Experimental 5G Setup Viability: QAM256 + MIMO + Bandwidth (N310, TDD, SCS 30kHz)

[MKSBarbosa]

Hello everyone,

I’m setting up an experimental 5G environment using srsRAN 5G with the goal of connecting 1 or 2 commercial smartphones under realistic TDD operation. My setup includes:

• USRP N310
• UHD 4.1
• Band n78
• SCS = 30 kHz
• Docker-based deployment

I’d like to evaluate the feasibility of the following configurations:

1. QAM256 + 40 MHz
2. QAM256 + 100 MHz
3. QAM256 + 2x2 MIMO + 40 MHz
4. QAM256 + 2x2 MIMO + 100 MHz
5. QAM256 + 4x4 MIMO + 20 MHz
6. QAM256 + 4x4 MIMO + 40 MHz
7. QAM256 + 4x4 MIMO + 100 MHz

---

🧪 My attempt with 40 MHz

I’ve tried the following gnb.yml configuration:

ru_sdr:
  device_driver: uhd
  device_args: type=n3xx
  srate: 61.44
  tx_gain: 35
  rx_gain: 60

cell_cfg:
  dl_arfcn: 632628
  band: 78
  channel_bandwidth_MHz: 40
  common_scs: 30
  plmn: "PLMN"
  tac: 1
  pci: 1

I launched the gNB with:

gnb -c /etc/srsran/gnb.yml -c /etc/srsran/qam256.yml -c /etc/srsran/mimo_usrp.yml

However, I consistently get the following runtime errors:

srsgnb  | ==== gNB started ===
srsgnb  | Type <h> to view help
srsgnb  | Late: 1404; Underflow: 273; Overflow: 4;
srsgnb  | Late: 1237; Underflow: 143; Overflow: 2;
srsgnb  | Late: 1447; Underflow: 200; Overflow: 1;
...

Sometimes the UE connects; other times it does not.

---

🧪 My attempt with 100 MHz

For the 100 MHz case, I just updated the sampling rate and bandwidth:

srate: 122.88
channel_bandwidth_MHz: 100

Unfortunately, I get the same “late” and “underflow” errors, and the UE never connects.

---

🙋‍♀️ Questions

• Are these configurations (especially with 40 MHz or 100 MHz + MIMO/QAM256) supported in the current release?
• Are there additional configurations or tuning steps required for stable operation?
• Is there a known limitation with MIMO support or sample rates in the Docker environment?

Any guidance or suggestions would be greatly appreciated.

Best regards,
Maria Katarine