Exploring Ultra-Wideband Technology for Micro-Location-Based Services
Ultra-wideband (UWB) is an IEEE 802.15.4a/z standard radio technology that can measure distance and location with unprecedented accuracy—within a few centimeters—by calculating the time it takes radio signals to travel between devices. It is uniquely suited to a new generation of micro-location-based systems that require secure real-time positioning information, indoors or outdoors. The standard was also designed with low-power and low-cost in mind and with the requirement to support large numbers of connected devices. UWB operates in regulated unlicensed spectrum and coexists with other wireless technologies using the same spectrum. UWB is on the brink of mass adoption; it has been incorporated into leading smartphones and many other devices and, ultimately, may become as ubiquitous as Wi-Fi and Bluetooth® Low Energy (BLE).
Today, it’s hard to imagine life without easily navigating anywhere in the world—both indoors and outdoors. GPS, popularized in the 1990s, was a huge advance in location technology and changed our lives. It allowed users to electronically locate the nearest gas station, track fitness, map out travel plans and find the way home. It has helped companies increase efficiency and build new business models. Without GPS, how would e-commerce companies efficiently navigate deliveries to your doorstep?
Ten years later, another breakthrough brought navigation inside, aptly called indoor navigation or positioning. Think Google Maps for malls, airports and other large buildings. When designing these first indoor location systems, engineers used the technologies widely available at the time, usually Wi-Fi and BLE. Though these technologies are excellent for data communications, they’re only capable of determining location within a few meters.
Now we’re seeing the rise of micro-location-based systems that have much greater precision. People and businesses want to be able to locate and find pretty much anything in real-time, whatever its size. Let’s say you’re at home and have misplaced your car keys or TV remote control, or you’re in a grocery store and can’t find your favorite brand of coffee, or you’re in a hospital urgently trying to find the infusion pump in an emergency. UWB is uniquely capable of supporting these micro-location applications because it was specifically designed for precise, secure, real-time measurement of location, distance and direction while concurrently supporting two-way communication. It is 50x faster than GPS, with updates up to 1000x per second, which is 3000x faster than a standard BLE beacon. It is also extremely reliable, with high immunity to interference, including reflected signals or multipath effects common indoors.
UWB technology is being incorporated into leading smartphones and many other devices and is poised for mass adoption worldwide, with a potential market of billions of units. It is already being used in more than 40 different industries, in consumer and business systems for healthcare, factory automation, automotive and others. But its greatest potential is in new generations of micro-location-based applications. Just as Wi-Fi and Bluetooth enable many applications that extend far beyond the original uses of those technologies, UWB will become ubiquitous and enable applications that haven’t yet been conceived.
HOW UWB WORKS
UWB has unique characteristics that enable it to determine distance and location more accurately than other technologies, even in the presence of noise and multipath interference. One of UWB’s key strengths is using time-of-flight (ToF) information to calculate distance and direction. Using timestamped signals, UWB calculates the time for signals to travel between devices, then multiplies that time by the signal speed (i.e., the speed of light) to obtain the distance between them.
In contrast, Wi-Fi and BLE rely primarily on the received signal strength indicator (RSSI) method. This measures the strength of received signals to determine the distance from a transmitter, since a radio signal’s strength varies according to the inverse square of the distance from the transmitter in free space. A key problem using the RSSI method is signal strength being affected by other factors, such as whether the signal is passing through walls or reflected by objects. A weak signal strength would lead the receiver to estimate the transmitting object is farther away - when, in fact, the signal has been attenuated only because it passed through a wall. Technologies that rely on RSSI can yield misleading distance and location measurements in indoor environments.
Figure 1 shows the advantage of using ToF indoors to calculate distance. In the diagram, a UWB signal transmitted by the blue device on the right reaches the gray device on the left via several different paths. One path reaches the gray device directly through an intervening wall; the other paths involve reflections and are longer. Because the direct path is the shortest, it reaches the gray device first and is used to calculate the ToF. The multipath signals can be ignored because the system relies on ToF to determine distance. This method works even if the direct signal is weaker than the reflected signals. Note that UWB only requires a single measurement to determine position accurately and reliably, while other RF technologies require multiple samples with filtering to determine location.
Because radio signals travel at the speed of light, extremely accurate measurement of ToF is necessary to determine distance within centimeters. The UWB signal is designed to help achieve this goal. Unlike other radio technologies, UWB does not encode information using amplitude or frequency modulation. Instead, UWB communicates information with short sequences of brief pulses using binary phase-shift keying and/or burst position modulation to encode the data. UWB signals also use much greater bandwidth than narrowband technologies, typically 500 MHz. As a result, each pulse is extremely short - only 2 ns - due to the inverse relationship between time and bandwidth. These pulses have much faster rise and fall times than narrowband signals, making it possible to precisely measure the time of arrival (ToA) of the signal. This also helps UWB signals maintain their integrity and structure in the presence of noise and multipath. As shown in Figure 2, because the UWB pulse is so short, it is separate from and unaffected by a reflected signal. Even under noisy conditions, the time is barely affected.
The ToF-based approach has also been tried with narrowband radio technologies; however, as shown in Figure 3, a narrowband signal is very sensitive to multipath. A reflected signal may combine destructively with the direct signal to cause errors at the receiver. Destructive interference shifts the time when the signal crosses the threshold, which is used to measure the signal’s ToA, resulting in poor accuracy. Noise also adds uncertainty to the ToA of the signal.
Knowing where people and assets are in real-time can also provide new methods of security. If physical presence cannot be faked, a person’s location can be used as a security credential, restricting access to areas and protecting physical assets, data and communications. Effectively, secure location information can be used to create virtual walls and boundaries for wireless networks. For example, because UWB uses ToF instead of RSSI to determine distance, it guards against relay attacks. In a relay attack, a malicious actor picks up a signal and amplifies it to trick the receiver into concluding a transmitting device is closer than it really is.
UWB technology can be implemented in different ways to address a wide range of needs. Depending on the implementation, UWB can be used to measure distance, 2D or 3D location and direction. The principal topologies are:
- Two-way ranging (TWR)
- Time difference of arrival (TDoA)
- Reverse TDoA
- Phase difference of arrival (PDoA).
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.
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.
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 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).
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.
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.