1

Bluetooth 5.1 introduced special direction finding signals, where a constant tone extension (CTE) is appended at the end of a certain packet. The CTE itself consists of only digital ones, so the whole CTE is transmitted on the same frequency and same wavelength, which of course boosts the accuracy of the localization.

I have 2 questions about this process and I cannot find answers in literature or Bluetooth specifications:

  1. Having two connected devices A and B, is it possible to do two-way direction finding in a time-division duplex manner.

Example: let's say we configure the CTE exchange to happen over multiple packets, can we do the following:

1 - A sends CTE to B (B estimates the location of A)

2 - B sends CTE to A (A estimates the location of B)

3 - A sends CTE to B (B estimates the location of A)

4 - B sends CTE to A (A estimates the location of B)

and so on?

  1. Does the devices perform frequency hopping during the CTE exchange?

Example: Instead of sending a single CTE on a single frequency (in step 1 and 3 from the previous question), is it possible that A sends multiple CTEs over multiple frequency (Same for device B in steps 2 and 4)?

Any suggestions/information is welcome.

Miroslav
  • 13
  • 3
  • Looking at [this](https://www.nordicsemi.com/Products/Low-power-short-range-wireless/Direction-finding) link from nordicsemi I understood direction findig to be used with either a low cost receiver or sender which would result in the usage of either angle of arrival (AoA) or angle of departure (AoD). I don't see why you couldn't use the same hardware on both ends and do the direction finding on both sides. But maybe I'm missing something :) anyway: great question! – Michael Kotzjan Jun 18 '21 at 04:26
  • 1
    Hi @M.Kotzjan, I believe so, too. Regarding my question 1: it is definitely possible, but I am not completely sure if devices have to change settings, which will cause extra delay, or they could easily switch between transmit and receive once CTE is enabled. In detail, step 1 from my questions goes as follows: once both devices have enabled direction finding: **A** sends **LL_CTE_REQ** packet as a request and **B** replies with **LL_CTE_RSP** which is the packet used for direction finding. So, can **B** send **LL_CTE_REQ** directly after that, or some settings must be changed? – Miroslav Jun 18 '21 at 09:19

1 Answers1

1

well technically df could be bidirectional. BUT it requires multiple antennas at all receivers.

the CTE enables phase shift detection at the receiver. it 'knows' what the waveform 'should' look like (constant signal value at a known frequency) so it can sample multiple antennas to detect the differences

but no general purpose devices (phones, computers, laptops) have antenna arrays. that is why AoA is the 'easiest' to implement. add a few specialized receivers, and voila!...

currently the antenna array I have for testing is just under 4 inches square.

I don't know the signal processing limits on miniaturizing such arrays while still having them be effective

Sam
  • 51
  • 1
  • 8
  • Thanks @Sam. Do you know how fast one can switch the role of devices: step 0 device A transmits a packet (so B estimates the location of A); step 1 device A receives a packet (so A estimates the location of B), then we repeat step 0-step 1 and so on. How fast we can go from 0 to 1? – Miroslav Apr 13 '22 at 09:52
  • sorry, didn't see that you asked a question. don't know . I am not aware of an AOA receiver that ALSO can be a sender. I added a transmitter at some receivers but still was unable to get clear AOA for any device – Sam Nov 08 '22 at 20:39