Contactless Vital-Sign Radar

Subject Vitals (ground truth)

Breathing rate 14 br/min

Heart rate 72 bpm

Radar-to-chest distance 1.2 m

Signal Conditions

Radar-Extracted Estimate

Est. respiration

— br/min

Est. heart rate

— bpm

Ground truth — resp: 14.0 br/min · HR: 72.0 bpm
Round-trip phase φ = 4πd/λ, λ = 3.9 mm

Chest displacement

0.0 mm

Signal SNR

high

How it works

This is the core mechanism behind ambient, contactless patient monitoring (FMCW / continuous-wave radar and Wi-Fi-sensing systems already deployed in ICU rooms and sleep labs): a 77 GHz radar beam bounces off the chest wall and its phase shift encodes sub-millimetre motion — no wearable, no contact, coverage of the whole room at once.

Chest position: d(t) = d0 + A_r sin(2π f_r t) + A_h sin(2π f_h t)
Round-trip phase: φ(t) = 4π d(t) / λ     (λ ≈ 3.9 mm at 77 GHz)
Recovered motion: x(t) = λ φ(t) / 4π

Breathing (f_r, amplitude A_r ≈ 4–12 mm) and the heartbeat's mechanical impulse against the chest wall (f_h, amplitude A_h ≈ 0.1–0.5 mm) sum into one phase signal. The demodulator here separates them the way a real system does:

Low-pass branch — smooths the raw signal, then times successive peaks to estimate the respiration rate (large, slow component).
High-pass branch — subtracts the smoothed respiration curve to isolate the tiny cardiac ripple, then times its zero-crossings to estimate heart rate (small, fast component).
Motion artifact — a random body-shift injects a large, non-periodic phase jump that swamps the cardiac ripple (SNR collapses) exactly as it does in real ambient-sensing hardware, which is why these systems fuse multiple sensors and reject outlier frames.

Real-world relevance: this is the working principle of contactless vital-sign monitors (Emerald Innovations, Google Soli-based sensing, hospital radar bed-sensors) — the point of "omnipresent" medical monitoring: vitals read continuously from the room itself, not from a device the patient must wear or remember.

Radar wavefront Chest micro-motion (exaggerated)

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Contactless Vital-Sign Radar: Ambient Medical Monitoring