An overview of how real time location systems (RTLS) operating in the ultra-wideband domain are becoming more common in many applications – ranging from automated student attendance verification to tool tracking for major aircraft manufacturers

by Zebra Technologies

A reason that UWB (ultra-wideband) is gaining in popularity is that it has many attributes that allow it to provide new capabilities that are not possible with traditional RTLS. The choice of UWB RTLS architecture will impact the ability to achieve the greatest ROI from these new capabilities.

Some system architectures are designed to only provide locating in specific areas of cov¬erage. This is a result of architecture choices that make the system too expensive for continuous coverage. Typically these architectures use discrete, multi sensor cells with inward focused antennas for each area of coverage, with each cell being a complete subsystem. Discrete cell architecture can affect not only the cost of coverage, but the cost of installa¬tion. Systems built with discrete cell architecture must rely on sophisticated radio channel schemes, which increase the cost of site design and increase the complexity of post installation tuning. Although there can be some benefits, these same results can be achieved by other means without adding the complexity of additional long range radio links.

Cost of ownership can also be affected by architectural fundamentals. The addition of two way radio communication using non-UWB channels draws significantly higher power from the tag and can thwart one of the prime advantages of UWB: Ultra long battery life at very fast update rates.

By choosing a system architecture that is designed to provide complete coverage over large areas and utilizes simple beacon UWB radio links, assets or people can be constantly tracked throughout an entire enterprise in a cost effective manner, over a long period of time, without interruption.

Blink rate

A system that is designed for continuous coverage can utilize receivers that do not view a single direction, but look in all directions at once. This allows a single location sensor, (radio receiver) to replace 4 cell type location sensors that only have a 90 degree field of view. The result is a greatly reduced number of sensors, and a corresponding reduction in costly cabling. Additionally, it is possible to have tags that blink at a rate of once per second, but will still blink continuously for seven years without the complexity of motion sensors and other sources of field failures. Since the update of rate of these tags is so high, a second radio to manipulate that update rate is not required. This simplifies installation, virtually eliminates post installation tweaking, and minimizes maintenance.

Ultra wideband RTLS can maximize its ROI when it is able to provide its primary function: accurate tracking of resources in real time throughout the factory, store, office building, or even sports arena. One of the key reasons for an RTLS system vs. an RFID system is to replace sporadic portal level information with a continuous and persistent flow of location information. The value of RTLS is realized by knowing the exact location of any resource, at any moment in time, and taking immediate action based on that knowledge. As an example, if a system is tracking tools, knowledge of the exact location of those tools when they are in the wrong place can be as important as the location data provided when matching them to a target process and work cell.

Continuous stream

UWB RTLS has unique attributes that make it specifically qualified to provide this continuous stream of information. These unique attributes are: precision Locating, fast update rates, long battery life and high capacity Because of its superior capability to discriminate direct signals from reflected ones (the ‘multipath’ problem inherent with narrowband RTLS), UWB RTLS can provide location data with levels of accuracy not possible with other technologies. Systems are available that can consistently provide accuracy of less than 1 m and even as precise as 15cm under certain conditions.

Resource tracking tags can provide multiple updates per second and still yield battery life measured in years. Additionally, the high data rate utilized in most standardized designs allow thousands of tags to be utilized in a rather compact area and still be able to be located at high update rates.

The key to receiving maximum benefit from these attributes is to use the unique data they can provide to make better quality decisions. It is an acknowledged fact, that better data yields better decisions. Therefore, as an essential requirement of any system that is going to be used to streamline a business process or to change an application paradigm, this location data must be available everywhere throughout a facility or area of interest. Additionally, in order for it to be financially viable, it must be essentially maintenance free.

In order to provide maximum value, and be cost effective, an RTLS architecture should not target discrete areas of coverage. The architecture must allow for shared radio coverage across multiple cells. Additionally, it should have the ability to provide redundancies so that failure of a component or a cable does not cause the entire system to fail. It must also be as simple as possible to make it robust and long lasting. Therefore, it should minimize the use of multiple radio technologies that add cost to the components, reduce battery life, and add complexity to the system installation.
There are many trade-offs that a system architect must consider in order to develop the optimum system for the broadest variety of applications. There is no doubt that particular design choices can be made for a specific narrow application, but for most applications, there are some clear advantages to certain design choices.

The first of these is to use systems designed with new generation, simple one-way beacon architecture. This saves tag cost, and greatly re¬duces downtime and maintenance costs by virtually eliminating battery changes. The complexity of having a second traditional radio can be elimi¬nated with very small losses in function by adding a simple low frequency Exciter function. An exciter is a low frequency electromagnetic field (EMF) device with very short, well-defined area of coverage, for which receivers can be made which consume extremely low power. Exciters do not require data connections and have been used in RTLS systems for over 15 years. The receiver side in the tag operates on micro-amps of current and does not have a significant impact on battery life. This technology can be used to create a transactional event in areas where there is only presence detection, and they can also be used to reprogram blink rates.

An architectural design that utilizes contiguous cells can create an uninterrupted field of coverage. The radio design of the receiver on the sensor can be optimized to provide the maximum margin and allow a choice of multiple antenna patterns. This allows the site designer/installer to choose the best combination of both directional and omni-directional antennas. If the sensor receiver is designed to use omni-directional antennas, and provide only slightly less range then directional antennas, then in multi-cell installations 1 sensor with an omni-directional antenna can replace 3 to 4 sensors for all interior reception points. Sensors with antenna options may be more expensive than sensors that only host directional antennas, but this is much more than compensated for by reducing the total number of sensors (and required cabling).

The choice of a continuous architecture utilizing multiple antenna types eliminates cellular handoff delays and saves significant cost if complete areas are covered. The entire facility can be covered and tags will not be out of sight between cells. Although utilizing single point omni-directional antennas to provide continuous coverage will not support Angle of Arrival AoA locating, this is an easy tradeoff to make given the low confidence level of AoA locates. In fact, the large savings in infrastructure density from the 1-to-4 sensor tradeoff can easily allow additional sensors in suspected trouble spots such as areas with mobile or multiple blockers. This will result in locates throughout the facility with minimum variation in accuracy. One of the attributes of the discrete cell architecture that is claimed superior is the ability to have a single sensor in a cell fail without affecting other cells. If the cable to the timing generator is damaged, then the entire cell is affected creating a complete blackout.

Design techniques

An installation using continuous cell architecture can be made much more robust than this by employing good site design techniques. The continuous cell architecture simplifies installation by using one CAT5 cable to supply power, timing, and data return to the sensors. However, there are multiple chains of these cables that are used to distribute the three functions. A customer should select a system that supports the maximum number of these chains in order to minimize the length of any chain. Therefore, by designing the installation to use the maximum number of chains, the minimum sensor count per chain; and cable runs designed to reach out to the widest dispersion area possible, a system using continuous coverage architecture can provide a completely soft failure. A system deployed in this manner will allow the entire system to continue to operate even if an entire chain of sensors fail.