[srslte-users] Offset in the round trip time

[ALEJANDRO BLANCO PIZARRO]

Dear srsLTE community,

We are doing a localization system measuring the Round Trip Time (RTT)
between eNodeB and UE. The idea is to use the Sounding Reference Signal
(SRS), which is sent in uplink, and process the autocorrelation in the
eNodeB side in order to measure the RTT, as timing in advance does.

To do so, we have a USRP x310 running the srsenb code and a bladeRF running
the srsue code. We configure the SRS in the eNodeB and the UE is properly
able to generate and send every 2 ms .

But unfortunately, when we measure the RTT we have a repeatable offset and
we do not know what is the cause. *This offset is more or less 420ns*.

I would like to ask if the tx_adv_sec variable could affect in our problem?
Depending on the devices, this variable can have different values. In the
case of USRP x310 is 1.953125 us.

And the purpose of this variable is to compensate the delay from antenna to
timestamp insertion, so, if we modify this variable , will the offset
change? It could be that we have to do a more strict calibration in our
environment to achieve the goal.

Any further information is really appreciated.

Best regards,
Alejandro
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