Synchronize multiple gNBs by aligning the slot boundaries

Currently, we derive frame and slot number based on proc->timestamp_rx using proc->frame_rx = (proc->timestamp_rx / (fp->samples_per_subframe*10))&1023; and proc->tti_rx = get_slot_from_timestamp(proc->timestamp_rx, fp);

This ignores the samples proc->timestamp_rx % fp->samples_per_frame while computing the frame and slot number creating a mis-alignment with the slot boundaries. A detailed description of the slot misalignment in a pictorial representation can be seen in the attachment below.

timing_offset_multi_gNB.pdf

We fix this alignment issue by compensating these samples during the first reception.

We can test the synchronization of the multiple gNBs from the estimated ToAs from the PRS in the Downlink and the estimated ToAs from the SRS in the Uplink using rfsim.

In order to test this, make sure you build gNB and nrUE along with the telnetsrv flag using the option --build-lib telnetsrv.

Also, enable the following log in the function nr_est_srs_timing_advance_offset() in openair1/PHY/NR_ESTIMATION/nr_measurements_gNB.c to observe SRS ToAs

LOG_I(NR_PHY, "SRS estimatd ToA %d [RX ant %d]: TA offset %d, TA offset ns %d (max_val %ld, mean_val %ld, max_idx %d)\n", srs_toa, ant, *ta_offset, *ta_offset_nsec, max_val, mean_val, max_idx);

Furthermore, update the option Active_gNBs = 2; in targets/PROJECTS/GENERIC-NR-5GC/CONF/ue.nr.prs.fr1.106prb.conf to enable decoding PRS transmitted from two gNBs

To run:

nrUE:

sudo ./nr-uesoftmodem -r 106 --numerology 1 --band 78 -C 3619200000 --uicc0.imsi 001010000000001 --rfsim --phy-test -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/ue.nr.prs.fr1.106prb.conf --rfsimulator.serveraddr server

gNB 0:

sudo ./nr-softmodem -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb0.prs.band78.fr1.106PRB.usrpx310.conf --gNBs.[0].min_rxtxtime 6 --rfsim --phy-test --rfsimulator.[0].serveraddr 127.0.0.1 --rfsimulator.[0].options chanmod --rfsimulator.[0].modelname AWGN --telnetsrv

gNB 1:

sudo ./nr-softmodem -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb1.prs.band78.fr1.106PRB.usrpx310.conf --gNBs.[0].min_rxtxtime 6 --rfsim --phy-test --rfsimulator.[0].serveraddr 127.0.0.1 --rfsimulator.[0].options chanmod --rfsimulator.[0].modelname AWGN --telnetsrv

When we run the nrUE, gNB0 and gNB1, we can see the following logs

nrUE:

[PHY] [gNB 0][rsc 0][Rx 0][sfn 976][slot 2] DL PRS ToA ==> 0.0 / 2048 samples, peak channel power -16.4 dBm, SNR +4.0 dB, rsrp -42.2 dBm

[PHY] [gNB 1][rsc 0][Rx 0][sfn 976][slot 3] DL PRS ToA ==> 0.0 / 2048 samples, peak channel power -16.4 dBm, SNR +4.0 dB, rsrp -42.2 dBm

gNB 0:

[NR_PHY] SRS estimatd ToA 0 [RX ant 0]: TA offset 31, TA offset ns 0 (max_val 99820162, mean_val 80176, max_idx 0)

gNB 1:

[NR_PHY] SRS estimatd ToA 0 [RX ant 0]: TA offset 31, TA offset ns 0 (max_val 98784562, mean_val 79369, max_idx 0)

From the above logs, when we run the rfsim and when no delays introduced, we can observe the following results

In the Downlink at the UE, the estimated ToAs using PRS from both gNB0 and gNB1 are 0 and in the uplink, the ToA estimated from the SRS transmitted by the UE at both the gNBs are also 0. This indicates that both the gNBs are synchronized along with their slot boundaries. We also highlight that this is not the case in the current develop branch since the slot boundaries are not aligned.

We also can test the multi gNB synchronization by introducing delays using telnet as follows,

Introduce new_offset (delay) of 4 samples to the gNB 0 from UE in the uplink as follows,

openairinterface5g$ echo rfsimu setdistance rfsimu_channel_enB0 20 | ncat --idle 1 127.0.0.80 9080
softmodem_gnb> rfsimu_setdistance_cmd: new_offset 4, new (exact) distance 19.518 m, new delay 0.000065 ms
Ncat: Idle timeout expired (1000 ms).

We can oberve this delay of 4 samples being reflected in the estimated SRS ToAs at gNB 0 while not changing the ToA in gNB 1

[NR_PHY] SRS estimatd ToA 4 [RX ant 0]: TA offset 31, TA offset ns 65 (max_val 98803636, mean_val 79430, max_idx 8)

similarly, Introduce new_offset (delay) of 2 samples to the gNB 1 from UE in the uplink as follows,

openairinterface5g$ echo rfsimu setdistance rfsimu_channel_enB0 10 | ncat --idle 1 127.0.0.81 9081
softmodem_gnb> rfsimu_setdistance_cmd: new_offset 2, new (exact) distance 9.759 m, new delay 0.000033 ms
Ncat: Idle timeout expired (1000 ms).

We can oberve this delay of 2 samples being reflected in the estimated SRS ToAs at gNB 1 while not changing the ToA in gNB 0

[NR_PHY] SRS estimatd ToA 2 [RX ant 0]: TA offset 31, TA offset ns 32 (max_val 98962768, mean_val 79552, max_idx 4)

Closes #897 (closed)