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15 Qorvo 2024 Figure 3: Secure two-way ranging between UWB tag and anchor. (Source: Qorvo) To understand distance and location measurement with UWB, it's essential to know the terms "anchor" and "tag." An anchor is generally a fixed UWB device with a known location. A tag generally refers to a mobile UWB device. An anchor and tag exchange information to establish the distance between them. The exact location of a tag can be determined by communication with multiple anchors. Some devices can act as an anchor, tag, or both. For example, when two mobile phones use UWB to calculate the distance between them, they may switch roles during the process, alternately acting as tag and anchor. Two-Way Ranging The TWR method calculates the distance between a tag and an anchor by determining the time it takes for UWB RF signals to pass back and forth between them (ToF) and then multiplying that time by the speed of light. A keyless car entry system is an example of an application that uses TWR for secure and accurate distance determination. As Figure 3 shows, the tag initiates TWR by sending a poll message with the known address of an anchor. The anchor records the time that it receives the poll message and sends a response. When the tag receives the response, it calculates the signal ToF based on the signal round-trip time (T round ) and the time it took for the anchor to process and reply to the initial poll message (T reply ). The distance is calculated by multiplying the ToF by the speed of light. If required, the tag can then pass the calculated distance to the anchor in a final message. With multiple anchors each requiring its own antenna, TWR can be used to determine the absolute position of mobile devices or other tags. By determining the distance to three or more anchors in known locations, the device can estimate its location with great accuracy. It can then communicate the distance to location- based applications or gateways using UWB or other wireless technologies, as shown in Figures 4 and 5. The disadvantage of using TWR for location measurement in this way is that the tag must carry out a lot of communication, which increases its power consumption and limits scalability. Figure 4: Two-way ranging with 2D/3D assets and listener. (Source: Qorvo)