Introduction to real-time location systems (RTLS) using ultra-wideband technology
RTLS stands for Real Time Location System. As you may guess, such a system aims to provide real-time information about the location of “an object” with respect to “a coordinate system”. This loose definition raises three questions:
– Provide information to whom?
– What is “an object”?
– What is “a coordinate system”?
We will address all these questions in this article and answer the more important question: “HOW is it done?”. Many radio technologies use some of the RTLS principles explained below.
However, we will focus specifically on UWB RTLS systems and specifically on indoor RTLS systems. Additionally, we’ll explain why a UWB Sniffer is a key element you should use while building such systems.
Understanding the basics of UWB positioning
As mentioned in the introduction, the goal of an RTLS system is to know where the object is in real time. The object may be a human in the wild, a parcel in a warehouse, a car on the road, or a smartphone or key forgotten somewhere in the apartment.
We can freely define what our “object” is and what our “coordinate system” is.
Differences between outdoor and indoor RTLS
Typically, RTLS are divided into outdoor RTLS and indoor RTLS. While the radio technologies used are different, the technique is mostly common in both kinds of systems.
It is worth noticing that the key differences between outdoor and indoor RTLS are precision, range, and the different properties of radio wave penetration.
Components of RTLS software: Tags and anchors
Examples of outdoor RTLS are well known. Nowadays, a GPS module is installed in every smartphone, in most smartwatches, cars, and other “smart” devices.
GPS allows us to localize “an object” on the Earth using satellites in space. If we delve deeper into understanding what “an object” is, we can conclude that we are localizing a GPS module in a phone or other device. So, in simple terms, our “object” is a GPS module.
In indoor RTLS, we call such an object a tag. The second half of the system is the satellites. Again, if we consider it a bit deeply, the fact that localization is done on the Earth is kind of a “coincidence”.
The system actually localizes a tag to satellites and only then maps the coordinate system of satellites to the globe.
In indoor RTLS, we call the system modules that play the satellite role in GPS anchors or similarly as in GPS satellites.
Another example is localization in a cellular network. The principle is the same. A tag is a mobile phone logged into the cellular network, and anchors are base transceiver stations (BTS) that reach a tag via radio communication.
These two parts (tag and anchors) are essential in RTLS. However, UWB localization can do more and is able to break this status quo. Read till the end to know how.
UWB indoor localization
The importance of mapping in indoor localization
As we mentioned above, the key parts of the system are the tag, anchors, and map. Technically, a map is not required if we expect an output in anchor coordinates.
However, from an application point of view, it’s useless. Simply put, the user wants to know that the tag is in the middle of the room; the user doesn’t care that it is 142 centimeters from the anchor.
Therefore, the third essential element is mapping, which answers the question of where the anchors are located with respect to the environment.
Key principles of UWB RTLS positioning
UWB RTLS primarily relies on the following principles: Two-Way Ranging (TWR), Downlink Time Difference of Arrival (DL-TDoA), Uplink Time Difference of Arrival (UL-TDoA), and radar (a.k.a. sensing).
All techniques utilize multilateration algorithms.
TWR-based UWB RTLS
TWR assumes that the anchor and tag are able to transmit and receive UWB signals. Depending on the exact configuration, communication may be initiated by the anchor or tag and may consist of two or more UWB messages.
The core idea of this approach is to sequentially measure the precise distance between the tag and each anchor. As we know the exact distances between each anchor and the tag, the minimum required number of anchors is three.
The multilateration algorithm may be executed by the tag or a supervisor system that gathers values from anchors.
We will not consider all the advantages, disadvantages, and limitations of TWR in the context of indoor navigation. I would only emphasize that the implementation of this is pretty simple compared to other methods.
However, it requires both sides to be able to transmit and receive UWB frames, which may not be very desirable from a power consumption point of view. Moreover, this solution is less scalable than others because of the number of required messages.
DL-TDoA-based UWB RTLS
DL-TDoA allows the tag to self-track without notifying the anchors about it. In this approach, anchors communicate with each other, and the tag only listens for these messages. This brings a few advantages.
First, the tag is less power-consuming as transmitting frames is not required.
Second, the anchor infrastructure doesn’t track the tag. In simple words, the tag knows where it is, but the anchors don’t.
Third, the number of tags doesn’t influence the anchors’ behavior, which means there is no limit to the number of tags using the same anchor infrastructure.
Based on the known map of anchors and the time difference of anchors frames arrivals, the tag is able to compute its own position on the map.
UL-TDoA-based UWB RTLS
UL-TDoA is a kind of reversed version of downlink time difference of arrival. In this case, the tag is not required to listen for any other messages. The tag emits its own message, typically called a _blink_.
Anchors know their relative positions to each other, and their clocks are synchronized. Based on the time difference of arrival, an “upper layer” can collect information from anchors and compute the tag’s position on a map.
An “upper layer” may be a cloud server, for example, or one of the anchors in the infrastructure with additional functionality.
Advantages and disadvantages of UL-TDoA for power-efficient tags
UL-TDoA allows further reducing the power consumption of a tag compared to other methods. However, this solution has scalability limits. The more tags using the same infrastructure, the more collisions happen in the medium.
Moreover, while DL-TDoA allows all computation inside one device without any additional message exchange, it requires an additional application layer before it can locate a tag. This may involve additional UWB message exchanges between anchors or utilizing other technologies (e.g., WiFi, BLE, etc.) for collecting measurement results.
However, if we’re talking about tracking assets or other industrial indoor navigation solutions, we need an “upper layer” anyway. So, maybe it’s not a big problem then.
Radar-based UWB RTLS
Capabilities and limitations of UWB Radar
It is possible to use a UWB device as a radar. So, theoretically, it is possible to utilize the radar capabilities of anchors. Unfortunately, UWB radar has the lowest precision of all the mentioned approaches and is poorly scalable in the context of industrial indoor navigation, but locating one or few people in the living is not a problem for UWB radar.
Radar allows tracking objects that are not UWB-capable. Basically, we eliminate half of the system, and now we are able to truly localize people and any other moving object.
And this is a game changer!
Well, after we took a look to approaches used by indoor UWB RTLS, one question still needs to be answered: Provide real-time localization information to whom? Well, it’s totally up to the solution. In DL-TDoA, only the tag knows about its localization.
Only the infrastructure knows where the tag is, while the tag doesn’t even know if some infrastructure exists around it.
In the Two-Way Ranging approach, depending on the configuration, both sides may know or calculate the tag’s position. Using radar, the trackable object may not even know about the existence of UWB radio technology.
How UWB Sniffer can speed up building UWB indoor localization systems
If you build an indoor RTLS solution or only part of it, a UWB Sniffer will be a very useful tool for you, and here are five reasons why:
1. Effective debugging. You may assume that your device is well configured and supposed to work as expected, but something is wrong, and it is not able to communicate with its counterpart. Using a UWB Sniffer, you can check if the radio parameters are set correctly. For example, if your device emits UWB frames using the correct channel, preamble code index, STS packet configuration, PRF mode, and others.
2. Discovering competitors’ implementations. Bringing a UWB Sniffer to an environment where indoor RTLS is used, you can discover what settings have been utilized by this system. It may involve discovering radio parameters as well as timing parameters and standards implemented.
3. Discovering custom implementations and solving compatibility issues. Let’s face the truth – often manufacturers use custom implementations and shortcuts within known standards, which may break compatibility with other devices.
If you want your device to be compatible with as many systems as possible, a UWB Sniffer allows you to discover such hacks and shortcuts in other manufacturers’ implementations.
4. Building an effective anchor network and discovering how it is done by others. You can examine your own anchor network to see if it utilizes the medium effectively. You can do the same trick with an existing competitor network to get an idea of what an “effective network” means.
5. Complex analysis of indoor UWB RTLS usage. With a UWB Sniffer, you can monitor an existing network from the inside. Leave the sniffer for a long run and have a comprehensive log of the network usage, including transmitted frames by anchors and tags.
To see more details about UWB Sniffer, check our page – UWB Sniffer.
