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Bluetooth Direction Finding: AoA vs AoD Explained

By RF Wireless Expert Team

This page describes Bluetooth Direction Finding, specifically focusing on the Angle of Arrival (AoA) and Angle of Departure (AoD) methods.

Introduction to Bluetooth Direction Finding

Historically, Bluetooth location services relied on Received Signal Strength Indication (RSSI) to estimate distance. RSSI can tell you roughly how far away a device is based on signal degradation, but it cannot tell you what direction the signal is coming from.

To solve this, Bluetooth introduced Direction Finding. Instead of just measuring signal strength, it measures the phase of the radio signal to calculate the exact angle at which a signal arrives or departs. When combined with the known locations of several reference points, this angle data allows systems to calculate a device’s precise 3D position down to sub-meter accuracy.

As per the Bluetooth Core Specification, this feature operates over the LE Uncoded PHYs (1M and 2M) and relies on transmitting special “direction finding enabled packets” that include a Constant Tone Extension (CTE). The CTE provides a continuous, unmodulated radio tone that gives the receiving device enough time to measure the phase of the signal.

To capture these measurements, Bluetooth uses two distinct architectures: Angle of Arrival (AoA) and Angle of Departure (AoD). Both rely on capturing In-phase and Quadrature (IQ) samples using an array of antennas and an RF (Radio Frequency) switch.

1. Angle of Arrival (AoA) Method

The setup of this AoA method is as below.

  • Transmitter: A simple device with a single antenna.
  • Receiver: A more complex locator device equipped with an RF switch and an antenna array (multiple antennas arranged in a specific pattern).

How it Works:

  1. The transmitter (e.g. a small tracking tag) sends a direction finding enabled packet containing the Constant Tone Extension (CTE).
  2. As the receiver picks up this unmodulated tone, its RF switch rapidly toggles between the different antennas in its array.
  3. Because the antennas in the array are physically separated by a known distance (dd), the radio wave reaches each antenna at a slightly different time, resulting in a phase difference (ψ\psi).
  4. The receiver captures IQ samples of these phase differences.
  5. Using trigonometry (the wavelength of the signal, the distance between antennas, and the phase difference), the receiver calculates the angle (θ\theta) from which the signal arrived.

Primary Use Case: Asset Tracking

AoA is heavily used for tracking items. Cheap, low power tags (single antenna) are placed on assets (e.g., warehouse pallets, hospital equipment). Fixed locators on the ceiling (antenna arrays) receive the signals, calculate the angles, and send the data to a central server to determine the exact location of the asset.

2. Angle of Departure (AoD)

The setup of this AoD method is as below.

  • Transmitter: A complex bluetooth beacon device equipped with an RF switch and an antenna array.
  • Receiver: A device with a single antenna (e.g. a smartphone).

How it Works:

  1. The transmitter (e.g. a fixed beacon on a ceiling) sends a direction finding enabled packet. While transmitting the CTE portion of the packet, it rapidly switches transmission across its array of antennas.
  2. The receiver (with its single antenna) listens to this signal. Because the signal is being emitted from slightly different physical locations (the different antennas in the array), the receiver detects phase shifts in the incoming signal.
  3. The receiver captures IQ samples of these phase shifts.
  4. To make sense of these shifts, the receiver must know the physical layout of the transmitter’s antenna array. This is shared via profile level information.
  5. Using the captured IQ samples and the beacon’s antenna layout, the receiver calculates the angle from which the signal departed the beacon.

Primary Use Case: Indoor Navigation / Wayfinding

AoD is designed for scenarios where a user needs to know their own location. Fixed beacons on the ceiling (antenna arrays) broadcast signals. A user’s smartphone (single antenna) receives the signals from multiple beacons, calculates the Angles of Departure, and computes its own location on a map. This keeps the complex calculations on the user’s device and allows an unlimited number of smartphones to use the beacons simultaneously without overloading a central server.

Comparison between Bluetooth AoA vs. AoD

FeatureAngle of Arrival (AoA)Angle of Departure (AoD)
TransmitterSingle Antenna (Simple)Antenna Array (Complex)
ReceiverAntenna Array (Complex)Single Antenna (Simple)
Who switches antennas?The Receiver (while listening)The Transmitter (while broadcasting)
Who calculates the angle?The Receiver (Locator)The Receiver (User Device)
Additional Data NeededNoneReceiver needs the layout of the transmitter’s antenna array (via profiles).
Primary ApplicationAsset Tracking (finding a moving item).Indoor Navigation (finding your own location).
Device ExamplesTags (TX) and Ceiling Locators (RX)Ceiling Beacons (TX) and Smartphones (RX)

References & Further Reading

  1. Bluetooth SIG : Bluetooth Core Specification Version 6.3, May 5, 2026.
  2. Bluetooth SIG : Bluetooth Technology Overview

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