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Running NR PRS with OAI gNB and nrUE |
This page is valid on tags starting from 2022.w37.
After you have built the softmodem executables, go to the build directory openairinterface5g/cmake_targets/ran_build/build/ and start testing the Rel16 PRS usecases.
| Mode | gNB config | nrUE 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.fr2.64PRB.usrpx310.conf gnb1.prs.band261.fr2.64PRB.usrpx310.conf |
ue.nr.prs.fr2.64prb.conf |
Under gNB and nrUE config files, parameters are configured under prs_config section. nrUE is capable to receive downlink PRS signal from multiple gNB simultaneously and therefore nrUE config file contains multiple prs_config sections, corresponding to each gNB. These parameters can be changed as per the test scenario.
As of now, PRS Comb size of 2 and 4 is supported and validated with R&S Spectrum analyzer. Also Muting is NOT supported yet. Sample configuration as 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;
}
);
To TURN OFF PRS, set NumPRSResources=0 in gNB prs_config section. nrUE config has Active_gNBs to specify number of active gNBs transmitting PRS signal simultaneously. Find the help string for PRS parameters in openair2/COMMON/prs_nr_paramdef.h
Note that numactl is only needed if you run on a NUMA architecture with more than 1 CPU. In this case it should be installed on Linux using command sudo apt-get install -y numactl
Also check the numa nodes USRP’s are connected to, using the following command:
cat /sys/class/net/eth_if/device/numa_node
Where eth_if has to be replaced with the name of the network interface the USRP is connected to.
In our case the output is 0 and hence we use numactl --cpunodebind=0 --membind=0
Open a terminal on the host machine, and execute below command to launch gNB with X310 USRPs
sudo numactl --cpunodebind=0 --membind=0 ./nr-softmodem -E -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb0.prs.band78.fr1.106PRB.usrpx310.conf --phy-test
If N310 USRPs are used, then run above command without -E option i.e without 3/4 sampling rate.
To run using rfsimulator, execute following command:
sudo ./nr-softmodem -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb0.prs.band78.fr1.106PRB.usrpx310.conf --noS1 --rfsim --phy-test
In FR2 mode, we need RF beamforming module to transmit signal in mmWave frequency range. X310 USRPs can be used with BasicTx daughtercard to transmit baseband signal at Intermediate Frequncy(IF) and then RF beamforming module would perform beamforming and the upconversion to FR2 frequencies. IF can be specified using if_freq in the RU section of gNB config.
If RF beamforming module is NOT present, gNB can still be launched with USRP alone; to transmit at supported if_freq.
sudo numactl --cpunodebind=0 --membind=0 ./nr-softmodem -E -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb0.prs.band261.fr2.64PRB.usrpx310.conf --phy-test
To run using rfsimulator, execute following command:
sudo ./nr-softmodem -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb0.prs.band261.fr2.64PRB.usrpx310.conf --noS1 --rfsim --phy-test
PRS is primarily used for positioning and localization of the UE with multiple gNBs transmitting simultaneously. OAI PRS implementation supports multiple gNB transmission provided all the gNBs are tightely synchronized using GPSDO clock. Therefore before running this scenario, make sure the USRPs has built-in GPSDO and 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 USRP fails to lock with GPSDO, try again until its locked.
To use GPSDO, make sure to change clock_source and time_source to gpsdo in RU section of gNB config.
sudo numactl --cpunodebind=0 --membind=0 ./nr-softmodem -E -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb0.band78.fr1.106PRB.prs.usrpx310.conf --phy-test
sudo numactl --cpunodebind=1 --membind=1 ./nr-softmodem -E -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb1.band78.fr1.106PRB.prs.usrpx310.conf --phy-test
While running gNB and nrUE on the same host machine, reconfig.raw and rbconfig.raw files would be generated with the launch of gNB and and then nrUE would automatically source it from build directory. However, if gNB and nrUE are running on two different host machines, then run gNB first with the corresponding config and exit after few seconds. This would generate reconfig.raw and rbconfig.raw files.
After this, nrUE can be launched using one of the below commands depending on the test scenario. If UE is NOT able to connect to the gNB, then check the USRP connections or try increasing --ue-rxgain in steps of 10dB.
Also check the instructions on numactl in gNB test section as it applies for nrUE execution as well.
Once gNB is up and running, open another terminal and execute below command to launch nrUE with X310 USRPs. Make sure to specify IP_ADDR1 and IP_ADDR2(optional) correctly as per USRPs IP address
sudo numactl --cpunodebind=0 --membind=0 ./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, then run above command without -E option i.e without 3/4 sampling rate.
To run using rfsimulator with local ETH IF 127.0.0.1, execute following command:
sudo ./nr-uesoftmodem --rfsim --phy-test --noS1 -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/ue.nr.prs.fr1.106prb.conf --rfsimulator.serveraddr 127.0.0.1
Like gNB, RF beamforming module is receiving at mmWave frequencies and then X310 USRPs with BasicRx daughtercard to receive the signal at intermediate frequncy(IF) from RF beamforming module. IF can be specified using --if_freq option in nrUE command line.
If RF beamforming module is NOT present, nrUE can still be launched with USRP alone; to receive at if_freq and validation can be done. Make sure to specify if_freq in the range supported by USRP nrUE is running with.
sudo numactl --cpunodebind=0 --membind=0 ./nr-uesoftmodem -E --phy-test -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/ue.nr.prs.fr2.64prb.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 rfsimulator with local ETH IF 127.0.0.1, execute following command:
sudo ./nr-uesoftmodem --rfsim --phy-test --noS1 -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/ue.nr.prs.fr2.64prb.conf --rfsimulator.serveraddr 127.0.0.1
In nrUE prs config file, change Active_gNBs to the actual number of gNBs launched. Also verify the parameter in prs_config sections of nrUE config is matching with that of gNB config used. And launch nrUE using one of the above commands depending on FR1/FR2 test scenario.
After successful connection, UE starts estimating channel based on the downlink PRS pilots using Least-Squares(LS) method. In the frequency domain, linear interpolation is used to reconstruct the channel over entire PRS bandwidth using LS estimates at pilot locations. UE also measures Time of Arrival(ToA) based on the time domain impulse response. On the console, ToA measurement is printed for each PRS resource.
[PHY] [gNB 0][rsc 0][Rx 0][sfn 314][slot 2] DL PRS ToA ==> 1 / 1024 samples, peak channel power 15.6 dBm, SNR +4 dB
[PHY] [gNB 0][rsc 1][Rx 0][sfn 314][slot 12] DL PRS ToA ==> 1 / 1024 samples, peak channel power 15.6 dBm, SNR +4 dB
[PHY] [gNB 0][rsc 2][Rx 0][sfn 314][slot 22] DL PRS ToA ==> 1 / 1024 samples, peak channel power 15.6 dBm, SNR +4 dB
[PHY] [gNB 0][rsc 3][Rx 0][sfn 314][slot 32] DL PRS ToA ==> 1 / 1024 samples, peak channel power 15.6 dBm, SNR +4 dB
At UE side, T tracer is used to dump PRS channel estimates, both in time and frequency domain using UE_PHY_DL_CHANNEL_ESTIMATE and UE_PHY_DL_CHANNEL_ESTIMATE_FREQ respectively. These dumps can be enabled using options --T_stdout 0 without console prints or --T_stdout 2 with console prints; in above nrUE launch command.
Once nrUE is launched with --T_stdout 0 or 2 option, open another terminal. Navigate to T tracer directory common/utils/T/tracer/ and build the T tracer binary using make
Once the build is successful, execute following command to start recording the PRS channel estimates 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, else it will keep running and take lot of disk space. Generally running it for 1-2 minutes should collect sufficient dumps.
To check the contents of recorded .raw file, replay it by executing:
./replay -i prs_dumps.raw
and textlog it on another terminal with following command:
./textlog -d ../T_messages.txt -ON -no-gui
Once T tracer dumps are recorded, PRS channel estimates can be extracted from .raw file using bash script extract_prs_dumps.sh in T tracer directory common/utils/T/tracer/
./extract_prs_dumps.sh -g <num_gnb> -n <num_resources> -f <recorded .raw file> -c <count>
In the end, the script will zip all the extracted dumps to prs_dumps.tgz. Make sure to check help in running script using -h option:
./extract_prs_dumps.sh -h
We have developed plot_prs_Ttracer_dumps.m script to visualize the extracted PRS dumps offline in Matlab/Octave. Location of the script is openair1/PHY/NR_UE_ESTIMATION/plot_prs_Ttracer_dumps.m
Make sure to enter the parameters script asks 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 respurces: <NumPRSResources>
Enter number of active gNBs: <Active_gNBs>
This script will read the IQ data from extracted PRS dumps(chF_gnbX_Y.raw and chT_gnbX_Y.raw) and plot them like below
Fig.1 - FR2 100MHz test
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