5G SA srsUE

Introduction

The 22.04 release of srsRAN brought 5G SA (Standalone) support to srsUE. This application note shows how srsUE can be used with a third-party 5G SA network. In this example, we use the Amari Callbox Classic from Amarisoft to provide the network.

Limitations

The current 5G SA UE application has a few feature limitations that require certain configuration settings at both the gNB and the core network. The key feature limitations are as follows:

  • Limited to 15 kHz Sub-Carrier Spacing (SCS), which means only FDD bands can be used.

  • Limited to 10 MHz Bandwidth (BW)

Hardware Requirements

For this application note, the following components are used:

  • Amari Callbox with 5G SA support as gNB and core

  • AMD Ryzen5 3600X Linux PC as UE compute platform

  • Ettus Research USRP X310 connected over 10GigE as UE RF front-end

The Amari Callbox is an LTE/NR SDR-based UE test solution from Amarisoft. It contains an EPC, a 5GC, an eNodeB, a gNodeB, an IMS server, an eMBMS server and an Intel i7 Linux PC with PCIe SDR cards. The gNodeB is release 15 compliant and supports both NSA and SA modes. A further outline of the specifications can be found in the data sheet. This test solution was chosen as it’s widely available, easily configurable, and user-friendly.

Hardware Setup

Tests may be carried out over-the-air or using a cabled setup. For this example, we use a cabled setup between the UE and the eNB/gNB (i.e from the X310 to the PCIe SDR cards on the Callbox). These connections run through 30dB attenuators as shown in the figure above. The PPS inputs for the accurate clocking of both the UE and Callbox are also shown. Both UE and Callbox require accurate clocks - in our testing we provide PPS inputs to both.

Configuration

To set-up and run the 5G SA network and UE, the configuration files for both the Callbox and srsUE must be changed.

All of the modified configuration files have been included as attachments to this App Note. It is recommended you use these files to avoid errors while changing configs manually. Any configuration files not included here do not require modification from the default settings.

UE files:

Callbox files:

srsUE

The following changes need to be made to the UE configuration file to allow it to connect to the Callbox in SA mode.

Firstly the following parameters need to be changed under the [rf] options so that the X310 is configured optimally:

[rf]
tx_gain = 3
freq_offset = 0

nof_antennas = 1

srate = 11.52e6

device_name = uhd
device_args = type=x300,serial=30B8658,clock=external,sampling_rate=23.04e6,lo_freq_offset_hz=23.04e6,None

The next set of changes need to be made to the [rat.eutra] options. The LTE carrier is disabled, to force the UE to use a 5G NR carrier:

[rat.eutra]
dl_earfcn = 2850
nof_carriers = 0

Finally the [rat.nr] options need to be configured for 5G SA mode operation:

[rat.nr]
nof_carriers = 1
bands = 3

Callbox

The amarisoft_enb.cfg file is responsible for the configuration of the gNB needed for a 5G SA network.

The main changes to the default config are as follows:

  • Enable NR support

  • Enable NR cell and configure NR cell

  • Modify the PRACH configuration

  • Modify the PUCCH configuration

Enable NR Support

This is done on line 47. by setting the corresponding flag to true:

nr_support: true,

NR Cell

Firstly the Band and ARFCN must be set. This is done on lines 61 and 62:

nr_cell_list: [
{
  rf_port: 0,
  cell_id: 1,
  band: 3,
  dl_nr_arfcn: 368500,
},

],

The band and dl_nr_afcn are chosen based on the known limitations of srsRAN.

Next, the SCS, BW and other configuration parameters can be changed from line 68:

nr_cell_default: {
  subcarrier_spacing: 15, /* kHz */
  ssb_subcarrier_spacing: 15, // only supported in FDD bands
  bandwidth: 10, /* MHz */
  n_antenna_dl: 1,
  n_antenna_ul: 1,


  ssb_pos_bitmap: "1000",

  ssb_period: 10, /* in ms */
  n_id_cell: 500,

Here the subcarrier_spacing is set to 15 KHz and the bandwidth to 10 MHz, the n_antenna_dl is set to 1 and the ssb_period is set to 10.

PRACH

For the PRACH config options (line 105) the following is used:

prach: {
  prach_config_index: 0,
  msg1_fdm: 1,
  msg1_frequency_start: 1,
  zero_correlation_zone_config: 0,
  preamble_received_target_power: -110, /* in dBm */
  preamble_trans_max: 7,
  power_ramping_step: 4, /* in dB */
  ra_response_window: 10, /* in slots */
  restricted_set_config: "unrestricted_set",
  ra_contention_resolution_timer: 64, /* in ms */
  ssb_per_prach_occasion: 1,
  cb_preambles_per_ssb: 8,
},

The changes made to the above include the setting of prach_config_index to 0, setting msg1_frequency_start to 1 and setting ra_response_window to 10.

PUCCH

Lastly, the PUCCH config must be changed. This is done from line 353:

pucch: {
  pucch_group_hopping: "neither",
  hopping_id: -1, /* -1 = n_cell_id */
  p0_nominal: -90,
    pucch1: {
      n_cs: 3,
      n_occ: 3,
      freq_hopping: false,
    },
    pucch2: {
      n_symb: 2,
      n_prb: 1,
      freq_hopping: false,
      simultaneous_harq_ack_csi: false,
      max_code_rate: 0.25,
    },
},

The only change here is that freq_hopping is set to false in both pucch1 and pucch2.

The gNB is now configured correctly. All other config files associated with the gNB and 5GC can be left in their default states.

Running the Network

The following order should be used when running the network:

  1. 5GC

  2. gNB

  3. UE

5GC

To run the 5GC the following command is used:

sudo ltemme mme.cfg

gNB

Next the eNB/ gNB should be instantiated, using the following command:

sudo lteenb gnb-nsa.cfg

Console output should be similar to:

      Base Station version 2021-03-15, Copyright (C) 2012-2021 Amarisoft

RF0: sample_rate=61.440 MHz dl_freq=1836.740 MHz ul_freq=1741.740 MHz (band n3) dl_ant=1 ul_ant=1

UE

To run the UE, use the following command:

sudo srsue ue.conf

Once the UE has been initialized you should see the following:

Opening 1 channels in RF device=uhd with args=type=x300,serial=30B8658,clock=external,sampling_rate=23.04e6,lo_freq_offset_hz=23.04e6,None

This will be followed by some information regarding the USRP. Once the cell has been found successfully you should see the following:

Found Cell:  Mode=FDD, PCI=1, PRB=50, Ports=1, CFO=0.1 KHz
Found PLMN:  Id=00101, TAC=7
Random Access Transmission: prach_occasion=0, preamble_index=0, ra-rnti=0xf, tti=3851
Random Access Complete.     c-rnti=0x4601, ta=3
RRC Connected
RRC NR reconfiguration successful.
PDU Session Establishment successful. IP: 192.168.4.2
RRC NR reconfiguration successful.

To confirm the UE successfully connected, you should see the following on the console output of the gNB:

PRACH: cell=01 seq=0 ta=3 snr=29.1 dB

Console Trace

srsUE

The following is an example console trace output when running bi-direction traffic with iPerf3:

---------Signal-----------|-----------------DL-----------------|-----------UL-----------
rat  pci  rsrp   pl   cfo | mcs  snr  iter  brate  bler  ta_us | mcs   buff  brate  bler
 nr  500    -3    0   2.0 |  27   28   2.0    23M    0%    0.0 |  27     59    16M    0%
 nr  500    -3    0   1.6 |  27   28   2.1    23M    0%    0.0 |  27    30k    16M    0%
 nr  500    -3    0   2.0 |  27   28   2.1    23M    0%    0.0 |  27    44k    16M    0%
 nr  500    -3    0  824m |  27   28   2.1    23M    0%    0.0 |  27    26k    16M    0%
 nr  500    -3    0   1.1 |  27   28   2.1    23M    0%    0.0 |  27    10k    17M    0%
 nr  500    -3    0   1.3 |  27   28   2.0    23M    0%    0.0 |  27    0.0    16M    0%
 nr  500    -3    0  106m |  27   28   2.0    23M    0%    0.0 |  27   118k    16M    0%
 nr  500    -4    0   1.0 |  27   28   2.1    22M    0%    0.0 |  27    52k    21M    0%
 nr  500    -4    0   1.9 |  27   28   2.0    22M    0%    0.0 |  27    57k    21M    0%
 nr  500    -3    0  840m |  27   28   2.0    23M    0%    0.0 |  27    54k    19M    0%
 nr  500    -3    0  160m |  27   28   2.0    23M    0%    0.0 |  27    20k    18M    0%

To read more about the UE console trace metrics, see the UE User Manual.

Amarisoft gNB

The following console output is shown on the gNB for the same period:

               ----DL----------------------- --UL------------------------------------------------
UE_ID  CL RNTI C cqi ri  mcs retx txok brate  snr puc1  mcs rxko rxok brate     #its phr  pl   ta
    1 001 4601 1  15  1 27.9    0 1472 22.6M 39.5   -  27.9    0 1022 18.7M  1/1.9/3   -   -  0.3
    1 001 4601 1  15  1 27.9    0 1476 22.7M 39.3   -  27.9    0  987 17.8M  1/1.9/3   -   -  0.3
    1 001 4601 1  15  1 27.9    0 1512 23.1M 36.3   -  27.9    0  908 15.7M  1/1.9/3   -   -  0.3
    1 001 4601 1  15  1 27.9    0 1474 22.6M 38.0   -  27.9    0  977 17.1M  1/1.9/3   -   -  0.3
    1 001 4601 1  15  1 27.9    0 1488 22.8M 46.6   -  27.9    0  929 16.3M  1/1.9/3   -   -  0.3
    1 001 4601 1  15  1 27.9   28 1427 21.9M 38.0   -  27.9    0 1035 19.1M  1/1.9/3   -   -  0.2
    1 001 4601 1  15  1 27.9    5 1428 21.9M 39.8   -  28.0    0 1113 21.3M  1/1.9/3   -   -  0.2
    1 001 4601 1  15  1 27.9    3 1416 21.7M 38.2   -  28.0    0 1159 22.4M  1/1.9/3   -   -  0.2
    1 001 4601 1  15  1 27.9    0 1395 21.4M 38.7   -  28.0    0 1222 24.7M  1/2.0/3   -   -  0.2
    1 001 4601 1  15  1 27.9    0 1405 21.6M 39.0   -  28.0    0 1182 23.3M  1/2.0/3   -   -  0.2