Angle of Arrival (AoA): the receiver has an antenna array and measures phase differences across elements to determine the incoming signal direction. The tag is a simple single-antenna transmitter. Angle of Departure (AoD): the transmitter has the array and sends CTE from each element sequentially; the receiver measures phase at its single antenna. AoA puts complexity at the infrastructure; AoD puts it at the beacon/locator.

Direction finding achieves sub-meter positioning accuracy indoors (typically 0.5-1.0 m) using locator arrays with 4-16 antenna elements. RSSI-based positioning is 3-5 m accuracy. The improvement comes from measuring angle (phase) rather than amplitude (RSSI). Multiple locators triangulate position using AoA/AoD angles combined with RSSI for distance estimation.

Constant Tone Extension is a sequence of unmodulated RF cycles appended to a BLE packet. During CTE, the receiver samples IQ data while switching antenna elements, measuring phase differences. CTE length: 16-160 microseconds. The unmodulated tone ensures phase measurements are not corrupted by data modulation.

Wireless Protocols

Bluetooth 5.1

Bluetooth 5.1 (released January 2019) introduced direction finding capabilities using Angle of Arrival (AoA) and Angle of Departure (AoD) techniques. A Constant Tone Extension (CTE) appended to BLE packets enables IQ sampling across antenna array elements, providing sub-meter indoor positioning accuracy. Additional features include GATT caching, periodic advertising sync transfer enhancements, and randomized advertising channel indexing.

Category: Wireless Protocols

Released: January 2019

Accuracy: Sub-meter

Understanding Bluetooth 5.1

Direction finding works by measuring the phase difference of a CTE signal across antenna array elements spaced at half-wavelength (approximately 6.1 cm at 2.44 GHz). With 4-16 elements, the receiver computes the angle of the incoming signal using algorithms like MUSIC or ESPRIT. Multiple locators triangulate the tag's 2D or 3D position.

GATT caching reduces connection setup time by caching the service database hash: if unchanged, the client skips rediscovery. This benefits reconnecting devices like headphones and wearables.

Direction Finding

AoA phase difference:
Δφ = 2π·d·sin(θ)/λ
d = element spacing, θ = angle of arrival

At 2.44 GHz, d=λ/2=6.1 cm:
Δφ = π·sin(θ)

Positioning Technology Comparison

Technology	Accuracy	Infrastructure	Power
BLE RSSI	3-5 m	Beacons	Very low
BLE 5.1 AoA	0.5-1 m	Locator arrays	Low
UWB	10-30 cm	Anchors	Medium
Wi-Fi RTT	1-2 m	APs	High

Common Questions

Frequently Asked Questions

AoA vs AoD?

AoA: receiver has array, tag is simple. AoD: transmitter has array, receiver is simple. AoA puts complexity at infrastructure.

Accuracy?

Sub-meter (0.5-1 m) with 4-16 element arrays. Far better than RSSI (3-5 m). Multiple locators triangulate position.

What is CTE?

Constant Tone Extension: unmodulated RF appended to packets for IQ phase sampling. 16-160 μs. Enables clean phase measurement.

Related Terms

← Bluetooth 5.0 Bluetooth 5.2 →

Indoor Positioning

Request a Quote

Need BLE direction finding modules, locator arrays, or RTLS solutions? Contact our team.

Get in Touch