Wireless isn't the black art that many think it is, but it's still a complex process affected by a broad range of variables. Before you select a wireless technology to support a real-time location system (RTLS) in an industrial setting, make sure you know the technology's strengths and weaknesses. Compare the reliability and accuracy it promises to deliver against your application requirements. Only by knowing the application's technical stressors and the technology's capabilities, can you set yourself up for success.
Received Signal Strength-Based Asset Tracking
Numerous enabling techniques for RTLS are based on the strength of the signal received by the asset's radio. In this approach, you associate variations of received signal strength with changes in the distance between the gateway/access point (AP) and the asset's receiver. The further a receiver is from a transmitter, the weaker the received signal strength. In terms of communication systems, this means that the received signal strength indicator (RSSI) follows Equation 1:
|R||=||receiver-transmitter separation distance|
The RSSI technique is predicated on all other parameters remaining constant so that any change must be caused by a change in the distance between the receiver and transmitter. If the receiver knows the transmitter's output level, the distance R can be determined. This effect can be viewed as contour lines that reflect the base station's signal strength, as shown in Figure 9. It becomes an easy matter for the asset's wireless device to report its RSSI value and radio/tag ID to the AP it is associated with. That information is processed locally at the AP or by a location engine software application to determine the location of the tag.
Figure 9. Contour lines that show the correlation of received signal strength and separation distance between access points and asset receivers
By knowing the tag's RSSI value for a transmission from a single AP, you can determine the approximate distance between the tag and the AP. In radial coordinates, the distance is known but not the angle. To determine the X, Y, (or angular), and potentially Z coordinates of the tag, the tag must be in communications range of multiple APs.
RSSI-based techniques may use a number of different wireless technologies which may have different operational frequencies. This implies that the actual RSSI value—relative to the actual transmitted signal level—within a single facility will be different for different technologies. This may appear convoluted, but the general notion is depicted in Figure 10 for a typical RSSI-based RTLS system. For ease of discussion, Figure 10 and the accompanying descriptions are based on RSSI and an 802.11 (WiFi) infrastructure.
Figure 10. RSSI-based tag-positional information, which requires that the tag be within range of multiple APs
Notice that for improved location determination, a simple approach is to reduce the size of the APs' RF footprints (reduce transmission power) and deploy a higher density of APs.
A measurement based solely on intensity can be problematic because any change in the received signal strength can be interpreted as a change in separation distance between the AP and the tag. In reality, the variation can be caused by a number of factors, such as a decrease in the transmit power, decrease in the receiver sensitivity, or introduction of a new object that attenuates the signal. Other issues, however, can be associated with RSSI-based asset tracking. For example, RSSI is not measured in specific units. Each wireless device vendor uses an arbitrary set of numerical units. Therefore, it is incorrect to attempt to match an RSSI value with, say, a power unit (e.g., milliwatt), because serious problems can arise if devices from multiple vendors are expected to interoperate yet they don't rely on the same units of measurement.
While not strictly suited for RTLS, RSSI is of great interest for WiFi devices because much of the perceived performance of a wireless network is based on inferences made via the use of RSSI. For example, the higher the RSSI, the higher the transmit data rate (up to a maximum), the implication being that many WiFi client devices tend to monitor the RSSI on a channel (frequency). When this value drops below a threshold, the device assumes the channel is clear and transmits data. The association and disassociation of client roaming between multiple APs is almost entirely determined by RSSI. Therefore, while RSSI may not provide the optimum asset tracking capability, it is used for a wide range of client-host operations.
Another flavor of asset tracking technology is RuBee, or IEEE P1902. This variety of wireless is based on long-wave technology, which refers to the use of a 135 kHz carrier signal. To obtain objective information on the core details of the technology, refer to material from a number of sources including the World Intellectual Property Organization.
These systems use low frequencies to reduce cost (less costly than many RFID tags) and extend battery life (marketing claims list 10–15 years). As with active RFID, because the tags have batteries, you can add static RAM, sensors, and LED displays at very low cost. The only disadvantage of this approach is that baud rates will always be limited to <9600 baud and in most cases the tags operate at 1200 baud.
Addressing and data transmission of RFID and RuBee tags differ in a subtle yet significant manner. Manufacturers typically assign ID serial numbers to RFID tags during production. As a result, tags and tag networks depend on addressing schemes based on fixed, arbitrary numbers, often 128 bits long. This requires essential data to be stored on remote IT systems. For example, for a package identified by a unique number, you can access its shipping and content details (residing on a server) by using a key based on its unique ID.
In contrast, the same type of information can reside on the package's tag, either as human-readable or machine-readable data. This approach has proven to be expensive in current RFID tags because they must use EEPROM to store read/write data and the read/write cycles are slow and power hungry.
RuBee tags use low-cost static RAM. In a bit-by-bit comparison, batteries and static RAM are faster and less expensive than EEPROM. In addition, the ability to use RAM in tags delivers many unexpected advantages: tags can use addresses that are IPv4 computable and—in combination with a suitable router—IPv6 compatible. Each such tag becomes a Web server, with an IPv6 address and a subnet address. You can search all routers connected to the Internet to find a tag anywhere in the world, using the existing Internet infrastructure. It is this addressing and accessibility point that leads many in the RTLS world to accept RuBee's low data transfer rate.
Numerous statements have been made regarding how ultra-wide bandwidth (UWB)–based wireless systems can provide better RTLS performance. With no specific standards to follow, these systems tend to rely on proprietary communication-ranging techniques similar to those just described for WiFi-based RTLS. The use of spectrally wider bandwidth allows the transmission of more sophisticated and longer communications messages (leading to adaptive correlation-based receiver systems), as well as reduced multipath-induced performance variation (the attenuation and reflectivity values for materials change with frequency). The essence of UWB is based on temporally short pulses, which deliver higher spatial resolution in a classic radar sense (a 1 ns pulse is spatially about 0.33 m long). Therefore, shorter temporal pulses allow for higher resolution spatial measurements.
Two fundamental classes of short-pulse reception and identification arise in a UWB system: coherent and incoherent detection. In each case, a temporally short pulse, with its corresponding wide frequency distribution (hence the name), is used for information/ranging transmission.
Coherent detection (Figure 11) delivers the highest resolution for spatial information, typically providing submeter resolution. In this case, the infrastructure requirements call for gateways (base stations) that are connected to a backbone communications network. For accurate performance, the tag must be visible to a number of base stations for time-of-flight (radar) ranging. The base stations have to be fixed and wired.
Figure 11. A traditional coherent RTLS topology
By contrast, incoherent UWB detection does not examine the fine structure of the actual temporal pulse. Instead, it relies on envelope averaging, which is easier and less expensive to implement than the coherent method, but comes with the logical reduction in spatial resolution. The envelope-averaging, incoherent-pulse shapes are shown in Figure 12.
Figure 12. Pulse shapes and detection envelopes for incoherent UWB
The network topology for an incoherent UWB-RTLS implementation is shown in Figure 13. The relaxation of the receiver-detection effort inherent in the incoherent detection method reduces the spatial resolution determination but requires the infrastructure to have an accurate clock for system synchronization. Without having to rely on the phase information of the carrier/transmitted field, the incoherent design's system performance can be less than that of a coherent system, but it's also more robust in dealing with tag-transmitter path variations.
Figure 13. Network topology for an incoherent UWB-RTLS system
The primary benefit of an UWB-based system is frequency coexistence. The frequencies allocated to UWB tend to be in the ~3–10 GHz range, with local variations. The general emission mask (i.e., the power a legal UWB transmitter can emit as a function of frequency) is shown in Figure 14.
Figure 14. UWB channel assignments
In the IEEE 802.15.3 arena, UWB is used for high data rate transmission of information. As of January 2008, the IEEE group was considering establishing a channel-identification scheme for multiband orthogonal frequency division multiplexing. The situation is analogous to channel-hopping 802.15.4 radio operation.
By using UWB for RTLS, you combine a form of impulse radar with the accompanying benefit of the temporally short pulses. This provides a high degree of spatial resolution and distributes the pulse energy over such a wide range of frequencies that it mitigates many of the coexistence and multipath problems to which narrower wireless signals can be subjected.
That said, UWB faces significant challenges in terms of receiver complexity—specifically in coherent UWB. The following articles provide technical information that dives deeper into the world of UWB: Yang and Giannakis, "Ultra-Wideband Communications: An Idea Whose Time Has Come," IEEE Signal Processing magazine, Nov. 2004; Gezici et al., "Localization via Ultra-Wideband Radios," IEEE Signal Processing magazine, July 2005.
There are a number of methods available for determining the location of assets. Although only a few have been examined in this series, you should participate in the ISA100.21 People and Asset Tracking in Industrial Settings Work Group of ISA100. For more information on ISA100, in general, and ISA100.21 in particular, please visit the ISA Web site.
This article is extracted from a document submitted to ISA100 by the ISA100.21 Working Group
The figures in Parts 1–3 of this series are numbered consecutively.