Why the "first path index" is between 700 and 900?

the tag send data with FP_INDEX , this value in CLE is available between 700 and 900? why? In the CLE, may I change this value is between 740 and 760?thank you!

When receiving the signal the system sees the preamble and attempts to align itself to the incoming message. It then performs a correlation between the incoming signal and it’s initial guess. The results of that correlation are what you see in a CIR plot with the first path peak typically around 740-760.

The zero point for this process really doesn’t matter much as long as the correlation peak is somewhere within the time period covered. It doesn’t technically matter if it’s at sample 1 or sample 1000 as long as it’s included. Since where to put the start is based on an initial guess the logical choice would be to put the expected peak in the middle of the time period, this would allow for the largest possible error in that initial guess.

But we are looking at very weak signals. The chance that the initial guess is too early is very slim, the chances that it’s too late are far higher. Reflections are often stronger than the direct signal, if the initial guess ended up being based on a reflection then the true signal will be earlier. So rather than putting the initial guess in the middle of the time window it is instead put at a point roughly 3/4 of the way along.
It turns out that the initial guess is actually very accurate and so the FP_Index typically ends up being very close to 750.

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