nRF9160 GPS data upload - frequency and current consumption

[Anders Larsson]

Hello Nordic support team!

I have some basic questions regarding the GPS receiver in nRF9160 as input to our evaluation of a possible tracker:

1. Is it possible to send a GPS position over LTE every 3rd second? What are the implications? What is the maximum frequency to upload GPS data?

2. What will the current consumption for nRF9160 be if we upload GPS data every 5th second?

BR Anders

[Martin Lesund]

Hi Anders,

Since the modem cannot run the GPS and LTE concurrently, it needs to switch between either of the "modes" or that you run the GPS when the LTE modem is in PSM mode or between the eDRX intervals when the LTE modem is in RRC IDLE mode. (see figure 1)
 

To do the latter it is important to know that the network supports these features and that the SIM card in the device is approved by the network to use these features. (This can easily be tested by e.g. requesting some PSM interval from the network by AT commands and see if the network actually grants it, e.g. iBasis SIMs are not granted these features in all networks.)

However, there is something called "RRC inactivity timer" which is the time the network requires the device to stay in RRC Connected mode before going into RRC IDLE or PSM mode. This is decided by the network you are on, some networks can have this timer set to 1minute and other 6 seconds, and this has a impact on power consumption and using the GPS in eDRX intervals in "RRC IDLE mode" or in "PSM mode".

The purpose of the "RRC inactivity timer" is to avoid having to set up a new RRC connection immediately if there is a pending ACK or more data to be received from the network. This is decided by the network, and cannot be requested by the User Equipment.

Note: For NB-IoT you can actually avoid that the device is stuck in this "RRC inactivity timer" by using "Release Assistance Indication" (RAI) feature. (described in figure 2)

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With regards to the first option mentioned in the beginning of the post:

We've added support for GPS priority in NCS now: https://github.com/nrfconnect/sdk-nrf/pull/2711

So it should now be possible to get fix from cold start also without eDRX/PSM.

After first fix, the GPS should be able to produce fixes each second without eDRX,PSM and without priority

The problem with sending every 3 seconds is mainly this RRC inactivity time that prevents the GPS from running.

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Therefore, I think the best option to be able to achieve this would be to use NB-IoT and it RAI functionality (RAI is not yet supported on LTE-M in the mfw):

(ref)

This functionality can be enabled with the AT command AT%XRAI. 
Then you could switch fast between RRC Connected (sending the data) and RRC IDLE in eDRX interval.

2. A bit difficult to estimate the current consumption since there are specific parameters that is decided by which network you are using.
I would advice doing field testing and measurements.

Best regards,
Martin L.

[Anders Larsson]

Thanks for your answer Martin!

For the question number 1 you answer: "Then you could switch fast between RRC Connected (sending the data) and RRC IDLE in eDRX interval."

Can you please estimate how often GPS data can be uploaded (including the LwM2M communication?! We need to have a number to estimate if the solution can support the planned use case.

BR Anders

[Martin Lesund]

Hi Anders,

If you chose to use the NB-IoT + RAI solution I would think you would be able to handle that in your time frame of 3-5 seconds. However, you need to test this to see that your network supports this since this is dependable of your network..

In your application are you planing to have this constant change in gathering and sending GPS data every 5 seconds constantly, or will the device "go to sleep" e.g. putting the modem in PSM mode for several hours etc. to save current.

Best regards,
Martin L.
 

[Anders Larsson]

Hello Martin!

When do Nordic Semiconductor plan to implement RAI on LTE-M?

It is really needed for our implementation so we are eager to see it implemented as soon as possible.

BR Anders

[Martin Lesund]

Hi Anders,

For new features and roadmap questions you need to check your RSM.

If you do not know who that is, please send me a direct message on Devzone and I will provide the information there.

Best regards,
Martin L.