601 lines
41 KiB
Markdown
601 lines
41 KiB
Markdown
# ADR-042: Coherent Human Channel Imaging (CHCI) — Beyond WiFi CSI
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**Status**: Proposed
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**Date**: 2026-03-03
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**Deciders**: @ruvnet
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**Supersedes**: None
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**Related**: ADR-014, ADR-017, ADR-029, ADR-039, ADR-040, ADR-041
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---
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## Context
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WiFi-DensePose currently relies on passive Channel State Information (CSI) extracted from standard 802.11 traffic frames. CSI is one specific way of estimating a channel response, but it is fundamentally constrained by a protocol designed for throughput and interoperability — not for sensing.
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### Fundamental Limitations of Passive WiFi CSI
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| Constraint | Root Cause | Impact on Sensing |
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|-----------|-----------|-------------------|
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| MAC-layer jitter | CSMA/CA random backoff, retransmissions | Non-uniform sample timing, aliased Doppler |
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| Rate adaptation | MCS selection varies bandwidth and modulation | Inconsistent subcarrier count per frame |
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| LO phase drift | Independent oscillators at TX and RX | Phase noise floor ~5° on ESP32, limiting displacement sensitivity to ~0.87 mm at 2.4 GHz |
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| Frame overhead | 802.11 preamble, headers, FCS | Wasted airtime that could carry sensing symbols |
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| Bandwidth fragmentation | Channel bonding decisions by AP | Variable spectral coverage per observation |
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| Multi-node asynchrony | No shared timing reference | TDM coordination requires statistical phase correction (current `phase_align.rs`) |
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These constraints impose a hard floor on sensing fidelity. Breathing detection (4–12 mm chest displacement) is reliable, but heartbeat detection (0.2–0.5 mm) is marginal. Pose estimation accuracy is limited by amplitude-only tomography rather than coherent phase imaging.
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### What We Actually Want
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The real objective is **coherent multipath sensing** — measuring the complex-valued impulse response of the human-occupied channel with sufficient phase stability and temporal resolution to reconstruct body surface geometry and sub-millimeter physiological motion.
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WiFi is optimized for throughput and interoperability. DensePose is optimized for phase stability and micro-Doppler fidelity. Those goals are not aligned.
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### IEEE 802.11bf Changes the Landscape
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IEEE Std 802.11bf-2025 was published on September 26, 2025, defining WLAN Sensing as a first-class MAC/PHY capability. Key provisions:
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- **Null Data PPDU (NDP) sounding**: Deterministic, known waveforms with no data payload — purpose-built for channel measurement
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- **Sensing Measurement Setup (SMS)**: Negotiation protocol between sensing initiator and responder with unique session IDs
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- **Trigger-Based Sensing Measurement Exchange (TB SME)**: AP-coordinated sounding with Sensing Availability Windows (SAW)
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- **Multiband support**: Sub-7 GHz (2.4, 5, 6 GHz) plus 60 GHz mmWave
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- **Bistatic and multistatic modes**: Standard-defined multi-node sensing
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This transforms WiFi sensing from passive traffic sniffing into an intentional, standards-compliant sensing protocol. The question is whether to adopt 802.11bf incrementally or to design a purpose-built coherent sensing architecture that goes beyond what 802.11bf specifies.
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### ESPARGOS Proves Phase Coherence at ESP32 Cost
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The ESPARGOS project (University of Stuttgart, IEEE 2024) demonstrates that phase-coherent WiFi sensing is achievable with commodity ESP32 hardware:
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- 8 antennas per board, each on an ESP32-S2
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- Phase coherence via shared 40 MHz reference clock + 2.4 GHz phase reference signal distributed over coaxial cable
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- Multiple boards combinable into larger coherent arrays
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- Public datasets with reference positioning labels
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- Ultra-low cost compared to commercial radar platforms
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This proves the hardware architecture described in this ADR is feasible at the ESP32-S3 price point ($3–5 per node).
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### SOTA Displacement Sensitivity
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| Technology | Frequency | Displacement Resolution | Range | Cost/Node |
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|-----------|-----------|------------------------|-------|-----------|
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| Passive WiFi CSI (current) | 2.4/5 GHz | ~0.87 mm (limited by 5° phase noise) | 1–8 m | $3 |
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| 802.11bf NDP sounding | 2.4/5/6 GHz | ~0.4 mm (coherent averaging) | 1–8 m | $3 |
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| ESPARGOS phase-coherent | 2.4 GHz | ~0.1 mm (8-antenna coherent) | Room-scale | $5 |
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| CW Doppler radar (ISM) | 2.4 GHz | ~10 μm | 1–5 m | $15 |
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| Infineon BGT60TR13C | 58–63.5 GHz | Sub-mm | Up to 15 m | $20 |
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| Vayyar 4D imaging | 3–81 GHz | High (4D imaging) | Room-scale | $200+ |
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| Novelda X4 UWB | 7.29/8.748 GHz | Sub-mm | 0.4–10 m | $15–50 |
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The gap between passive WiFi CSI (~0.87 mm) and coherent phase processing (~0.1 mm) represents a 9x improvement in displacement sensitivity — the difference between marginal and reliable heartbeat detection at ISM bands.
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---
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## Decision
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We define **Coherent Human Channel Imaging (CHCI)** — a purpose-built coherent RF sensing protocol optimized for structural human motion, vital sign extraction, and body surface reconstruction. CHCI is not WiFi in the traditional sense. It is a sensing protocol that operates within ISM band regulatory constraints and can optionally maintain backward compatibility with 802.11bf.
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### Architecture Overview
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```
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┌─────────────────────────────────────────────────────────────────────────┐
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│ CHCI System Architecture │
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├─────────────────────────────────────────────────────────────────────────┤
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│ │
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│ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │
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│ │ CHCI Node │ │ CHCI Node │ │ CHCI Node │ │
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│ │ (TX + RX) │ │ (TX + RX) │ │ (TX + RX) │ │
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│ │ ESP32-S3 │ │ ESP32-S3 │ │ ESP32-S3 │ │
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│ └──────┬──────┘ └──────┬──────┘ └──────┬──────┘ │
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│ │ │ │ │
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│ └───────────┬───────┴───────────────────┘ │
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│ │ │
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│ ┌────────┴────────┐ │
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│ │ Reference Clock │ ← 40 MHz TCXO + PLL distribution │
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│ │ Distribution │ ← 2.4/5 GHz phase reference │
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│ └────────┬────────┘ │
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│ │ │
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│ ┌──────────────────┴──────────────────────────────┐ │
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│ │ Waveform Controller │ │
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│ │ ┌────────────┐ ┌────────────┐ ┌────────────┐ │ │
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│ │ │ NDP Sound │ │ Micro-Burst│ │ Chirp Gen │ │ │
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│ │ │ (802.11bf) │ │ (5 kHz) │ │ (Multi-BW) │ │ │
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│ │ └────────────┘ └────────────┘ └────────────┘ │ │
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│ │ │ │ │ │ │
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│ │ └──────────────┼───────────────┘ │ │
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│ │ ▼ │ │
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│ │ ┌─────────────────┐ │ │
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│ │ │ Cognitive Engine │ ← Scene state │ │
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│ │ │ (Waveform Adapt) │ feedback loop │ │
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│ │ └─────────────────┘ │ │
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│ └───────────────────────────────────────────────────┘ │
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│ │ │
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│ ▼ │
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│ ┌───────────────────────────────────────────────────┐ │
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│ │ Signal Processing Pipeline │ │
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│ │ ┌──────────┐ ┌───────────┐ ┌────────────────┐ │ │
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│ │ │ Coherent │ │ Multi-Band│ │ Diffraction │ │ │
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│ │ │ Phase │ │ Fusion │ │ Tomography │ │ │
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│ │ │ Alignment │ │ (2.4+5+6) │ │ (Complex CSI) │ │ │
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│ │ └──────────┘ └───────────┘ └────────────────┘ │ │
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│ │ │ │ │ │ │
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│ │ └──────────────┼───────────────┘ │ │
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│ │ ▼ │ │
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│ │ ┌─────────────────┐ │ │
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│ │ │ Body Model │ │ │
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│ │ │ Reconstruction │ ── DensePose UV │ │
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│ │ └─────────────────┘ │ │
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│ └───────────────────────────────────────────────────┘ │
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│ │
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└─────────────────────────────────────────────────────────────────────────┘
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```
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### 1. Intentional OFDM Sounding (Replaces Passive CSI Sniffing)
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**What changes**: Instead of waiting for random WiFi packets and extracting CSI as a side effect, transmit deterministic OFDM sounding frames at a fixed cadence with known pilot symbol structure.
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**Waveform specification**:
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| Parameter | Value | Rationale |
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|-----------|-------|-----------|
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| Symbol type | 802.11bf NDP (Null Data PPDU) | Standards-compliant, no data payload overhead |
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| Sounding cadence | 50–200 Hz (configurable) | 50 Hz minimum for heartbeat Doppler; 200 Hz for gesture |
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| Bandwidth | 20/40/80 MHz (per band) | 20 MHz default; 80 MHz for maximum range resolution |
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| Pilot structure | L-LTF + HT-LTF (standard) | Known phase structure enables coherent processing |
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| Burst duration | ≤10 ms per sounding event | ETSI EN 300 328 burst limit compliance |
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| Subcarrier count | 56 (20 MHz) / 114 (40 MHz) / 242 (80 MHz) | Standard OFDM subcarrier allocation |
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**Phase stability improvement**:
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```
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Passive CSI: σ_φ ≈ 5° per subcarrier (random MCS, no averaging)
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NDP Sounding: σ_φ ≈ 5° / √N where N = coherent averages per epoch
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At 50 Hz cadence, 10-frame average: σ_φ ≈ 1.6°
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Displacement floor: 0.87 mm → 0.28 mm at 2.4 GHz
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```
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**Implementation**: New ESP32-S3 firmware mode alongside existing passive CSI. Uses `esp_wifi_80211_tx()` for NDP transmission and existing CSI callback for reception. Sounding schedule coordinated by the Waveform Controller.
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### 2. Phase-Locked Dual-Radio Architecture
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**What changes**: All CHCI nodes share a common reference clock, eliminating per-node LO phase drift that currently requires statistical correction in `phase_align.rs`.
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**Clock distribution design** (based on ESPARGOS architecture):
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```
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┌──────────────────────────────────────────────────┐
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│ Reference Clock Module │
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│ │
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│ ┌──────────┐ ┌──────────────┐ │
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│ │ 40 MHz │────▶│ PLL │ │
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│ │ TCXO │ │ Synthesizer │ │
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│ │ (±0.5ppm)│ │ (SI5351A) │ │
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│ └──────────┘ └──────┬───────┘ │
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│ │ │
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│ ┌──────────────┼──────────────┐ │
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│ ▼ ▼ ▼ │
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│ ┌──────────┐ ┌──────────┐ ┌──────────┐ │
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│ │ 40 MHz │ │ 40 MHz │ │ 40 MHz │ │
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│ │ to Node 1│ │ to Node 2│ │ to Node 3│ │
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│ └──────────┘ └──────────┘ └──────────┘ │
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│ │
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│ ┌──────────┐ ┌──────────┐ ┌──────────┐ │
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│ │ 2.4 GHz │ │ 2.4 GHz │ │ 2.4 GHz │ │
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│ │ Phase Ref│ │ Phase Ref│ │ Phase Ref│ │
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│ │ to Node 1│ │ to Node 2│ │ to Node 3│ │
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│ └──────────┘ └──────────┘ └──────────┘ │
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│ │
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│ Distribution: coaxial cable with power splitters │
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│ Phase ref: CW tone at center of operating band │
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└──────────────────────────────────────────────────┘
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```
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**Components per node** (incremental cost ~$2):
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| Component | Part | Cost | Purpose |
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|-----------|------|------|---------|
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| TCXO | SiT8008 40 MHz ±0.5 ppm | $0.50 | Reference oscillator (1 per system) |
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| PLL synthesizer | SI5351A | $1.00 | Generates 40 MHz + 2.4 GHz references (1 per system) |
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| Coax splitter | Mini-Circuits PSC-4-1+ | $0.30/port | Distributes reference to nodes |
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| SMA connector | Edge-mount | $0.20 | Reference clock input on each node |
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**Acceptance metric**: Phase variance per subcarrier under static conditions ≤ 0.5° RMS over 10 minutes (vs current ~5° with statistical correction).
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**Impact on displacement sensitivity**:
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```
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Current (incoherent): δ_min ≈ λ/(4π) × σ_φ = 12.5cm/(4π) × 5° × π/180 ≈ 0.87 mm
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Coherent (shared clock): δ_min ≈ λ/(4π) × 0.5° × π/180 ≈ 0.087 mm
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With 8-antenna coherent averaging:
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δ_min ≈ 0.087 mm / √8 ≈ 0.031 mm
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```
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This puts heartbeat detection (0.2–0.5 mm chest displacement) well within the sensitivity envelope.
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### 3. Multi-Band Coherent Fusion
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**What changes**: Transmit sounding frames simultaneously at 2.4 GHz and 5 GHz (optionally 6 GHz with WiFi 6E), fusing them as projections of the same latent motion field in RuVector embedding space.
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**Band characteristics for coherent fusion**:
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| Property | 2.4 GHz | 5 GHz | 6 GHz |
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|----------|---------|-------|-------|
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| Wavelength | 12.5 cm | 6.0 cm | 5.0 cm |
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| Wall penetration | Excellent | Good | Moderate |
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| Displacement sensitivity (0.5° phase) | 0.087 mm | 0.042 mm | 0.035 mm |
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| Range resolution (20 MHz) | 7.5 m | 7.5 m | 7.5 m |
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| Fresnel zone radius (2 m) | 22.4 cm | 15.5 cm | 14.1 cm |
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| Subcarrier spacing (20 MHz) | 312.5 kHz | 312.5 kHz | 312.5 kHz |
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**Fusion architecture**:
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```
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2.4 GHz CSI ──▶ ┌───────────────────┐
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│ Band-Specific │ ┌─────────────────────┐
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│ Phase Alignment │────▶│ │
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│ (per-band ref) │ │ Contrastive │
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└───────────────────┘ │ Cross-Band │
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│ Fusion │
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5 GHz CSI ────▶ ┌───────────────────┐ │ │
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│ Band-Specific │────▶│ Body model priors │
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│ Phase Alignment │ │ constrain phase │
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│ (per-band ref) │ │ relationships │
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└───────────────────┘ │ │
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│ Output: unified │
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6 GHz CSI ────▶ ┌───────────────────┐ │ complex channel │
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(optional) │ Band-Specific │────▶│ response │
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│ Phase Alignment │ │ │
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└───────────────────┘ └─────────────────────┘
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│
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▼
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┌─────────────────────┐
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│ RuVector Contrastive │
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│ Embedding Space │
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│ (body surface latent)│
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└─────────────────────┘
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```
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**Key insight**: Lower frequency penetrates better (through-wall sensing, NLOS paths). Higher frequency provides finer spatial resolution. By treating each band as a projection of the same physical scene, the fusion model can achieve super-resolution beyond any single band — using body model priors (known human dimensions, joint angle constraints) to constrain the phase relationships across bands.
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**Integration with existing code**: Extends `multiband.rs` from independent per-channel fusion to coherent cross-band phase alignment. The existing `CrossViewpointAttention` mechanism in `ruvector/src/viewpoint/attention.rs` provides the attention-weighted fusion foundation.
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### 4. Time-Coded Micro-Bursts
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**What changes**: Replace continuous WiFi packet streams with very short deterministic OFDM bursts at high cadence, maximizing temporal resolution of Doppler shifts without 802.11 frame overhead.
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**Burst specification**:
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| Parameter | Value | Rationale |
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|-----------|-------|-----------|
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| Burst cadence | 1–5 kHz | 5 kHz enables 2.5 kHz Doppler bandwidth (Nyquist) |
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| Burst duration | 4–20 μs | Single OFDM symbol + CP = 4 μs minimum |
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| Symbols per burst | 1–4 | Minimal overhead per measurement |
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| Duty cycle | 0.4–10% | Compliant with ETSI 10 ms burst limit |
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| Inter-burst gap | 196–996 μs | Available for normal WiFi traffic |
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**Doppler resolution comparison**:
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```
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Passive WiFi CSI (random, ~30 Hz):
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Doppler resolution: Δf_D = 1/T_obs = 1/33ms ≈ 30 Hz
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Minimum detectable velocity: v_min = λ × Δf_D / 2 ≈ 1.9 m/s at 2.4 GHz
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CHCI micro-burst (5 kHz cadence):
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Doppler resolution: Δf_D = 1/(N × T_burst) = 1/(256 × 0.2ms) ≈ 20 Hz
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BUT: unambiguous Doppler: ±2500 Hz → v_max = ±156 m/s
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Minimum detectable velocity: v_min ≈ λ × 20 / 2 ≈ 1.25 m/s
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With coherent integration over 1 second (5000 bursts):
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Δf_D = 1/1s = 1 Hz → v_min ≈ 0.063 m/s (6.3 cm/s)
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Chest wall velocity during breathing: ~1–5 cm/s ✓
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Chest wall velocity during heartbeat: ~0.5–2 cm/s ✓
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```
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**Regulatory compliance**: At 5 kHz burst cadence with 4 μs bursts, duty cycle is 2%. ETSI EN 300 328 allows up to 10 ms continuous transmission followed by mandatory idle. A 4 μs burst followed by 196 μs idle is well within limits. FCC Part 15.247 requires digital modulation (OFDM qualifies) or spread spectrum.
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### 5. MIMO Geometry Optimization
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**What changes**: Instead of 2×2 WiFi-style antenna layout (optimized for throughput diversity), design antenna spacing tuned for human-scale wavelengths and chest wall displacement sensitivity.
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**Antenna geometry design**:
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```
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Current WiFi-DensePose (throughput-optimized):
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┌─────────────────┐
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│ ANT1 ANT2 │ ← λ/2 spacing = 6.25 cm at 2.4 GHz
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│ │ Optimized for spatial diversity
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│ ESP32-S3 │
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└─────────────────┘
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Proposed CHCI (sensing-optimized):
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┌───────────────────────────────────────┐
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│ │
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│ ANT1 ANT2 ANT3 ANT4 │ ← λ/4 spacing = 3.125 cm
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│ ●───────●───────●───────● │ at 2.4 GHz
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│ │ Linear array for 1D AoA
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│ ESP32-S3 (Node A) │
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└───────────────────────────────────────┘
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λ/4 = 3.125 cm
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Alternative: L-shaped for 2D AoA:
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┌────────────────────┐
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│ ANT4 │
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│ ● │
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│ │ λ/4 │
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│ ANT3 │
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│ ● │
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│ │ λ/4 │
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│ ANT2 │
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│ ● │
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│ │ λ/4 │
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│ ANT1──●──ANT5──●──ANT6──●──ANT7 │
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│ │
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│ ESP32-S3 (Node A) │
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└────────────────────┘
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```
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**Design rationale**:
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| Design parameter | WiFi (throughput) | CHCI (sensing) |
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|-----------------|-------------------|----------------|
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| Spacing | λ/2 (6.25 cm) | λ/4 (3.125 cm) |
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| Goal | Maximize diversity gain | Maximize angular resolution |
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| Array factor | Broad main lobe | Narrow main lobe, grating lobe suppression |
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| Geometry | Dual-antenna diversity | Linear or L-shaped phased array |
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| Target signal | Far-field plane wave | Near-field chest wall displacement |
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**Virtual aperture synthesis**: With 4 nodes × 4 antennas = 16 physical elements, MIMO virtual aperture provides 16 × 16 = 256 virtual channels. Combined with MUSIC or ESPRIT algorithms, this enables sub-degree angle-of-arrival estimation — sufficient to resolve individual body segments.
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### 6. Cognitive Waveform Adaptation
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**What changes**: The sensing waveform adapts in real-time based on the current scene state, driven by delta coherence feedback from the body model.
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**Cognitive sensing modes**:
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```
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┌───────────────────────────────────────────────────────────────┐
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│ Cognitive Waveform Engine │
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│ │
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│ Scene State ─────▶ ┌────────────────┐ ─────▶ Waveform Config │
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│ (from body model) │ Mode Selector │ (to TX nodes) │
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│ └───────┬────────┘ │
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│ │ │
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│ ┌──────────────┼──────────────────┐ │
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│ ▼ ▼ ▼ │
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│ ┌────────────┐ ┌────────────┐ ┌────────────┐ │
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│ │ IDLE │ │ ALERT │ │ ACTIVE │ │
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│ │ │ │ │ │ │ │
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│ │ 1 Hz NDP │ │ 10 Hz NDP │ │ 50-200 Hz │ │
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│ │ Single band│ │ Dual band │ │ All bands │ │
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│ │ Low power │ │ Med power │ │ Full power │ │
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│ │ │ │ │ │ │ │
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│ │ Presence │ │ Tracking │ │ DensePose │ │
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│ │ detection │ │ + coarse │ │ + vitals │ │
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│ │ only │ │ pose │ │ + micro- │ │
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│ │ │ │ │ │ Doppler │ │
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│ └────────────┘ └────────────┘ └────────────┘ │
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│ │ │ │ │
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│ ▼ ▼ ▼ │
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│ ┌────────────┐ ┌────────────┐ ┌────────────┐ │
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│ │ VITAL │ │ GESTURE │ │ SLEEP │ │
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│ │ │ │ │ │ │ │
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│ │ 100 Hz │ │ 200 Hz │ │ 20 Hz │ │
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│ │ Subset of │ │ Full band │ │ Single │ │
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│ │ optimal │ │ Max bursts │ │ band │ │
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│ │ subcarriers│ │ │ │ Low power │ │
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│ │ │ │ │ │ │ │
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│ │ Breathing, │ │ DTW match │ │ Apnea, │ │
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│ │ HR, HRV │ │ + classify │ │ movement, │ │
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│ │ │ │ │ │ stages │ │
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│ └────────────┘ └────────────┘ └────────────┘ │
|
||
│ │
|
||
│ Transition triggers: │
|
||
│ IDLE → ALERT: Coherence delta > threshold │
|
||
│ ALERT → ACTIVE: Person detected with confidence > 0.8 │
|
||
│ ACTIVE → VITAL: Static person, body model stable │
|
||
│ ACTIVE → GESTURE: Motion spike with periodic structure │
|
||
│ ACTIVE → SLEEP: Supine pose detected, low ambient motion │
|
||
│ * → IDLE: No detection for 30 seconds │
|
||
│ │
|
||
└───────────────────────────────────────────────────────────────┘
|
||
```
|
||
|
||
**Power efficiency**: Cognitive adaptation reduces average power consumption by 60–80% compared to constant full-rate sounding. In IDLE mode (1 Hz, single band, low power), the system draws <10 mA from the ESP32-S3 radio — enabling battery-powered deployment.
|
||
|
||
**Integration with ADR-039**: The cognitive waveform modes map directly to ADR-039 edge processing tiers. Tier 0 (raw CSI) corresponds to IDLE/ALERT. Tier 1 (phase unwrap, stats) corresponds to ACTIVE. Tier 2 (vitals, fall detection) corresponds to VITAL/SLEEP. The cognitive engine adds the waveform adaptation feedback loop that ADR-039 lacks.
|
||
|
||
### 7. Coherent Diffraction Tomography
|
||
|
||
**What changes**: Current tomography (`tomography.rs`) uses amplitude-only attenuation for voxel reconstruction. With coherent phase data from CHCI, we upgrade to diffraction tomography — resolving body surfaces rather than volumetric shadows.
|
||
|
||
**Mathematical foundation**:
|
||
|
||
```
|
||
Current (amplitude tomography):
|
||
I(x,y,z) = Σ_links |H_measured(f)| × W_link(x,y,z)
|
||
Output: scalar opacity per voxel (shadow image)
|
||
|
||
Proposed (coherent diffraction tomography):
|
||
O(x,y,z) = F^{-1}[ Σ_links H_measured(f,θ) / H_reference(f,θ) ]
|
||
Where:
|
||
H_measured = complex channel response with human present
|
||
H_reference = complex channel response of empty room (calibration)
|
||
f = frequency (across all bands)
|
||
θ = link angle (across all node pairs)
|
||
Output: complex permittivity contrast per voxel (body surface)
|
||
```
|
||
|
||
**Key advantage**: Diffraction tomography produces body surface geometry, not just occupancy maps. This directly feeds the DensePose UV mapping pipeline with geometric constraints — reducing the neural network's burden from "guess the surface from shadows" to "refine the surface from holographic reconstruction."
|
||
|
||
**Performance projection** (based on ESPARGOS results and multi-band coverage):
|
||
|
||
| Metric | Current (Amplitude) | Proposed (Coherent Diffraction) |
|
||
|--------|--------------------|---------------------------------|
|
||
| Spatial resolution | ~15 cm (limited by wavelength) | ~3 cm (multi-band synthesis) |
|
||
| Body segment discrimination | Coarse (torso vs limb) | Fine (individual limbs) |
|
||
| Surface vs volume | Volumetric opacity | Surface geometry |
|
||
| Through-wall capability | Yes (amplitude penetrates) | Partial (phase coherence degrades) |
|
||
| Calibration requirement | None | Empty room reference scan |
|
||
|
||
### Acceptance Test
|
||
|
||
**Primary acceptance criterion**: Demonstrate 0.1 mm displacement detection repeatably at 2 meters in a static controlled room.
|
||
|
||
**Full acceptance test protocol**:
|
||
|
||
| Test | Metric | Target | Method |
|
||
|------|--------|--------|--------|
|
||
| AT-1: Phase stability | σ_φ per subcarrier, static, 10 min | ≤ 0.5° RMS | Record CSI, compute variance |
|
||
| AT-2: Displacement | Detectable displacement at 2 m | ≤ 0.1 mm | Precision linear stage, sinusoidal motion |
|
||
| AT-3: Breathing rate | BPM error, 3 subjects, 5 min each | ≤ 0.2 BPM | Reference: respiratory belt |
|
||
| AT-4: Heart rate | BPM error, 3 subjects, seated, 2 min | ≤ 3 BPM | Reference: pulse oximeter |
|
||
| AT-5: Multi-person | Pose detection, 3 persons, 4×4 m room | ≥ 90% keypoint detection | Reference: camera ground truth |
|
||
| AT-6: Power | Average draw in IDLE mode | ≤ 10 mA (radio) | Current meter on 3.3 V rail |
|
||
| AT-7: Latency | End-to-end pose update latency | ≤ 50 ms | Timestamp injection |
|
||
| AT-8: Regulatory | Conducted emissions, 2.4 GHz ISM | FCC 15.247 + ETSI 300 328 | Spectrum analyzer |
|
||
|
||
### Backward Compatibility
|
||
|
||
**Question 1: Do you want backward compatibility with normal WiFi routers?**
|
||
|
||
CHCI supports a **dual-mode architecture**:
|
||
|
||
| Mode | Description | When to Use |
|
||
|------|-------------|-------------|
|
||
| **Legacy CSI** | Passive sniffing of existing WiFi traffic | Retrofit into existing WiFi environments, no hardware changes |
|
||
| **802.11bf NDP** | Standard-compliant NDP sounding | WiFi AP supports 802.11bf, moderate improvement over legacy |
|
||
| **CHCI Native** | Full coherent sounding with shared clock | Purpose-deployed sensing mesh, maximum fidelity |
|
||
|
||
The firmware can switch between modes at runtime. The signal processing pipeline (`signal/src/ruvsense/`) accepts CSI from any mode — the coherent processing path activates when shared-clock metadata is present in the CSI frame header.
|
||
|
||
**Question 2: Are you willing to own both transmitter and receiver hardware?**
|
||
|
||
Yes. CHCI requires owning both TX and RX to achieve phase coherence. The system is deployed as a self-contained sensing mesh — not parasitic on existing WiFi infrastructure. This is the fundamental architectural trade: compatibility for control. For sensing, that is a good trade.
|
||
|
||
### Hardware Bill of Materials (per CHCI node)
|
||
|
||
| Component | Part | Quantity | Unit Cost | Purpose |
|
||
|-----------|------|----------|-----------|---------|
|
||
| ESP32-S3-WROOM-1 | Espressif | 1 | $2.50 | Main MCU + WiFi radio |
|
||
| External antenna | 2.4/5 GHz dual-band | 2–4 | $0.30 each | Sensing antennas (λ/4 spacing) |
|
||
| SMA connector | Edge-mount | 1 | $0.20 | Reference clock input |
|
||
| Coax cable | RG-174 | 1 m | $0.15 | Clock distribution |
|
||
| PCB | Custom 4-layer | 1 | $0.50 | Integration (at volume) |
|
||
| **Node total** | | | **$4.25** | |
|
||
| Reference clock module | SI5351A + TCXO + splitter | 1 per system | $3.00 | Shared clock source |
|
||
| **4-node system total** | | | **$20.00** | |
|
||
|
||
This is 10× cheaper than the nearest comparable coherent sensing platform (Novelda X4 at $50/node, Vayyar at $200+).
|
||
|
||
### Implementation Phases
|
||
|
||
| Phase | Timeline | Deliverables | Dependencies |
|
||
|-------|----------|-------------|--------------|
|
||
| **Phase 1: NDP Sounding** | 4 weeks | ESP32-S3 firmware for 802.11bf NDP TX/RX, sounding scheduler, CSI extraction from NDP frames | ESP-IDF 5.2+, existing firmware |
|
||
| **Phase 2: Clock Distribution** | 6 weeks | Reference clock PCB design, SI5351A driver, phase reference distribution, `phase_align.rs` upgrade | Phase 1, PCB fabrication |
|
||
| **Phase 3: Coherent Processing** | 4 weeks | Coherent diffraction tomography in `tomography.rs`, complex-valued CSI pipeline, calibration procedure | Phase 2 |
|
||
| **Phase 4: Multi-Band Fusion** | 4 weeks | Simultaneous 2.4+5 GHz sounding, cross-band phase alignment, contrastive fusion in RuVector space | Phase 1, Phase 3 |
|
||
| **Phase 5: Cognitive Engine** | 3 weeks | Waveform adaptation state machine, coherence delta feedback, power management modes | Phase 3, Phase 4 |
|
||
| **Phase 6: Acceptance Testing** | 3 weeks | AT-1 through AT-8, precision displacement rig, regulatory pre-scan | Phase 5 |
|
||
|
||
### Crate Architecture
|
||
|
||
New and modified crates:
|
||
|
||
| Crate | Type | Description |
|
||
|-------|------|-------------|
|
||
| `wifi-densepose-chci` | **New** | CHCI protocol definition, waveform specs, cognitive engine |
|
||
| `wifi-densepose-signal` | Modified | Add coherent diffraction tomography, upgrade `phase_align.rs` |
|
||
| `wifi-densepose-hardware` | Modified | Reference clock driver, NDP sounding firmware, antenna geometry config |
|
||
| `wifi-densepose-ruvector` | Modified | Cross-band contrastive fusion in viewpoint attention |
|
||
| `wifi-densepose-wasm-edge` | Modified | New WASM modules for CHCI-specific edge processing |
|
||
|
||
### Module Impact Matrix
|
||
|
||
| Existing Module | Current Function | CHCI Upgrade |
|
||
|----------------|-----------------|-------------|
|
||
| `phase_align.rs` | Statistical LO offset estimation | Replace with shared-clock phase reference alignment |
|
||
| `multiband.rs` | Independent per-channel fusion | Coherent cross-band phase alignment with body priors |
|
||
| `coherence.rs` | Z-score coherence scoring | Complex-valued coherence metric (phasor domain) |
|
||
| `coherence_gate.rs` | Accept/Reject gate decisions | Add waveform adaptation feedback to cognitive engine |
|
||
| `tomography.rs` | Amplitude-only ISTA L1 solver | Coherent diffraction tomography with complex CSI |
|
||
| `multistatic.rs` | Attention-weighted fusion | Add PLL-disciplined synchronization path |
|
||
| `field_model.rs` | SVD room eigenstructure | Coherent room transfer function model with phase |
|
||
| `intention.rs` | Pre-movement lead signals | Enhanced micro-Doppler from high-cadence bursts |
|
||
| `gesture.rs` | DTW template matching | Phase-domain gesture features (higher discrimination) |
|
||
|
||
---
|
||
|
||
## Consequences
|
||
|
||
### Positive
|
||
|
||
- **9× displacement sensitivity improvement**: From 0.87 mm (incoherent) to 0.031 mm (coherent 8-antenna) at 2.4 GHz, enabling reliable heartbeat detection at ISM bands
|
||
- **Standards-compliant path**: 802.11bf NDP sounding is a published IEEE standard (September 2025), providing regulatory clarity
|
||
- **10× cost advantage**: $4.25/node vs $50+ for nearest comparable coherent sensing platform
|
||
- **Through-wall preservation**: Operates at 2.4/5 GHz ISM bands, maintaining the through-wall sensing advantage that mmWave systems lack
|
||
- **Backward compatible**: Dual-mode firmware supports legacy CSI, 802.11bf NDP, and native CHCI — deployable incrementally
|
||
- **Privacy-preserving**: No cameras, no audio — same RF-only sensing paradigm as current WiFi-DensePose
|
||
- **Power-efficient**: Cognitive waveform adaptation reduces average power 60–80% vs constant-rate sounding
|
||
- **Body surface reconstruction**: Coherent diffraction tomography produces geometric constraints for DensePose, reducing neural network inference burden
|
||
- **Proven feasibility**: ESPARGOS demonstrates phase-coherent WiFi sensing at ESP32 cost point (IEEE 2024)
|
||
|
||
### Negative
|
||
|
||
- **Custom hardware required**: Cannot parasitically sense from existing WiFi routers in CHCI Native mode (802.11bf mode can use compliant APs)
|
||
- **PCB design needed**: Reference clock distribution requires custom PCB — not a pure firmware upgrade
|
||
- **Calibration burden**: Coherent diffraction tomography requires empty-room reference scan — adds deployment friction
|
||
- **Clock distribution complexity**: Coaxial cable distribution limits deployment flexibility vs fully wireless mesh
|
||
- **Two-phase deployment**: Full CHCI requires Phases 1–6 (~24 weeks). Intermediate modes (NDP-only, Phase 1) provide incremental value.
|
||
|
||
### Risks
|
||
|
||
| Risk | Likelihood | Impact | Mitigation |
|
||
|------|-----------|--------|------------|
|
||
| ESP32-S3 WiFi hardware does not support NDP TX at 802.11bf spec | Medium | High | Fall back to raw 802.11 frame injection with known preamble; validate with `esp_wifi_80211_tx()` |
|
||
| Phase coherence degrades over cable length >2 m | Low | Medium | Use matched-length cables; add per-node phase calibration step |
|
||
| ETSI/FCC regulatory rejection of custom sounding cadence | Low | High | Stay within 802.11bf NDP specification; use standard-compliant waveforms only |
|
||
| Coherent diffraction tomography computationally exceeds ESP32 | Medium | Medium | Run tomography on aggregator (Rust server), not on edge. ESP32 sends coherent CSI only |
|
||
| Multi-band simultaneous TX causes self-interference | Medium | Medium | Time-division between bands (alternating 2.4/5 GHz per burst slot) or frequency planning |
|
||
| Body model priors over-constrain fusion, missing novel poses | Low | Medium | Use priors as soft constraints (regularization) not hard constraints |
|
||
|
||
---
|
||
|
||
## References
|
||
|
||
### Standards
|
||
|
||
1. IEEE Std 802.11bf-2025, "Standard for Information Technology — Telecommunications and Information Exchange between Systems — Local and Metropolitan Area Networks — Specific Requirements — Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications — Amendment: Enhancements for Wireless Local Area Network (WLAN) Sensing," IEEE, September 2025.
|
||
2. ETSI EN 300 328 V2.2.2, "Wideband transmission systems; Data transmission equipment operating in the 2.4 GHz band," ETSI, July 2019.
|
||
3. FCC 47 CFR Part 15.247, "Operation within the bands 902–928 MHz, 2400–2483.5 MHz, and 5725–5850 MHz."
|
||
|
||
### Research Papers
|
||
|
||
4. Euchner, F., et al., "ESPARGOS: An Ultra Low-Cost, Realtime-Capable Multi-Antenna WiFi Channel Sounder for Phase-Coherent Sensing," IEEE, 2024. [arXiv:2502.09405]
|
||
5. Restuccia, F., "IEEE 802.11bf: Toward Ubiquitous Wi-Fi Sensing," IEEE Communications Standards Magazine, 2024. [arXiv:2310.05765]
|
||
6. Pegoraro, J., et al., "Sensing Performance of the IEEE 802.11bf Protocol," IEEE, 2024. [arXiv:2403.19825]
|
||
7. Chen, Y., et al., "Multi-Band Wi-Fi Neural Dynamic Fusion for Sensing," IEEE ICASSP, 2024. [arXiv:2407.12937]
|
||
8. Samsung Research, "Optimal Preprocessing of WiFi CSI for Sensing Applications," IEEE, 2024. [arXiv:2307.12126]
|
||
9. Yan, Y., et al., "Person-in-WiFi 3D: End-to-End Multi-Person 3D Pose Estimation with Wi-Fi," CVPR 2024.
|
||
10. Geng, J., et al., "DensePose From WiFi," Carnegie Mellon University, 2023. [arXiv:2301.00250]
|
||
11. Pegoraro, J., et al., "802.11bf Multiband Passive Sensing," IEEE, 2025. [arXiv:2507.22591]
|
||
12. Liu, J., et al., "Monitoring Vital Signs and Postures During Sleep Using WiFi Signals," MobiCom, 2020.
|
||
|
||
### Commercial Systems
|
||
|
||
13. Vayyar Imaging, "4D Imaging Radar Technology Platform," https://vayyar.com/technology/
|
||
14. Infineon Technologies, "BGT60TR13C 60 GHz Radar Sensor IC Datasheet," 2024.
|
||
15. Novelda AS, "X4 UWB Radar SoC Datasheet," https://novelda.com/technology/
|
||
16. Texas Instruments, "IWR6843 Single-Chip 60-GHz mmWave Sensor," 2024.
|
||
17. ESPARGOS Project, https://espargos.net/
|
||
|
||
### Related ADRs
|
||
|
||
18. ADR-014: SOTA Signal Processing (phase alignment, coherence scoring)
|
||
19. ADR-017: RuVector Signal + MAT Integration (embedding fusion)
|
||
20. ADR-029: RuvSense Multistatic Sensing Mode (multi-node coordination)
|
||
21. ADR-039: ESP32 Edge Intelligence (tiered processing, power management)
|
||
22. ADR-040: WASM Programmable Sensing (edge compute architecture)
|
||
23. ADR-041: WASM Module Collection (algorithm registry)
|