RFID, NFC & UWB · Study deck

How UWB Measures Distance

A forklift and a worker badge exchange precisely timed UWB packets so the system can estimate separation.

Radio Remi is your guide for this deck.

channel-impulse-responsesecure-rangingtime-of-flight

Radio Remi, the module guide, in a scene from this chapter.

After studying this chapter

Learning objectives

You will be able to:

Explain why UWB's wide bandwidth improves timing resolution without implying higher transmit power, given that regulators cap power spectral density rather than total power
Convert a timing error into a range error using the c is approximately 0.3 m/ns rule for one-way and round-trip timing
Trace the UWB evidence chain from wideband signal to timing to range to position to application policy, and identify what each layer does and does not prove
Distinguish first-path timing evidence from a stronger multipath reflection when reviewing channel impulse response quality

Major section

Follow the First Path Into a Range Estimate

A broad signal supports finer separation of closely spaced arrivals, helping the receiver distinguish an early direct path from later reflections.

A metal obstruction may weaken the direct arrival until a reflected path looks dominant; choosing that later peak inflates range.
Distance from one-way time of flight is $d=c\Delta t$ .

Wide occupied bandwidth sharpens a signal's correlation peak and reduces timing uncertainty without implying high transmit power.

Major section

Follow the First Path Into a Range Estimate (continued)

A measured 10 ns flight gives $3.0\times10^8\ \mathrm{m/s}\times10\times10^{-9}\ \mathrm{s}=3.0\ \mathrm{m}$ .

Two-way ranging uses several timestamps to reduce clock-offset effects, so its exact formula must match the packet exchange.
Multiple well-positioned anchors provide intersecting constraints for location.
Poor geometry can amplify small range errors, especially when anchors lie nearly on one line or one side of the tag.

Major section

Start With the Story

Bandwidth is the span of radio frequencies used by a signal.

The system must decide which door it passed, not just report that some radio energy was heard.
The first scene defines the decision the range must support, while the obstruction scene shows how a body, metal cart, wall, or reflected path can bias timing.

Major section

Start With the Story (continued)

Recording tag identity, anchor positions, timing quality, allowed error, and difficult scenes makes uncertainty visible.

A body, metal shelf, wall, poor clock, or reflected path can shift the result.
More fixed points can improve shape, yet they add cost, setup work, and more places for bad data.
This door story cannot make location automatic.

Major section

Start With the Story (continued)

The final review route prevents a weak door estimate from becoming a confident business event.

The installed scene needs calibration and known failure rules.
The deeper work explains the distance claim; it does not turn radio timing into magic.
UWB is useful when distance evidence matters.

Major section

Eddie's Math Bridge: Budget UWB First-Path Margin

Wide bandwidth is useful here because it changes time resolution, not because it promises a stronger transmitter.

At 6,489.6 MHz and 10 m, free-space loss is 68.7 dB, leaving −83.0 dBm at the receiver: about 2 dB above a −85 dBm sensitivity.

Major section

Overview: UWB Is Timing Evidence, Not Magic Location

The wideband signal exposes fine timing features, but placement, obstruction, and antenna delay affect what is measured.

Several usable ranges then support a position estimate only when anchor geometry is sufficient.
The final application decision needs freshness and confidence rather than a raw coordinate.

UWB fundamentals should keep the evidence chain visible from timing features through range, geometry, confidence, and policy.

Major section

Under the Hood: Timing, Multipath, Confidence, and Drift

Choosing the later peak adds propagation delay and inflates range.

At approximately $0.30\ \text{m/ns}$ , a 10 ns one-way flight time gives $10\times0.30=3.0\ \text{m}$ , while a 1 ns timing error contributes about 0.30 m.

Channel impulse response evidence helps reviewers distinguish first-path timing from stronger reflections before consuming a range or position estimate.

Major section

Checkpoint: Summary

Wide bandwidth supports fine timing evidence, but channel quality, antenna delay, mounting, obstruction, and firmware still shape the result.

Range evidence becomes position only through geometry, calibration, synchronization or peer exchange, filtering, confidence handling, and retest discipline.
Security-sensitive UWB decisions need secure-ranging context, freshness, confidence thresholds, fallback, logs, and ownership.
A useful review record names the evidence boundary and the changes that reopen the decision.

Deck summary

Key takeaways

A broad signal supports finer separation of closely spaced arrivals, helping the receiver distinguish an early direct path from later reflections.

A measured 10 ns flight gives $3.0\times10^8\ \mathrm{m/s}\times10\times10^{-9}\ \mathrm{s}=3.0\ \mathrm{m}$ .
Bandwidth is the span of radio frequencies used by a signal.
Recording tag identity, anchor positions, timing quality, allowed error, and difficult scenes makes uncertainty visible.
The final review route prevents a weak door estimate from becoming a confident business event.

Retrieval practice

Recall check 1 of 5

Radio Remi says: answer from memory, then check your reasoning.

Q1A round-trip timing error is 2 ns. With radio travel at 0.30 m/ns, about how large is the range error?

A0.60 m

B0.30 m

C0.15 m

D3.0 m

Show answer

Answer: B The round-trip path is halved, so the range error is 0.30 m.

Retrieval practice

Recall check 2 of 5

Radio Remi says: answer from memory, then check your reasoning.

Q2A team says UWB is ready because one tag produced a precise-looking distance value during a demo. What should the review say?

AApprove the installation because the demo distance has enough displayed precision for the intended threshold.

BTreat it as one range demo; request timing, channel, geometry, confidence, and retest evidence.

CUse the radio's wide bandwidth as the channel evidence because it helps separate reflected paths.

DUse repeated distance readings as the access fallback because averaging can reduce measurement noise.

Show answer

Answer: B One precise-looking value proves little; the review still needs timing, channel quality, calibration, geometry, confidence, policy, owner, and retest evidence.

Retrieval practice

Recall check 3 of 5

Radio Remi says: answer from memory, then check your reasoning.

Q3A UWB asset tracker works in an open lab, but the production site has metal racks, moving people, and different tag mounting. What evidence is missing before approval?

ANothing. UWB range values are independent of placement, obstruction, and installation geometry.

BOnly a new application dashboard, because radio evidence cannot affect the position quality.

COnly a longer retention period for the logs, because logs replace real field validation.

DInstalled geometry and channel evidence: anchor positions, tag mounting, line-of-sight quality, and retest triggers.

Show answer

Answer: D The gap is installed geometry and channel evidence: anchor positions, tag mounting, line-of-sight quality, multipath indicators, confidence handling, fallback, owner, and retest triggers.

Retrieval practice

Recall check 4 of 5

Radio Remi says: answer from memory, then check your reasoning.

Q4A range trace has an early weak arrival and a later strong peak. Which timing should be reviewed?

AEarliest valid path, with quality and obstruction evidence.

BChoose the strongest peak because signal strength identifies the tag's arrival event.

CFinal map coordinate alone, without a timing trace.

DLast stored range after the anchor has moved.

Show answer

Answer: A A stronger reflection can arrive later and bias range.

Retrieval practice

Recall check 5 of 5

Radio Remi says: answer from memory, then check your reasoning.

Q5A UWB system reports a plausible position after one anchor is moved, but the confidence logic still uses the old geometry record. Which review action is strongest?

AKeep the old approval, because a plausible coordinate is already enough proof that the geometry still works fine.

BIgnore channel evidence, because anchor geometry changes only ever affect the user interface.

CApprove access-control decisions as long as the displayed coordinate is inside the expected room.

DTreat the anchor move as a retest trigger: refresh geometry, rerun range checks, and hold affected policy until evidence is current.

Show answer

Answer: D A moved anchor invalidates the geometry record, so treat it as a retest trigger: refresh geometry, rerun channel and range checks, review confidence, and keep affected policy decisions unapproved until the evidence is current.

Print reference

Answers 1 of 2

Answer key.

B · The round-trip path is halved, so the range error is 0.30 m.
B · One precise-looking value proves little; the review still needs timing, channel quality, calibration, geometry, confidence, policy, owner, and retest evidence.
D · The gap is installed geometry and channel evidence: anchor positions, tag mounting, line-of-sight quality, multipath indicators, confidence handling, fallback, owner, and retest triggers.
A · A stronger reflection can arrive later and bias range.

Print reference

Answers 2 of 2

Answer key.

D · A moved anchor invalidates the geometry record, so treat it as a retest trigger: refresh geometry, rerun channel and range checks, review confidence, and keep affected policy decisions unapproved until the evidence is current.