# Angle of Arrival Localization for Single-Chip Micro Mote

Manjiang Cao, Filip Maksimovic, David Burnett, Thomas Watteyne, Kris S J Pister, Tengfei Chang

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Manjiang Cao, Filip Maksimovic, David Burnett, Thomas Watteyne, Kris S J Pister, et al. Angle of Arrival Localization for Single-Chip Micro Mote. CrystalFreeIoT 2025 - The 2025 Workshop on Crystal-Free/-Less Radio and System-based Research for IoT, IEEE, Sep 2025, Leuven, Belgium. hal-05288172

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Distributed under a Creative Commons Attribution 4.0 International License
# Angle of Arrival Localization for Single-Chip Micro Mote

Manjiang Cao

HKUST(Guangzhou)
Guangzhou, China

mcao999@connect.hkust-gz.edu.cn

Thomas Watteyne

Inria

Paris, France

thomas.watteyne@inria.fr

Filip Maksimovic

Inria

Paris, France

filip.maksimovic@inria.fr

Kris S.J. Pister

UC Berkeley

Berkeley, United States

ksjp@berkeley.edu

David Burnett

Portland State University

Portland, United states

dburnett@pdx.edu

Tengfei Chang*

HKUST(Guangzhou)

Guangzhou, China

tengfeichang@hkust-gz.edu.cn

## Abstract

The Single-Chip Micro Mote ($SC\mu M$) is a 2x3mm² single die, featuring a 2.4 GHz radio supporting IEEE 802.15.4 and Bluetooth Low Energy (BLE). Because of its small size, it is an ideal platform for tracking and localizing wild animal or insect. The existing localization method for $SC\mu M$ is based on lighthouse laser, and uses the chip's built in light sensor. While this technique works, the chip needs to be less than 5 m from a lighthouse base station, and this type of localization can only be used indoors. We develop a localization method for $SC\mu M$ based on the direction-finding feature of BLE 5.1. $SC\mu M$ is flexible enough that we can, in software, add a Constant Tone Extension (CTE) portion to a BLE frame, which is key to use BLE's direction finding feature. With an antenna array and CTE, frequency offset can be estimated through the phase samples, assisting to improve the direction finding performance over $SC\mu M$. We built a prototype Angle of Arrival (AoA) localization system and achieves a median angle estimation error of ±5° inside of an anechoic chamber.

## CCS Concepts

* **Networks** → Location based services; * **Hardware** → Sensor applications and deployments.

## Keywords

Localization, $SC\mu M$, Angle of Arrival, BLE

## 1 Introduction

The Single-Chip Micro Mote ($SC\mu M$) is the first crystal-free mote-on-chip that can interoperate with off-the-shelf BLE and IEEE 802.15.4 radios. On a 2 × 3 mm² single die, $SC\mu M$ integrates a 32-bit ARM Cortex-M0 core and a 2.4 GHz IEEE 802.15.4/BLE radio. The traditional crystal used in System-on-Chip (SoC) designs for clock generation is replaced by an internal oscillator circuit, allowing $SC\mu M$ to operate with just an antenna and a power source. This innovation further reduces the size of IoT nodes, facilitating the development of compact tags for tracking applications, including warehouse management and wildlife tracking.

Alvarado et al. have developed a solution to localize $SC\mu M$ chip; it is based on the lighthouse system, in which $SC\mu M$ uses its built-in optical receiver to detecting a laser that a basestation periodically sweeps across a room [1]. This work achieving an RMS error of

7.25 mm in 2D, 11.2 mm in 3D. $SC\mu M$ can calculate its own position [4]. The main limitations of light-house localization is that $SC\mu M$ needs to be at most 5 m from a basestation, and that it doesn't work outside, as direct sunlight blinds the optical receiver.

The BLE 5.1 standard recently introduced a direction-finding feature, using AoA [5]. Each anchor node has an antenna array; its radio chip measures the phase offset of each antenna's received frame. By knowing the relative positions of the antennas, it determines the signal's angle of arrival. A central computation unit then determines the location of the mobile node through triangulation. In this system, $SC\mu M$ is the mobile node. BLE AoA only works if the mobile node emits frames that have a Constant Tone Extension (CTE), a unmodulated and unwhitened tone for 16-160 µs inserted into the frame. $SC\mu M$ can do this because its whitening is software-defined. Although $SC\mu M$'s internal oscillator has frequency error without calibration, the frequency error can be estimated based on the CTE reference period and won't impact the performance of angle estimation. Compared to lighthouse localization, AoA localization consumes less energy, and as it can work outdoors, this technique is suitable for wildlife tracking.

## 2 BLE Angle of Arrival

If the transmitter is sufficiently far away, the signal arriving at the receiver can be treated as a plane wave [2] (Fig. 1, upper right). The angle of signal can be computed using (1), where $\Delta_{\phi}$, $d$, $\lambda$ and $\theta$ represent the phase difference between two antennas, the distance between two antennas, the wave length of signal and the arrival angle of the signal, respectively.

$$ \theta = \arcsin\left(\frac{\Delta_{\phi}\lambda}{2\pi d}\right) \qquad (1) $$

The BLE AoA receiver uses an RF switch to lower costs and energy consumption. It connects to an RF switch chip that allows sampling from one antenna at a time, resulting in discarded signals from others. To address this, BLE introduced the CTE packet field, enabling recovery of the full signal from each antenna using fewer samples.

The CTE structure, shown in the upper left corner of Fig. 1, consists of a sequence of unwhitened 1 bits following a standard BLE packet, with a length ranging from 16us to 160us. After GFSK modulation, this produces a sinusoidal signal at a constant frequency. The CTE starts with a 4us guard period, followed by an 8us reference period, and then features alternating switch and sample slots, each lasting 1us in our experiment. During the reference period, I/Q

<footnote>*Corresponding Author.</footnote>
samples are generated every 1us, allowing the SoC to estimate the
carrier frequency offset (CFO) from the slope of the black solid line
from 4us to 12us. The unstable I/Q samples during switching slot
are discarded. The phase difference between antennas can then be
measured by compensating one I/Q sample to another, as illustrated
by the red dashed line and circle in Fig. 1.

<figure><img src="image_2.png" /><figcaption>Figure 1: The structure of CTE and The phase difference between two antennas in BLE AoA system</figcaption></figure>

3 $\mathcal{SC}_{\mu}$M AoA Localization Evaluation

**3.1 Experiment Setup**

To evaluate the AoA localization performance for the SCμM, a pro-
totype is built using the SCμM, nRF5340-DK board, and a CHW
antenna array. In system, the SCμM transmits a BLE packet with
CTE, while the nRF5340-DK receives the packet. During the recep-
tion of the CTE portion, the RF switch mechanism is activated to
generate I/Q samples from different antennas. All the I/Q samples
are sent to a PC via serial communication, where the angle of arrival
is calculated.

To reduce multipath interference, the experiment is conducted
in an anechoic chamber, as shown in Fig. 2. The distance between
the SCμM and the antenna array is set to 1m, with both the SCμM
and the antenna array positioned on two tripods, 1m above the
ground. Below the antenna array, a tripod head with a dial gauge
allows for rotating to different angles, simulating various angle of
arrival scenarios. The antenna array is rotated from -50° to 50° in
10° interval. At each position, 50 angle estimations are calculated,
with beamforming and grid search algorithm [3] to improve the
accuracy.

<figure><img src="image_3.png" /><figcaption>Figure 2: The SC<sub>μ</sub>M AoA experiment setup</figcaption></figure>

**3.2 Results**

Fig. 3 illustrates the angle estimation results. The boxplot displays the angle estimates at different positions, with the blue line representing the ground truth. The red dashed line and yellow dashed line indicate the ±10° and ±5° boundaries, respectively. At position ±50°, the estimated angle distribution shows significant errors compared to the true angle, indicating that the AoA localization for the SCμM has a limited effective range. Within the range of -40° to 40°, most estimated angle errors fall within the ±10° boundary. When calculating the estimated angle at a given position multiple times, the median value remains within the ±5° boundary, except at the positions of ±40°. Despite the limited effective range and some outliers, the main distribution and median provide valuable information for locating the SCμM.

<figure><img src="image_4.png" /><figcaption>Figure 3: SC<sub>μ</sub>M AoA estimation results in anechoic chamber</figcaption></figure>

4 Conclusion

In this paper, we proposed a new localization method for SCμM, based on the direction-finding features of BLE 5.1. Compare to lighthouse localization, our method is energy efficient and suitable for outdoor environment. By modifying the whitening step on SCμM, we successfully use SCμM to transmit a BLE packet with CTE. The CFO of SCμM in each packet is estimated based on CTE so that it is possible to use AoA for SCμM localization while the frequency offset of SCμM is unstable. An AoA-based localization system for the SCμM is presented and tested in an anechoic chamber. The results indicate that the receiver accurately determines the direction of the SCμM within ±40°, with most angle errors under 10° and median errors under 5°.

**References**

[1] ALVARADO-MARIN, S., HUIDOBRO-MARIN, C., BALBI, M., SAVIĆ, T., WATTEYNE, T., AND MAKSIMOVIC, F. Lighthouse localization of miniature wireless robots. *IEEE Robotics and Automation Letters* (2024).

[2] HUANG, C., LI, J., AND LIU, H. A closed-form phase difference estimation method for ble direction finding using the best linear unbiased estimator. In *2023 IEEE 23rd International Conference on Communication Technology (ICCT)* (2023), IEEE, pp. 1048–1052.

[3] KRIM, H., AND VIBERG, M. Two decades of array signal processing research: the parametric approach. *IEEE signal processing magazine* 13, 4 (1996), 67–94.

[4] WANG, C., CHANG, T., ALVARADO-MARIN, S., BURNETT, D., MAKSIMOVIC, F., WATTEYNE, T., AND PISTER, K. S. Simultaneous localization and clock calibration for crystal-free mote. In *2024 IEEE Workshop on Crystal-Free/-Less Radio and System-Based Research for IoT (CrystalFreeloT)* (2024), IEEE, pp. 36–37.

[5] WOOLLEY, M. Bluetooth core specification v5.1. *Bluetooth Special Interest Group* (2019).