这些天来，随着他们走动时，需要跟踪其位置。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?
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First, it’s important to distinguish iBeacons and their role, namely proximity detection, from the more general problem of measuring location and distance. Proximity sensing is when a device detects that it’s within range of another device, such as when a phone enters range of a beacon. Detecting proximity doesn’t necessitate measurement of distance; rather, it only requires detecting a steady signal. Some of these systems distinguish being “in range” of a beacon and being “close” to a beacon by distinguishing weak and strong signal strengths. However, this still isn’t the same as measuring precise distance or tracking location.
Some companies have taken BLE to the next step by using BLE signals to measure distance between a phone and a beacon. They measure the signal strength and convert that signal strength to a distance estimate. The problem with such an approach, of course, is that signal strength is a poor indicator of distance. If signal strength is low, does that mean the phone is far from the beacon or that there’s a huge pillar between the beacon and the phone?
解决最后一个问题的一种解决方案涉及一种称为“指纹识别”的方法。信标安装在固定位置，每隔几米进行测量，以记录每个位置的每个信标的信号强度。这些信号强度集保存在指纹数据库中。然后，如果设备要确定其位置，则可以将当前可测量的信号强度与指纹数据库进行比较，并且假定最接近的匹配项已接近当前位置。
指纹具有许多变体，它们使用具有广泛复杂性的算法。不过，重要的是要记住，这些系统可围绕蓝牙无法准确测量距离，这并不能真正解决距离测量问题。
使用Wi-Fi的位置跟踪
在苹果在接近检测中将BLE带到最前沿之前，室内定位的最常见无线电信号是Wi-Fi。如今，它们是在Android手机上运行的室内地点技术最常见的信号，通常与BLE结合使用。Wi-Fi的主要优势是它在大多数公共或私人场所的盛行。
The common approach to estimating distance or location with Wi-Fi signals is to measure signal strength, as discussed above for Bluetooth. Signal strength is the most readily available Wi-Fi data on smartphones and mobile devices. But estimating distance by signal strength suffers from the same challenges as described for Bluetooth. Some companies have developed alternative algorithms, attempting to measure distance more precisely using the time of flight (ToF) or time of arrival (ToA) of Wi-Fi signals, but this can’t be done in a straightforward manner using standard Wi-Fi hardware.
当wi - fi或难以精确测量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（图3）.
使用超宽带的位置跟踪
新的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（图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.
无线电结构上的这种差异意味着可以在定义明确的启动和停止中更精确地测量UWB无线电波，而多路径效应几乎没有损坏。
实际上，与Wi-Fi和蓝牙的5米精度相比，UWB信号能够有效地测量具有5到10厘米精度的两个设备之间的距离。当在固定信标跟踪标签位置的系统中实现时，可以将位置计算为10厘米的精度。
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?
现代UWB系统提供的数据速率为6至8 MB/s，介于Wi-Fi和蓝牙之间。前一代UWB提供了更高的数据速率，最多100 MB/s，但是功率要求和距离范围使在移动设备中使用不切实际。尽管今天的UWB可能不足以流式传输高清视频，但足以用于传感器数据，安全摄像机流，说明等。
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.
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以.pdf格式下载本文 This file type includes high resolution graphics and schematics when applicable. |
Precise location tracking is also critical for next-generation robot devices. A robot moving through a doorway needs to be sure that it’s going to make it through without bumping and damaging the sides of the door. Meter-level location accuracy simply isn’t good enough for moving through a doorway. The same parameters hold true for the budding market of aerial drones.
结论
For electronics designers that need very precise location measurement, accurate to 5 to10 cm with small size and power requirements, few choices exist on the market beyond modern UWB systems. UWB is currently at a disadvantage of not being handled by today’s smartphones and mobile devices, but this will likely change over the next several years. For Internet of Things devices, wearable devices, and robots, UWB offers the promise of much more location precision versus other radio systems.
Ciaran Connell, CEO ofDecawave，在通信和半导体行业拥有20多年的经验，并在战略，运营，业务发展和公司投资和收购方面获得了500强的经验。在DeCawave之前，他在法国Freescale领导战略和业务发展（EMEA），然后在法国和美国的摩托罗拉和Cornel工作之前。
