The Foundation for Accurate Ranging
Oct 2024
Understanding Round-Trip CIR: The Foundation for Accurate Ranging
In wireless communications, precise ranging is crucial for applications like indoor navigation, asset tracking, and proximity-based services. One of the greatest challenges to achieving accurate distance measurements lies in dealing with the complexities of the communication channel itself. This is where Channel Impulse Response (CIR) becomes key.
What Is Channel Impulse Response?
The Channel Impulse Response (CIR) represents how a signal interacts with the environment as it travels between a transmitter and receiver. In real-world scenarios, signals don’t travel along a single, clear path. Instead, they bounce off walls, reflect off surfaces, and scatter through objects, creating multiple copies of the signal that arrive at the receiver at different times. This phenomenon, known as multipath propagation, can make it challenging to determine the correct distance.
For ranging applications, it is critical to isolate the direct path of the signal—the straight, line-of-sight transmission between the transmitter and receiver. The CIR captures both this direct path and additional reflected signals.
The Concept of Round-Trip CIR in 2WR
In two-way ranging (2WR), a signal travels from the transmitter to the receiver and then back again, resulting in what is called round-trip CIR. This method allows distance measurements based on the signal’s travel time over both directions.
Unlike one-way ranging (1WR), which would involve analyzing the CIR on a single leg of the journey (and which may be a part of future Bluetooth evolutions), 2WR CIR is already fully supported by the current Bluetooth 6.0 Channel Sounding specification. This is a significant advancement for accurate distance measurement, offering far greater precision than traditional RSSI-based methods, which rely only on signal strength rather than the detailed channel characteristics provided by CIR.
The challenge with 2WR lies in understanding the convoluted CIR—the combination of the signal’s forward and return trips. Since the signal interacts with the environment twice, the effects of multipath propagation are compounded. This makes it even more important to separate the direct path from reflected paths to achieve precise distance measurements.
However, due to the limited bandwidth of 80 MHz in BLE (Bluetooth Low Energy) radios, the CIR does not consist of ideal Dirac impulses, as it would with unlimited bandwidth. Instead, it contains sinc functions, which spread the signal energy over time. These sinc functions, combined with the bandwidth limitation, merge together, making it harder to separate individual paths clearly. This can have a noticeable impact on ranging precision.
A Real-World Demonstration: Cable Car Test Setup
To visualize how the CIR behaves in a real environment, Metirionic conducted a cable car test over a track spanning 11.5 meters. We measured the round-trip CIR at every 1 cm position along the track, creating a comprehensive view of how the environment influences the signal at each point. Using this data, we produced a stop-motion video that illustrates how the CIR evolves along the track.
In the video, the red line represents the reference distance, which ideally should align with the first peak in the CIR, indicating the direct path of the signal. However, due to the effects of limited bandwidth and multipath propagation, several phenomena can be observed. In some cases, the first peak appears after the red line, while in rare instances, there is a peak before the red reference line. These discrepancies highlight how the merged sinc functions, reflections, and environmental interactions affect the CIR and challenge the identification of the direct path.
The video reveals that, as the signal encounters obstacles and surfaces, the CIR changes significantly. Each centimeter of movement produces a slightly different CIR, demonstrating how dynamic the environment can be and why it’s crucial to address the combined forward and return paths when calculating the distance.
Why Understanding 2WR CIR Matters for Accurate Ranging
With the current Bluetooth 6.0 specification supporting 2WR CIR, the potential for precise distance measurements is significantly enhanced compared to older methods, such as RSSI-based approaches. However, to fully leverage the power of Channel Sounding, it is essential to understand and mitigate the impact of the convoluted CIR, which combines the signal’s interactions during both forward and return trips.
The Pathfinder algorithm, developed by Metirionic, is designed to tackle this challenge. By carefully analyzing the round-trip CIR, Pathfinder can separate the direct path from the multipath components, ensuring that distance measurements are highly accurate, even in environments with significant reflections and scattering.
As Bluetooth Channel Sounding continues to evolve, future updates may enable 1WR CIR, offering even more potential for fine-tuning distance calculations. For now, however, 2WR CIR represents a major leap forward, enabling precise distance measurements that surpass traditional methods.
Conclusion
The introduction of Bluetooth 6.0 Channel Sounding is a game-changer for accurate distance measurements. By focusing on understanding and mitigating the effects of 2WR, Metirionic’s Pathfinder algorithm delivers high precision even in challenging environments. While future evolutions of the Bluetooth specification may enable 1WR and offer even greater potential, today’s 2WR already provides a solid foundation for precise and reliable ranging. In the next article, we’ll explore how Phase-Based Ranging (PBR) compares to MARS Pathfinder based on round-trip CIR and how it outperforms traditional methods.