Ciholas is often asked to define the number of DWUSBs needed for an application. The descriptions below should help to identify the appropriate number of DWUSBs to purchase.
Two Way Ranging (TWR)
Developers who wish to test ranging between devices using TWR need a minimum of two units. One unit can be powered by battery, allowing it to be moved around an environment for testing. The second unit should be attached to a Linux PC via the USB port. The CUWB server software can be configured to operate in TWR mode displaying a time-based graph of distance and signal strength between the devices. In addition, the system can compute the average and standard deviation values for the distance readings.
To test TWR between two mobile battery-powered devices, three DWUSB devices are needed. Two units can be powered via battery, while the third unit should be attached to a Linux PC to collect the data needed by the CUWB server. The CUWB server can then be configured to show range data between any pair of devices, including both of the battery-powered units.
Real Time Location Service (RTLS)
In order to locate a tag in 2D or 3D space in a field of anchors for an RTLS, it is necessary to have a minimum of six DWUSBs. This allows for four devices to be used as anchors, one device as a tag, and one device as a master. The master node should be attached to a Linux PC to manage the network of devices. The master serves to hand out 'roles' to the other devices, assign and control time slots to avoid collisions, and collects data for the CUWB server location calculations. The primary role of the master node is network management and synchronization. Data from the master unit is not used in the location algorithm to locate tags.
Four anchors is the minimum set necessary to locate a tag by the location algorithm. Systems that require higher accuracy should consider additional anchors. The use of more than four anchors increases overall accuracy of the location algorithm and allows it to handle occlusions. Occlusions, or loss of any one anchor's data, result in position errors that can be fairly large. To handle this issue it is generally recommended that applications use six to ten anchors.
A system of ten anchors, shown in the demonstration videos below, works well in the presence of data loss, or bad data, from an anchor.
Tracking More Tags
Tracking more than a single tag requires an additional DWUSB unit for each tag desired. The CUWB server supports up to 48 tags with each tag being tracked at 20Hz. This results in a total of 960 locates per second (LPS) from the CUWB server. The primary limitation in LPS is the RF air-time necessary for the anchors to backhaul data to the master node for location calculations.
In summary, for RTLS:
- 4-10 DWUSB units are needed as anchors
- 1 DWUSB for the master node
- 1 to 48 DWUSB units can be used as tags
The DWUSB firmware supports all roles (anchor, tag, and master) and can be configured through the CUWB server.