I have a question regarding the First Path Index in my dataset, which was collected using the DWM1001 module. One of the features in the dataset is the First Path Index. As shown in the attached plot, the first path index consistently falls between 740 and 750 (most of the time) for ranges spanning from 50 cm to 14 meters.
Is this behavior expected? If so, could someone explain how the time of flight (ToF) is calculated in this case?
Here’s what confuses me: If each CIR sample (from the total of 1015 samples) corresponds to a timestamp, shouldn’t the first path index reflect the distance? For instance:
For a 3-meter range, I would expect the first path index to be around 600.
For a 6-meter range, it should be closer to 1200.
I am struggling to understand how these values align with ToF calculations. Any insights or explanations would be greatly appreciated!
Time of flight is not calculated from CIR directly, but calculated using the timestamps of message TX and RX.
In the case of ToF, CIR (and other diagnostic data) can be used to determine if the ToF calculation is reliable or not. For example, you can see (guess) if it is a LOS or NLOS scenario and then adjust or discard your ToF calculation based on the diagnostic data.
It is normal that the FP indices are mostly between 730 and 750. FP index serves as a reference point for multipath component analysis rather than an absolute value which is proportional to ToF. In the CIR plot, you can see multiple peaks except for the peak of LOS. They correspond to multipath components caused by reflections, scatterings or diffractions. By checking the index difference between these peaks wrt the first path, you can roughly calculate the path difference between these multipath components and the first path, though it is not crucial information for LOS ranging.
