What does “channel estimation” mean in the IQxel WLAN analysis?
Typical “real world” receivers use channel estimation methods to improve the receiver’s ability to accurately demodulate data. To help receivers perform this function the 802.11 standard defines specific “Training Fields” that are transmitted in the packets. The receiver pre-processes the received signal and removes components, such as frequency and phase shift, before attempting to demodulate data.
When measuring device transmitter quality, the IQxel family test systems have the ability to do similar pre-processing. There are processing settings that are recommended to use by the IEEE spec, which include PPDU (packet) detection and frequency estimation as well as LTF (training field) channel estimation. Using these settings ensures that you are measuring the actual quality of the device under test (DUT). There are additional optional settings that can be used to isolate transmitter errors for debug purposes, which include DATA (the complete packet) channel estimation, amplitude tracking, as well as noise correction. When using these settings, the resulting EVM calculation will typically be “better” by 2 to 3 dB, but can mask potential actual DUT issues. These optional correction methods should only be used in the characterization and debug process to identify areas of the design where there are DUT-specific transmitter impairments.