What’s The Difference Between Measuring Location By UWB, Wi-Fi, and Bluetooth?

More devices being made these days need to track their location as they move around. Though best known in smartphone applications, particularly in terms of GPS and now the onset of indoor location, it’s making its way into a wide variety of other devices—from wireless toys to consumer drones, from keychains that want to be found to shopping carts tracked around a store. Buzzwords used to describe these devices include Internet of Things, wearable-computing smart devices, smart-home devices, connected appliances, and others. The common element among them all is the desire to track their position.

Designers making devices that need to track location face the challenge of selecting the right technology. Is Wi-Fi or Bluetooth best due to their track records in mobile-device deployments? Or do newer radio technologies hold an advantage? What tradeoffs are involved?

Location Tracking Using Bluetooth

Bluetooth-based location tracking has received lots of attention since Apple announced its iBeacon technology. If Apple is using Bluetooth for indoor location (more specifically, Bluetooth Low Energy, or BLE), wouldn’t that be the best approach?

Difficulty in measuring precisely when a Wi-Fi or Bluetooth radio signal arrives at its destination is due in part to the inherent structure of narrowband radio waves. As it crosses a given threshold, a narrowband radio wave with typical noise (Fig. 1) demonstrates how difficult it can be to measure precisely. As the signal goes up and down in a SIN-wave-like manner, selecting a threshold to measure becomes hard enough; the tiny perturbations from noise only makes this more difficult to measure consistently. Needless to say, narrowband systems aren’t the easiest to use when trying to achieve consistent direct measurement of signal arrival.

This ratchets up even further in the presence of signal reflections and multi-path effects, which are commonly found in everyday environments. The reflection causes the radio band pulse to reverse itself and partially cancel out the original signal (Fig. 2). This means that interference and multi-path effects will not only weaken the signal, but also damage its integrity. This will likely not damage the data payload, but it will hurt the ability to measure the start and stop of the signal (Fig. 3).

Location Tracking Using Ultra-Wideband

New standards for ultra-wideband (UWB) radio signals have been designed to address this problem. The new IEEE 802.15.4-2011 standard uses radio waves with very short impulse transmissions (Fig. 4). The short bursts of signals, with sharp rises and drops, makes the signals’ starts and stops inherently easier to measure. This means that the distance between two UWB devices can be measured precisely by measuring the time that it takes for a radio wave to pass between the two devices. It delivers much more precise distance measurement than signal-strength estimation.

More importantly, UWB signals maintain their integrity and structure even in the presence of noise and multi-path effects. As shown in Figure 5, the nature of noise doesn’t hurt the distinctiveness of the short burst of signal. Moreover, due to the shortness of the radio pulse, multi-path effects will typically not overlap with the true signal, thus not damaging the integrity and strength of the true signal.

This difference in radio structure means that UWB radio waves can inherently be measured much more precisely, with well-defined start and stop and with little damage by multi-path effects.

In practice, UWB signals are able to effectively measure distance between two devices with 5- to 10-cm accuracy, compared to roughly 5-m accuracy for Wi-Fi and Bluetooth. When implemented in a system of fixed beacons tracking tag locations, the locations can be calculated to within 10-cm accuracy.

Examining The Tradeoffs

The accuracy advantages of UWB are clear: UWB can measure distance and location to an accuracy of 5 to 10 cm, while Wi-Fi, Bluetooth, and other narrowband radio systems can only reach an accuracy of several meters. What are the other tradeoffs between the different systems?

Modern-day UWB systems provide a data rate of 6 to 8 MB/s, which falls between Wi-Fi and Bluetooth. Previous-generation UWB delivered much higher data rates, up to 100 MB/s, but power requirements and distance range made it impractical for use in mobile devices. Though today’s UWB may not be sufficient for streaming HD video, it’s enough for sensor data, security camera streams, instructions, and the like.

UWB consumes significantly less power than Wi-Fi, although Bluetooth 4.0 also uses extremely low power. That, of course, is a key factor for mobile devices and other small electronics in terms of battery life and practical usability.

Obviously, one tradeoff against UWB is that Wi-Fi- and Bluetooth-enabled devices are better able to interact with today’s smartphones and tablets. Nonetheless, some companies are implementing UWB, building devices that use both UWB and either Wi-Fi or Bluetooth to get the best of both worlds.

What’s Next

While smartphones and tablets garner most of the attention these days, electronics design is moving more and more to the next generation of smaller devices, which comprises wearable devices and the Internet of Things. Devices are steadily shrinking, from appliance-sized to pocket- or wristwatch-sized, and correspondingly, power and size requirements are getting increasingly important. At the same time, data-rate requirements are often less significant—smaller devices tend to send and receive sensor data and instructions, not the HD video that typically goes to bigger screens.

Increasingly, designers of wearable and Internet of Things devices also want to be able to precisely track location. Some devices are designed to be found by other devices; some move around (like robots) and need to know where they are so as not to bump into walls; some carry out various actions based on their location (such as turning on a radio when entering a room); some are designed to know how far they are from other devices; and some want to track their locations for other new and innovative reasons. Whatever the reason, UWB technology gives devices the ability to track their locations more precisely than Bluetooth or Wi-Fi, with lower power and size requirements.