RFID, NFC & UWB · Study deck
UWB Ranging Techniques
Picture a cart moving between two marked points while four systems report different distances.
Radio Remi is your guide for this deck.
After studying this chapter
Learning objectives
You will be able to:
- Explain: If the reply delay stretches to 1 ms, the same 20 ppm offset becomes 20 ns, or roughly 3 m of distance bias before multipath, antenna-delay, or timestamp-noise effects.
- Explain: In SS-TWR, device A sends a poll, device B replies after a known processing delay, and A computes distance from the round-trip time minus B's reply delay.
- Explain: The design tension is that fine time resolution turns tiny clock errors into real distance errors, so each method is a different answer to "whose clock do we trust?".
Major section
In 60 Seconds · Eddie's Math Bridge: Bandwidth And Ranging Resolution
Two-way ranging (TWR) measures a round trip between two devices and subtracts responder delay.
- Double-sided TWR adds a reverse exchange so clock-rate error has less influence.
- Angle of arrival (AoA) uses antenna arrays to estimate direction and is usually combined with range or TDoA evidence.
Major section
Ranging Is Evidence
UWB does not directly output truth.
- It produces timestamp, channel, and geometry evidence that a positioning engine or application policy may use.
- The comparison reaches geometry evidence.
- The running argument in ranging is evidence therefore stays bounded.
- TDoA: A tag blink is timestamped by synchronized anchors.
Major section
Method Choice Is a Clock and Airtime Budget
That number explains why UWB ranging is powerful but not automatic truth.
- UWB measures distance by timing radio pulses, and the named methods trade accuracy, airtime, and clock requirements differently.
- The design tension is that fine time resolution turns tiny clock errors into real distance errors, so each method is a different answer to "whose clock do we trust?".
- A useful range record carries more than distance.
Major section
Method Choice Is a Clock and Airtime Budget (continued)
In a two-way exchange, a timing error in the round-trip estimate is divided by two when it becomes one-way distance.
- A 2 ns round-trip error is therefore about 0.30 m of one-way range error: 0.30 m/ns x 2 ns / 2 = 0.30 m.
- If a tool cabinet only needs to decide whether a device is within about 1.5 m, a well-calibrated DS-TWR exchange with confidence fields may be enough.
- A bare "3.2 m" is a number with hidden assumptions.
Major section
Two-Way Ranging
The responder delay is $T_3: T_2$.
- The initiator's elapsed time is $T_4: T_1$.
- where $c$ is the speed of light in air for the required engineering precision.
- Both: UWB Two-Way Ranging (TWR) Process and: Receive Poll at T2 need evidence.
- Reopen two-way ranging whenever: Receive Poll at T2 changes.
Major section
Double-Sided TWR · Time Difference of Arrival
Single-sided TWR assumes the responder delay measured on one device's clock is directly comparable with the elapsed time measured on the other device's clock.
- Small clock-rate differences can matter when reply delays are long or timing quality is weak.
- DS-TWR does not make measurements perfect.
- TDoA changes the architecture.
Major section
Angle of Arrival · Measurement Quality
It also creates new failure modes: array calibration, device orientation, polarization, body blockage, and reflections can bias the angle estimate.
- Every ranging technique needs a quality gate before an application acts.
- Good systems do not only store a distance or coordinate; they store enough evidence to explain why the measurement was accepted, down-weighted, retried, or rejected.
Major section
Technique Selection · Example Selection Reasoning
A tool cabinet only needs to know whether one authorized device is near the correct side of the cabinet at checkout time.
- A factory-wide asset map with many tags may fit TDoA if the site can maintain synchronized anchors and confidence monitoring.
- A robot docking system might combine range with AoA if direction helps resolve approach angle.
- None of these choices is complete until the installed environment proves the evidence quality.
Major section
Capacity and Failure-Mode Review
Acceptance testing should mirror the decision.
- The practical split is two-way versus one-way timing.
- TWR needs a back-and-forth with each anchor, which costs airtime per tag but needs no synchronized infrastructure.
- A design review that names those checks is stronger than one that only cites an accuracy headline.
Major section
Clock Drift Error Budget
In SS-TWR, device A sends a poll, device B replies after a known processing delay, and A computes distance from the round-trip time minus B's reply delay.
- The catch is that the reply delay is measured on B's clock while the round trip is measured on A's clock.
- Those crystals never run at exactly the same frequency.
- The cost is one more message and more energy per range.
Major section
Clock Drift Error Budget (continued)
That frequency offset multiplies the reply delay into a ranging error, and reply delays are long enough that small parts-per-million offsets become centimeters.
- The apparent reply-delay error is about 100 microseconds x 20 / 1,000,000 = 2 ns.
- In a single-sided range, that becomes about 0.30 m/ns x 2 ns / 2 = 0.30 m of distance bias.
- DS-TWR still leaves the rest of the error budget.
Major section
Clock Drift Error Budget (continued)
If the reply delay stretches to 1 ms, the same 20 ppm offset becomes 20 ns, or roughly 3 m of distance bias before multipath, antenna-delay, or timestamp-noise effects.
- Antenna delay can shift timestamps, non-line-of-sight paths can arrive late, and filtering can publish a fresh-looking range from stale evidence.
- TDoA has a different budget: if anchor synchronization is off by 0.5 ns, the distance-difference evidence is biased by about 0.15 m.
- With a geometry factor of 2, that can become about 0.30 m of position uncertainty before environment bias.
Major section
Common Pitfalls
Claiming automatic centimeter accuracy.: UWB can support fine timing evidence, but installed accuracy depends on line of sight, calibration, geometry, clock quality, antenna placement, and filtering.
- Using TDoA without clock evidence.: Time-difference systems are only as good as anchor synchronization, timestamping, and drift monitoring.
- Ignoring non-line-of-sight bias.: A reflected path can arrive later than the direct path and make the range look too long.
- Optimizing update rate before airtime.: More ranges per second can increase collisions, stale processing, and energy use.
- Turning range into authorization.: Secure ranging can strengthen proximity evidence, but authorization, freshness, confidence, boundary policy, and fallback still decide the action.
Major section
Summary
TWR estimates distance from a bidirectional exchange and does not require synchronized anchors.
- TDoA can support managed multi-tag systems when synchronized anchors and backend quality monitoring are available.
- AoA adds direction evidence, usually as a complement to range or time-difference measurements.
- UWB ranging results should be handled as evidence with confidence, freshness, geometry, and fallback rules.
Major section
Key Takeaway · Concept Relationships
UWB ranging quality depends on timing method, anchor geometry, line of sight, multipath, calibration, and validation against the required accuracy.
- Wide bandwidth supports precise timestamp evidence.
- AoA adds direction evidence through antenna geometry.
- CIR review helps distinguish first-path evidence from reflections.
- Application policy decides whether evidence is strong enough to accept, retry, down-weight, or fall back.
Deck summary
Key takeaways
Two-way ranging (TWR) measures a round trip between two devices and subtracts responder delay.
- UWB does not directly output truth.
- That number explains why UWB ranging is powerful but not automatic truth.
- In a two-way exchange, a timing error in the round-trip estimate is divided by two when it becomes one-way distance.
- The responder delay is $T_3: T_2$.
Retrieval practice
Recall check 1 of 3
Radio Remi says: answer from memory, then check your reasoning.
Q1What does double-sided two-way ranging (DS-TWR) add over basic two-way ranging?
Show answer
Answer: A DS-TWR adds a reverse exchange so clock-rate error has much less influence on the distance estimate.
Retrieval practice
Recall check 2 of 3
Radio Remi says: answer from memory, then check your reasoning.
Q2In a UWB TWR exchange, the initiator records T1 = 0 ns and T4 = 220 ns. The responder records T2 = 10 ns and T3 = 210 ns. Using c = 0.3 m/ns, what distance should the initiator calculate?
Show answer
Answer: A TWR subtracts responder delay, then divides the remaining round-trip flight time by two before converting to distance.
Retrieval practice
Recall check 3 of 3
Radio Remi says: answer from memory, then check your reasoning.
Q3An SS-TWR system shows a ranging error that grows with the responder's reply delay. What is the cause, and which method removes it?
Show answer
Answer: D SS-TWR error comes from inter-device clock frequency offset scaling the responder's reply delay; DS-TWR times a round trip on each side and cancels the first-order offset, while TDoA instead needs synchronised anchors.
Print reference
Answers
Answer key.
- A · DS-TWR adds a reverse exchange so clock-rate error has much less influence on the distance estimate.
- A · TWR subtracts responder delay, then divides the remaining round-trip flight time by two before converting to distance.
- D · SS-TWR error comes from inter-device clock frequency offset scaling the responder's reply delay; DS-TWR times a round trip on each side and cancels the first-order offset, while TDoA instead needs synchronised anchors.