What is BLE, how it works and what it is for

BLE technology

What is BLE technology?

Among radiofrequency technologies that revolutioned tracking and asset flow management, Bluetooth Low Energy (BLE) is definitely worth a mention. Published ufficially in 2010 as a part of Bluetooth 4.0 by Bluetooth Special Interest Group (SIG), BLE met the needs of the market right-away: smaller but also long-running devices were required. In fact, classic Bluetooth is high-consuming in terms of energy, due to the continuous connection between devices.

Since the publication of Bluetooth 4.0 standard, the technology evolved substantially. The latest innovations are enclosed in standard Bluetooth 5.1 / 5.2 / 5.3 (the latest published in 2022), with the introduction of Angle of Arrival (AoA) and Angle of Departure (AoD) and multi-stream connection support.

Bluetooth vs BLE

"Classic" Bluetooth (also referred to as BR/EDR - Basic Rate/Enhanced Data Rate) is designed for high-speed, continuos communication of a data stream; as a consequence, its energy consumption is very high.

BLE is born to be energetically efficient, and at the same time reliable, scalable without compromising on communication distance.

Data elaboration

Information about tags can be retreived with BLE: presence, distance, direction of movement. The fact that this information can be obtained is an intrinsic feature of BLE technology.

Network topology

BR/EDR bluetooth works with only one network topology, a point-to-point data exchange (1:1 device communication), while bluetooth LE also allows broadcast communication (one-to-many 1:m) and mesh networking (many-to-many m:m).

Communication efficiency

While BR/EDR bluetooth uses a continuous communication to exchange a stream of data, Bluetooth LE communicates packages of data intermittently, saving on power.

How it works

Bluetooth Low Energy is a radiofrequency technology that operates in 2.40GHz ISM (Industrial, Scientific and Medical) band, transmitting in 2MHz intervals over 40 channels. Similarly to RFID, BLE systems consist of a device that collects data - gateway - and active tracking elements, called tags or beacons, that are associated to an asset and can have different size or shape depending on the application. Data can then be processed by a software to detect or locate tags.

Components

A BLE system consists of the following elements:

  • TAG - a tag is any active radiofrequency device that in classic BLE and AoA systems transmits data to the gateway, while receiving in AoD systems. In the former case, tags can be beacons - bracelets or plates - while in the latter they can be smartphones and devices that can process data. Every tag is uniquely identified by a unique code and can be integrated with built-in sensors and buttons.
  • Anchor (or gateway or locator) - it is the component that in classic BLE and AoA receives data from the tag, while receiving in AoD. They can integrate a single BLE antenna or an antenna array (AoA/AoD).
  • Location server - it is the software that processes the collected data to detect the tags and evaluates the trilateration, relying on different algorithms based on the BLE system.

RTLS with Bluetooth LE

One of the mail advantages of BLE technology is the implementation of Real-Time Location Systems (RTLS) and the possibility to locate tags within a certain precision range.

The most common method for location is trilateration of the signal of at least 3 gateways, that must be placed in a specific geometry. The quantity that is processed is the strength of the signal (RSSI).

The most innovative location BLE technology is AoA (and AoD), that allows to evaluate the direction line with respect to the anchor, on which the tag can be found. The intersection of lines processed by different gateways allow to obtain a fairly accurate point in space, with a higher precision compared to that of RSSI method. There are two possible configurations for AoA, based on the antenna array geometry in the gateway:

  1. ULA (Uniform Linear Array – 1D): antenna are placed in a linear arrangement;
  2. URA (Uniform Rectangular Array – 2D): antennas are placed in a bidimensional matrix.

Types of RTLS with BLE

RSSI trilateration and direction evaluation with AoA are widely applied methods for bulk tracking, while AoD is ideal for location and indoor navigation applications. Therefore, we will take the first two into consideration.

RSSI

A trilateration algorithm allows to locate the beacon with a maximum precision of about 3m (increasing the precision is hard due to reflection of the signal). To do so, RSSI from at least 3 gateways is processed so that the distance from the anchors can be evaluated.

PROs: Integrating classic BLE gateways and processing RSSI data is much cheaper than using AoA. 

CONs: RSSI is very sensitive to multipath fading, which is a consequence of interference and reflection with objects in the environment, and to tags orientation.

AoA

Calculating the direction of the ray gateway/tag, a much more precise location can be obtained. Since Time-of-Flight - ToF - cannot be evaluated with BLE, a pure sine signal called CTE (Constant Tone Extension) is added to the modulated one, that lasts about 16 µs – 160 µs. The gateways elaborates the CTE and calculate the azimuth angle of the signal, and therefore, the direction where the tag can be found.

If the gateway is ULA, it can only evaluate the azimuth angle (and therefore the direction of the ray gateway/tag), while if it is URA, it can evaluate both the azimuth angle and the elevation.

PROs: AoA can reach 0.3 m – 1,5 m of location precision, crossing signals from 2 or more AoA gateways. It is less affected by the environment.

CONs: it is a very expensive technology and the infrastructure density is higher than a RSSI-based RTLS. Furthermore, configuration is more difficult and requires maintenance.

AoD

Come i gateway AoA, i gateway Angle of Departure hanno un array di antenne che possono essere disposte in geometria lineare (ULA) o bidimensionale (URA). Ciò che li distigue da  AoA, è che il processo di elaborazione è invertito: nel AoD è il gateway a trasmettere il CTE al tag, che potrebbe essere ad esempio uno smartphone e che si occupa calcolare l’angolo di partenza.