Sync-free TDoA for UWB

We’ve completed designing, implementing and testing UWB A-TDoA (Asynchronous TDoA) algorithm, which enables low-power and scalable UWB RTLS without advanced clock synchronizations.

 

Test Case #1

Test Case #2

 

The above pictures demonstrate the location accuracy, which is 7-8 centimeters for test case #1 and 15-20 centimeters for test case #2. Intersections of hyperbolas show the calculated positions and markers show the actual positions of UWB tags.

 

Before jumping into the details of this feature, which will become available in Release R12.1 soon, let’s review the basic concepts of UWB RTLS and its popular algorithms.

 

Traditional UWB RTLS architectures assume one of the following algorithms:

 

TWR: Two-way Ranging

TDoA: Time Difference of Arrival

 

Each algorithm has its own disadvantages, which result in some extra complexities and compromises during the RTLS installations and operations:

 

TWR 


  • Battery life for UWB tags is measured in hours or days for each recharging cycle.

  • Scalability (tag density) is very limited.

 TDoA

  • Advanced clock synchronizations are required by default – for all RTLS anchors.

  • Extra calibrations are needed for any changes to UWB blink rates or cabling.

Battery life is the main concern for UWB TWR systems, because the task of recharging / replacing batteries might become a major issue from both operational and financial perspectives. 

At the same time, reducing UWB transmissions and data rates for battery saving purposes might not be accepted as an optimal solution. For example: our UWB TWR devices have the so-called “adaptive blink rates” based on accelerometers and motion recognition. Still, some projects require 24×7 location monitoring for continuously moving assets, so the adaptive blink rates will not help to resolve the power consumption issue.

TDoA-based systems normally require advanced clock synchronizations with sub-nanosecond accuracy. By default, clock synchronization signals are delivered via Ethernet cables connected to RTLS anchors and the central clock source. Sometimes the wireless synchronization methods are used, but both of them require all RTLS anchors to be accurately synchronized.

Introducing changes to the TDoA RTLS environment, such as new cabling or additional UWB tags, might result in the re-calibration process for RTLS coefficients and clock synchronization settings.

In the end, any system dependent upon sub-nanosecond synchronization is prone to errors and instabilities over time. That’s why a safer approach might be about removing such clock synchronizations completely.

During the last months we’ve been experimenting with several algorithms with / without clock synchronizations for UWB TDoA.

The resulting algorithm has the following advantages: 

  • Just one message between UWB tag and anchors – instead of 9 messages for UWB TWR ranging. Result: much better battery life and scalability (tag density) than for the case of TWR.

  • Minimum 3 RTLS anchors – some other TDOA / TWR systems require 4 or even 6 RTLS anchors as a minimum. Result: less RTLS anchors for separate rooms.

  • No need for any clock synchronizations – wireless or cable-based. Result: less complexities for both installations and operations.

The code for the resulting algorithm has been integrated into the firmware of UWB tags and anchors, as well as PowerGate (RTLS server) and CVO Portal. It is available for pilot projects and RTLS demo kits, which you can purchase starting from today.

    

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