White Paper

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

Ron Borsato, Spirent CommunicationsDr. Michael D. Foegelle, ETS-Lindgren w

ww.spirent.com

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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.

Introduction

Figure 1. Conducted RF Signal Figure 2. Over-The-Air RF Signal

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Conventional Standalone GPSWith 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.

Figure 3. GPS User in Urban Environment

Conventional Standalone GPS vs. Assisted GPS

Assisted GPSUsing 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.

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 1993. 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)

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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.

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

likely adopters of the new version of the CTIA OTA Test Plan.

It is also likely that other industry

bodies will adopt similar methods in

the future.

Industry organizations are not

the only ones interested in mandating

A-GPS OTA requirements. Many

network operators also believe this

testing is very important and some

already have A-GPS OTA test programs

in place. Now that version 3.0 of

the CTIA OTA Test Plan is finalized,

many of these network operators are

expected to adopt the methodology

in this specification to help ensure

the performance of the A-GPS-capable

devices they offer their customers.

About CTIA - The Wireless Association

CTIA-The Wireless Association is an

industry consortium representing the

wireless communications industry in

the United States. Founded in 1984,

this organization represents network

operators, device manufacturers, wireless

data/networking companies, and other

contributors to the wireless sector. In

addition to lobbying the U.S. Congress and

FCC on behalf of the wireless industry and

operating one of the industry’s largest trade

shows, CTIA maintains a wireless device

certification program intended to ensure a

high standard of quality performance for

consumers.

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

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.

Until recently, all A-GPS testing

to industry standards was performed

using a cabled RF connection. As a

consequence, devices that pass all

tests in the existing conformance

standards may perform poorly in

the real world. This results in an

inferior end-user experience of LBS

applications and has led some

industry leaders to create their own

A-GPS OTA solutions.

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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

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.

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.

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.

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.

In all likelihood, the device will be operating in a highly

scattered environment when operating near the limit of

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.

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. 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

A-GPS Over-The-Air Test Method

movement of the MA relative to the DUT requires some form of

spherical positioning system.

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.

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.

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

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

is a fully anechoic chamber where all of the walls, the floor,

and the ceiling are lined with RF absorber.

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

range path loss must be applied to correct the test equipment

reading to correspond to what is occurring at the DUT. To

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.

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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.

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.

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.

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.

A-GPS Over-the-Air Test Method (cont’d)

Figure 4. Typical OTA Equipment Setup (Diagram used with permission of ETS-Lindgren)

DUT

Multi-AxisPositioner

Dual PolarizedMeasurement

Antenna

Fully Anechoic Chamber

RF AbsorberMaterial

CommunicationAntenna

MeasurementSignal Path

CommunicationSignal Path

Base StationEmulator

Dynamic Range Signal Conditioning

RF SwitchMatrix

BroadbandSignal Analyzer

PositioningController

SatelliteSimulator

PC Running TestAutomationSoftware

Fiber Optic Control Lines

Network Emulator

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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

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.

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

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Figure 6 illustrates the impact of a human head on a GPS

antenna pattern. Note the large valley at the location of the

head.

Figure 6. Impact of Human Head on Antenna Pattern

Figure 7 stresses the importance of GPS antenna location.

In this case, the antenna is at the bottom of the device (notice

the peaks facing downwards) when the device is held upright.

Since the device clearly fails to effectively receive GPS signals

from directly overhead, it is likely to be a relatively poor

performer.

Figure 7. Poorly Performing Antenna Pattern

While it’s not uncommon to perform passive tests to

evaluate an antenna radiation pattern by feeding it with an

RF cable, this is avoided for OTA testing for several reasons.

The RF cable itself can drastically change the radiation

pattern of a device, especially for electrically small devices

like a typical mobile phone. In addition, the results from

interactions between the radio, antenna, and device platform

are not the same as the performance of the individual system

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).

Additionally, two orthogonal antenna polarizations (e.g.

parallel to the theta (q) and phi (f) directions of motion) must

be measured to determine the total power received at each

point, for a total of 120 measurements. See Figure 8 for an

illustration.

Figure 8. Dual Axis Rotation with Two Antenna Polarization

Eight GPS satellites are simulated during the antenna

pattern measurement. The C/N0 ratio is measured by the

device under test for each individual satellite, and the average

C/N0 is used as the metric for each discrete antenna pattern

measurement.

A-GPS Over-the-Air Test Method (cont’d)

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LinearizationThe 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/N0 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

6.16.2 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 http://www.ctia.org/certification for further

information.

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.

The GPS performance of mobile devices is closely

correlated with the user experience of location-based applica-

tions. 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.

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/N0 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 34.171 for UMTS, 3GPP TS

51.010-1 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.

Figure 9. Average GPS Signal -130 dBm at Surface of Earth

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TIS, UHIS, PIGS CalculationOnce 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).

A-GPS Over-the-Air Test Method (cont’d)

Figure 13. Good UHIS Performance

Figure 10. TIS

Figure 14. Poor UHIS Performance

Figure 11. UHIS

Figure 15. Indoors with Reflections from Floor

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.

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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.

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 inthe states!

Figure 16. Performance Problem while Roaming

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/N0 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/N0 measurements

are repeated at various intermediate channels for that

particular operating band. The final ICD measurement is

defined as the difference between the C/N0 measurement

at the mid channel and the lowest C/N0 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.

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Anechoic ChamberThe 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.

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 turn-

table/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 Chamber EquipmentThe 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 linearly-

polarized antenna capable of independently transmitting two

orthogonal polarizations and supporting the GPS L1 frequency

of 1575.42 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

Figure 17. DUT and Head Phantom Inside an Anechoic Chamber (Photo used with permission of ETS-Lindgren)

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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/N0 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.

14www.spirent.com

Typical Test Solution (cont’d)

GPS Satellite SimulatorEmulation 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.

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 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

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 SoftwareThe 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.

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 www.spirent.com

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/N0

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

16www.spirent.com

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© 2009 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

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.

For the past fourteen years, he has worked at major wireless companies including Verizon

Wireless, Motorola and Lucent Technologies before joining Spirent Communications in 2008.

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 Ron.Borsato@spirent.com.

Dr. Michael D. Foegelle is the Director of Technology Development at ETS-Lindgren in

Cedar Park, TX.

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 802.11 Task Group T for wireless

performance prediction of 802.11 devices and many more.

He can be contacted at foegelle@ets-lindgren.com.

Ronald BorsatoSpirent Communications

Dr. Michael D. FoegelleETS-Lindgren