AN1195: Antenna Array Design Guidelines

for Direction Finding

In January 2019, the Bluetooth SIG announced support for Angle of Arrival (AoA) and

Angle of Departure (AoD) in the Bluetooth 5 specification. AoA and AoD can be used

for building RF-based real-time locationing systems and applications, based on phase-

based angle estimation algorithms. Application use cases for the Internet of Things

(IoT) are tracking of assets and people, as well as indoor locationing and wayfinding.

The purpose of this application note is to provide hardware design guidelines specific

to the antenna arrays required for the direction finding implementations.

Note: This document applies to 2.4 GHz BLE standards for EFR32MG22 and

EFR32BG22 devices.

KEY POINTS

• Describes general direction finding use

cases

• Provides general antenna

recommendations for a direction finding

antenna array

• Details 4x4 antenna array properties and

insight into antenna tuning and items

which affect the antenna

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1. Introduction

AoA relies on a single-antenna transmit beacon with continuous tone extension appended to a Bluetooth packet transmission and a

locator receiver device to measure the arrival angle of the signal using an array of antennas. Each antenna in the array sees phase

differences due to different line of sight distances to the beacon device. The antennas in the array are switched during continuous tone

extension, resulting in IQ samples with phase information for each antenna. This IQ-data is fed to an angle estimator algorithm. In this

AoA use case, the receiving device tracks arrival angles for individual transmit beacon objects.

AoD is similar to AoA but the beacon and receiver roles are swapped. The AoD use case relies on a single-antenna mobile receiver

and multi-antenna transmitter beacons. The mobile receiving device can calculate its own position in space using angles from multiple

beacons and their positions.

Angle calculation is based on phase information from the individual antenna elements of the antenna array. The arrays utilized for this

purpose are uniform rectangular arrays. The number of channels/antennas affects the overall angle estimation accuracy. A larger num-

ber of antennas also helps with multipath effects. Other types of arrays could be used for AoA/AoD, such as linear arrays, circular ar-

rays, and non-uniform arrays. However, custom array development requires significant simulation and test efforts. For this reason, we

recommend utilizing the array discussed in detail in this application note. The sections to follow outline general and specific recommen-

dations related to the antenna array design.

AN1195: Antenna Array Design Guidelines for Direction Finding

Introduction

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