What Is Bluetooth Low Energy?

Bluetooth Low Energy is a short-range wireless communication technology defined by the Bluetooth Special Interest Group for devices that must run for months or years on a coin cell. Introduced in version 4.0 of the Bluetooth Core Specification in 2010, it is a separate radio and protocol stack from Bluetooth Basic Rate and Enhanced Data Rate, sharing only the 2.4 GHz ISM band and the brand. Where the classic stack was designed for continuous streams such as audio, the low energy variant is designed for brief exchanges of small data payloads separated by long periods in which the radio is off.

The design centers on duty cycle rather than raw efficiency. A low energy device spends most of its life in a sleep state, wakes to transmit a packet lasting a few hundred microseconds, and returns to sleep. Peak current during transmission is comparable to that of other 2.4 GHz radios, but average current falls to microamperes, which is what makes battery lifetimes of several years possible in sensors, tags, and wearables.

Radio and Link Layer

The band is divided into 40 channels of 2 MHz each, using Gaussian frequency shift keying at a symbol rate of 1 Msym/s in the original specification. Three of those channels are reserved for advertising and are placed to avoid the most heavily used Wi-Fi channels. Version 5.0 added a 2 Msym/s option for higher throughput and coded physical layers that trade data rate for roughly four times the range through forward error correction. Connections use adaptive frequency hopping across the 37 data channels, with the hop sequence and connection interval negotiated between the two devices. A survey of the technology's characteristics and limits in vehicular settings documents measured throughput, latency, and connection setup behavior against these parameters.

Advertising, Connections, and GATT

A peripheral device broadcasts advertising packets at a configurable interval; a central device scans, and either connects or simply harvests the broadcast payload. Connectionless operation is why the technology underpins beacons, asset tags, and the exposure notification systems deployed during the COVID-19 pandemic. Once connected, data exchange runs through the Generic Attribute Profile, which organizes state as a table of attributes grouped into services and characteristics, each identified by a UUID. A heart rate monitor exposes a heart rate service with a measurement characteristic that a client reads or subscribes to for notifications. This attribute model, rather than a stream socket, is the normal programming abstraction for the technology.

Security and Recent Extensions

Pairing establishes a long-term key used for AES-CCM encryption of the link. LE Secure Connections, added in version 4.2, replaced the original pairing scheme with elliptic curve Diffie-Hellman key agreement to resist passive eavesdropping during pairing, and address randomization limits tracking of a device by its MAC address. NIST Special Publication 800-121 Revision 2, the Guide to Bluetooth Security, sets out the threat model and configuration recommendations for organizations deploying these links. Later releases extended function considerably: direction finding through angle of arrival and angle of departure in version 5.1, LE Audio with the LC3 codec and broadcast Auracast in version 5.2, and channel sounding in the Bluetooth Core 6.0 specification, which uses phase-based ranging and round-trip timing to measure distance far more accurately than the received signal strength methods it replaces.

Applications

Bluetooth Low Energy is used in a wide range of products and systems, including:

• Fitness trackers, smartwatches, and medical wearables
• Hearing aids and wireless audio through LE Audio and Auracast broadcast
• Indoor positioning, asset tracking, and proximity beacons
• Industrial and building sensor networks, including Bluetooth mesh lighting
• Keyless entry and secure ranging for vehicles and door locks
• Configuration and commissioning interfaces for Internet of Things devices