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.
Figure 1 UWB is resistant to multipath because it uses ToF to calculate distance.
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.
Figure 2 UWB pulses are not affected by reflections or noise.
Figure 3 Impact of reflections and noise on measuring ToA with narrowband signals.
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 TOPOLOGIES
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).
