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The next big leap in Bluetooth Low Energy (BLE) enhancements is on our doorstep. The ratification of the Channel Sounding (CS) enhancement, formally known as High Accuracy Distance Measurement (HADM), is expected to be released in the upcoming months and will pave the way for several new wireless use cases.
The Bluetooth Special Interest Group, or simply the “Bluetooth SIG,” is the standards organization that administers the licensing of Bluetooth Technologies and trademark, as well as the qualification process (a.k.a certification) of products to ensure cross compatibility and compliance to the various specifications. Historically, each new Bluetooth specification release consists of three to four enhancements but there is usually one enhancement that is most compelling and drives the release of the new specification. Such is the case with channel sounding in what most people in the industry have been referring to as Bluetooth 6.0(*) for over the past year and a half.
(*) The Bluetooth SIG selects the official versioning number scheme at the time that the new specification is released publicly.
So Why Channel Sounding?
The Bluetooth SIG, in conjunction with ABI Research, releases an annual Bluetooth Market Update every March. The 2024 report forecasts the total annual Bluetooth device shipments to increase from 5.4 billion in 2024 to 7.5 billion in 2028, a healthy 8% compounded annual growth rate (CAGR) over this five-year period. Of the various market segments defined in the report, “Location Services” device shipments are forecasted to grow by 2.68x annually, which equates to a whopping 22% CAGR over this same time. Here at Synopsys, we see the new channel sounding distance measurement feature as one of the key catalysts for this explosive growth.
Figure 1: Explosive growth in annual shipments of “Location Services” devices is expected
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Prior to channel sounding, the only coarse method of approximating the distance between two devices was via the Received Signal Strength Indicator (RSSI). This crude method of distance measurement was affected by multi-path signal reflections and absorption variations caused by the surrounding environment, resulting in highly inaccurate distance approximations. Due to wide fluctuation in approximate distance estimations the best-case results were that something was “kind of close” or “kind of far away.”
To date, a few BLE silicon vendors have offered propriety methods of distance measurement which only worked when the same vendor was on both ends of the link. This is why standardization is so important to ensure all devices, regardless of vendor, will work together. The new channel sounding enhancement will provide a significant improvement in radio-based distance measurement with upgraded security and accuracy.
So how does channel sounding work?
The intent of the paper is to give a basic understanding of how channel sounding works, the various modes defined in the specification, and considerations that will affect the distance measure accuracy. Detailed technical information can be found in various webinars posted on the Bluetooth SIG website.
The specification defines four modes of operation:
Mode 0 is basically a “set-up mode” for lack of better terminology when two devices exchange their capabilities used to measure the frequency offset between the initiator (the initial sending device) and the reflector (the device which then responds). During Mode 0 “handshakes” the two devices are calibrated with each other in terms of frequency and timing.
Mode 1 is the first mode that does actual distance measurement. It calculates the round-trip timing (RTT) of a packet between the initiator and the reflector. Packet transmission time or time-of-flight (ToF) is measured on both the initiator and reflector sides using Time-of-Arrival (ToA) and Time-of-Departure (ToD). Remember the old submarine war movies where a sonic “ping” is sent out and the time was measured on how quickly the reflected sound wave returned to estimate distance? RTT is basically the same idea, but done not at the speed of sound, but at the speed of light of radio waves.
Figure 2: Mode 1 does an actual distance measurement between the initiator and the reflector
Mode 2 is the second method for calculating distance and has proven to be a bit more accurate than Mode 1. Mode 2 is called Phase-Based Ranging (PBR) and uses an exchange of channel sounding tones to measure phase and amplitude of the communication channel between the initiator and the reflector in the form of in-phase and quadrature-phase (I and Q) measurements. To visualize this, a simple analogy is throwing a stone into a lake and watching the ripple (phase) spread out, bounce off an object and then return out of phase with the original ripples.
Figure 3: Mode 2 uses phase-based ranging to calculate distance
Mode 3 in an optional mode in the specification where both Mode 1 and Mode 2 are used in conjunction with each other for additional measurement accuracy.
What is the accuracy of channel sounding?
The answer to this question is not completely straight forward. It is like the question of what gas mileage can be expected with a car. The answer is “it depends.” The new channel sounding specification from the Bluetooth SIG defines the methodology of how round-trip timing (RTT) and phase-based ranging (PBR) data is derived and passed up through the protocol stack. Bluetooth qualification will ensure that devices from various vendors are compatible with the specification to exchange this data. What is not specified is what to do with the data to get your actual distance measurement and how accurate it must be.
In general, channel sounding is expected to achieve 5-20cm accuracy but there are three key elements that will ultimately determine the accuracy:
- Like the saying “trash-in, trash-out,” which means the quality of output is determined by the quality of the input, the accuracy of channel sounding devices starts with the quality of the Bluetooth radio sub-system. This is where Synopsys Bluetooth IP comes into play. You must start with a good radio transceiver with at least -98dBm to receive sensitive and a flat response across the entire 2.4GHz band. The Synopsys BLE solution includes both the RFPHY (Transceiver + Digital PHY) and the Bluetooth Link Layer Controller (LLC). These two components are highly dependent on each other and must be designed, integrated, and tested together to ensure a properly working, accurate sub-system. Sourcing these two critical components from different vendors can be problematic. A final consideration with the Synopsys solution is that the RFPHY has continuous auto calibration. This means any frequency drift caused by thermal or voltage supply fluctuations is compensated for real-time, thus allowing phase measurements discussed in mode 2 (PBR) to be more accurate.
- After good quality data from the radio sub-system is obtained, the next question that arises is how well do you deal with the data generated from the radio sub-system. The mathematical calculations that result in the actual distance measurement are done by algorithms. These are complex mathematical models that may vary from vendor to vendor in their accuracy. This is a highly specialized field and Synopsys is working with the three leading algorithm vendors in the industry to ensure the best, most accurate solution is provided.
- The final variable is the quality and number of antennas chosen by the end-product designer/manufacturer. There are no perfect antennas that have a consistent response in all three axes. The response of antenna can be seen in the 2D polar charts below. The results shown are in a 2D plane so both the azimuth and elevation axis must be observed to visualize 3D performance. A hypothetical perfect antenna is called isotropic where it radiates and receives RF signals equally in all directions (think of a basketball). In real life, antennas generally have a good response in one plane but not the other. To visualize this, think of a doughnut or bagel. Slicing it horizontally will yield a fairly symmetrical circle shape, but slicing it vertically will end up with a cross section with “lobes” radiating in different direction and voids in other areas
Figure 4: Antennas have a good response in one plane but not the other. In the 2D plane, both the elevation and the azimuth axes must be observed to visualize 3D performance
In the real world we seldom experience a “free space” environment where there are no surfaces or objects that either absorb or reflect the radio waves causing multi-path conditions where signals arrive at different times.
To compensate for this, two antennas can be used that are placed 90 degrees out of phase with each other (one pointing up and one pointing across). This is called polar diversity, and it helps minimize the multi-path affects, resulting in better distance measurement accuracy. This comes at the expense of complexity and additional BOM costs for the device manufacturer. Hopefully this gives you some appreciation for how antenna implementations can affect the quality of the RF signal transmitted and received, thus impacting distance estimations.
There are a multitude of applications for channel sounding including automotive digital keys, security access via smart door locks, indoor wayfinding, real-time location services, logistics and proximity detection to name a few. The distance measurement accuracy required for these various applications will help define the trade-off between higher unit cost with better accuracy vs lower cost devices at the expense of distance accuracy.
Synopsys offers production-proven Bluetooth Low Energy RFPHY and controller IP that support the low-power and security requirements for wireless connectivity. Using standards-compliant IP can reduce integration risks and help accelerate time to market for wireless design SoCs. The expansive Synopsys IP portfolio includes Bluetooth-qualified RFPHYs and link layer controllers with industry leading PPA and supports the latest BLE enhancements, including the current channel sounding enhancement. The IP also supports Zigbee, Thread, and Matter wireless protocols. As the wireless protocol continues to evolve, we can expect to see many more ways for BLE to enrich our lives.
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