Our partnership with Decawave has enabled us to gain expertise in several UWB-based methodologies used in the determination of location. Every methodology has its own set of dependencies and requirements and thus might not be suitable for every use-case. That’s why we have brought you this article from our own experts, which will help you decide the right methodology by weighing up all of the pros and cons.
Time Difference of Arrival (TDoA)
In this method, the tags send out data packets that are received by the anchors. The difference in the time of reception by the anchors is the basis of the distance calculation and, ultimately, the calculation to locate the object.
- Tag design is simple and efficient; the tag sends only a blink message.
- TDoA requires just one-way communication from the ‘Tags’ to the ‘Anchors’. This saves time and enabled the system to track more numbers of tags.
- One-way communication also implies lower power requirements when compared to two-way communication.
- Needs nanosecond clock synchronization; any miss in clock synchronization between two anchors is translated directly to a miss in the tag location. Typically, this requirement is addressed by wiring the anchors, which ensures a solid synchronization between them.
- Anchors need to be in listening mode all the time. RX is a power-hungry operation, and keeping the receivers open constantly will drain battery-powered devices quickly.
- The tag signal needs to be received by at least 3 anchors. Thus, at least three anchors at a time should be in the line of sight of the tag.
Angle of Arrival (AoA) Triangulation
In AoA, the delay of arrival at each element is measured directly and converted to an AoA measurement. Triangulation is the process of determining the location of a point by forming triangles to it from known points
- It only needs two measuring units for 2D positioning and three for 3D.
- It doesn’t need synchronization between the measuring units.
- Susceptible to Multipath interferences, and works with complete precision only in clear Line of Sight (LOS)
- Large and complex hardware
- Accuracy also decreases when the mobile target moves further from the measuring units.
Some insight on the advantages of using Ultra-wideband for Indoor Positioning.
Time of Flight (ToF)
ToF measures how long it takes for the radio signal to travel from the tag to the anchor and back to the tag, thus requiring two-way communication.
Time synchronization is not required, which reduces the hardware requirements.
The requirement of two-way communication makes it power-hungry.
Two-Way Ranging (TWR)
This methodology uses two delays that naturally occur in signal transmission to determine the range between two stations. The two delays are: signal propagation delay between two wireless devices & processing delay of acknowledgements within a wireless device.
- TWR eliminates any error due to imperfect synchronisation.
- TWR is an enhancement that eliminates the need for phase tracking.
- Frequency offsets of off-the-shelf crystal oscillators can result in time measurement errors and hence decrease ranging accuracy.
Symmetrical Double Sided – Two-Way Ranging (SDS-TWR)
This two-way ranging method is called symmetrical and double sided because there are only two stations involved and the measurements from one to the other are mirror images of each other.
- The effect of high inaccuracy generated from subtracting two large numbers measured with different clocks can be avoided by using Symmetric Double Sided-Two Way Ranging (SDS -TWR).
- Reduces the effect of crystal offset.
- Ranging time is twice as long as Simple TWR due to additional reply delay time.
Asymmetric Double Sided - Two-Way Ranging (ADS-TWR)
ADS-TWR has some difference from the above method in that the replies from the two stations are not synced, i.e., one station doesn’t wait for another station’s reply to send its own.
- Errors due to crystal offset are halved when compared to TWR.
- Ranging delay is also halved when compared to SDS-TWR; hence, results can be obtained more quickly.
Near-Field Electromagnetic Ranging (NFER)
NFER Technology relies on the following property of electro-magnetic theory. “Close to a small antenna the electric and magnetic components of a radio wave, are ninety degrees out of phase. Far from a transmitting antenna, these components converge and are in phase.”
- No signal modulation is required, so baseband signals with an arbitrarily small bandwidth may be used for ranging.
- Precise synchronization is not required between different receivers; in fact, a local range measurement can be made with just a single receiver.
- Electro-magnetic phase differences are preserved when a signal is down-converted to baseband, and high-range precision may be achieved with relatively low time precision.
- Low frequencies have better penetrating power and long wavelengths are less susceptible to multi-path.
- Higher frequency antennas are smaller in size, whereas lower frequency antennas need to be bigger. Large antennas are one practical disadvantage of this methodology.
RSS Based on Trilateration
The target point receives the RSS of three different specific Access Points (Typically Wi-Fi routers) whose positions are known, and the RSS is then converted into the distances between the target and the corresponding APs in accordance with the transmission loss model of wireless signals.
- Wireless signals are commonly affected by path loss, shadow fading, and so on in the transmission process.
- This includes factors such as attenuation due to the number of obstructions present, orientation differences between location receiver antennas and the client device antennas, reflections due to multipath, and so on.
- The signal transmission model is error-prone and may not be applicable to all environments; thus, it has limited implementation and is used as auxiliary means.
We hope this comparison helps you in identifying the right technology for your Indoor Positioning system. Want some expert suggestions? Don’t hesitate to write to us.