srsRAN UE: "Could not lock reference clock source" when using external clock

[marshoepial]

We are trying to use srsRAN UE for 5G communication similar to this documentation: https://docs.srsran.com/projects/project/en/latest/tutorials/source/srsUE/source/index.html. We have two USRP B205mini SDRs connected to an external Leo Bodnar GPSDO, and the gNB and UE running in properly privileged Docker containers with access to USB devices. We had a lot of timing issues before adding the GPSDO, but after adding it, synchronization on the gNB side has been much better.

However, we are getting a curious error from the UE stdout: /usr/src/srsRAN_4G/lib/src/phy/rf/rf_uhd_imp.cc:833: Could not lock reference clock source. Sensor: ref_locked=false. Checking the USRP status using uhd_usrp_probe, both report being locked to the clock source. No matter which of the two SDRs we run the UE with, it will report this error. We've also confirmed that the GPSDO is outputting a properly stable 10MHz signal. Communication is still somewhat unstable, and from the log files it does seem that the UE has issues with keeping synchronization.

Attached is our UE configuration and log file from an example run (setup, connection with the gNB, and subsequent desync/disconnection.) Is there some extra configuration, other than clock=external, to make the UE lock to the reference clock? Thanks for your help.

ue.conf:

[rf]
freq_offset = 0
tx_gain = 50
rx_gain = 70
srate = 23.04e6
nof_antennas = 1

device_name = uhd
device_args = clock=external
time_adv_nsamples = 300

[rat.eutra]
dl_earfcn = 2850
nof_carriers = 0

[rat.nr]
bands = 3
nof_carriers = 1
max_nof_prb = 106
nof_prb = 106

[pcap]
enable = none
mac_filename = /tmp/ue_mac.pcap
mac_nr_filename = /tmp/ue_mac_nr.pcap
nas_filename = /tmp/ue_nas.pcap

[log]
all_level = info
phy_lib_level = none
all_hex_limit = 32
filename = /tmp/srslogs/ue.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

ue.log