Wireless and Optical Sensing for IoT · Study deck
Presence and Vital-Sign Sensing with Radar and Wi-Fi
Radio Remi is helping a care-home team check whether a room is occupied at night.
Radio Remi is your guide for this deck.
After studying this chapter
Learning objectives
You will be able to:
- Explain how small chest displacement changes radar phase and Wi-Fi channel measurements.
- Separate simulated breathing and heartbeat bands while detecting motion contamination.
- Bound presence and wellness claims by geometry, people count, consent, and validation evidence.
- Explain: Radio Remi is helping a care-home team check whether a room is occupied at night.
Major section
Start With the Story
Radio Remi is helping a care-home team check whether a room is occupied at night.
- A camera would reveal too much.
- A simple motion sensor may miss a sleeping person.
- Remi asks the team to make a limited claim: detect a steady breathing-like pattern, reject large motion, and call for another check when the signal is unclear.
Major section
Overview
A radar receiver can track phase in one range bin; Wi-Fi CSI can track changes across subcarriers and antenna paths.
- Slow chest motion often places breathing energy near 0.1–0.5 Hz, while a smaller heartbeat cue may appear near 0.8–2 Hz.
- These bands are useful teaching ranges, not universal diagnostic limits.
- Geometry, clothing, posture, motion, other people, noise, and hardware drift can move or hide the peaks.
Major section
Turn displacement into phase
For a monostatic radar, a target displacement x(t) changes the round-trip path by 2x(t).
- The corresponding phase change is approximately Δϕ(t)=4πx(t)/λ, where λ is wavelength.
- Shorter wavelengths create more phase change for the same motion, but wrapping, noise, multipath, and poor target selection still matter.
- Wi-Fi CSI offers a related multipath cue across communication links, yet its phase also carries clock and hardware errors that need calibration.
Major section
Turn displacement into phase (continued)
An FMCW radar first selects a range bin before following phase over slow time.
- That range step can separate people at different distances, but it cannot always separate people at the same distance or overlapping angles.
- A single-tone Doppler sensor has no range bins, so multiple subjects mix more easily.
- The sensing layout, antenna field of view, update rate, and no-decision rule are part of the measurement contract.
Major section
Read the displacement and spectrum
The broad blue swing is breathing, while the smaller green ripple is the heartbeat component.
- The amber burst is deliberate gross motion, so its window must not produce a vital-sign estimate.
Major section
Filter without hiding artefacts
A band-pass filter can separate slow breathing energy from faster heartbeat energy in a clean simulated trace.
- Large motion spreads energy across frequencies and can shift phase by many cycles.
- A robust pipeline first detects motion or low signal quality, then abstains before calculating rates.
- Frequency resolution depends on observation time.
Major section
Handle more than one person
Two people may occupy different radar range bins, but equal range, side lobes, reflections, or a narrow Wi-Fi path can mix them.
- Beamforming and several antennas can add angle evidence; multiple links can add spatial diversity.
- Neither removes the need to validate the room, poses, bedding, pets, fans, and everyday movements.
- A detector trained on one adult lying still must not silently become a monitor for children, patients, or crowded rooms.
Major section
Place products inside the evidence boundary
Consumer sleep and elder-care products may use radar to support presence, motion, respiration-rate, or sleep-pattern features.
- Product labels and authorisations determine the claim; the sensing method alone does not.
- A wellness trend is not a diagnosis, and an unauthorised monitor must not replace supervision, clinical assessment, or an approved safety device.
- The lab in this chapter teaches phase and filtering with synthetic signals only.
Major section
Summary
Phase can reveal tiny periodic displacement, but the final claim depends on target selection, observation time, motion rejection, geometry, and validation.
- Radar phase changes with round-trip displacement; Wi-Fi CSI changes with multipath.
- Breathing and heartbeat bands help separate a clean fixture, while motion and multiple people can defeat the estimate.
- Synthetic filtering practice does not validate a medical device, consumer product, or occupied room.
Deck summary
Key takeaways
Radio Remi is helping a care-home team check whether a room is occupied at night.
- A radar receiver can track phase in one range bin; Wi-Fi CSI can track changes across subcarriers and antenna paths.
- For a monostatic radar, a target displacement x(t) changes the round-trip path by 2x(t).
- An FMCW radar first selects a range bin before following phase over slow time.
- The broad blue swing is breathing, while the smaller green ripple is the heartbeat component.
Retrieval practice
Recall check 1 of 4
Radio Remi says: answer from memory, then check your reasoning.
Q1Why should a large-motion window be rejected before estimating breathing or heart rate?
Show answer
Answer: D Large movement can create broadband or wrapped phase changes that overwhelm breathing and heartbeat cues, so the safe output is no estimate.
Retrieval practice
Recall check 2 of 4
Radio Remi says: answer from memory, then check your reasoning.
Q2What does a 0.25 Hz peak prove here?
Show answer
Answer: D Only the known synthetic breathing cue was recovered.
Retrieval practice
Recall check 3 of 4
Radio Remi says: answer from memory, then check your reasoning.
Q3Small chest motion occurs while a person rests. What change can radar or Wi-Fi sensing look for?
Show answer
Answer: A Displacement changes reflected and multipath signals, which can appear in phase and channel measurements.
Retrieval practice
Recall check 4 of 4
Radio Remi says: answer from memory, then check your reasoning.
Q4A lab model finds a breathing-like cue from a still volunteer. Can it be sold as a health alarm for crowded rooms?
Show answer
Answer: B Claims must follow geometry, validation population, privacy permission, and the approved purpose.
Print reference
Answers
Answer key.
- D · Large movement can create broadband or wrapped phase changes that overwhelm breathing and heartbeat cues, so the safe output is no estimate.
- D · Only the known synthetic breathing cue was recovered.
- A · Displacement changes reflected and multipath signals, which can appear in phase and channel measurements.
- B · Claims must follow geometry, validation population, privacy permission, and the approved purpose.