How to design a good UWB product
UWB technology is now advanced enough to enable applications which were not possible earlier, such as auto-follow camera, accurate finding solutions for logistics, and highest security automatic doors, etc.
All of this is based on the special capability of new UWB radio chips which can accurately measure the time of flight ToF of the impulse radio signal, allowing accurate distance measurement and additionally can detect the direct portion of the signal, and separate it from the reflected portion of the signal, which also allows accurate angle of arrival detection, AoA.
Both are fundamental features which the other radio technologies do not offer, thus UWB is creating a new market of new products and services.
As we now take steps to new markets with an innovative technology, it is important to pay special attention to issues which are different compared to more conventional radio designs. In this first article we focus on UWB system design more from a radio performance point of view.
Design UWB as a system with attention on four issues
Most of the new UWB systems are designed for applications such as indoor positioning, access management, logistics-finding, industrial safety solutions, and most likely the systems are optimized for one or more of the following UWB specific features:
- accurate distance measurement
- directional awareness
- high bit rate with lowest power
Previous features and application environments need the same attention as any other radio design, such as coverage range, power budget, wearability, UI and connectivity, but UWB design needs particular attention to the following four issues. Even those are only portions of the system, it is best to review them before looking at the entire system as the architecture, performance and feasibility relates to those.
1 - Flatness of radiation pattern
This is a particularly important aspect to guarantee a good coverage range in all desired directions specified by application. International regulatory limit of maximum transmitted power of a UWB signal is reasonably low and as the direct portion the UWB signal plays the main role for accuracy, you need to pay special attention to avoid any high or low power directions of radiation pattern. The target to have a flat radiation pattern is our most important lesson from a vast number of implemented UWB designs, not forgetting the total radiation efficiency. Another aspect related to radiation pattern is group delay variation, which is in direct relation to distance measurement accuracy. Usually, artefacts in radiation pattern flatness and high group delay variations are in the same directions of each antenna system design. Therefore, it is important to use an experienced UWB radio designer or partner to optimize performance of a UWB product.
2 - Flatness of spectrum over the pass band
As the technology is ultra-wide band the TX and RX band is wide, usually 500MHz, it means that the TX and RX pass band need to be wide enough. But additionally, because of tight regulation limits, it needs to be flat over the pass band to avoid spikes up or down to guarantee a good coverage range. This feature is dictated mainly by the UWB chip itself, but again, special attention is needed for professional radio antenna system design, including PWB, radio FE components, and mechanics. One additional aspect is to use high quality reference oscillators to avoid harmonic spikes on the pass band.
3 - Accuracy and stability of reference oscillators
As mentioned above, the reference oscillators used, whether it be crystal TCXO, VCXO or else, have an impact on pass band flatness, but also on the accuracy of distance measurement, especially if time synchronized TDoA base station nodes are used for positioning.
4 - Receiver sensitivity
All three previous design aspects are related to one of the most important design targets: the coverage range. The fourth way to improve the range is by improving receiver sensitivity. Again, the most crucial factor is the choice of the UWB chip, but in many cases, it can be improved by optimal front-end design, selection of filters, passives, and by using a low noise amplifier design in RX path of front end. In practice it is possible to get around +4dB improvement by optimal LNA design.
UWB system architecture
We cannot cover all known application alternatives in this article, but we can mention some evident ones.
Positioning solutions with fixed base stations
If you design a positioning system with UWB, you can use a system topology with fixed UWB base stations with fixed known positions. The position of the tag is calculated from the distance to the base stations by triangulation calculation. The advantage of this method compared to, for example, Bluetooth based system, are better multi-path signal rejection, easier installation by possible self-positioning methods, fewer and cheaper base stations.
Another widely used positioning topology is based on TDoA, the time difference of arrival TDoA method, where base stations are time synchronized by a wired or a wireless method. An advantage of this is a simple tag solution, which can work as a pure TX-device without RX and with reduced power consumption, and minimal time resources to allow a vast number of parallel active tags. This method includes a disadvantage of a somewhat lower positioning accuracy.
As UWB offers both the distance and direction as point-to-point measure, it also allows point-to-point positioning capability. This allows solutions such as positioning systems based on a single ceiling locator or an auto-follow camera with directional and focus controls, which are not possible with any other single technology. The directional awareness can be based on the phase difference of arrival PDoA or direction of arrival method DoA, developed by Sharpeye Systems Oy. The choice of TDoA or DoA depends on application needs.
Design methodologies and other remarks
Key questions in the start phase of a new product are related to possible performance, structure, architecture, and the cost of the outcome. Nowadays, as there is already a lot of experience of real implementations, there are many parties who can do a feasibility study for your specific application to find out what is possible and how.
Regulations and certification
A part of feasibility study is to look for an application, whether it be for indoor and/or outdoor use, the target market and countries in relation to regulations and certifications needed. In the beginning of the development phase, it is easier to guarantee success by using suitable UWB radio parameters, frequency bands and time division protocol of RX and TX.
Antenna system design and simulations
The easiest and fastest way to reach optimal performance of the UWB system is by an antenna system and user environment simulations in an early phase of the project. It is not only the choise of the right antenna topologies, but also to take radio ground paths, eq. entire other electronic, mechanics and user presence into account.
UWB is now an advanced and proven technology for enhancing old products and creating new products for new markets, and new chips and chip sets are still coming for higher performance and higher integration, with lower cost. Let’s make it happen! We are happy to discuss further, and please be free to comment and contact us.
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