Figure 4 Secure two-way ranging between UWB tag and anchor.
The concepts “anchor” and “tag” are important to understand distance and location measurement with UWB. 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 communicating with multiple anchors. Some devices can act as either an anchor or tag. For example, when two mobile phones use UWB to calculate the distance between them, they may switch roles during the process, alternating between tag and anchor.
TWR - This method calculates the distance between a tag and an anchor by determining the time it takes for the UWB RF signals to pass between them (ToF), then multiplying that time by the speed of light. A keyless car entry system is an application that uses TWR for secure and accurate distance determination (see Figure 4). As shown in the figure, the tag initiates TWR by sending a poll message with the known address of an anchor. The anchor records the time 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 (Tround) and the time for the anchor to process and reply to the initial poll message (Treply). The distance is calculated by multiplying the ToF by the speed of light. The tag can then pass the calculated distance to the anchor in a final message, if required.
With multiple anchors, TWR can 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 via UWB or other wireless technologies to location-based applications or gateways (see Figures 5 and 6). The disadvantage of using TWR for location measurement in this way is the tag does frequent communication, which increases its power consumption and limits scalability.
Figure 5 Two-way ranging with 2D/3D assets and listener.
Figure 6 Two-way ranging with 2D/3D assets and data tag backhaul.
TDoA - This method is extremely scalable for determining the location of tags within a venue. Because tags only transmit once during the process, they use very little power and have a very long battery life. Multiple anchors are deployed in fixed and known locations and are tightly time synchronized. When a mobile device sends a “beacon” or “blink” signal, each anchor that receives the signal “time stamps” its arrival based on the common synchronized time base. The timestamps from multiple anchors are then forwarded to a central location engine, which runs multilateration algorithms to determine the device’s location based on the differences in arrival times at each anchor (see Figure 7). The result is a 2D or 3D position for the mobile device.
Figure 7 Determining location with TDoA.
Figure 8 Reverse TDoA.
Figure 9 Using PDoA to calculate direction and distance.
RTDoA - It is also possible to implement a reverse TDoA system, which works a bit like GPS. The anchors transmit synchronized blinks with fixed or known offsets to avoid collisions, and the mobile devices use TDoA and multilateration algorithms to compute their respective locations (see Figure 8).
PDoA - This method enables two devices to calculate their relative positions without needing any other infrastructure by using a combination of distance and directional information. This is important for peer-to-peer applications or to reduce the infrastructure to be deployed. For PDoA, one of the devices must have at least two antennas (see Figure 9). When this device receives a signal from the other device, it measures the difference in the phase of the arriving signal at each antenna. Based on this difference, it calculates the angle from which the incoming signal arrived. The receiving device now knows both the direction and the distance of the transmitting device.
For simplicity, Figures 5 through 9 only show one tag; however, UWB applications can support many tags.
UWB FREQUENCIES
UWB operates in regulated unlicensed spectrum, so anyone can implement UWB communications without a telecommunications license if the system operates within the regulated frequency and power range. The Federal Communication Commission (FCC) defines the UWB frequency range from 3.1 to 10.6 GHz and UWB systems as those operating with 1) an absolute bandwidth larger than 500 MHz at a maximum power density at a central frequency (fc) above 2.5 GHz or 2) a fractional bandwidth greater than 0.2 with fc lower than 2.5 GHz. UWB spectrum is divided into channels; not all channels are used in all regions (see Table 1).
Figure 10 Spectrum used by common wireless technologies.
Although UWB’s large bandwidth is very useful, it means the frequencies used overlap with those of other communications technologies (see Figure 10). The FCC and other regulatory organizations therefore limit the power of UWB transmissions to avoid interference (see Table 2). The FCC limits the radiated power to -41.3 dBm from 3.1 to 10.6 GHz, with tighter restrictions in other frequency ranges.
THE FUTURE OF UWB
UWB is on the brink of mass adoption, now used in more than 40 market verticals for a range of applications, including:
- Secure keyless entry to cars
- Locating essential supplies in hospitals
- Improving operational efficiencies and safety in factories
- Controlling smart devices in homes, based on user’s location.
Integrating UWB into smartphones is a key step to the use of UWB in our daily lives. UWB-enabled smartphones will trigger the development of a broad ecosystem of new devices and applications that cannot be implemented with other technologies. UWB is a potentially revolutionary technology that will ultimately become ubiquitous— impossible to imagine today all the ways that it might be used in the future.
However, it typically takes time to realize the full potential of a new technology and have it adopted into mainstream use. It is therefore difficult to predict the future of UWB adoption. Yet history gives us some hints about its possible trajectory. For example, Wi-Fi started as a proprietary wireless communications solution for cash registers in the early 1990s. Apple’s endorsement of Wi-Fi in 1999 helped spur its rapid adoption, with development of a rich ecosystem of devices and a network effect that led to annual shipments of billions of units.
Interoperability is key to mass adoption, as is the development of full-featured software stacks and hardware solutions developers can use as application building blocks. Several industry consortia are working on interoperability, UWB use cases and regulation. Participants include a wide range of companies, from semiconductor suppliers to device manufacturers, carmakers, test equipment vendors and app developers. The FiRa Consortium™ is developing use cases across many industries, including hands-free access control, indoor location and navigation, as well as peer-to-peer applications. The consortium’s mission includes developing test specifications, certification programs and events to ensure interoperability between UWB products. The Car Connectivity Consortium (CCC) is working on smartphone-to-car connectivity solutions. CCC is developing the Digital Key, a new open standard that enables smart devices like smartphones and smartwatches to act as vehicle keys. The UWB Alliance is working with global regulation bodies and organizations to ensure a favorable regulatory and spectrum landscape to maximize UWB’s market growth.
CONCLUSION
UWB is uniquely capable of calculating location, distance and direction with unprecedented accuracy, indoors and outdoors, securely and in real-time. These capabilities will lead to a new wave of micro-location-based applications delivering new experiences and capabilities, no doubt many that weren’t previously possible.
