AoA vs AoD architecture?

AoA: tags are simple single-antenna transmitters; receivers (locators) have antenna arrays. The locator measures phase across its array to determine direction. AoD: transmitters (beacons) have antenna arrays and sequentially transmit from each element; the receiver (phone) computes direction from IQ samples at its single antenna. AoA is preferred for asset tracking; AoD for wayfinding.

How many locators needed?

2D positioning: minimum 3 locators with overlapping coverage (triangulation). 3D positioning: minimum 4 locators at different heights. Each locator determines a direction line; the intersection gives position. More locators improve accuracy and coverage. Deploy at ceiling height (3-4 m) with 10-15 m spacing for warehouse environments.

Array design requirements?

Element spacing: lambda/2 = 6.1 cm at 2.44 GHz (avoids grating lobes). Typical arrays: 4 elements (1D angle), 4x4 or 3x3 (2D angle). Switching speed: 1-2 microseconds per element during CTE. Higher element count improves angular resolution: 4 elements give approximately 10 degree resolution; 16 elements give approximately 3 degrees.

Wireless Protocols

Bluetooth Direction Finding

Bluetooth Direction Finding (BLE 5.1) enables sub-meter indoor positioning by measuring the angle of incoming BLE signals using antenna arrays. Angle of Arrival (AoA) uses arrays at fixed locators to determine direction to simple tags. Angle of Departure (AoD) uses arrays at beacons for phone-based wayfinding. A Constant Tone Extension (CTE) appended to packets provides unmodulated RF for clean IQ phase measurements across 4-16 antenna elements.

Category: Wireless Protocols

Accuracy: 0.5-1 m

Elements: 4-16

Understanding Direction Finding

The CTE is a period of unmodulated carrier appended to a BLE packet. During CTE transmission, the receiver switches through antenna elements at 1-2 μs intervals, sampling IQ data at each element. The phase difference between elements is proportional to sin(θ), where θ is the arrival angle. Super-resolution algorithms (MUSIC, ESPRIT) extract angles more accurately than simple phase comparison.

In a warehouse deployment: tags on pallets transmit BLE advertising with CTE every 1-10 seconds. Ceiling-mounted locators with 4x4 arrays compute AoA. A positioning engine combines AoA from 3+ locators to triangulate 2D position.

AoA Phase Relationship

Δφ = 2π·d·sin(θ)/λ
d = element spacing (λ/2 = 6.1 cm)
θ = angle of arrival

Angular resolution:
4 elements: ~10°
16 elements (4×4): ~3°
Position accuracy: 0.5-1 m at 10 m range

Direction Finding Deployment Comparison

Architecture	Array Location	Tag Complexity	Use Case
AoA	Locator (RX)	Simple (1 antenna)	Asset tracking
AoD	Beacon (TX)	Simple (1 antenna)	Wayfinding
AoA + RSSI	Locator (RX)	Simple	3D positioning

Common Questions

Frequently Asked Questions

AoA vs AoD?

AoA: array at locator, simple tag. AoD: array at beacon, simple receiver. AoA for tracking; AoD for wayfinding.

Locators needed?

2D: min 3 locators. 3D: min 4 at different heights. Deploy at 3-4 m height, 10-15 m spacing in warehouses.

Array design?

Spacing: λ/2 = 6.1 cm. 4 elements: 10° resolution. 4×4: 3°. Switch 1-2 μs per element during CTE.

Related Terms

← Bluetooth Classic BLE →

RTLS

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