Why UWB Is Suddenly Everywhere
Ultra‑Wideband (UWB) quietly moved from labs into pockets. Phones ship with UWB radios. Digital car keys use it for walk‑up unlock. Smart tags guide you a few centimeters from a lost item. The reasons are straightforward: UWB measures time more precisely than Bluetooth or Wi‑Fi, and time-of-flight translates into distance with 10–30 cm accuracy under the right conditions. For indoor positioning, that is a big deal.
In this guide, we go deep on how UWB ranging works, how to plan a small anchor network, how to integrate with iOS and Android APIs, and how to keep distance measurements trustworthy. You do not need RF design experience to follow along; we will stay practical and explain the few bits of radio physics that matter.
The Core Idea: Time Becomes Distance
UWB radios do not measure signal strength to guess distance. They measure how long a radio pulse takes to arrive. Because radio signals move at the speed of light, every nanosecond corresponds to ~30 cm of distance. That is why UWB needs very short pulses and wide bandwidth: to locate the first path among reflections with fine time resolution.
What 802.15.4z Adds
Modern UWB devices implement IEEE 802.15.4a/z. The “z” update strengthens ranging security with Scrambled Timestamp Sequences (STS) so attackers cannot easily replay or slow down frames. It also refines the pulse shapes used for high rate pulse (HRP) modes, improving time-of-flight estimates in cluttered rooms.
FiRa Profiles in Plain Terms
The FiRa Consortium defines how devices interoperate. Think of FiRa as the “Bluetooth SIG” equivalent for UWB. Its profiles specify packet timings, security, and what a “ranging session” looks like. If you want your accessory to range with phones reliably, FiRa certification smooths the rough edges across vendors.
Ranging Modes You Can Actually Use
There are two main ways to turn UWB timing into a position you can use in an app. You can either measure the distance to one peer at a time (ranging) or compute your position relative to a set of fixed anchors (localization). Each has trade‑offs for battery life, infrastructure, and latency.
Two‑Way Ranging (TWR)
TWR is a handshake between two devices:
- Device A sends a poll.
- Device B replies with the exact time it received the poll and when it sent the response.
- Device A calculates the round‑trip delay and subtracts processing times to estimate distance.
Single‑sided TWR is fastest but sensitive to clock drift. Double‑sided TWR sends a second exchange so both sides can average out clock errors. TWR shines for access control (phone-to-lock), robot docking, and pointing use cases where you want a distance to one thing right now.
Time Difference of Arrival (TDoA)
With TDoA, a mobile tag transmits short “blink” messages. Several anchors hear the blink and record its arrival time. Because the anchors are time‑synchronized, they can compute hyperbolas of equal time difference. Intersect the hyperbolas and you get the tag’s 2D or 3D position.
TDoA is perfect for tracking many tags because the tags do little work. The tag battery lasts longer, and the infrastructure carries the math. The trade‑off: you must synchronize your anchors carefully and wire them to a backend that solves for positions.
Angle of Arrival (AoA)
Some chipsets support AoA by using multiple antennas to sense the phase difference of the received pulse. AoA gives you a bearing in addition to distance. Combined with TWR or TDoA, AoA helps in long halls or when reflections confuse simple time-of-flight.
Hardware That Won’t Fight You
You will find two families of UWB solutions in the wild today: Qorvo/Decawave DW3000‑series and NXP SR1xx‑series. Both are widely supported by software stacks and compatible with Android and iOS UWB devices within FiRa profiles.
Modules to Consider
- Qorvo DW3000 family (e.g., modules built around DW3110/DW3120): popular, well‑documented, and found inside many dev kits. Good for TWR and TDoA labs.
- NXP SR150/SR040: supports AoA and multi‑antenna configurations, often chosen for infrastructure anchors where direction helps.
- Combo modules with UWB + BLE + MCU: reduce integration effort when you need pairing over BLE and ranging over UWB.
Antenna and Enclosure Tips
- Keep the UWB antenna clear of large metal surfaces. Metal detunes and creates reflections close to the radio.
- Use a short RF path. Long coax runs between module and antenna introduce loss and distort pulses.
- Pick enclosures with low dielectric loss at ~6.5–8.0 GHz. Thin plastics are better than thick glass or ceramic bezels.
- Mind the human factor. Bodies absorb UWB energy. For wearable tags, offset the antenna from the body with a small standoff if possible.
Clocking and Synchronization
TWR handles clock drift by design. For TDoA networks, you need synchronized anchors. There are two practical approaches:
- Wired sync: use Ethernet with PTP (Precision Time Protocol) to keep anchors within sub‑microsecond alignment. You do not need picoseconds; tens of nanoseconds can still yield good positioning with calibration.
- Over‑the‑air sync: designate a master anchor that broadcasts timing beacons. Slave anchors discipline their clocks to the master. Easier to deploy but more sensitive to RF conditions.
How to Plan a Small UWB Anchor Network
If you want room‑level precision across a home, studio, or small warehouse, anchor layout matters more than raw chip specs. Here’s a simple plan you can adapt.
Start With Geometry
- Height: Mount anchors higher than head level to reduce body occlusion and to improve line-of-sight across furniture.
- Spread: Place at least three anchors per zone for 2D (four for 3D). Avoid placing anchors in a straight line; favor triangles or tetrahedra.
- Overlap: Aim for zones where each tag can see 3–4 anchors. Overlap zones at doorways to avoid position jumps.
Distance and Density
Typical indoor UWB range is 10–30 meters depending on walls and materials. For a 200 m² space with rooms, expect 4–6 anchors. For an open floor of the same size, you can often do it with 3–4 anchors and careful placement.
Power and Backhaul
Anchors like power and Ethernet. PoE (Power over Ethernet) simplifies installs, gives you PTP for sync, and avoids local wall warts. If you cannot run Ethernet, use Wi‑Fi backhaul plus a clean DC supply and over-the-air sync, but plan for periodic recalibration.
Calibrate Once, Then Check
Survey anchor coordinates with a laser distance meter or tape and sanity‑check with a few known tag positions. Most UWB SDKs include a least-squares solver. Save the anchor map and run a weekly drift check by comparing calculated distances between anchors to the known geometry. Building temperature changes and enclosure creep can shift timing very slightly over months.
From Ranging to Position: The Math Without Tears
UWB gives you raw measurements: timestamps, channel impulse responses, and maybe AoA. You convert those into positions using lightweight estimation methods. The two that work well on embedded hardware are extended Kalman filters (EKF) and particle filters.
EKF for Smooth Tracking
Model the tag as a point with position, velocity, and acceleration. The EKF predicts where the tag should be next, then corrects the prediction with new UWB distances. Fuse IMU data (accelerometer + gyro) to keep motion smooth when UWB drops for a second due to occlusion. The result is stable motion that still responds quickly to real movement.
Outlier Rejection
Multipath creates occasional “too short” distances when a strong reflection looks like the first path. Use the channel impulse response (CIR) quality metrics many chipsets provide. If the first-path amplitude is low or late compared to the strongest path beyond a threshold, downweight or drop that measurement. Simple rules remove most bad points.
Security and Privacy That Hold Up
Location features expand your attack surface: relay attacks, replay attacks, and spoofed anchors can make a device “believe” it is closer than it is. That is unacceptable for car unlocks or secure spaces. Fortunately, 802.15.4z and FiRa give you tools to defend distance.
Distance Bounding in Practice
- Cryptographic STS: Use session keys to generate pseudorandom timestamp sequences. The receiver correlates only the right sequence and rejects delayed copies.
- Tight time budgets: Cap maximum processing delays. If a reply comes back too slow or too perfectly spaced, treat it as suspicious.
- Auxiliary channels: Pair with BLE or NFC to bootstrap identities and share keys, then range over UWB. This reduces man‑in‑the‑middle risk.
Protecting People’s Movements
Use ephemeral identifiers for tags. Encrypt ranging sessions. Let users pause or disable real‑time sharing. When you log positions for analytics, aggregate and blur the data. If your anchors leave the home or factory, treat location data like any other sensitive telemetry and apply retention limits.
Integrating With Phones and OS APIs
Developers do not need to write every radio detail from scratch. Modern operating systems expose UWB features in safe, high‑level APIs.
iOS: Nearby Interaction
Apple’s Nearby Interaction framework gives you access to distance and direction with tight OS integration. You pair your accessory over BLE or NFC, then range over UWB. iOS abstracts the session setup, security, and low‑level messaging. Access typically requires that your accessory supports the relevant profiles and you enroll for the entitlement. The upside: a predictable user experience and first‑class background behavior.
Android: UWB Manager
Android’s UWB APIs let you start ranging sessions with supported devices, configure update rates, and receive ranging results with quality indicators. Hardware support varies by phone model. The Pixel Pro line and many recent flagships support UWB. On Android, you also control more of the data plane, which is handy when building custom accessories or anchor networks.
Bridging to Your Backend
For TDoA systems, anchors forward timestamps to a server that solves for positions. Use a compact binary format and include per‑anchor clock quality indicators so the solver can ignore an anchor that went out of sync. For TWR, the phone or device typically has the answer locally; send only what the app needs for history and visualization.
Power Budget and Update Rates
UWB ranging is not free. Every pulse train and reply burns energy, and battery‑powered tags must balance snappy updates and long life.
TWR Battery Math
A double‑sided TWR session can take a few milliseconds end‑to‑end at common data rates. Budget a few millijoules per measurement. At 1 Hz updates, small coin‑cell tags may last months. At 10 Hz, life can drop to days. This is why TWR is best for on‑demand interactions: wake, measure fast, sleep.
TDoA Battery Math
TDoA tags transmit short blinks, often under 1 ms airtime each. At a 1–5 Hz burst rate, year‑long life on a coin cell is feasible if you keep the MCU and sensors asleep between blinks. For motion‑triggered behavior, add a wake on movement accelerometer and transmit only when something changes.
Dealing With Real‑World RF
On paper, UWB has centimeter‑level promise. In real rooms, reflections, people, and furniture will test your patience. Plan for it.
Materials Matter
- Concrete and brick attenuate and scatter. Expect shorter ranges and more multipath compared to drywall.
- Metal shelving causes strong reflections. Use higher placement and aim antennas into the aisles, not at the shelves.
- Water and people absorb. Crowded spaces need more anchors or lower update rates to maintain quality.
Channel and Preamble Choices
UWB has multiple channels (e.g., ch5 around 6.5 GHz and ch9 around 8 GHz) and preamble configurations. If you see interference or neighbors, switch channels or use distinct preamble codes. For high density, schedule ranging windows. UWB’s low duty cycles help, but collisions still waste power and time.
NLOS Detection
Non‑line‑of‑sight (NLOS) often looks like a distance that is too long because the first detectable path bounced. Track per‑measurement quality (first‑path/peak ratio, leading edge bias). If your filter sees a sudden increase in residual errors, downweight that anchor or require more anchors before publishing a position.
Putting It All Together: Example Scenarios
Hands‑Free Door With Anti‑Relay
You want a door to unlock as an authorized person approaches, but only when they are truly nearby. Use BLE for pairing and presence, then UWB double‑sided TWR at a modest rate (2–3 Hz) in the last 2 meters. Enable 802.15.4z STS and enforce tight response timing. If a relay tries to echo frames, the delay gives them away. Only then enable the latch.
AR Finds What You Lost
A phone app guides users to tagged objects with an on‑screen arrow. At launch, use a quick TWR to pick the nearest tag. As the user walks, fuse IMU and short bursts of UWB to keep the arrow stable. Use the phone’s camera and on‑device SLAM to line up the arrow with the real world. Update at 5–10 Hz only while the app is visible. Suspend ranging in the background to save battery.
Robot Navigation in Aisles
Install TDoA anchors every 10–15 meters along the ceiling. Robots transmit blinks at 10 Hz while moving, 1 Hz while idle. Combine UWB positions with wheel odometry and a 2D lidar. If UWB quality drops (few anchors visible), rely more on odometry until a better view returns. This yields steady navigation even in reflective environments.
Testing Without Fancy Equipment
You do not need a spectrum analyzer to validate a UWB system. A handful of measurements and simple plots go far.
What to Log
- Timestamp and position estimate per update.
- Per‑anchor distance and quality flags (first‑path SNR, NLOS indicators).
- Update rate and dropped packet ratio.
- Battery voltage and temperature (for tags).
How to Interpret
Plot a walking path around the space. If it jumps, check for zones with fewer than three anchors or persistent NLOS. Plot residuals (measured vs predicted distances). A single anchor with consistently high residuals suggests a placement or sync issue. Correlate issues with time of day to find human‑driven interference (e.g., metal doors opening).
Regulatory and Coexistence Notes
UWB is globally permitted but subject to regional power and mask limits. Device makers must conform to local rules (e.g., FCC in the U.S. and ETSI in Europe). Modules and dev kits often ship with certified reference designs that save time. At the application layer, also plan for coexistence with other radios on the board (BLE, Wi‑Fi). Good PCB layout and antenna isolation reduce self‑interference.
What’s Next for UWB
Two trends are pushing UWB further into mainstream products:
- Standardized secure ranging: FiRa 2.x profiles and the Car Connectivity Consortium’s digital key specs make it easier to launch interoperable, anti‑relay solutions.
- Hybrid positioning: UWB fused with vision and inertial sensors yields reliable tracking in tough spaces. Expect phones and wearables to expose more combined signals via high‑level APIs.
As modules drop in price and anchor networks get easier to install with PoE and over‑the‑air sync, teams can add room‑scale awareness to devices that once could only guess. The constraint is no longer range math—it is product design: where to put antennas, how to conserve milliamps, and how to earn user trust with privacy‑first defaults.
Practical Checklist Before You Ship
- Define your mode: TWR for peer‑to‑peer interactions; TDoA for many tags over an area; AoA where direction helps.
- Place anchors smartly: height, spread, and overlap. Use PoE if you can.
- Secure ranging: enable 802.15.4z STS, enforce time budgets, pair over BLE/NFC.
- Handle RF reality: design for NLOS, pick channels thoughtfully, test with people moving.
- Optimize power: duty‑cycle aggressively; wake on motion; keep update rates user‑driven.
- Integrate cleanly: use iOS Nearby Interaction or Android UWB APIs; log quality; fuse IMU.
- Respect privacy: ephemeral IDs, encrypted sessions, clear user controls, and minimal retention.
Summary:
- UWB measures time-of-flight, turning nanoseconds into centimeter‑level distance indoors.
- Pick TWR for instant peer‑to‑peer ranging; pick TDoA for tracking many tags with low tag power.
- Anchor placement, synchronization, and calibration matter more than raw chip specs.
- Use 802.15.4z STS and tight timing to defend against relay and replay attacks.
- Fuse UWB with IMU (and vision if available) for smooth, robust positions.
- Integrate via iOS Nearby Interaction and Android UWB APIs to leverage phone hardware securely.
- Design for power: duty‑cycle tags, adjust update rates to actual needs, and wake on motion.
- Plan for coexistence and regional rules; prefer PoE anchors and PTP for reliable sync.
