Collision decode: AIS SIC + ADS-B findings (sighting-based ICAO clock)#97
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- AIS successive interference cancellation: a confirmed FCS-valid burst is reconstructed exactly (known bits through the modulator's own GMSK synthesis), least-squares scaled with per-burst CFO, subtracted, and the residual re-hunted/decoded for a colliding burst. Measured: no gain on the (sparse, inland) bench fixture - the missing 17 never anchor at any threshold; they are deep-weak, not collisions. Machinery in place for dense traffic. - ADS-B in-frame collision scanning falsified on modes1 (-7 unique: mid-frame false DF11 candidates pollute the ICAO cache); reverted. - Lasting fix from that experiment: the ICAO cache staleness clock now ticks on sightings, not candidate attempts (attempt-based aging thrashes under near-floor gates). - hunt refactored around a shared slice-based template metric. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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| let dphi = (c2 * c1.conj()).arg(); | ||
| let cfo_step = Complex::from_polar(1.0, dphi / (n as f32 / 2.0)); |
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For AIS bursts with ordinary carrier offsets, this whole-burst phase-slope estimate aliases because arg() wraps the phase difference between the two halves; at 48 kHz and a typical ~1-slot waveform, the unambiguous range is only on the order of 40 Hz, while the decoder accepts offsets up to ±1200 Hz. In those cases SIC subtracts the confirmed frame with the wrong per-sample rotation, leaving the original burst in the residual (or adding interference), so collision recovery only works for near-zero CFO bursts even though try_decode already found a usable coarse/fine CFO.
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What
The collision-decode round, honestly reported:
Gates
All green at current floors (ADS-B 323 ≥ 310, AIS 51 ≥ 48, VDL2 44 ≥ 42); full workspace passing.
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