[srslte-users] UE replacing signal with 0's during UL transmission.

[Raghav Rathi]

Hi,

We are having some trouble with using srsUE. We are trying to transmit a signal of length 9216 complex floats.

The experiment is setup in the following way.
We have configured the eNB in such a way that it transmits a beacon signal of length 9216 complex float every half a second. The sample rate we are using here is 0.5 msps.
Now, at the UE side, once it receives and decodes the beacon signal, it starts transmitting the same beacon signal once every half a second at a sample rate of 0.5 msps.
eNB is running on USRP x310 and UE is running on USRP B210.

The problem we are facing is as follows,
After UE receives the first beacon signal, it starts making transmission on the UL channel and since it is transmitting the same beacon signal which is of length 9216, the eNB should also receive the same and which it does but only for the first transmission from UE.
After the first transmission, the received signal at eNB has the 2040 samples being replaced by 0's.
After debugging at a few places, we come to the rf_uhd_imp.cc where rf_uhd_send_timed_multi is defined. In there, the function calls handler->uhd->send() in a do while loop until all the samples are transmitted.
Since we are using a sample rate of 0.5 msps and we are making the UE transmit every 20 ms, the program passes a buffer of size 10000 (500 *20) samples to the handler->uhd->send() and since the driver cannot transmit the entire 10000 samples at once, it breaks it down based on "handler->tx_nof_samples".
Now, we observed that that the segments were divided as follows, 2040 | 2040 | 2040 |2040 | 1840 || 10000 total.

After the first transmission from UE, the signal received at the eNB is exactly missing the first 2040 samples and we came to this conclusion by shifting the beacon signal between the 10000-sample buffer such that we split the beacon signal between 2 transmissions and the received signal was missing the beginning of the second half, and by missing, I mean that part of beacon signal was replaced by 0's.
Since we are reading the beacon signal from a file while transmitting from the UE, it is independent from the received beacon signal from eNB.
We have also tested the setup with running a listener node listening the UL frequency and it is observing the same behaviour i.e. the first 2040 samples are being replaced with 0's.
At this point we are uncertain what can cause this issue.
We have already checked the buff_ptr which is passed in "(handler->uhd->send(buffs_ptr, tx_samples, md, RF_UHD_IMP_TRX_TIMEOUT_S, txd_samples)" and it contains the entire signal. The program is not generating any error for transmission as well.

Does anyone know what can go wrong here? We would appreciate any help in solving this issue.

Thank You,
Raghav

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