Passive Antenna Measurements vs. Over-The-Air Active Measurements and Associated Metrics for Wi-Fi...

Passive Antenna Measurements

vs.

Over-The-Air Active Measurements

and Associated Metrics for

Wi-Fi Testing

By Dr. Michael D. Foegelle

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Copyright 2004, ETS-Lindgren, L.P.

Visit www.ets-lindgren.com for more information!

Overview

Introduction Terminology Conducted Testing Passive Antenna Testing Active Antenna Testing & Over-The-Air

(OTA) Performance Metrics Summary & Conclusion

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Introduction

This presentation will give an overview of current radiated performance testing concepts and methods.

It will describe existing metrics. It will list a range of obstacles and

issues to be addressed in the standardization of a test requirement.

Future work will propose solutions.

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Terminology

For the purposes of this presentation, I will use the following definitions: Component – represents a network system level

component or device such as an AP, Wi-Fi enabled computer, NIC (in a computer), etc. Implies a consumer level product in its usage configuration.

Sub-component – represents pieces that go together to make an operational wireless device, including antennas, NIC or AP without antenna, radio, PC without wireless NIC, etc.

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Terminology Obviously there are shades of gray between

these definitions, but metrics are only finally important at the component level.

May or may not be possible to combine sub-component metrics into component metrics.

Can divide metrics into categories: 1. MAC layer metrics unaffected by PHY layer. 2. PHY layer metrics unaffected by OTA

performance or directly correlatable to simpler OTA metrics.

3. PHY layer metrics including OTA performance. 4. OTA sub-component performance

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Conducted Testing

Best Solution for Categories 1 and 2. Only practical solution for MAC related

performance metrics. Assumes 50 test connector available.

Problem for built-in antennas. Modify circuit for test of most metrics.

Fastest way to measure metrics. Most data can correlate to OTA metrics or antenna performance.

No need to measure twice.

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Conducted Testing

Important metrics related to radiated performance: Communication performance

Throughput, etc.

- vs. - TX/RX signal strength

Total Radiated PowerSensitivity

Frequency (Channel)

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Passive Antenna Testing

Assumes Conducted Performance + Antenna Pattern = OTA Performance

+ = ?

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Cabled Connection to Antenna Under Test (AUT) Assumes 50 input

impedanceActual antenna impedance could

be different Cable introduces

contributions to pattern

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Antenna alone doesn’t represent real world condition. “Hotwiring” cable to antenna

on complete component is a first step.

Still missing radiated interactions with active components.

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Passive testing IS practical and even necessary for remote mount antennas with specific mounting requirements.

Always useful as an R&D tool. Typically faster than equivalent OTA

tests. Current interim solution until OTA

metrics are developed.

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Active Antenna Testing & OTA Metrics

Why test Active Devices Over-The-Air? Reproduces real-world behavior that cannot

be determined by other means.Determine antenna interaction with near-field

environment, including packaging.Determine behavior of RF circuitry under load of

de-tuned antenna impedance.Determine behavior of circuitry under exposure

to radiated RF and surface currents. Tests entire RF signal path at one time.

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OTA Testing is SLOW Repeat testing of same sub-metric at each

angular position. Sub-metrics are defined as the equivalent

RF performance metric from a cabled test. OTA metrics add magnitude and direction

(pattern) information to sub-metrics. Need to identify only most important metrics

for OTA testing.

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Need metrics that represent RF behavior and performance of device. Metrics may not actually represent

communication quality directly, but allow prediction when coupled with site model and OTA metrics for other components.

Can be coupled with conducted metrics vs. signal level (attenuation) to provide all necessary communication quality information.

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Common metrics for Wireless Mobile Station testing are: Total Radiated Power (TRP)

Sum of energy radiated in all directions.Mobile broadcasts at full power. Only tests reverse channel (mobile to base)

performance.Blanket metric for course inter-comparison. Weighted metrics (Near Horizon Partial Radiated

Power) help predict usage performance. Pattern information available for further analysis.

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TRP Testing Configuration

Fiber Optics for MAPS system

Receive Antenna

GPIB-Bus

Spectrum Analyzer

Universal Radio Communication Tester

Relay Switch Unit

MAPS Controller

Communication Antenna on ceiling

Communication Antenna on MAPS

Mobile Phone

Measurement Signal Path

Communication Path

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Common metrics for Wireless Mobile Station testing (continued) are: Total Isotropic Sensitivity (TIS)

Inverse sum of sensitivity from any one direction. (Response to isotropic incoming wave.)

Measure forward (base to mobile) power required to generate a given bit/frame error rate (sensitivity level).

Mobile broadcasts at full power to simulate condition when far from base station. Provides test for interaction between broadcast signal and circuitry (self jammers).

Different frequency from TRP (reverse channel) test. Considerably longer test at coarser resolution. Same availability of total, weighted, and pattern metrics.

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TIS Testing Configuration


Transmit Antenna

GPIB-Bus

Spectrum Analyzer

Universal Radio Communication Tester

Relay Switch Unit

MAPS Controller

Communication RX Antenna on ceiling

Communication RX Antenna on MAPS

Mobile Phone

Communication Return Path

Measurement Signal Path

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Through these two metrics, performance of a mobile station on a network can be predicted. Use of empirical evidence based on known

performance of other devices on the network. Establishment of link budgets to determine

required density of base stations for a given level of mobile performance.

General performance comparison between different models for device selection.

Selection of pass/fail criteria to meet minimum performance requirements.

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Current Wi-Fi OTA System uses combined signal communication performance metric: Throughput vs. Attenuation (path loss)

Variable attenuator inserted between AP and measurement antenna directed to DUT.

Attenuation plus range path loss simulates total free-space distance.

Times packet throughput on either communication leg (AP to DUT or DUT to AP)

Measures attenuation effects on both forward and reverse link simultaneously.

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Wi-Fi Throughput Testing Configuration


Transmit Antenna

GPIB-Bus

Wi-Fi Access Point

Programmable Variable Attenuator

Relay Switch Unit

MAPS Controller

Wi-Fi DUTMeasurement Signal Path

Network Cable

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Typical Throughput vs. Attenuation(Single Position)

Throughput vs. Attenuation

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Attenuation (dB)0 505 10 15 20 25 30 35 40 45

0

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Current Wi-Fi OTA System limitations: Unable to distinguish between RF effects at either

endpoint or on either link direction. Transmit power of DUT vs. sensitivity of AP. Transmit power of AP vs. sensitivity of DUT. Forward vs. Reverse link.

Link loss a problem. Slow to recover.

AP Un-calibrated. No correlation to other sites.

No software control of channel. Unable to automate frequency dependence measurements.

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Wide variety of throughput curve shapes

Need better understanding of RF interaction.


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Attenuation (dB)0 405 10 15 20 25 30 35

0.8

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Attenuation (dB)0 101 2 3 4 5 6 7 8 9

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Attenuation (dB)0 505 10 15 20 25 30 35 40 45

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Broad range of products to test:

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Broad range of products to test: Need to define standardized test

methods for each type of product. Need to keep in mind limitations of

test equipment (positioners) and system (range length vs. EUT size vs. max frequency)

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Are these all the same product?

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Are these all the same product? Certainly they have different

patterns and may have significantly different overall performance.

Need to define standardized orientations and configurations for given product types.

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Just how many variations do we test?

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What about these? Component or

Sub-component?

How do you standardize testing of (sub)components that can’t be tested alone?

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Don’t forget real world usage! We don’t have this:

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We have this!

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Need to define standardized simulations of real-world conditions. Phantom objects for simulating blockage and

loading of antenna. Antenna behavior is always different. Active RF circuitry may behave different due to extreme

mismatch of antenna load. Cabling specifications.

Current on cables can have huge effects. Don’t need to cover every possible condition, but

ideally cover extrema to determine expected range of performance.

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Crossover Devices. PDAs and SmartPhones will have Cellular,

Bluetooth, and Wi-Fi. Laptops and Desktops may as well. Need to avoid conflicts with CTIA OTA Test

Plan. (Ideally leverage from it.) What about inter-op testing?

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Summary & Conclusion We have a LOT of work to do!

OTA testing is the only way to get real world results.

Need to determine what (sub-)metrics really need repeating in OTA test.

Requires determining conducted RF metrics first.

Need to determine repeatable test methods given available equipment.

Need to leverage existing efforts and avoid onerous requirements that would compete or conflict with existing tests and systems.

I have some ideas…

Passive Antenna Measurements vs. Over-The-Air Active Measurements and Associated Metrics for Wi-Fi...

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