 16 illustrates this potential problem. I m not in the states! Figure 16. Performance Problem while Roaming 11

12 Typical Test Solution Application-specific test equipment is needed to accurately perform this procedure per the CTIA OTA Test Plan. A typical test solution for A-GPS OTA consists of the following components: Anechoic chamber Specialized chamber equipment such as device turntable/positioner, GPS antenna, cellular antenna, phantom head/hand, and RF switch matrix Cellular network emulator GPS satellite simulator SMLC (UMTS) or PDE (CDMA) software server for A-GPS capability Automation software to control equipment, automate test procedure, and present results Anechoic Chamber The anechoic chamber is a critical piece of equipment for Over-The-Air testing. It serves two purposes; it isolates the DUT from outside signal sources that could interfere with radiated measurements while the special RF absorbing material inside the chamber prevents signal reflections inside the chamber from corrupting measurements. An area around the DUT, known as the quiet zone, represents a qualified test volume where signals from the measurement antenna produce a field with a known level of uniformity. As long as the device is contained within this test volume, differences in the location of the antenna on the device will have only a small impact on the resulting TIS measurement. The quality of the quiet zone is affected by the range length and the overall chamber size. Ideally the quiet zone would be located such that the measurement antenna is in the radiating far field of the device and vice versa. The chamber must be large enough to cover the required test range, and, depending on the positioning system arrangement used, may need to be over twice the range length. In addition, larger chambers achieve better anechoic performance by increasing the angle of incidence with the absorber covered walls, thereby reducing the strength of reflections. Most anechoic chambers used for OTA testing are quite large, extending 3-4 meters in each dimension. 12 Figure 17. DUT and Head Phantom Inside an Anechoic Chamber (Photo used with permission of ETS-Lindgren) Chamber Equipment The chamber equipment consists of many components in addition to an anechoic chamber. A measurement antenna is required to transmit the simulated GPS signals to the DUT. For CTIA-defined A-GPS OTA testing, this must be a linearlypolarized antenna capable of independently transmitting two orthogonal polarizations and supporting the GPS L1 frequency of MHz. The path loss from the satellite simulator through all switching, cabling, the measurement antenna, and the space inside the chamber to the center of the quiet zone is calibrated to allow the GPS satellite levels to be referenced at the DUT level rather than the satellite simulator. In addition to the measurement antenna, at least one communication antenna is needed to wirelessly transmit and receive the GSM, (Wideband Code Division Multiple Access) WCDMA, or CDMA communication signals between the DUT and a cellular network emulator. A positioning system is required to move the measurement antenna relative to the device in order to perform a spherical measurement around the device. Two orthogonal axes of motion, corresponding to the theta (q) and phi (f) coordinates of the spherical coordinate system, are required to move the antenna and/or DUT allowing measurements to be made at discrete points around the device. Most systems start with a turntable that rotates the DUT in one axis. The second axis can then be mounted on top of the first to allow rotation of the DUT in two axes within the chamber. This requires more support structure than just

13 a simple turntable (which can use an expanded polystyrene foam column for DUT support) since the second axis must have an axle and bearings, etc., sufficient to support the weight of the DUT. Another alternative for spherical measurement is to rotate the measurement antenna up and down around the device. However, this requires a larger chamber to accommodate the same range length and test volume, since the measurement antenna must move to the same distance above and below the DUT. A variant on this would be to place multiple measurement antennas within the chamber, eliminating the need for one or both axes of motion in favor of electrically switching between measurement antennas. This adds complexity and limitations to the system including a fixed angular resolution, but can reduce test time due to the ability to rapidly switch between MAs as opposed to physically moving the DUT or MA. The device positioner is generally controlled by test automation software, allowing automated testing of the A-GPS OTA procedure. Typically, a head and/or hand phantom is used while testing mobile phones to simulate the impact of having the phone next to a human ear or in a human s hand. For A-GPS testing, the most likely real-world usage is with the device held in a person s hand. Having the phone hand-held next to a person s head is more likely when making an emergency call (e.g. 911 in the United States or 112 in Europe), or possibly when using turn-by-turn direction services. The CTIA organization has very specific requirements for the characteristics of these head and hand phantoms. An RF switch matrix is also required outside the chamber in order to connect the test equipment to the appropriate antennas inside the chamber. In addition to switching the correct RF signals, the switch matrix enables automated switching between the two polarizations of the measurement antenna. Network Emulator A-GPS requires a wireless radio communication link in order to operate. A cellular network emulator is a required component of this solution. This instrument emulates all network components required to establish mobile calls, exchange necessary messages for A-GPS sessions, and retrieve GPS C/N 0 measurements from the phone. The CTIA OTA Test Plan applies to all cellular devices, whether they are UMTS, GSM, or CDMA. All North American frequency bands supported by a device must be tested. A network emulator used to test such a device must be able to support WCDMA, GSM, or CDMA air interfaces at all supported North American frequency bands, at a minimum. However, the device s A-GPS performance may also need to be evaluated in other bands of interest (e.g. GSM 900 MHz, GSM 1800 MHz, and UMTS 2100 MHz). For example, to enable roaming, a typical UMTS device may support three WCDMA operating bands (850 MHz, 1900 MHz, and 2100 MHz) plus four GSM frequency bands (850 MHz, 900 MHz, 1800 MHz, and 1900 MHz). For this reason, it is desirable that the network emulator is able to support all of the frequency bands supported by the device under test. Additionally, a high level of synchronization is required between the cellular and GPS emulators to meet the timing requirements for UMTS solutions. Specific timing requirements include coarse time accuracy delivery (<200 ms uncertainty) and calculation of time-to-first fix (accuracy <300 ms uncertainty). The bar is set even higher for CDMA solutions, which require nearly perfect synchronization. The minimum requirement is <30 ns, but any timing uncertainty will result in degraded device performance. It is essential that timing synchronization accuracy in A-GPS OTA test solutions is high enough to prevent unnecessary device performance degradation. 13

14 Typical Test Solution (cont d) GPS Satellite Simulator Emulation of the GPS satellite constellation is an essential requirement in this solution. The CTIA OTA Test Plan requires up to eight GPS satellites to be broadcast simultaneously. The simulator must accurately control the power level of each satellite. The Horizontal Dilution of Precision (HDOP) of the satellite constellation is required to be <1.5 for UMTS, <1.6 for CDMA GPS Accuracy, and <2.1 for CDMA GPS sensitivity, which is very precise. This may imply the need to automatically rewind the GPS scenario at periodic intervals. position fixes. With a flexible SMLC, test execution time can be reduced by over 60% without having a significant impact on the A-GPS OTA test results. The Position Determination Entity (PDE) is a CDMA network entity that serves the same purpose as the SMLC in UMTS networks. The PDE software server must also work in conjunction with the satellite simulator and network emulator to provide the required assistance data correctly. Automation Software The industry is also preparing for the adoption of other GNSS constellations, such as GLONASS and Galileo, as well as regional systems such as QZSS. With the expectation that mobile devices will soon adopt these technologies, it will become increasingly important for satellite simulators to also support these additional constellations. SMLC (UMTS) and PDE (CDMA) Software Server The automation software controls the entire test solution. This software provides a single user interface for setting up test sessions, executing tests, and analyzing results. At a minimum, the CTIA-defined test method for A-GPS OTA should be automated in this software. Additionally, it may allow parameters to be modified for customized test scenarios. A key benefit of good automation software is that it reduces the complexities of the test procedures and instrumentation control, making user interaction with the solution intuitive and easy-to-use. The Serving Mobile Location Centre (SMLC) is a UMTS network entity that manages several important tasks for A-GPS positioning. Firstly, the SMLC captures assistance data from a network of GPS reference receivers and delivers this data to the mobile device during a positioning session. Secondly, the SMLC helps to calculate position accuracy during MS-assisted positioning sessions. The SMLC software server must work in conjunction with the satellite simulator and network emulator to provide the required assistance data correctly. The CTIA OTA Test Plan defines very specific assistance data parameters and it is necessary that the SMLC server complies with the plan. For those looking to test A-GPS OTA beyond the requirements of the CTIA OTA Test Plan, flexibility and programmability of the SMLC software server is essential. Fully characterizing the sensitivity of a device in the real-world requires different levels of assistance data. Sensitivity, when tested with the maximum level of assistance data, is much greater than sensitivity tested with no assistance data. There will be a large spectrum of performance when tested between those two extremes. Test time can be minimized by configuring the SMLC in a way that reduces the time it takes for devices to return 14 To save time and cost, the software should control all equipment in the system once a test session is started, reducing the need for user intervention and increasing the repeatability of tests. A major advantage is that customized test sessions can be saved and tested again at any time. This allows an understanding as to how device performance changes as hardware/software modifications are made, and to understand how performance varies across different devices. Critically, automation software also stores and recalls results data from the tests that have been executed, providing the ability to view and analyze these results. For A-GPS OTA tests, it should be possible to carry out all the analyses discussed in the OTA Test Method section within the automation software itself. The antenna pattern graphs are particularly important with this test methodology. Finally, testing inevitably goes wrong at some point. Whether it is call set up, or understanding a particular protocol error, unexpected problems can always occur. Automation software should also allow debugging of unexpected problems. In most cases, this is accomplished by providing tools such as event, instrument communication and protocol logs.

15 Conclusion The arrival of A-GPS OTA testing is a very significant event for the cellular industry. Industry bodies clearly recognize the need to test A-GPS OTA performance in the manner described above and are in the process of making this a mandatory test procedure. Companies that best understand how to make and interpret these measurements have an advantage in selling Location Based Services or the platforms that deliver them. Ultimately, A-GPS OTA testing helps to assure the consumer of a superior end-user experience of LBS applications. Glossary of Terms Acronyms Description 3GPP 3rd Generation Partnership Project AFLT Advanced Forward Link Trilateration A-GPS Assisted Global Positioning System C/N 0 Carrier-to-Noise Ratio CDMA Code Division Multiple Access CTIA Cellular Telecommunications and Internet Association DUT Device-Under-Test EIS Effective Isotropic Sensitivity E-OTD Enhanced Observed Time Difference FCC Federal Communications Commission GNSS Global Navigation Satellite System GPS Global Positioning System GSM Global System for Mobile (GSM) communications HDOP Horizontal Dilution of Precision ICD Intermediate Channel Degradation LBS Location Based Services MA Measurement Antenna NE Network Emulator OTA Over-The-Air PDE Position Determination Entity PIGS Partial Isotropic GPS Sensitivity PND Personal Navigation Device PTCRB PCS Type Certification Review Board QZSS Quasi-Zenith Satellite System SMLC Serving Mobile Location Centre TIS Total Isotropic Sensitivity UHIS Upper Hemisphere Isotropic Sensitivity UMTS Universal Mobile Telecommunications System U-TDOA Up-Link Time Difference of Arrival WCDMA Wideband Code Division Multiple Access WLAN Wireless LAN 15

16 Ronald Borsato is a Solutions Architect at Spirent Communications in Eatontown, NJ and is the chair of the CTIA s GPS OTA Subgroup and the chair of the CTIA s MIMO Anechoic Chamber Subgroup. Ronald Borsato Spirent Communications For the past fourteen years, he has worked at major wireless companies including Verizon Wireless, Motorola and Lucent Technologies before joining Spirent Communications in He is a recognized subject matter expert in the development of radiated sensitivity testing for CDMA and GPS and has been a contributor and auditor of the CTIA Test Plan for Mobile Station Over-The-Air Performance. In addition, he has participated in many other wireless standards/certification working groups including the 3GPP RAN4 and RAN5 Groups, the 3GPP2 Electro-Acoustic Ad-Hoc Group, the ATIS HAC Incubator, the CTIA CDMA Sub-Working Group, the CTIA Audio Sub-Working Group, and the CTIA Bluetooth Sub-Working Group. He can be contacted at [email protected]. Dr. Michael D. Foegelle is the Director of Technology Development at ETS-Lindgren in Cedar Park, TX. Dr. Michael D. Foegelle ETS-Lindgren He is the industry recognized subject matter expert in radiated RF testing with numerous publications in the areas of Electromagnetics, EMC, Wireless Performance Testing, and Condensed Matter Physics. He is co-chair of the CTIA s Converged Devices Ad-hoc Group, has served as vice-chair of its MIMO Anechoic Chamber Subgroup and Wi-Fi Alliance Wi-Fi/ Mobile Convergence Group, and is the editor and principal contributor for the WiMAX Forum Radiated Performance Tests (RPT) for Subscriber and Mobile Stations Test Plan. In addition, he has been involved in numerous standards committees on EMC and wireless, including the ANSI ASC C63 working groups, the CTIA Certification Program Working Group on Over-The- Air performance testing of wireless devices, the IEEE Task Group T for wireless performance prediction of devices and many more. He can be contacted at [email protected]. Spirent Communications Performance Analysis, Wireless 541 Industrial Way West Eatontown, NJ USA ETS-Lindgren Corporate Headquarters 1301 Arrow Point Drive Cedar Park, TX USA Spirent Communications 1325 Borregas Avenue Sunnyvale, CA USA SALES AND INFORMATION [email protected] Americas T: Europe, Middle East, Africa T: SALES AND INFORMATION [email protected] Americas T: Europe T: Asia Pacific T: Asia T: Spirent Communications, Inc. All of the company names and/or brand names and/or product names referred to in this document, in particular the name Spirent and its logo device, are either registered trademarks or trademarks pending registration in accordance with relevant national laws. All rights reserved. Specifications subject to change without notice. Rev. D 07/09 16