Short-range radio, Bluetooth and UWB
The short-range radios inside phones, wearables and smart-home devices. Bluetooth Classic and LE, UWB ranging, the IEEE 802.15.4 radios under Zigbee and Thread, Matter, and NFC.
Overview
Short-range radios connect devices over centimetres to tens of metres, usually from a battery. They mostly share the 2.4 GHz band with Wi-Fi, so coexistence is a design problem, not an afterthought. This hub covers the families you will meet in phones, wearables, trackers and smart-home products, and what each radio layer does. It is for engineers who choose a radio for a product, debug a link, or need to read the specifications behind a feature such as ranging.
Key ideas
Bluetooth Classic and Bluetooth LE
Bluetooth comes in two radios, both specified by the Bluetooth SIG in one Core Specification. The latest version on the SIG’s specification page at the time of writing is 6.3. According to the SIG’s technology overview, both use 2.402 to 2.480 GHz. Bluetooth Classic (BR/EDR) uses 79 channels at 1 MHz spacing and is mainly used for audio streaming. Bluetooth Low Energy uses 40 channels at 2 MHz spacing, three for advertising and 37 for data, and is designed for very low power operation. LE modulates with GFSK. Classic uses GFSK, π/4 DQPSK and 8DPSK.
LE offers three main PHYs: LE 1M at 1 Mbit/s, LE 2M at 2 Mbit/s, and LE Coded at 500 kbit/s or 125 kbit/s. The coded PHY spends rate on forward error correction to gain range. Version 6.0 of the Core Specification added Channel Sounding, according to the SIG’s Core 6.0 feature overview. It measures distance between two devices with phase-based ranging across many channels and with round-trip timing, described in the SIG’s Channel Sounding technical overview. Accuracy depends on the implementation and the environment, so treat vendor ranging figures as claims until you measure them.
UWB ranging
Ultra-wideband (UWB) radios send very short pulses over a wide channel. The FiRa Consortium’s explainer gives 500 MHz channels and pulses of about 2 ns, and says FiRa devices support channels 5 to 14, with channel 9 mandatory. A wide bandwidth gives fine time resolution, which is why UWB is used for ranging. Distance comes from time of flight, measured by timing the round trip of a challenge and response between two devices. IEEE 802.15.4z-2020, approved in June 2020, added enhanced UWB PHYs and MAC support for ranging, and FiRa credits it with PHY-level protection against known attacks on older UWB ranging. Coppens and co-authors compare the IEEE, FiRa and Apple variants in An Overview of UWB Standards and Organizations (IEEE Access, 2022).
IEEE 802.15.4, Zigbee, Thread and Matter
Several smart-home stacks share one radio. Zigbee is based on the IEEE 802.15.4 MAC and PHY, and Zigbee PRO uses the 2003 edition, according to Silicon Labs’ Zigbee documentation. The CSA’s Zigbee page says the PHY supports 2.4 GHz and sub-GHz bands and describes a self-healing mesh. Thread is also built on the 802.15.4 MAC and PHY, but runs IPv6 above it, according to the Thread Group. Matter is an application layer that runs over IP. Nordic Semiconductor’s Matter page summarises it as using Thread, Wi-Fi and Ethernet for transport, and Bluetooth LE for commissioning.
NFC
NFC operates at 13.56 MHz, and the NFC Forum gives a typical range of up to 2 cm in its technology overview. It defines three modes: card emulation, reader/writer and peer-to-peer. The short range is a feature: a tap is a deliberate act, which makes NFC useful for payments, access control and pairing other radios.
Where to start
Read What is a radio link and Decibels and link budgets to estimate range for a given transmit power and receiver sensitivity. Digital modulation covers the frequency-shift keying family that includes Bluetooth’s GFSK. The synchronisation and estimation hub explains the timing and phase estimation behind ranging. For long-range, low-rate links, move to the LPWAN and IoT hub.
A typical development part is the Nordic nRF52840. Nordic’s product page lists a 2.4 GHz transceiver with Bluetooth LE, IEEE 802.15.4, Thread and Zigbee support, +8 dBm transmit power, a 64 MHz Cortex-M4, 1 MB of flash and 256 KB of RAM.
Tools and platforms
| Tool | What for | Licence |
|---|---|---|
| Zephyr RTOS | Embedded OS with a Bluetooth LE stack | Apache-2.0 |
| OpenThread | Open-source Thread stack | BSD-3-Clause |
| Matter SDK | Reference Matter implementation | Apache-2.0 |
| Wireshark | Decoding captured Bluetooth LE and 802.15.4 frames | GPL-2.0 |
Platform hubs: Nordic Semiconductor and open-source tools.
Common mistakes
- Quoting a range from a datasheet without the link budget behind it. Body loss, antenna placement and the enclosure matter more than the chip.
- Testing in a quiet lab, then shipping into a home full of 2.4 GHz Wi-Fi traffic.
- Assuming the LE Coded PHY is free. Lower rate means longer airtime, more energy per bit and more collisions.
- Treating received signal strength as a distance measurement. Multipath and body shadowing make it unreliable. Use a ranging method designed for it.
- Mixing up Zigbee, Thread and Matter. Two of them share a radio, and Matter is an application layer.
- Checking UWB rules too late. Emission limits and allowed channels are set by each national regulator.
Going deeper
- Bluetooth SIG, Core Specification and the Channel Sounding technical overview.
- IEEE, 802.15.4z-2020, enhanced UWB PHYs and associated ranging techniques.
- Coppens and co-authors, An Overview of UWB Standards and Organizations (IEEE 802.15.4, FiRa, Apple), IEEE Access, 2022.
- The Thread Group and the Connectivity Standards Alliance for Thread, Zigbee and Matter.
- NFC Forum, NFC technology overview.