How do you capture WLAN packets over-the-air (OTA) with LitePoint testers?

OTA RF measurements have a few unique challenges such as:

• Many signals in the RF band that are not the intended signals to be captured.
• Low signal power compared to direct conducted measurements.
• Changing packet data rates.
• Packet power levels are not uniform and can change from one packet to another.

To maximize success in capturing and measuring OTA environment:

• Use RF enclosure to limit the stray RF signals that may incorrect be received.
• Ensure the RF power from the DUT is as high as possible. Use conducted or coupled measurements if possible.
• If DUT is cabled to the access point, use a directional couple to isolate signals from the intended device.
• Use RF triggering (not immediate triggering) so that the capture begins at the beginning of the RF burst.
• Use “Single” capture. Under traffic, it is not known which packet gets captured at a given time Therefore, using “Single” capture allows you to decide whether to keep or discard the current capture. Keep pressing the start button until an intended packet is captured.
• Do not use “Auto Range”. “Auto Range” is useful only if the DUT transmits consistently the same packet over and over again. This is not the case for the OTA environment. Therefore it is necessary to set the “Max Sig Level” manually. One rule of thumb is to use peak power. Peak power can be calculated by adding peak-to-average ratio to the DUT transmit power. Peak-to-average ratio is usually 10 for OFDM and 3 for DSSS and CCK. If the DUT transmit power is unknown, simply make a test capture and use the power reading to set the “Max Sig Level” accordingly, then capture again to verify the setting is correct.