The information covered here is based on Silicon Labs’ 2023 Tech Talk: “The Latest in HADM with Bluetooth LE.”
As our world grows more connected every day, location services are becoming increasingly vital. Whether it’s tracking goods in a warehouse, navigating within a complex building, or ensuring security in a restricted area, the ability to measure distances between devices accurately is critical.
Historically, the Received Signal Strength Indicator (RSSI) has been the go-to method for estimating these distances. However, RSSI has its drawbacks. Its measurements can vary wildly based on environmental factors, especially in indoor settings with multiple signal paths. This environmental sensitivity often leads to a lack of reliability and accuracy, sometimes as much as a 4-5 meter discrepancy.
Security is another concern with RSSI. Its levels can be manipulated, potentially leading to breaches in high-security scenarios. For instance, in a security-sensitive setup like an automated door system, an attacker can increase the RSSI level to make the system believe someone is closer than they actually are. Such security vulnerabilities render RSSI unsuitable for high-security applications.
Given RSSI’s limitations, the need for a superior, reliable distance measurement solution is clear. Such a solution should fulfill the following criteria:
Accuracy and reliability: Less affected by environmental variables, ensuring consistent distance measurements.
Simplicity: A low-cost, single-antenna design is essential. Other technologies like Angle of Arrival (AOA) and Ultra-Wideband (UWB) offer improved accuracy but at the cost of increased complexity.
Security: Robust against signal manipulation and fit for use in high-security applications.
Interoperability: Ideally, the solution should not be proprietary and should integrate seamlessly with various systems.
In response to these challenges, the Bluetooth Special Interest Group (SIG) has introduced a new feature Channel Sounding. This innovative feature potentially ticks all the boxes for a next-generation distance measurement solution, setting the stage for improved location services.
Bluetooth LE’s Channel Sounding Feature
Channel sounding is a technique used in wireless communications to understand and analyze the characteristics of a radio channel. It involves transmitting a known signal, or “tone,” through the channel. The receiving device then examines how this tone changes as it travels, providing insights into the channel’s properties and potential interference.
Bluetooth LE uses this technique to make significant improvement over previous distance measurement methods like RSSI, which struggled with accuracy and reliability in complex RF environments.
Bluetooth LE’s Channel Sounding feature provides two modes for enhanced distance measurement: Phase-based Ranging (PBR) and Round Trip Timing (RTT).
Phase-based Ranging (PBR)
In channel sounding, the two devices engaged in communication are referred to as the initiator and the reflector. The initiator transmits a specific signal, and the reflector receives this signal, performs some processing, and transmits it back.
In PBR mode, the two devices involved—the initiator and the reflector—exchange information about the phase and amplitude of signals using Amplitude Shift Keying (ASK). This exchange forms the basis for distance measurement.
We know that the phase of an RF signal is a function of the carrier frequency and the distance it has traveled. For instance, Bluetooth LE signals have a wavelength of about 12.5 cm. Hence, if the phase difference between two devices is π (pi), the distance between the devices would be half the wavelength, which is about 6.25 cm.
Phase-based ranging (PBR)
So, by comparing the phase of the transmitted and received signals, the relative phase difference provides a measure of the distance between the initiator and the reflector.
PBR offers significant improvements over older techniques like RSSI, and AOA. For one, the phase of a signal is much harder to manipulate, offering enhanced security.
Additionally, PBR provides a higher level of accuracy. Before making distance estimates, the In-phase and Quadrature (IQ) samples from the initiator and the reflector undergo signal analysis and filtering. This process enables distance measurement with sub-meter level accuracy.
Moreover, PBR does not require multiple antennas or an antenna array, contributing to the simplicity of the system and reducing design complexity.
Round Trip Timing (RTT) Mode
In RTT mode, packets are sent between the initiator and the reflector. The time taken for these packets to travel, known as the Time of Flight (TOF), is calculated. This TOF can be used to determine the distance between the devices using the speed formula.
Round Trip Timing (RTT)
The round-trip time formula is given as:
RTT = 2TOF = (Time of Arrival – Time of Departure) – (Time of Arrival – Time of Departure).
This equation basically describes the total time taken for a signal to travel from the initiator to the reflector and back again.
The accuracy of TOF and thus the calculated distance is dependent on the precision of the devices’ sampling clocks. Fractional timing techniques are often used to enhance this accuracy. These techniques involve the use of smaller time units than those provided by the clock’s ticks to measure the TOF, thus resulting in a more precise distance estimate.
A significant advantage of RTT mode is the inherent security it provides. Since time cannot be reversed or easily manipulated, it’s harder for potential attackers to interfere with the timing information, thereby offering a layer of security against signal manipulation.
However, RTT is not as accurate as PBR. The requirement for high-precision clocks and the need for fractional timing techniques make it a more complex solution than PBR.
Performance of Silicon Lab’s HADM Solution
Silicon Labs rigorously tested its High Accuracy Distance Measurement (HADM) solution within an office environment mimicking real-world usage conditions.
The HADM test environment was set up within an office corridor, complete with surrounding conference rooms, a kitchen, and other typical office features. A ceiling rail infrastructure was installed to allow automated, repeated testing at varying distances and configurations, both in Line of Sight (LOS) and Non-Line of Sight (NLOS) scenarios.
Embedded in wall and ceiling locations were stationary EFR32 devices replicating typical access point positions. A mobile EFR32 device on the rail system facilitated dynamic measurements, assessing the system’s performance with moving targets and providing static measurements up to 30 meters.
The performance trials primarily utilized the Phase-Based Ranging (PBR) mode. The results (illustrated in the provided graph) demonstrated impressive performance.
Silicon Lab’s HADM PBR distance estimation
With the x-axis representing the actual distance and the y-axis indicating the median absolute error, measurements were taken every meter up to a distance of 21 meters. In general, the observed error was below 1 meter—a highly promising result considering the demanding multipath environment.
Getting Started with Silicon Labs’ Solution
Silicon Labs currently offers an early access program to select customers, allowing them to begin experimenting with the HADM feature even while it’s still in the development phase.
For a smooth start, they provide a ranging kit, based on the EFR32MG4 SoC, alongside the 23Q2 GSDK software package, complete with essential ranging APIs and libraries for HADM applications. Also included is a pre-built HADM demo for both the initiator and reflector, offering a quick way to experiment with HADM in your own environment.
Furthermore, a visualization tool offers real-time graphical views of estimated distances, enhancing understanding and facilitating development. An option to construct an RSSI-based ranging system is available, useful for comparing its performance with PBR and RTT.
Aligning with the Spec
As Silicon Labs’ HADM solution remains under development, its results and capabilities are set to further improve. Additionally, the Bluetooth LE Channel Sounding feature, which is central to their offering, is also currently in draft. You can review the feature in its current form on the Bluetooth SIG website. Silicon Labs continues to monitor its progress, ensuring their product aligns seamlessly with the final Bluetooth specification once ratified.
All images used courtesy of Silicon Labs