nRF9160 Bluetooth coexistence Insufficient Documentation

[optical]

Hello,

We are in process of FCC certification and we would like to use LTE and Bluetooth Coexistence to reduce number of measurement needed for the certification.

We are aware of SoftDevice support for the coexistence, we would like to use zephyr Opensource Bluetooth controller. We are willing to help with the Opensource implementation on the controller side.  However to do so we encountered an issue. There is a large gap in documentation of the COEX pin functionality. So there are several questions i would like you to clarify.

1. COEX0 functionality - Input to the LTE modem from external device.
   1. Does this effect only Transmission or also Receiving ?
   2. Is it currently implemented inside the Modem firmware ?
   3. Diagram shows on COEX0 State x State y State z, What exactly does that mean ? It is single GPIO which can only be 'one', 'zero' or 'high impedance' ?
2. COEX2 functionality - Shows the pin in '1' during whole operation of the LTE Modem.
   1. How does it cycle when the modem is NOT using PSM ? Doest it even cycle to allow Bluetooth to operate ?
   2. Is the COEX2 active during LTE receiving ? I found later that indeed the COEX can be setup using AT Command. This resolve this question
      
   3. Can the timing of the COEX2 pin some how combine with '4 – COEX2 pin toggling enabled, TX indicator. Pin ON when RF is transmitting.' ? What is the default timing for this option "as late as possible timing, approximately 50–100 μs anticipation."? 

Maybe you could give us an insight into operation of the coexistence in SoftDevice controller to help us.

[optical]

This is result of %CONEVAL when the LTE signal is poor and the modem is taking too much "Air" time for TX operations. You can evaluate the behavior of the COEX pin of nrf9160 down on the screenshots from the analyzer. COEX2=4 Mode.

%CONEVAL: 0,1,5,8,7,19,"000D951F","23003",306,6447,20,0,1,23,32,8,160
%CONEVAL: 0,1,5,8,7,19,"000D951F","23003",306,6447,20,0,1,23,8,8,160
%CONEVAL: 0,1,5,9,9,20,"000D951F","23003",306,6447,20,0,1,24,8,8,159
%CONEVAL: 0,1,5,8,7,18,"000D951F","23003",306,6447,20,0,1,24,8,8,160
%CONEVAL: 0,1,5,8,7,19,"000D951F","23003",306,6447,20,0,1,23,8,8,160
%CONEVAL: 0,1,5,9,9,19,"000D951F","23003",306,6447,20,0,1,24,8,8,159

These are when the modem has good signal.

%CONEVAL: 0,1,5,36,19,29,"000D951F","23003",306,6447,20,0,0,23,32,8,132
%CESQ: 36,1,18,2
%CONEVAL: 0,1,5,37,19,29,"000D951F","23003",306,6447,20,0,0,23,32,1,131
%CONEVAL: 0,1,7,37,21,29,"000D951F","23003",306,6447,20,0,0,23,1,1,131
%CONEVAL: 0,1,7,37,21,29,"000D951F","23003",306,6447,20,0,0,23,1,1,131
%CONEVAL: 0,1,7,37,19,29,"000D951F","23003",306,6447,20,0,0,23,1,1,131

What can be done to ensure that the BT will have enough "Air" time to work ?

[helsing]

Hi Tomas,

Thank you for the update. The signal in the bad scenario is barely any signal at all. The signal in the "good" scenario is quite poor as well. The poor signal conditions are causing CE level 1 to be used, which means worse conditions for coexistence.

I am also discussing this issue with another team and will get back to you over the weekend.

[ovrebekk]

Hi Tomas

Håkon is currently out of office, so I will help out in the mean time. 

optical said:

What can be done to ensure that the BT will have enough "Air" time to work ?

When conditions are this poor you have limited options other than considering whether you can delay the LTE transmission until conditions improve, or somehow reduce the amount of data transmitted to reduce the radio time used by the modem. 

As for the Bluetooth link you can adjust the connection parameters in order to make it more robust to these interruptions. 

If you use a small connection interval and a long connection supervising timeout then you should be able to keep the link alive even if the radio time is limited. 

Data throughput and latency will still be affected though, you will obviously not get the same performance for the Bluetooth link when the radio time is limited. 

Best regards
Torbjørn

[optical]

We are developing drone tracking device, which needs to report its position.
It simply cannot stop reporting the position whenever the modem decides that it whats whole air time.

Is there some configuration of the modem to reduce the amount of data transmitted or retries ? 

[ovrebekk]

Hi 

Unfortunately not, this is decided by the network, not the device. 

Best regards
Torbjørn