[srsran-users] RX and TX time management

[Anil Gurses]

Hi all,

Of curiosity, I am trying to understand the TX and RX time management on
the "txrx.cc" file. There is something that I need help figuring out. Let
me briefly describe what I understood from the code,

1- I/Q samples are received from hardware along with the RX time
information. I couldn't see any stream_cmd(for USRP) time command.
Therefore, the signal is received at the time it's requested.
2- After obtaining the receive time information, 4ms is added for
calculating the transmission time.
3- TX time info is shared with hardware along with the signal and
transmitted at that exact hardware time.
4- TX_dev function inside "radio.cc" calculates the next TX time based on
the previous transmission time and the number of transmitted samples.
5- Since steps 1-4 is a loop, these steps are repeated over time.

In step 4, the TX_dev function checks the difference between the TX time
from the receive time + 4ms and the last TX time + num_samples /
sample_rate. Based on the difference, zeroes might be transmitted to close
the transmit gap or overlap. I checked on the actual hardware, and this
difference is always 0, which is good. However, I am trying to understand
how this happens without setting the time for receiving the signal. If the
receive time is not set, the RX time should shift and cause gaps in the
transmission. This never happens and works perfectly fine. I am probably
missing something. Can someone give more details about how this is
achieved?

Please correct me if I understood something wrong.

Thanks,
Anil
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