Our UWB project originated from the need to use precise location in a customer's project.
We have examined various location methods, including UWB, Bluetooth, Zigbee, Wifi, etc., and finally we chose to use UWB. Other location methods basically rely on signal strength to determine distance. In addition to the influence of distance on signal strength, there are many factors, so the location effect is not ideal.
UWB uses the flight time of radio waves in the air to calculate distance, which is of course more accurate.
At the beginning of the project, we need to determine the pattern of location. At the beginning, the only UWB chip that could be purchased in large quantities on the market was DecaWave's DW1000. There are two options for location mode: TOF and TDOA.
TOF refers to time of flight. Device A sends a data packet to device B, and the time it takes for this data packet to fly in the air. The time that radio waves travel through the air is about the same as the speed of light. Through the flight time, we can determine the distance between two devices. If there are 3 anchors and 1 tag, and we know the distance between the tag and the 3 anchors, we can calculate the coordinates of the tag.
TDOA refers to time difference of arrival. The tag sends a data packet, and three anchors receive the data packet. Because of the inequality between the tag and each anchor, the time when the data packet is received will be different (time difference). The system uses these time differences to calculate the coordinates of the tag. .
There are a large number of TOF routines in DecaWave's example code. In fact, the development kit sold by DecaWave includes a complete TOF location application. Because it is for demonstration, it is omitted in terms of functionality and scale.
After investigation, we finally decided to use the TODA model.
TDOA has many advantages, the biggest one being that there is less data packet interaction. The tag only needs to send out a data packet and it can be located. In TOF, at least 2 data packets are required for interaction between the tag and each anchor, and most designs may require 3 to 4 data packets for interaction. Once there are too many tags and there are many areas that need to be positioned (large number of anchors), many UWB data packets will be spread in the air, which will inevitably lead to communication conflicts and also affect the location rate and the number of tags that can be accommodated. In addition, tags are usually powered by batteries. If TOF is used, at least 6 data packets are exchanged for one location, which causes the tag to spend a lot of time in working state, which is a test of power consumption. The TDOA tag is in a dormant state most of the time. It wakes up regularly to send a location data packet and immediately goes to sleep again, which saves power.
In subsequent articles, we will introduce more details of TDOA.