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Some details about localization systems (RTLS), Ultra-wideband (UWB) and it's use for tracking positions.
RTLS are used to continuously track the location of objects or people in real time, usually within a building but possibly outdoors. Wireless RTLS tags are attached to objects or worn by people, and in most RTLS, fixed reference points (called anchors) receive wireless signals from tags to determine their location using a positioning algorithm/technique.
Different technologies can be used to implement a RTLS. Ultra-wideband (UWB) localization can provide more accurate ranging results than other RF technologies, but (for now?) can be more costly. Check out the chart below for a comparison of some techniques:
Ultra-wideband has, as detailed in the IEEE 802.15.4 standard, a number of frequency channels of at least 500MHz wide around the 3-6GHz range. Signalling over a wide frequency range, as opposed to narrowband RF technologies such as WiFi and Bluetooth which signal at a certain frequency (2.4GHz), means using a narrow pulse in time-domain.
Because of it’s wide frequency range (and inversely related, it’s narrow pulse width), UWB has unique characteristics and capabilities when compared to other technologies:
The position of an object can be determined in many different ways. It can be based on the time difference of arrival of the signals (TDOA) or by calculating the distance between the tags and the anchors via a method called Two Way Ranging (TWR). Also, measuring the Received Signal Strength (RSS) gives you an idea of the distance between the sender and receiver. When the angle of the propagation of the signal is measured, Angle of Arrival (AOA) could be applied. For each of the methods, different implementations and variations exist. All algorithms have their pros and cons, which we will discuss below.
The most relevant algorithms for us are TWR and TDOA. TWR is more accurate than TDOA, but it is more CPU hungry and needs more signals in the air. By default, the engine runs on the PC and the tag itself is not aware of its position (TDOA1 and TWR1). For both schemes, there is however an alternative in which the positions are calculated on the tag itself (TDOA2 and TWR2).
Two Way Ranging is a Time of Flight (TOF) method. With this method, positions will be calculated using trilateration. The time of the propagation of the signal is measured between the transmitter and the receiver. A signal is send from a tag to an anchor and back (hence the ‘two way’). It needs to be sent back because the clocks of the anchors and tags are not synchronized. This means that the timestamps taken at the tag are on a different timebase than those taken on the anchor.
With the 4 timestamps (2x transmission and 2x reception) we can accurately calculate how long the signal has traveled back and forth between the nodes. Dividing that time by two yields the TOF and thus the distance.
The airtime of the nodes is higher than for TDOA, so less positions can be calculated.
Time differences yield hyperbolas. The position of the tag is calculated by calculating the intersection of the hyperbolas. This technique is also called multilateration. Two anchors yield one TD (as opposed to only one anchor for one distance). This means a minimum of 3 anchors are necessary to calculate one 2D position and 4 anchors for a 3D position. The clocks of the nodes need to be synchronized. The better the synchronization, the higher the accuracy of the positions. Tags don’t need to be active very often so this method is power consumption friendly. We differentiate between 2 schemes (TDOA1 and TDOA2) based on where the time differences are measured.
Angle of Arrival is a method in which the direction of the propagation of the RF-signal is calculated on the receiving node, for example based on the phase difference measured on 2 different antennas. A 2D position can be derived from 2 angles with some simple triangulation.