I am developing a solution with the nrf9160 that should use LTE and GPS at the same time. But unfortunately I am having some problems. Perhaps somebody from Nordic or the community already has solved some of the problems.
At the moment I am in Switzerland using LTE-M with Swisscom and the link works well and accepts PSM and eDRX.
The GPS also works better than expected in standalone-mode considering the antenna (I am using a similar design as the DK with the Ethertonics LTE antenna and a separate GPS antenna with a Skyworks frontend).
I am using the latest released Zephyr 2.1 and modem Firmware 1.1.0.
Encountered problems:
[1] There seems to be no GPS PPS signal on COEX1
According to the Product Specification v1.1 on page 364:
> When internal GPS is used, COEX1 delivers the GPS 1PPS (one pulse per second) time mark pulse.
I am unable to confirm that - I tried all possible GPS interval / power save modes with no success
[2] COEX10 should enable Frontend
According to the Product Specification v1.1 on page 364:
> When internal GPS is used, COEX0 can be used as active high control for the external LNA component.
This seems to work but leaves the LNA active also if the RF is switched off because then it seems that the output inverts.
I am using the AT\%XCOEX0=1,1,1574,1577 command to activate the output.
There is a remark in the AT command reference guide v1.1 on page 56:
> When RF is turned off, the given COEX0 state is inverted.
But on the other had if the modem enters power save mode it will switch off the output.
Therefore it seems that I need an AND between COEX0 and COEX2. Is this really the intended behavior?
[3] LTE with GPS active with PSM BUT ...
Our protocol used sends an UDP packet every 30-60 seconds and receives an UDP reply. Packet loss is handled up to 50% with no additional traffic and is always handled on the next communication interval. The server never will contact the mobile devices on its own.
It seems that after sending any data the modem stays for 70 seconds in an idle state where GPS reception is not possible and then it enters eDRX and/or PSM.
The RAU / TAU and eDRX values can be successfully set but do not change the 70 seconds where the modems basically wakes up shortly every 1.2 seconds.
For my application it would be great to be able to send the package and receive the reply and enter immediately into eDRX and/or PSM cycle in order to receive GPS.
I tried also to play with AT%XRAI=3 or AT%XRAI=4 with no change of behavior.
Can this be changed from my side? Does the network influence that? Is this a firmware issue? Do I need an external GPS for this use case?
To add to the list there are some inconsistencies and possible improvements:
- Bug: +CEREG reports roaming network and %XMONITOR reports correctly a home network (Swisscom SIM on Swisscom LTE-M network 22801)
- Bug/Improvement: NMEA GGA is missing Geoidal separation. Hopefully the reported altitude has a correction applied?!
- Improvement: I much prefer the binary mode over NMEA but please add geoid seperation, leap second and vertical velocity. I am quite sure that all this data is available within the GPS engine.
Any help is welcome
- Martin
