BLE Direction Finding: AoA and AoD for Indoor Positioning
Angle of Arrival and Angle of Departure explained
Direction Finding
Bluetooth Advertising">Direction Finding, introduced in Bluetooth 5.1, adds angular position to RSSI-based proximity. Instead of estimating distance, Direction Finding systems determine the angle at which a signal arrives (AoA) or departs (AoD), enabling sub-meter accuracy without additional infrastructure protocols like UWB.
AoA vs AoD
Two complementary methods exist, differentiated by which side hosts the antenna array:
Angle of Arrival (AoA): The locator has a multi-element antenna array. The tag transmits a continuous tone (CTE — Constant Tone Extension) on a single antenna. The locator samples the signal on each array element sequentially (IQ Sampling) and computes the phase differences to derive the angle.
Angle of Departure (AoD): The locator transmits the CTE while switching through its antenna array. The tag receives the signal on a single antenna. Phase differences at the tag's receiver encode the departure angle. The tag must compute and report the angle or raw IQ data to the system.
| Property | AoA | AoD |
|---|---|---|
| Array location | Locator (fixed infrastructure) | Locator (fixed infrastructure) |
| Computing side | Locator | Tag (or tag + server) |
| Tag complexity | Low (single antenna) | Higher (angle computation) |
| Infrastructure cost | High (array per locator) | High (array per locator) |
| Privacy (tag tracking) | Tag passively transmits | Tag computes own position |
| Best for | Asset tracking, RTLS | Navigation, wayfinding |
Antenna Arrays and IQ Sampling
Direction Finding accuracy depends on array geometry and element count. A linear array of N elements provides one-dimensional angular resolution (azimuth only). An L-shaped or planar array supports 2D or 3D positioning.
Array spacing is standardized at λ/2 (half-wavelength ≈ 6.25 cm at 2.4 GHz) to avoid spatial aliasing. Larger arrays improve angular resolution: a 4-element array achieves ±5–10°; a 16-element array can reach ±1–2°.
The radio switches between antenna elements at 1 µs or 2 µs intervals during the CTE. IQ samples (8-bit I + 8-bit Q) are collected per element. Phase difference between adjacent elements:
Δφ = (2π × d × sin θ) / λ
where d is element spacing, θ is the angle of arrival, and λ is wavelength. The MUSIC or ESPRIT algorithms extract θ from the IQ matrix.
Indoor Positioning Integration
Angle measurements from multiple locators trilaterate a tag's 3D position. Accuracy depends on:
| Factor | Impact |
|---|---|
| Number of locators | 2 = 2D position, 3+ = 3D + redundancy |
| Array element count | More elements → sharper angle resolution |
| Multipath environment | Reflections degrade IQ coherence; open spaces are ideal |
| CTE length | Longer (up to 160 µs) → more IQ samples → better SNR |
| Sync / calibration | Locator positions must be surveyed accurately |
The Range Calculator models path loss for locator placement. Commercially, AoA indoor positioning systems achieve 0.3–1 m accuracy in typical office environments, versus 2–5 m for RSSI fingerprinting.
Channel Sounding
Bluetooth 6.0 (2024) introduced Channel Sounding as a complement to Direction Finding. While Direction Finding measures angle, Channel Sounding measures Time of Flight (ToF) using phase-based ranging across multiple channels. Together they enable centimeter-level positioning:
- Channel Sounding: 10–20 cm ranging accuracy
- Direction Finding AoA: 1–3° angular accuracy
- Combined: sub-30 cm 3D position
Channel Sounding uses the LE 1M PHY and requires both devices to support it (declared in LL features). It is expected to appear in production silicon through 2025–2026.
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