How to connect an UE to a RU in a CU/DU split scenario ? #1379

HeloisaJ · 2025-10-03T02:29:42Z

HeloisaJ
Oct 3, 2025

I have made a CU/DU split scenario using srsCU and srsDU, which I can add multiple DUs over time. The issue is that I'm trying to connect one of my DUs to a UE from srsRAN 4G. I'm using the Open5GS Docker container as the core and ZMQ. I found quite confusing to configure the TX and RX ports for ZMQ, one question I have is are they arbitrary ? I know I need to invert the TX and RX ports when configuring the UE and RU, but does they need to be a especific value or there is any other rule beside this?

The main error that appears for me is the UE not connecting to the RU. While I was testing two types of error occured, one with Attaching UE and other which appears Random Access Transmition message multiple times. Could someone help me with this problem ?

Code for the DU:

gnb_id: 411
gnb_du_id: 0                        # Optional UINT (0). Sets the gNB DU ID. Supported: [0 - 68719476735].

f1ap:
  cu_cp_addr: 127.0.10.1                    # The address of CU-CP
  bind_addr: 127.0.1.2

f1u:
  socket:
    -
      bind_addr: 127.0.1.2

ru_sdr:
  device_driver: zmq
  device_args: tx_port=tcp://127.0.0.1:2000,rx_port=tcp://127.0.0.1:2001,base_srate=11.52e6
  srate: 11.52
  tx_gain: 75
  rx_gain: 75

cell_cfg:
  sector_id: 0011
  dl_arfcn: 368500
  band: 3
  channel_bandwidth_MHz: 10
  common_scs: 15
  plmn: "00101"
  tac: 7
  pci: 1
  pdcch:
    common:
      ss0_index: 0                  # Set search space zero index to match srsUE capabilities
      coreset0_index: 6             # Set search CORESET Zero index to match srsUE capabilities
    dedicated:
      ss2_type: common              # Search Space type, has to be set to common
      dci_format_0_1_and_1_1: false # Set correct DCI format (fallback)
  prach:
    prach_config_index: 1           # Sets PRACH config to match what is expected by srsUE
  pdsch:
    mcs_table: qam64
  pusch:
    mcs_table: qam64

Code for the UE:

[rf]
freq_offset = 0
tx_gain = 50
rx_gain = 40
srate = 11.52e6
nof_antennas = 1

device_name = zmq
device_args = tx_port=tcp://127.0.0.1:2001,rx_port=tcp://127.0.0.1:2000,base_srate=11.52e6

[rat.eutra]
dl_earfcn = 2850
nof_carriers = 0

[rat.nr]
bands = 3
nof_carriers = 1
max_nof_prb = 52
nof_prb = 52
dl_nr_arfcn = 368500
ssb_nr_arfcn = 367930

[pcap]
enable = none
mac_filename = /tmp/ue1_mac.pcap
mac_nr_filename = /tmp/ue1_mac_nr.pcap
nas_filename = /tmp/ue1_nas.pcap

[log]
all_level = info
phy_lib_level = none
all_hex_limit = 32
filename = /tmp/ue1.log
file_max_size = -1

[usim]
mode = soft
algo = milenage
opc  = 63BFA50EE6523365FF14C1F45F88737D
k    = 00112233445566778899aabbccddeeff
imsi = 001010123456780
imei = 353490069873319

[rrc]
release = 15
ue_category = 4

[nas]
apn = srsapn
apn_protocol = ipv4

[gw]
netns = ue1
ip_devname = tun_srsue
ip_netmask = 255.255.255.0

[gui]
enable = false

Attaching UE error:

Active RF plugins: libsrsran_rf_zmq.so
Inactive RF plugins: 
Reading configuration file ./ue1.conf...

Built in Release mode using commit ec29b0c1f on branch master.

Opening 1 channels in RF device=zmq with args=tx_port=tcp://127.0.0.1:2001,rx_port=tcp://127.0.0.1:2000,base_srate=11.52e6
Supported RF device list: zmq file
CHx base_srate=11.52e6
Current sample rate is 1.92 MHz with a base rate of 11.52 MHz (x6 decimation)
CH0 rx_port=tcp://127.0.0.1:2000
CH0 tx_port=tcp://127.0.0.1:2001
Current sample rate is 11.52 MHz with a base rate of 11.52 MHz (x1 decimation) 
Current sample rate is 11.52 MHz with a base rate of 11.52 MHz (x1 decimation) 
Waiting PHY to initialize ... done!
Attaching UE...

Random Access Transmission error:

Active RF plugins: libsrsran_rf_zmq.so                                                                                                                      
Inactive RF plugins:                                                                                                                                        
Reading configuration file ./ue1.conf...                                                                                                     
                                                                                                                                                            
Built in Release mode using commit ec29b0c1f on branch master.                                                                                              
                                                                                                                                                            
Opening 1 channels in RF device=zmq with args=tx_port=tcp://127.0.0.1:2001,rx_port=tcp://127.0.0.1:2000,base_srate=11.52e6                                  
Supported RF device list: zmq file                                                                                                                          
CHx base_srate=11.52e6                                                                                                                                      
Current sample rate is 1.92 MHz with a base rate of 11.52 MHz (x6 decimation)                                                                               
CH0 rx_port=tcp://127.0.0.1:2000                                                                                                                            
CH0 tx_port=tcp://127.0.0.1:2001                                                                                                                            
Current sample rate is 11.52 MHz with a base rate of 11.52 MHz (x1 decimation)                                                                              
Current sample rate is 11.52 MHz with a base rate of 11.52 MHz (x1 decimation)                                                                              
Waiting PHY to initialize ... done!                                                                                                                         
Attaching UE...                                                                                                                                             
Random Access Transmission: prach_occasion=0, preamble_index=0, ra-rnti=0xf, tti=331                                                                        
Random Access Transmission: prach_occasion=0, preamble_index=0, ra-rnti=0xf, tti=361                                                                        
Random Access Transmission: prach_occasion=0, preamble_index=0, ra-rnti=0xf, tti=391                                                                        
Random Access Transmission: prach_occasion=0, preamble_index=0, ra-rnti=0xf, tti=421
Random Access Transmission: prach_occasion=0, preamble_index=0, ra-rnti=0xf, tti=451
Random Access Transmission: prach_occasion=0, preamble_index=0, ra-rnti=0xf, tti=481
Random Access Transmission: prach_occasion=0, preamble_index=0, ra-rnti=0xf, tti=511

Answered by HeloisaJ

Oct 7, 2025

I tested here and the RU address worked with a random IP and port. The configuration is actually similar to the GNB and is also good to remember the UE Limitations available at this link https://docs.srsran.com/projects/project/en/latest/tutorials/source/srsUE/source/index.html#zeromq-based-setup. While I was testing I found the following error on the UE log:

PDCCH DMRS Rx: Measure L=2; ncce=0; RSRP=-inf dBfs; EPRE=-inf dBfs; Corr=0.000 
2025-10-06T18:01:20.595426 [PHY_LIB] [I] [ 0] Discarded PDCCH candidate L=2;ncce=0; Invalid measurement; 
2025-10-06T18:01:20.595428 [PHY_LIB] [I] [ 0] PDCCH DMRS Rx: Measure L=2; ncce=0; RSRP=-inf dBfs; EPRE=-inf dBfs; Corr=0.000 
2025-10-06T18:01:20.595…

View full answer

HeloisaJ · 2025-10-07T19:32:20Z

HeloisaJ
Oct 7, 2025
Author

I tested here and the RU address worked with a random IP and port. The configuration is actually similar to the GNB and is also good to remember the UE Limitations available at this link https://docs.srsran.com/projects/project/en/latest/tutorials/source/srsUE/source/index.html#zeromq-based-setup. While I was testing I found the following error on the UE log:

PDCCH DMRS Rx: Measure L=2; ncce=0; RSRP=-inf dBfs; EPRE=-inf dBfs; Corr=0.000 
2025-10-06T18:01:20.595426 [PHY_LIB] [I] [ 0] Discarded PDCCH candidate L=2;ncce=0; Invalid measurement; 
2025-10-06T18:01:20.595428 [PHY_LIB] [I] [ 0] PDCCH DMRS Rx: Measure L=2; ncce=0; RSRP=-inf dBfs; EPRE=-inf dBfs; Corr=0.000 
2025-10-06T18:01:20.595428 [PHY_LIB] [I] [ 0] Discarded PDCCH candidate L=2;ncce=0; Invalid measurement; 
2025-10-06T18:01:20.595430 [PHY_LIB] [I] [ 0] PDCCH DMRS Rx: Measure L=2; ncce=4; RSRP=-inf dBfs; EPRE=-inf dBfs; Corr=0.000 
...

After this I discovered the following discussion with the same error message https://github.com/srsran/srsRAN_Project/discussions/297. The main change I did after this was putting the value 11.52 for the srate. To help other people, here is the code for the DU and the UE.

Code for DU:

gnb_id: 411
gnb_du_id: 0                        # Optional UINT (0). Sets the gNB DU ID. Supported: [0 - 68719476735].

f1ap:
  cu_cp_addr: 127.0.10.1                    # The address of CU-CP
  bind_addr: 127.0.1.2

f1u:
  socket:
    -
      bind_addr: 127.0.1.2

ru_sdr:
  device_driver: zmq
  device_args: tx_port=tcp://127.0.0.2:2000,rx_port=tcp://127.0.0.1:2001,base_srate=11.52e6
  srate: 11.52
  tx_gain: 80
  rx_gain: 40

cell_cfg:
  sector_id: 0011
  dl_arfcn: 368500
  band: 3
  channel_bandwidth_MHz: 10
  common_scs: 15
  plmn: "00101"
  tac: 7
  pci: 1
  pdcch:
    common:
      ss0_index: 0                  # Set search space zero index to match srsUE capabilities
      coreset0_index: 6             # Set search CORESET Zero index to match srsUE capabilities
    dedicated:
      ss2_type: common              # Search Space type, has to be set to common
      dci_format_0_1_and_1_1: false # Set correct DCI format (fallback)
  prach:
    prach_config_index: 1           # Sets PRACH config to match what is expected by srsUE
  pdsch:
    mcs_table: qam64
  pusch:
    mcs_table: qam64

The UE code:

[rf]
freq_offset = 0
tx_gain = 50
rx_gain = 40
srate = 11.52e6
nof_antennas = 1

device_name = zmq
device_args = tx_port=tcp://127.0.0.1:2001,rx_port=tcp://127.0.0.2:2000,base_srate=11.52e6

[rat.eutra]
dl_earfcn = 2850
nof_carriers = 0

[rat.nr]
bands = 3
nof_carriers = 1
max_nof_prb = 52
nof_prb = 52
dl_nr_arfcn = 368500
ssb_nr_arfcn = 368410

[pcap]
enable = none
mac_filename = /tmp/ue1_mac.pcap
mac_nr_filename = /tmp/ue1_mac_nr.pcap
nas_filename = /tmp/ue1_nas.pcap

[log]
all_level = info
all_hex_limit = 2048
filename = /tmp/ue1.log
file_max_size = -1

[usim]
mode = soft
algo = milenage
opc  = 63BFA50EE6523365FF14C1F45F88737D
k    = 00112233445566778899aabbccddeeff
imsi = 001010123456780
imei = 353490069873319

[rrc]
release = 15
ue_category = 4

[nas]
apn = srsapn
apn_protocol = ipv4

[gw]
netns = ue1
ip_devname = tun_srsue
ip_netmask = 255.255.255.0

[gui]
enable = false

0 replies

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

How to connect an UE to a RU in a CU/DU split scenario ? #1379

Uh oh!

{{title}}

Uh oh!

Replies: 1 comment

Uh oh!

{{title}}

Uh oh!

Select a reply

Uh oh!

How to connect an UE to a RU in a CU/DU split scenario ? #1379

Uh oh!

HeloisaJ Oct 3, 2025

Replies: 1 comment

Uh oh!

HeloisaJ Oct 7, 2025 Author

HeloisaJ
Oct 3, 2025

HeloisaJ
Oct 7, 2025
Author