1. Passive Antenna Measurements vs. Over-The-Air Active Measurements
and Associated Metrics for
Wi-Fi Testing
By Dr. Michael D. Foegelle

2. Overview Introduction Terminology Conducted Testing
Passive Antenna Testing
Active Antenna Testing & Over-The-Air (OTA) Performance Metrics
Summary & Conclusion

3. 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.

4. 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.

5. 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

6. 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.

7. Conducted Testing Important metrics related to radiated performance:
Communication performance
Throughput, etc.
- vs. -
TX/RX signal strength
Total Radiated Power
Sensitivity
Frequency (Channel)

8. Passive Antenna Testing
Assumes Conducted Performance + Antenna Pattern = OTA Performance +
= ?

9. Passive Antenna Testing
Cabled Connection to Antenna Under Test (AUT)
Assumes 50  input impedance
Actual antenna impedance could be different
Cable introduces contributions to pattern

10. Passive Antenna Testing
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.

11. Passive Antenna Testing
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.

12. 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.

13. Active Antenna Testing & OTA Metrics
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.

14. Active Antenna Testing & OTA Metrics
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.

15. Active Antenna Testing & OTA Metrics
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.

16. Active Antenna Testing & OTA Metrics
TRP Testing Configuration
Communication Antenna on ceiling
Spectrum Analyzer
Universal Radio Communication Tester
Measurement Signal Path
Mobile Phone
Relay Switch Unit
Receive Antenna 
Communication Antenna on MAPS
Communication Path
GPIB-Bus
Fiber Optics for MAPS system
MAPS Controller

17. Active Antenna Testing & OTA Metrics
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.

18. Active Antenna Testing & OTA Metrics
TIS Testing Configuration
Communication RX Antenna on ceiling
Spectrum Analyzer
Universal Radio Communication Tester
Measurement Signal Path
Mobile Phone
Relay Switch Unit
Transmit Antenna 
Communication RX Antenna on MAPS
Communication Return Path
GPIB-Bus
Fiber Optics for MAPS system
MAPS Controller

19. Active Antenna Testing & OTA Metrics
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.

20. Active Antenna Testing & OTA Metrics
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.

21. Active Antenna Testing & OTA Metrics
Wi-Fi Throughput Testing Configuration
Wi-Fi Access Point
Network Cable
Programmable Variable Attenuator
Wi-Fi DUT
Measurement Signal Path
Relay Switch Unit
Transmit Antenna 
GPIB-Bus
Fiber Optics for MAPS system
MAPS Controller

22. Active Antenna Testing & OTA Metrics
Typical Throughput vs. Attenuation
(Single Position)

23. Active Antenna Testing & OTA Metrics
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.

24. Active Antenna Testing & OTA Metrics
Wide variety of throughput curve shapes
Need better understanding of RF interaction.

25. Active Antenna Testing & OTA Metrics
Broad range of products to test:

26. Active Antenna Testing & OTA Metrics
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)

27. Active Antenna Testing & OTA Metrics
Are these all the same product?

28. Active Antenna Testing & OTA Metrics
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.

29. Active Antenna Testing & OTA Metrics
Just how many variations do we test?

30. Active Antenna Testing & OTA Metrics
What about these?
Component or Sub-component?
How do you standardize testing of (sub)components that can’t be tested alone?

31. Active Antenna Testing & OTA Metrics
Don’t forget real world usage!
We don’t have this:

32. Active Antenna Testing & OTA Metrics
We have this!

33. Active Antenna Testing & OTA Metrics
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.

34. Active Antenna Testing & OTA Metrics
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?

35. 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…