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Beamforming self-sounding: per-subcarrier CSI over the air (Jaguar-1 + Jaguar-3)#154
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802.11ac explicit sounding leaks per-subcarrier channel state: a beamformee
estimates H(k) from a sounding NDP, compresses the steering matrix V(k) into
Givens angles, and transmits it back over the air as a VHT Compressed
Beamforming report. On Jaguar silicon that pipeline is hardware-terminated
(a chip can't read back its own estimate), but the report is addressed to the
beamformer — so with devourer driving both link ends, a monitor RX captures
per-subcarrier CSI the chip otherwise hides.
Adds a generation-neutral BeamformingSounder helper: the MAC sounding registers
(0x718/0x6E4/0x6F4/0x714/0x42C) are byte-identical across the Realtek AC family,
so arm_sounder() is fully shared; per-generation beamformee differences (0xCB vs
0xDB, the 0x9B4 BB-CSI-content that is the narrowband clock divider on Jaguar-3,
the RX-filter/own-AID gates) sit behind BfeeConfig presets (kBfeeJaguar1,
kBfeeJaguar23). Recipes transcribed from the vendor hal_txbf_jaguar_enter /
hal_txbf_8822b_enter.
Env-gated, no-op by default (like the other DEVOURER_* debug knobs):
DEVOURER_BF_ARM_SOUNDER arm the MAC sounding engine (beamformer)
DEVOURER_TX_NDPA mark the injected frame as an NDPA in the TX desc
DEVOURER_TX_NDPA_RA=mac txdemo builds a VHT NDPA to this beamformee
DEVOURER_BF_ARM_BFEE=mac arm the hardware CSI responder (beamformee), no
association — wired for Jaguar-1 and Jaguar-3
DEVOURER_BF_DETECT_REPORT report detector in WiFiDriverDemo (1 summary /
2 frame survey / 3 CSI hexdump / 4 raw for decode)
Validated on hardware: 8812AU/8821AU (Jaguar-1) and 8822CU/8822EU (Jaguar-3)
beamformees reply with VHT Compressed Beamforming reports to an unassociated
sounder; unarmed control yields zero.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
tools/bf_report_decode.py decodes captured VHT Compressed Beamforming reports
into per-subcarrier CSI: parses the MIMO-control field (Nc/Nr/BW/Ng/codebook),
extracts per-stream avg SNR, unpacks the Givens angle bitstream, and
reconstructs the per-tone steering vector V(k). The angle bit-split is chosen
by cross-frame stability and validated against the standard phi=psi+2 Givens
structure; it also flags a flat (frequency-nonselective) channel. Realtek's
reports use a compact 10-bit/subcarrier codebook (phi=6, psi=4) rather than the
textbook 12/16-bit VHT sizes. The decoder is chip-agnostic — it reads
over-the-air standard frames, so the same tool decodes Jaguar-1 and Jaguar-3
reports.
Validation harnesses (out-of-band, like tests/regress.py):
bf_ndpa_probe.py / bf_ndpa_onair.sh B210 burst-pair probe: does the TX-desc
NDPA bit make the MAC emit a HW NDP?
bf_selfsound_onair.sh B210 burst-chain check of the responder
bf_report_sniff.sh 3-adapter decode-level confirmation
bf_selfsound_jaguar3.sh cross-generation (8822C/E) beamformee
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Documents the NDPA/NDP/report exchange, why the register recipe is generation-neutral, the per-generation beamformee differences, how to drive it via the demo env vars, and the measured 2x1 report byte layout (52 subcarriers x 10-bit compact codebook) that the decoder consumes. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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## What Follow-up to #154 (SU self-sounding). An SU beamforming report carries only the per-tone steering **direction** plus a per-stream **scalar** SNR. An **MU** report additionally appends the *MU Exclusive Beamforming Report* — genuine **per-subcarrier SNR** — which is the per-tone channel quality that maps to a per-tone modulation order (the textbook "256-QAM on clean tones, QPSK on faded tones" picture). ## How - **`arm_beamformee_mu()`** (`src/BeamformingSounder.h`) layers the MU group-table registers (`0x14C0` MU-TX-CTL, `0x14C4`/`0x14C8`/`0x14CC` group/user-position, entry `0x1684`) on top of the SU responder base. For *self*-sounding we program the group/user-position directly, so no over-the-air VHT Group ID Management handshake is needed. Recipe from the vendor `hal_txbf_8822b_enter()` MU branch (identical across 8822B/C/E). - Wired on Jaguar-3 via **`DEVOURER_BF_ARM_BFEE_MU=1`**; the NDPA MU feedback bit (STA-info bit 12) via **`DEVOURER_TX_NDPA_MU=1`** in `WiFiDriverTxDemo`. - **`tools/bf_report_decode.py`** parses the MU report. Realtek packs the SNR not as the spec's 4-bit deltas but as 8-bit values in pairs — series A (even bytes) is the per-tone SNR that swings with the channel. The decoder extracts it, maps to dB (`22 + 0.25·int8`) and to a modulation per tone, and trims devourer's trailing chip-FCS/RX bytes where the smooth series collapses. `--operating-snr N` re-centres the *measured* per-tone shape to a stated link budget, turning the real frequency-selectivity into the textbook QAM ladder even on a strong bench link. ## Result (8822CU MU-beamformee, ch100, 20 MHz) Reports grow 99 → **153 bytes**; the per-tone SNR series A is a smooth **45–53 dB** curve with ~8 dB of real frequency-selective swing: ``` as measured: SNR 53..45 dB → all 256-QAM (link is uniformly strong) --operating-snr 26 (same shape, weaker link): SNR: 30 29 29 28 28 28 27 27 27 26 26 26 25 24 24 24 24 23 23 22 dB QAM: + + + : : : : : : : : : : : : : : : : : (+ 16-QAM / : QPSK) ``` Note: `DEVOURER_TX_PWR` does **not** attenuate the link — the NDP is hardware-generated by the sounding engine, not sent through `send_packet` — hence the operating-point model rather than a physical power sweep. `ctest` green; all new code is env-gated and no-op by default. 🤖 Generated with [Claude Code](https://claude.com/claude-code) --------- Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
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What
Recovers per-subcarrier channel state from Realtek Jaguar silicon that the chip otherwise terminates in hardware, by driving both ends of a link with devourer as an 802.11ac beamforming sounder + beamformee.
A beamformee estimates the channel H(k) per subcarrier from a sounding NDP, compresses the steering matrix V(k) into Givens angles, and transmits it back over the air as a VHT Compressed Beamforming report. The chip can't read back its own estimate — but the report is addressed to the beamformer, so a monitor RX captures it. With two adapters we sound our own link on demand and decode per-subcarrier CSI.
How
src/BeamformingSounder.h— generation-neutral helper. The MAC sounding registers (0x718/0x6E4/0x6F4/0x714/0x42C) are byte-identical across the Realtek AC family, soarm_sounder()is fully shared. Per-generation beamformee differences (sounding-control0xCBvs0xDB; the0x9B4BB CSI-content register that is the narrowband clock divider on Jaguar-3 and must not be written there; the RX-filter + own-AID gates) sit behindBfeeConfigpresets. Recipes transcribed from the vendorhal_txbf_jaguar_enter/hal_txbf_8822b_enter.RtlJaguarDevice(Jaguar-1) andRtlJaguar3Device(Jaguar-3), env-gated and no-op by default like the otherDEVOURER_*debug knobs.tools/bf_report_decode.py— chip-agnostic decoder: parses the MIMO-control field, extracts per-stream avg SNR, unpacks the angle bitstream, reconstructs the per-tone V(k). Auto-selects the bit-split by cross-frame stability and flags a flat channel. (Realtek uses a compact 10-bit/subcarrier codebook — 6-bit φ, 4-bit ψ — not the textbook 12/16-bit VHT sizes.)docs/beamforming-self-sounding.md— mechanism, recipe, report byte layout.Env vars (opt-in)
DEVOURER_BF_ARM_SOUNDER=1DEVOURER_TX_NDPA=1DEVOURER_TX_NDPA_RA=<mac>WiFiDriverTxDemobuilds a VHT NDPA to this beamformeeDEVOURER_BF_ARM_BFEE=<mac>DEVOURER_BF_DETECT_REPORT=1..4WiFiDriverDemo(1 summary … 4 raw for the decoder)Validation (on hardware, ch100)
Nc=1 Nr=2; zero when unarmed.Nc=2 Nr=2; zero when unarmed.ctestgreen. All new library code is behind env gates and compiles into every per-chip build subset. Jaguar-2 (8822BU) is a straightforward follow-up (the recipe is already the sharedkBfeeJaguar23) once that port lands.🤖 Generated with Claude Code