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Recommendations for comments addressing Unknown Antenna Location in Wi-Fi devices

Recommendations for comments addressing Unknown Antenna Location in Wi-Fi devices. Date: 2007-11-14. Authors:. CID 37. The intent of the COAT calibration is to measure the path loss along the transmission route to a DUT/SUT.

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Recommendations for comments addressing Unknown Antenna Location in Wi-Fi devices

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  1. Recommendations for comments addressing Unknown Antenna Location in Wi-Fi devices Date: 2007-11-14 Authors: Anthony Maida, 3eTI, Inc.

  2. CID 37 • The intent of the COAT calibration is to measure the path loss along the transmission route to a DUT/SUT. • When the antenna location of a particular unit is unknown it will create some uncertainty in the measurements. • The impact of this uncertainty may be reduced upon specifying certain assumptions and adding language to the test section regarding how to handle these types of devices. Anthony Maida, 3eTI, Inc.

  3. Testing a Laptop as an STA • The XYZ location of the reference antenna should be noted while calibrating the environment. When replacing the calibration antenna with a laptop the XY location should match the center location of the Keyboard. • The Z location of the reference antenna should not exceed the height of the laptop screen by more than 1 inch. • According to research conducted of laptop radiation patterns, the entire laptop is a radiating isotropic antenna. Anthony Maida, 3eTI, Inc.

  4. Testing a Laptop as an STA • Using a laser pointer the tester should ensure the center of the reference antenna aligns with the center of the laptop screen. • When testing 2.4 GHz the laptop should be opened to an angle of 105 degrees as measured from the keyboard to the screen. • When testing 5.8 GHz the laptop should be opened to an angle of 90 degrees as measured from the keyboard to the screen. • The laptop should be rotated in 15 degree increments to capture performance metrics, this would most likely be a recommended modification rather than an actual requirement in this section unless section 5.8 is ultimately merged in with 5.3. • By listing a recommended orientation for laptops, the uncertainty is reduced significantly and the test becomes repeatable. Anthony Maida, 3eTI, Inc.

  5. Possible Problems • Some comments addressed antenna volume. • Antenna Volume has nothing to do with what the commenter probably intended. • The right concern would be the effective Area. • The proper way to do the test is to choose a calibration antenna with a closely equivalent antenna effective area as the laptop. • Here is the but…what is the antenna’s effective area of the laptop? • Next slide…. Anthony Maida, 3eTI, Inc.

  6. Effective Area of laptop STA • Another approach and methodology is to measure the “average” gain of the antenna installed on the laptop in an anechoic chamber. There are numerous ways to define and measure the average gain. The most comprehensive would be to measure the antenna pattern over 4 steradians (all of the radiated energy), average the results over all angles, and normalize the average with respect to an ideal isotropic radiator. In principle, this is straightforward, but in practice it is too tedious. This method determines the average gain from pattern measurements made in the horizontal (azimuth) plane for both polarizations of the electric field. The results are averaged over azimuth and elevation angles and normalized with respect to an ideal isotropic radiator. This average gain is used in the link budget model to determine whether the system performance is adequate. Anthony Maida, 3eTI, Inc.

  7. Do we really want to do this, or make assumptions? • 4 Steradians means the user must make 90 measurements. • All of the measurements would be input into an Excel (or similar) spreadsheet. • The user would use the included Statistics library within Excel to create a normalized graph. • Once this is done, each laptop from the same vendor with the same exact configuration will be equivalent to the calculated value. • This test would be performed once per newly introduced STA. • Assumptions would involve the original methodology I am proposing which is to simply measure at the center and replace with an STA at that exact center. Anthony Maida, 3eTI, Inc.

  8. PC Cards used in Laptops • Nearly all laptop computers are equipped with one or two PC card slots for extended applications. Communications-related PC cards, such as the Aironet card, use the slot for the WLAN. The performance of these cards is laptop-dependent. The antenna is placed at the outer end of the card to reduce the effect of the laptop itself on communication performance. Performance is particularly influenced by the effects of metal and the carbon-filled plastic laptop case. Anthony Maida, 3eTI, Inc.

  9. PC Cards used in Laptops • Since not all laptops are created equal as far as outer material is concerned, this test will need the 90 iteration investigation of antenna effective area. • It will also require the laptop card displacement from the slot opening be noted. • This displacement is measured from the opening to the inside edge of the PCMCIA card’s “nub”. • This displacement is important as signal strength is directly affected by this displacement distance. • The question is: How do we add this as a calibrated metric? Anthony Maida, 3eTI, Inc.

  10. PC Cards used in Laptops • Keeping consistency with the use of PC cards will prove to be difficult as one vendor may test with an IBM Thinkpad versus another vendor testing with a Dell D620. • Both have different profiles • Both are made of different materials. • The easiest way to achieve consistency is to require the use of a PCMCIA Extender for test purposes (shown below): Anthony Maida, 3eTI, Inc.

  11. PC Cards in Laptops • Using the PCMCIA extender would create the necessary distance needed where the body of the laptop is no longer a variable. • I have performed this test on numerous PCMCIA cards for interoperability testing of our products and have measured the relative output power with respect to d. When the antenna is located 4 or 5 mm outside the conducting surface of the laptop is when the maximum output power is seen and any further distance makes no difference. So the recommendation would be stated that d must be a minimum of 5 mm. • What about Multipath from the laptop body? • Good question…next slide. Anthony Maida, 3eTI, Inc.

  12. PC Cards in Laptops • My proposal to deal with multipath is to recommend an absorbent lining be used with a thin slit opening for the PCMCIA card. • Note I did not say cutout, we need a slit in the lining to push the extender through so it “hugs” the card and minimizes RF leakage. • The absorbent lining must be large enough to block out the entire laptop body, for example we don’t want one that is shorter than the screen or not exceeding the width of the body. • The RF Absorbent lining would take care of the multipath issue. • What about the antenna? Anthony Maida, 3eTI, Inc.

  13. PC Cards in Laptops • Some PCMCIA cards in use are only WLAN cards, but with the advent of multi-purpose cards from Cellular carries, a GSM antenna has been added. The following figure depicts the standard placement of these internal antennas: Anthony Maida, 3eTI, Inc.

  14. PC Cards in Laptops • The placement of the WLAN radiator is usually slightly off-center and the wiring portrudes to the left of the card (relative to looking down at a card with the PCMCIA insertion side away from you), which basically means the antenna is horizontally polarized. Anthony Maida, 3eTI, Inc.

  15. PC Cards in Laptops • The problem this creates is if the Reference Antenna is not in a horizontally polarized position and the environment is calibrated with the calibration antenna not in a horizontally polarized position is one of cross polarization. • When a horizontally polarized antenna attempts to receive signals transmitted by a vertically polarized antenna (hence, "cross-polarized antennas") a loss of up to 30dB or more can result. This means that a Client that could have connected (with properly aligned antennas) at a maximum bit rate (11 Mbps 802.11b or 54 Mbps 802.11g or 802.11a) would be forced down the the minimum bit rate (as low as 1 Mbps), and a connection that could have taken place at a minimum bit rate would not occur at all. Cross-polarization is a bad thing. • This is why the environment must change and this change must be recommended for these types of devices. The environment must be calibrated in a horizontally polarized manner. Anthony Maida, 3eTI, Inc.

  16. PC Cards in Laptops • During calibration, the calibration antenna should be a smaller SMA-type antenna. This would give a relatively equivalent effective antenna area as the PCMCIA card. • The laptop should be placed in the environment first to obtain an XYZ position of the center of the PCMCIA card when aligned with the center of the reference antenna. • Now the Calibration antenna is placed in the exact same XYZ location measured. • Once calibrated, the laptop is placed back into the environment and the test is run. • Recommended variations of this test would include the use of a turn-table, a cross-polarized environment, and attenuation to simulate obstacles. Anthony Maida, 3eTI, Inc.

  17. Testing a PDA/Pocket PC as an STA • Pocket PCs and PDAs would be similar in orientation recommendations as that of a laptop. • The calibration antenna should be a PDA-sized Omni or MIMO antenna depending on the available modes of the PDA. If the PDA is Draft N-compatible then the antenna would be the MIMO one. • The PDA would be measured in three tilt orientations: 0, 105, and 135 degrees. • The test aparatus would need to be customized. Anthony Maida, 3eTI, Inc.

  18. Testing a PDA/Pocket PC as an STA • The custom aparatus would be an investment of less than $200 for a company. • It would simply be a car PDA holder sent to a third party manufacturer to be coated with polystyrene (common name is styrofoam). • The degree measurement would be relevant to the ground and the screen of the PDA/Pocket PC. • The XYZ measure and placement would be identical to that of the laptop recommendation. Anthony Maida, 3eTI, Inc.

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