OAI nrUE: SA FDD DL performance issue

Hi,

I'm using latest develop branch (tag 2025.w14), Ubuntu 24.04.1 PC (Intel Ultra5 CPU, 16GB RAM) + USRP B210 as nrUE connected to a 3rd-party gNB emulator by RF cable, also 10MHz clock from gNB is used as external reference for USRP. After UE enters connected mode, for MAC layer throughput, although UL MCS can reach max. 27, the DL MCS can only reach max. 22 (MCS Table 256QAM) with 0% BLER, when MCS increased to 23, almost 100% BLER appears.

Test condition:

SA FDD band n3, SISO, 20M BW, SCS 15kHz, RB number 106, only 1 DL slot scheduled with 12 PDSCH symbols

Command used to run nrUE:

sudo ./nr-uesoftmodem -O ~/uxm_ue.conf --band 3 -C 1815000000 --CO -95000000 -r 106 --numerology 0 --ssb 6 --agc --clock-source 1

UE log:

nr_fdd_dl-mcs22.log

From UE log I can see measured frequency offset is only 10Hz:

[PHY]   [UE 0] Measured Carrier Frequency offset 10 Hz

I also tried to adjust Rx gain or increase gNB Tx power but it doesn't help much, the DL SNR at gNB side should be enough for higher MCS than 22.

Could someone please help with this issue? Is additional log information (DL EVM, CRC...) or IQ data capture on UE side needed for debugging?

Thanks!