Running NR PRS with OAI gNB and nrUE

July 29, 2026 ยท View on GitHub

After you have built the softmodem executables, go to the build directory build/ and start testing the Rel16 PRS use cases.

PRS parameters and config files

ModegNB confignrUE config
FR1 40MHz
30kHz SCS		gnb0.prs.band78.fr1.106PRB.usrpx310.conf
gnb1.prs.band78.fr1.106PRB.usrpx310.conf		ue.nr.prs.fr1.106prb.conf
FR2 100MHz
120kHz SCS		gnb0.prs.band261.fr2.66PRB.usrpx310.conf
gnb1.prs.band261.fr2.66PRB.usrpx310.conf		ue.nr.prs.fr2.66prb.conf

In both the gNB and nrUE config files, the PRS parameters are configured under the prs_config section. The nrUE can receive downlink PRS from multiple gNBs simultaneously, so the nrUE config contains one prs_config section per gNB. These parameters can be changed to suit your test scenario.

As of now, PRS comb sizes 2 and 4 are supported and validated with an R&S spectrum analyzer.

Note: Muting is NOT supported yet.

A sample PRS configuration is shown below:

prs_config = (
{
  NumPRSResources       = 1;
  PRSResourceSetPeriod  = [20, 2];
  SymbolStart           = [7];
  NumPRSSymbols         = [6];
  NumRB                 = 106;
  RBOffset              = 0;
  CombSize              = 4;
  REOffset              = [0];
  PRSResourceOffset     = [0];
  PRSResourceRepetition = 1;
  PRSResourceTimeGap    = 1;
  NPRS_ID               = [0];
  MutingPattern1        = [];
  MutingPattern2        = [];
  MutingBitRepetition   = 1;
}
);

The nrUE config has Active_gNBs to specify the number of active gNBs transmitting PRS simultaneously. Help strings for all PRS parameters are documented in openair2/COMMON/prs_nr_paramdef.h.

Note: PRS transmission and reception can only be validated in phy-test mode.

gNB in phy-test mode

FR1 test

Open a terminal on the host machine and execute the command below to launch the gNB with X310 USRPs:

sudo ./nr-softmodem -E -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb0.prs.band78.fr1.106PRB.usrpx310.conf --phy-test --gNBs.[0].min_rxtxtime 6 -D 0 -U 0

If N310 USRPs are used, run the above command without the -E option (i.e. without the 3/4 sampling rate).

To run using the rfsimulator, execute the following command:

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

Note: -D 0 -U 0 disables downlink and uplink data in the phy-test mode

FR2 test

In FR2 mode, an RF beamforming module is needed to transmit the signal in the mmWave frequency range. X310 USRPs can be used with a BasicTx daughtercard to transmit the baseband signal at an intermediate frequency (IF); the RF beamforming module then performs beamforming and the upconversion to FR2 frequencies. The IF can be specified using if_freq in the RU section of the gNB config.

If no RF beamforming module is present, the gNB can still be launched with the USRP alone to transmit at a supported if_freq:

sudo ./nr-softmodem -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb0.prs.band261.fr2.66PRB.usrpx310.conf --phy-test --gNBs.[0].min_rxtxtime 6 -D 0 -U 0

To run using the rfsimulator, execute the following command:

sudo ./nr-softmodem -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb0.prs.band261.fr2.66PRB.usrpx310.conf --phy-test --gNBs.[0].min_rxtxtime 6 -D 0 -U 0 --rfsim --rfsimulator.[0].serveraddr 127.0.0.1

Multiple gNB scenario

PRS is primarily used for positioning and localization of the UE, with multiple gNBs transmitting simultaneously. The OAI PRS implementation supports multi-gNB transmission provided all gNBs are tightly synchronized using a GPSDO clock. Therefore, before running this scenario, make sure the USRPs have a built-in GPSDO and that the GPS antennas are connected with good satellite visibility. Also, every time a gNB is launched, wait until GPS LOCKED is printed on the terminal during gNB startup. If a USRP fails to lock to the GPSDO, try again until it locks.

To use the GPSDO, change clock_source and time_source to gpsdo in the RU section of the gNB config.

FR1

gNB0:

sudo ./nr-softmodem -E -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb0.prs.band78.fr1.106PRB.usrpx310.conf --phy-test --gNBs.[0].min_rxtxtime 6 -D 0 -U 0

gNB1:

sudo ./nr-softmodem -E -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb1.prs.band78.fr1.106PRB.usrpx310.conf --phy-test --gNBs.[0].min_rxtxtime 6 -D 0 -U 0

To run using the rfsimulator, execute the following commands:

gNB0:

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

gNB1:

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

Note: In rfsim, the multiple gNBs are automatically synchronized using the system timestamp.

nrUE in phy-test mode

When the gNB and nrUE run on the same host machine, the reconfig.raw and rbconfig.raw files are generated with the launch of the gNB, and the nrUE then sources them automatically from the build directory. However, if the gNB and nrUE run on two different host machines, first run the gNB with the corresponding config and exit after a few seconds. This generates the reconfig.raw and rbconfig.raw files; copy them to the machine that runs the nrUE.

Note: If the UE is NOT able to connect to the gNB, check the USRP connections or try increasing --ue-rxgain in steps of 10 dB.

FR1 test

Once the gNB is up and running, open another terminal and execute the command below to launch the nrUE with X310 USRPs. Make sure to specify IP_ADDR1 and IP_ADDR2 (optional) to match the USRP IP addresses:

sudo ./nr-uesoftmodem -E --phy-test --usrp-args "addr=IP_ADDR1,second_addr=IP_ADDR2,time_source=internal,clock_source=internal" -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/ue.nr.prs.fr1.106prb.conf --ue-rxgain 80 --ue-fo-compensation --non-stop

If N310 USRPs are used, run the above command without the -E option (i.e. without the 3/4 sampling rate).

To run using the rfsimulator (UE as server), execute the following command:

sudo ./nr-uesoftmodem -E --phy-test -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/ue.nr.prs.fr1.106prb.conf --rfsim --rfsimulator.[0].serveraddr server

FR2 test

Like the gNB, the RF beamforming module receives at mmWave frequencies, and X310 USRPs with a BasicRx daughtercard receive the signal at the intermediate frequency (IF) from the RF beamforming module. The IF can be specified using the --if_freq option on the nrUE command line.

If no RF beamforming module is present, the nrUE can still be launched with the USRP alone to receive at if_freq and perform validation. Make sure if_freq is within the range supported by the USRP the nrUE is running with:

sudo ./nr-uesoftmodem --phy-test -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/ue.nr.prs.fr2.66prb.conf --usrp-args "addr=IP_ADDR1,second_addr=IP_ADDR2,time_source=internal,clock_source=internal" --ue-rxgain 80 --ue-fo-compensation --if_freq 50000000 --non-stop

To run using the rfsimulator (UE as server), execute the following command:

sudo ./nr-uesoftmodem --phy-test -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/ue.nr.prs.fr2.66prb.conf --rfsim --rfsimulator.[0].serveraddr server

Understanding UE logs

Before testing, make sure that:

  • In the nrUE PRS config file, Active_gNBs is set to the actual number of gNBs launched.
  • The parameters in the prs_config sections of the nrUE config match those of the gNB config used.

Then launch the nrUE using one of the commands above, depending on the FR1/FR2 test scenario.

After a successful connection, the UE starts estimating the channel from the downlink PRS pilots using the Least-Squares (LS) method. In the frequency domain, linear interpolation reconstructs the channel over the entire PRS bandwidth from the LS estimates at the pilot locations. The UE also measures the Time of Arrival (ToA) from the time-domain impulse response. The ToA measurement is printed on the console for each PRS resource.

The unit of ToA printed in the console is in samples. To convert the samples to time in seconds (s), use the following equation,

ToA (s) = ToA(samples) / sampling rate

The UE logs can be seen as follows:

FR1: 2 gNBs, 1 PRS resource per gNB

[PHY]    [gNB 0][rsc 0][Rx 0][sfn 433][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 433][slot 3] DL PRS ToA ==> 0.0 / 2048 samples, peak channel power -16.4 dBm, SNR +4.0 dB, rsrp -42.2 dBm

FR2: 1 gNB, 8 PRS resources

[PHY]    [gNB 0][rsc 0][Rx 0][sfn 689][slot 2] DL PRS ToA ==> 0.0 / 1024 samples, peak channel power -14.4 dBm, SNR +4.0 dB, rsrp -39.1 dBm
[PHY]    [gNB 0][rsc 1][Rx 0][sfn 689][slot 12] DL PRS ToA ==> 0.0 / 1024 samples, peak channel power -14.4 dBm, SNR +4.0 dB, rsrp -39.1 dBm
[PHY]    [gNB 0][rsc 2][Rx 0][sfn 689][slot 22] DL PRS ToA ==> 0.0 / 1024 samples, peak channel power -14.4 dBm, SNR +4.0 dB, rsrp -39.1 dBm
[PHY]    [gNB 0][rsc 3][Rx 0][sfn 689][slot 32] DL PRS ToA ==> 0.0 / 1024 samples, peak channel power -14.4 dBm, SNR +4.0 dB, rsrp -39.2 dBm
[PHY]    [gNB 0][rsc 4][Rx 0][sfn 689][slot 42] DL PRS ToA ==> 0.0 / 1024 samples, peak channel power -14.4 dBm, SNR +4.0 dB, rsrp -39.1 dBm
[PHY]    [gNB 0][rsc 5][Rx 0][sfn 689][slot 52] DL PRS ToA ==> 0.0 / 1024 samples, peak channel power -14.4 dBm, SNR +4.0 dB, rsrp -39.1 dBm
[PHY]    [gNB 0][rsc 6][Rx 0][sfn 689][slot 62] DL PRS ToA ==> 0.0 / 1024 samples, peak channel power -14.4 dBm, SNR +4.0 dB, rsrp -39.1 dBm
[PHY]    [gNB 0][rsc 7][Rx 0][sfn 689][slot 72] DL PRS ToA ==> 0.0 / 1024 samples, peak channel power -14.4 dBm, SNR +4.0 dB, rsrp -39.1 dBm

Note : verify that peak channel power, SNR and rsrp have reasonable values

On the UE side, the T tracer dumps the PRS channel estimates in both the time and frequency domains, using UE_PHY_DL_CHANNEL_ESTIMATE and UE_PHY_DL_CHANNEL_ESTIMATE_FREQ respectively. These dumps can be enabled by adding --T_stdout 0 (without console prints) or --T_stdout 2 (with console prints) to the nrUE launch command above.

Recording T tracer dumps

Once the nrUE is launched with the --T_stdout 0 or --T_stdout 2 option, open another terminal, navigate to the T tracer directory common/utils/T/tracer/, and build the T tracer binary using make.

Once the build is successful, execute the following command to start recording the PRS channel-estimate dumps:

./record -d ../T_messages.txt -on LEGACY_PHY_INFO -on UE_PHY_DL_CHANNEL_ESTIMATE -on UE_PHY_DL_CHANNEL_ESTIMATE_FREQ -o prs_dumps.raw

Exit using Ctrl+C to stop recording; otherwise it will keep running and take up a lot of disk space. Running it for 1-2 minutes generally collects sufficient dumps.

To check the contents of a recorded .raw file, replay it by executing:

./replay -i prs_dumps.raw

and textlog it in another terminal with the following command:

./textlog -d ../T_messages.txt -ON

Extracting PRS channel estimates

Once the T tracer dumps are recorded, the PRS channel estimates can be extracted from the .raw file using the bash script extract_prs_dumps.sh in the T tracer directory common/utils/T/tracer/:

./extract_prs_dumps.sh -g <num_gnb> -n <num_resources> -f <recorded .raw file> -c <count>

For example: FR1 with 2 gNBs, 1 PRS resource per gNB and 100 samples:

./extract_prs_dumps.sh -g 2 -n 1 -f prs_dumps.raw -c 100

For example: FR2 with 1 gNB, 8 PRS resources (rsc 0 to 7) and 100 samples:

./extract_prs_dumps.sh -g 1 -n 8 -f prs_dumps.raw -c 100

In the end, the script zips all the extracted dumps into prs_dumps.tgz. Check the script's help with the -h option:

./extract_prs_dumps.sh -h

MATLAB/Octave script to visualize PRS channel estimates

We have developed the plot_prs_Ttracer_dumps.m script to visualize the extracted PRS dumps offline in MATLAB/Octave. The script is located at common/utils/T/tracer/.

Enter the parameters the script asks for as input, like below:

Enter the directory path to T tracer dumps: '<workspace>/openairinterface5g/common/utils/T/tracer'
Enter the OFDM FFT size used for file parsing: <frame_parms->ofdm_symbol_size>
Enter number of PRS resources: <NumPRSResources>
Enter number of active gNBs: <Active_gNBs>

This script reads the IQ data from the extracted PRS dumps (chF_gnbX_Y.raw and chT_gnbX_Y.raw) and plots them as shown below:

PRS CFR FR2

PRS CIR FR2