LTE: Passive Network Scan

[Thanson]

Greetings,

For my company's product, we have strict requirements around when we are permitted to begin transmitting LTE (CAT-M1), and we also have a hard limit on how long we can transmit for. For this reason, it would be advantageous to us if we could perform a scan for networks in a passive way so that once we were clear to begin non-passive operations we could connect to the network and begin sending right away.

Does the nRF9160 support any kind of LTE network scan that is totally passive? If so, any relevant resources would be appreciated.

Thank you,

Thanson

[Elfving]

Hello Thanson,

We will have to get back to you on this.

Regards,

Elfving

[Charlie]

Hi Thanson,

Does the nRF9160 support any kind of LTE network scan that is totally passive?

I am not sure if I understand you correctly. It sounds like you want to connect with LTE-M as soon as possible and shut down the LTE connection immediately after data transmission.

You can manually assign the network you want to connect using PLMN selection +COPS commands, but this means you must know which network you want to use and it is available in your device operation area.

Best regards,

Charlie

[Thanson]

Thank you for your quick reply! I will clarify our situation more.

Our device will be traveling, and so will frequently be in a new location where it does not yet know what networks are available. We have restrictions on when we are allowed to perform any kind of RF transmissions. Before we are allowed to begin RF transmissions, we would like to be able to scan for networks (so that we know which are available). Once it is permitted for us to transmit we could then immediately connect to whichever network(s) we found. However, I do not know if there is a network scan feature that works without any RF transmissions, that is, a passive way to scan for networks.

For example, my understanding is the %COPS and +COPS AT commands can be used to scan for networks. I suspect they might work without any RF transmission. Can you confirm whether this is the case?

Hopefully this clarifies the situation better.

Thank you,

Thanson

[Charlie]

Hi Thanson,

The current network scan(AT%COAP=?) command needs to run with CFUN=1, and CFUN=2(receive only) will report an error.

I need to check with our development team if it is possible. I will let you know once I get a reply.

Best regards,

Charlie

[Charlie]

Hi Thanson,

Here you can find very detailed advice from our modem firmware developer:

I assume the question concerns %COPS=? (or +COPS=?) command. Unfortunately, it is not currently possible to use the commands without activating modem first with +CFUN=1. Even if the manual network search was possible in +CFUN=2 state (RX only), this approach would increase the power consumption a lot comparing to the normal functionality where modem is just activated in automatic network selection mode with +CFUN=1. This is because %COPS=? triggers a full network scan to all supported bands whereas after activation modem is able to follow Nordic proprietary enhancements for finding a suitable network/cell faster hence consuming power much less. But maybe power consumption is not a problem in customers use case.

Another problematic thing is that even though it was possible to manually scan the networks before activating modem (or activating modem first to "RX only mode"), the scan would return just a list of available networks (PLMN IDs) to application. Modem does not store any information about the completed manual network search, but just returns the results to the application. Afterwards, using manual network selection would just tell modem which network to search for (because it was available earlier) but it does tell where to search it for. Therefore, in the worst case modem would still need to scan a lot of channels and bands before finding a suitable cell, which can take time. After finding one, Attach could still be rejected for some reason in which case a new manual network selection command to another network would be needed. So, I don't say it is impossible but application should be prepared for many kind of scenarios.

Possible workaround:

%CONEVAL would be the first command I would use. The command can be used in "RX only mode" i.e. +CFUN=2 mode and it returns evaluation of LTE connection if the connection was triggered immediately. It is just an evaluation, but it scans channels that have been stored to modem's channel history for the last registered network. If the evaluation succeeds, then the same network is still available and can be used i.e. no need to scan further. The evaluation should be a very fast scan.

If the connection evaluation did not return wanted results and customer still wants to continue with the manual network search approach, i.e. no concerns on power consumption nor possible rejects from network (i.e. possible rejects can be handled appropriately in application), they could try the following:

1. Before modem activation, command modem to manual network selection mode and use a network code that surely cannot be found, e.g. +COPS=1,2,"00000"
2. Activate modem: +CFUN=1. Modem starts scanning networks but won't find anything suitable due to manual network selection. So no TX used at all.
3. Trigger forced network search: %COPS=? . Note: Only the proprietary command will override the ongoing network selection.
4. Modem returns a list of all available networks after the scan has been completed.
5. Deactivate modem: +CFUN=0 or +CFUN=4 (does not trigger writing to NVM hence does not cause NVM wear).
6. Send the manual network selection command with a valid network code that was found in the previous search.  +COPS=1,2,"PLMN_ID"
7. When the time is right for data transmission, activate modem normally and initiate the transmission: +CFUN=1

I would still like to remind that this consumes a lot more power than just activating the modem in automatic network selection mode. And it cannot be guaranteed that it would always make the network selection faster. This is something that could be tried out and has not been tested.

The customer can also consider using %XBANDLOCK or %XCOUNTRYDATA in addition. The first one can be used for limiting the number of supported bands (i.e. how many bands area scanned) to make it faster to find the network. So if there are bands that are known to be "empty", those could be left out from the list of supported bands to make the scan faster (also in automatic network selection mode). The latter one is beneficial in devices that move a lot, because it makes it possible to configure bands and channels to be prioritized in a certain country (defined with MCC). Even if the device moves only inside one country, it is still useful to configure known bands and channels for this country.

Best regards,

Charlie

[Thanson]

Thank you for the very detailed and thorough answer. It is incredibly helpful.

We will have to consider this option and see if it is a good fit for our product.

I'll mark the given answer as verified.