Question for CSI

[ZouZangyuan]

Dear Author,

I have recently been working on the openwifi project, and I have successfully run the monitor mode following the documentation, obtaining a large amount of frequency offset and CSI plots. However, I am a bit unclear about what they represent. So, I would like to ask for your guidance: does the frequency offset plot reflect the frequency offset of a single device, or of all devices within the channel? In the CSI plots, why do I sometimes see only one line, while other times there are multiple curves, and occasionally the scale on the vertical axis also changes? What do these curves represent—are they the CSI of different devices? Also, what is the meaning of the equalizer plot? How should the information in these plots be extracted and utilized?

Is there any documentation or paper that explains these issues in detail? If so, could you please direct me to it? I would greatly appreciate your help!

Thank you!

[HavingaThijs]

does the frequency offset plot reflect the frequency offset of a single device, or of all devices within the channel?

It is the frequency offset of your device compared to the transmitter of the packet.

What do these curves represent—are they the CSI of different devices?

Each curve is a separate packet, and it represents the CSI of that received packet (so the channel between the transmitter and you at the moment the packet was sent).

Also, what is the meaning of the equalizer plot?

It is the constellation of some of the OFDM symbols of the data part of the packet. Each point represents the real and complex value of a subcarrier after equalization in the IQ plane. The clearer the constellation, the better the demodulation will perform.

See also: https://github.com/open-sdr/openwifi/blob/master/doc/app_notes/csi.md