We are developing firmware for the dwm1000 modules to be able to conduct TWR from a number of tags (<20) to a number of anchors (=<6).
We have encountered a strange issue that gives us spurious “bad” ranges when we turn more tags on.
Example of a good set of range messages (Tag side)
This issue happens randomly across all Anchors giving range messages of the region of -50 to + 50m. We observe that this happens for frequently the more tags we have on the network. The network seems relatively stable with 3 tags and 6 anchors.
What we did notice was that these errors coincide with PHY /Receive errors flagged up by the dwm1000. Looking at the datasheet we found this snippet that could account for our problem.
We have followed the soft reset instructions which helped a bit but the anomalous ranges still occur. Are the symptoms that we are seeing the ones related to the above paragraph? Or are we seeing something completely different?
Any comments + guidance is appreciated.
Some technical info about our setup.
MCU - STM32F10
Medium access control - ALOHA random access
Channel - 4
Preamble - 1024
Pulse Freq - 64Mhz
Data Rate - 850Kbs
Preamble code - 18
Speed - Tags ranging at 5Hz
Can tag/anchors transmissions overlap? If DWs transmit at the same time (overlapping preambles) then the unit receiving will receive both (if their clock offset/xtal is very small) and the resultant CIR will have both FPs, thus the RX timestamp will be form an earlier FP, and the received message data from either device, so you can have timestamp corresponding to one device and message from the other.
Please use TDMA (will remove these collisions) or add filtering based on e.g. FP index (not as good).
Tag/Anchor transmissions can overlap as we are not using the TDMA scheme. Our assumption was that should two transmit at the same time we would encounter a collision and that would be detected by the various CRC checking that the DWM1000 employs.
Is the CRC not that strong? Is there the possibility that they are getting past those checks and we are therefore seeing previous timestamps?
Could you elaborate on how we would filter by the FP Index?
No, CRC is correct as you have received the correct frame from one of the devices. However the CIR is from both preambles… as the clocks/carrier freq. of both overlapping devices are very close (e.g. couple of ppms) then both preambles will accumulate in the CIR, so the receiver is basically receiving both transmissions and reflections from both transmission etc., depending how these accumulate you may get two strong peaks, or just one, etc… the FP will be triggered on the 1st peak after noise region.
If you read the FP index, you may discover that in bad cases e.g. the FP is < 600, but in good cases it is always > e.g. 650 … so you could use this rough estimate to remove errors…
It will not always be the case but may work in majority of your tests… as I have said the best way to remove interference is to have only one unit transmitting at any one time.