Handover between 2 DUs when using CU-DU split

[HaoxinSEU]

Hi community,

We are trying to trigger handover between 2 DUs in the CU-DU split setup.

Our setup is like: one CU connects to two DUs, and each DU is connecting to its own SDR, and with pci 1 and 2 respectively.

If we trigger the handover from the CU console, i.e., by running ho 1 4601 2, we can see that the UE is indeed pushed to the other DU. And in the log we can see

[CU-CP   ] [I] ue=2: Trigger inter DU handover from source_du=1 to target_du=0

However, we cannot make the handover work by measurement. In the RRC message, we see that the UE only reports the serving cell measurements, but nothing for the neighboring cells. So if we change the SDR gain to make the neighboring cell's signal very strong, the UE just disconnects from the original cell, and reconnects to the neighbor, as there is no logs about handover in CU.

We are wondering what could be the issue for this? Thank you for your help!

Here are the configurations for the CU and two DUs:

CU:

gnb_id: 531                       
gnb_id_bit_length: 32              
ran_node_name: cu_cp_01     

slicing:                           
  -                                   
    sst: 1                               
    sd: 0                              

amf: 
  addr: open5gs-amf-ngap.open5gs
  bind_addr: 10.42.1.75

cu_cp:
  inactivity_timer: 7200     
  plmns: "00103"
  tacs: "1"           
 
  mobility:
    trigger_handover_from_measurements: true  
    cells:                                    
      - nr_cell_id: 0x2130                
        periodic_report_cfg_id: 1            
        ncells:                               
          - nr_cell_id: 0x2131                 
            report_configs: [ 2 ]                
      - nr_cell_id: 0x2131                    
        periodic_report_cfg_id: 1            
        ncells:                             
          - nr_cell_id: 0x2130                  
            report_configs: [ 2 ]               
    report_configs:                           
      - report_cfg_id: 1                      
        report_type: periodical               
        report_interval_ms: 480                 
      - report_cfg_id: 2                        
        report_type: event_triggered            
        a3_report_type: rsrp                    
        a3_offset_db: 2                      
        a3_hysteresis_db: 0                     
        a3_time_to_trigger_ms: 100             
  f1ap: 
    bind_addr: 10.42.1.75               
   
cu_up: 
  nru:                               
    bind_addr: 10.42.1.75               
    udp_max_rx_msgs: 256                  

log:
  filename: /tmp/cu.log
  all_level: debug

DU1:

gnb_id: 531
gnb_id_bit_length: 32
gnb_du_id: 0

f1ap:
  cu_cp_addr: 10.42.1.75
  bind_addr: 10.42.1.115

nru:                                # Interface between CU and CU-UP
  queue_size: 2048                      # Optional INT (2048). Sets the F1-U PDU queue size in bytes. 
  bind_addr: 10.42.1.115

ru_sdr:
  device_driver: bladerf
  device_args: "rx_gain=manual"
  # clock:
  # sync:
  srate: 23.04
  otw_format: sc16
  tx_gain: 66
  rx_gain: 15

cell_cfg:
  dl_arfcn: 640000
  band: 78
  channel_bandwidth_MHz: 20
  common_scs: 30
  plmn: "00103"
  tac: 1
  pci: 1

log:
  filename: /tmp/du.log
  all_level: info

DU2:

gnb_id: 531
gnb_id_bit_length: 32
gnb_du_id: 1

f1ap:
  cu_cp_addr: 10.42.1.75
  bind_addr: 10.42.0.182

nru:                            
  queue_size: 2048                   
  bind_addr: 10.42.0.182

ru_sdr:
  device_driver: uhd
  device_args: type=b200,num_recv_frames=64,num_send_frames=64
  srate: 23.04
  otw_format: sc12
  tx_gain: 80
  rx_gain: 40

cell_cfg:
  dl_arfcn: 640000
  band: 78
  channel_bandwidth_MHz: 20
  common_scs: 30
  plmn: "00103"
  tac: 1
  pci: 2

log:
  filename: /tmp/du.log
  all_level: debug

[FabianEckermann]

Hi @HaoxinSEU,
can you provide logs and PCAPs for this?
See https://docs.srsran.com/projects/project/en/latest/user_manuals/source/outputs.html on how to enable them.

[HaoxinSEU]

Hi Fabian,

Thank you for the hint. We tried to launch both DUs first, then connect the UE. The result is the same as before, and the UE does not do handover...

Here are the logs and pcaps.
logs_and_pcap_launch_all.zip

[HaoxinSEU]

We notice one potential issue here.

In our setup, we set nr_cell_id to be gnb_id (HEX) concatenated with 0 or 1. So that CU can consider that both DUs are managed by itself. Then in the mobility configuration, we don't specify fields such as ssb_arfcn, ssb_period... But then we see in the CU log, the configured cell seems to be incomplete for DU2:

2024-07-09T07:43:18.881526 [CU-CP   ] [D] Measurement objects:
2024-07-09T07:43:18.881527 [CU-CP   ] [D]  - ssb_freq=639648: ssb_freq=639648 ssb_scs=30kHz smtc1: periodicity=10 offset=0 dur=5
2024-07-09T07:43:18.881527 [CU-CP   ] [D] Configured cells:
2024-07-09T07:43:18.881528 [CU-CP   ] [D]  - nci=0x2130 complete=yes gnb_id=531 pci=1 band=78 ssb_arfcn=639648 ssb_scs=30kHz ncells=[ 0x2131 ]
2024-07-09T07:43:18.881529 [CU-CP   ] [D]  - nci=0x2131 complete=no gnb_id=0 pci=? band=? ssb_arfcn=? ssb_scs=? ncells=[ 0x2130 ]

But if we add these fields back, but the CU cannot run, and gives the error:

--== srsRAN CU (commit 4e8dec77c) ==--

cell=0x2131: For cells managed by the CU-CP the gnb_id_bit_length, pci, band, ssb_arfcn, ssb_scs, ssb_period, ssb_offset and ssb_duration must not be configured in the mobility config
srsRAN ERROR: Invalid configuration detected.

So we are wondering if these fields can cause some problems?

[HaoxinSEU]

Hi Fabian,

And also, we see that in the RRC message, reportConfigToAddModList only contains report_cfg_id: 1, but the second part report_cfg_id: 2 is missing in the message.... Do you think this could also be a problem? Thank you so much! :)

"reportConfigToAddModList": [
                    {
                      "reportConfigId": 1,
                      "reportConfig": {
                        "reportConfigNR": {
                          "reportType": {
                            "periodical": {
                              "rsType": "ssb",
                              "reportInterval": "ms480",
                              "reportAmount": "infinity",
                              "reportQuantityCell": {
                                "rsrp": true,
                                "rsrq": true,
                                "sinr": true
                              },
                              "maxReportCells": 4,
                              "reportQuantityRS-Indexes": {
                                "rsrp": true,
                                "rsrq": true,
                                "sinr": true
                              },
                              "maxNrofRS-IndexesToReport": 4,
                              "includeBeamMeasurements": true,
                              "useAllowedCellList": false
                            }
                          }
                        }
                      }
                    }
                  ],

[FabianEckermann]

Hi @HaoxinSEU,
we found the issue and fixed it. It will soon be in the main branch

[HaoxinSEU]

Hi Fabian,

Thanks for the reply! Look forward to the fix. :)

Best,
Haoxin