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wfb-ng efficient configuration & on-air TX throughput

This documents (a) the most efficient wfb-ng configuration for the RTL8812AU and (b) an SDR-measured on-air TX throughput comparison between devourer (userspace libusb) and wfb-ng (kernel svpcom/rtl8812au driver).

Results

On-air channel occupancy measured with a USRP B210 (tests/sdr_duty.py) on a clean 5 GHz channel (ch149), 1500 B frames. on_air_Mbps = duty × PHY_rate:

Config devourer wfb-ng (svpcom + wfb_tx)
MCS1 / 20 MHz 94.6 % duty → 12.3 Mbps 94.5 % duty → 12.3 Mbps
MCS7 / 20 MHz 80.1 % duty → 52.1 Mbps 79.8 % duty → 51.9 Mbps
MCS7 / 40 MHz 62.7 % duty → ~85 Mbps —

devourer and wfb-ng deliver the same on-air injection throughput. wfb-ng's useful goodput is then × the FEC ratio k/n = 8/12 ≈ 0.67, so MCS1/20 ≈ 8 Mbps and MCS7/20 ≈ 35 Mbps — consistent with wfb-ng's ~7 Mbps default and OpenIPC's ~52 Mbps-total / 36 Mbps-video real-world figures.

Two regimes are visible: at low MCS the link is airtime-limited (≈95 % duty, the channel is nearly saturated); at high MCS the host feed becomes the limit (duty drops to ~80 % at 20 MHz, ~63 % at 40 MHz) while absolute throughput keeps rising. Larger frames (up to the 3993 B max payload) raise duty at high MCS by amortising per-frame overhead.

Bare-metal vs VM: the same svpcom driver + injector, run bare-metal and inside the libvirt VM via qemu-xhci USB passthrough, give identical occupancy (80.5 % vs 80.4 %). USB passthrough adds no throughput cost here — the limit is airtime / chip TX, not the USB transport.

Most efficient wfb-ng config (RTL8812AU)

  • Driver: github.com/svpcom/rtl8812au (module 88XXau_wfb, sudo ./dkms-install.sh). It is the wfb-ng injection-tuned driver. Set rtw_tx_pwr_idx_override 30–45 (≤63; higher needs active cooling). The in-tree rtw88 driver's monitor injection is much slower (~6 Mbps) — use svpcom for wfb-ng. It builds on modern host kernels as well as the pinned 5.15.
  • Throughput levers (/etc/wifibroadcast.cfg, or wfb_tx -M/-B/-G/-S/-L):
    • mcs_index — the primary lever (MCS1 ≈ 7 Mbps → MCS5–7 + 40 MHz ≈ 36–52 Mbps).
    • bandwidth = 40 — ~doubles capacity.
    • short_gi = True — ~+11 %.
    • ldpc = 1 — RTL8812AU supports it; better FEC robustness.
    • stbc = 1 — TX diversity on dual-antenna cards.
  • Channel: a clean 5 GHz channel (ch149/165). 2.4 GHz is congested, so mac80211 CSMA backoff sharply lowers injection rate.
  • FEC: fec_k = 8, fec_n = 12 (33 % overhead) is the common default.
  • MTU: radio_mtu / MAX_PAYLOAD_SIZE = 3993 is wfb-ng's max single-frame payload.

Measuring on-air throughput

Counting frames at a Wi-Fi monitor sniffer caps around ~2900 fps, so it undercounts a fast transmitter. tests/sdr_duty.py measures the fraction of time the (clean) channel's received power is above the idle noise floor = the transmitter's airtime occupancy (duty cycle), which has no such ceiling: on_air_Mbps ≈ duty × PHY_rate(MCS, BW, GI). Calibrate the idle noise floor once (--noise-db; ≈ −62 dB is a typical quiet-bench value) — a percentile auto-floor mis-reads once the channel is ~saturated because the low tail becomes signal.

Reproduce

# kernel side: build + load the wfb-ng driver, build wfb_tx
git clone https://github.com/svpcom/rtl8812au && cd rtl8812au && make && \
  sudo insmod 88XXau_wfb.ko rtw_tx_pwr_idx_override=30
git clone https://github.com/svpcom/wfb-ng && cd wfb-ng && make
# devourer side: build/txdemo with DEVOURER_TX_RATE=MCS7/20 + DEVOURER_TX_PAYLOAD_BYTES + DEVOURER_TX_GAP_US=0
# measure (ceiling-free) while either side floods a clean 5 GHz channel:
sudo python3 tests/sdr_duty.py --freq 5745e6 --secs 4 --mcs 7 --bw 20 --noise-db -62

Measured with an RTL8812AU (0bda:8812) and a USRP B210; the bare-metal-vs-VM comparison ran the same driver inside a libvirt VM (kernel 5.15) over qemu-xhci USB passthrough.