Speed up first connection to cell?

[Peter Horauer]

I am developing on the nrf9151 with SDK v3.4.0.

The device I am developing will be moving trough all of europe . My goal is to speed up the `AT+CFUN=1` conenction part of the modem.
I found the `AT%XCOUNTRYDATA` for specifying a certain Band Technology and other parameters for up to 70 countries.
Can 70 countries be set with multiple sets of data for LTE-M and NB-IoT. Or are 70 entries ment with this in the AT command documentation?

This works very well as I am assuming the modem takes a shortcut and does not do a full band scan before connecting.
Is there some documentation on how the modem connection process works? Or how many cells are cached?

Is there some documentation for which Bands are used in the european countries with LTE-M and NB-IoT? And which technology is best supported?
I am aware of the GSMA map and this wikipedia list, but could not find other decent sources.

Currently I have only tested `AT%XCOUNTRYDATA` by first doing a slow connection with full band sweep and then taking these parameters and
setting them in `AT%XCOUNTRYDATA`. After this even with doing a `AT%XFACTORYRESET` the connection can be speed up significantly.

Are there other ways to speed up to connection process?

When `AT%XSYSTEMMODE=1,1,1,3` is used LTE-M is prefered but can be overriden by network selection. Is it recommended to overwrite the network preference or use the network preference in the field to reduce battery usage in mainly the connection process (I am using PSM anyways so some higher power consumption while sending does not matter as much as a longer power on would consume)?

[stak]

Hi Peter,

I’ve gone through your questions and here's what I've found:

Regarding AT%XCOUNTRYDATA, the limit is 70 stored entries, not 70 countries with both technologies in one record. Because Band Technology is set per entry, LTE-M and NB-IoT for the same country each need their own slot. So both technologies everywhere only fit about 35 countries; LTE-M-only can cover up to 70. You can confirm this with AT%XCOUNTRYDATA? after adding two entries for the same MCC with different Band Technology values.

For LTE-M and NB-IoT in Europe, Nordic’s general guidance is that LTE-M is preferable when the device has good to reasonable radio conditions and requires mobility. LTE-M supports mobility through automatic cell handover, whereas NB-IoT is generally better suited to stationary devices, particularly where deep indoor or underground coverage and long range are important. Since your device will be moving throughout Europe, LTE-M would therefore most likely be the preferred technology.
Unfortunately, I’m not currently aware of an authoritative country-by-country source listing the LTE-M and NB-IoT bands used throughout Europe beyond the GSMA coverage map and the other (web/Wikipedia) list you have already found.

There are a few additional mechanisms that may help reduce connection time. AT%FEACONF=0,3,1 enables PLMN Selection Optimization, which improves the initial PLMN selection after modem activation. Note that this setting can only be changed while the modem is inactive.
AT%FREQRANGES can also be used to configure a specific list of frequency ranges for the modem to try before resorting to a full scan. This provides another way to reduce the amount of frequency searching required. Note: this is a proprietary AT command for the nRF91x1 NTN modem firmware. May not be applicable in your case.
There is also a modem domain event for search status 1. This indicates that the modem has completed its initial “light search” using its intelligent search mechanisms. Your application could use this information to decide, for example, when to stop waiting and turn the modem off if coverage is poor, rather than allowing it to proceed into a potentially lengthy full-band sweep. 

Regarding AT%XSYSTEMMODE=1,1,1,3 (note: the third 1 also enables GPS): preference 3 means PLMN/home-network selection is prioritized, with LTE-M preferred over NB-IoT when both are available on the selected network. For a mobile device where connection time matters more than TX power (especially with PSM), keeping this setting is recommended. Switching to preference 1, forces LTE-M over PLMN selection and can lengthen search/attach while roaming. If NB-IoT isn't needed, disabling it (e.g. 1,0,1,3) is a stronger optimization than forcing LTE-M. I cannot currently confirm that LTE-M generally has lower energy consumption for data transmission than NB-IoT. Its higher throughput can, however, result in a shorter radio-active period. 

Let me know if this answered all your questions or if you need any further assistance.

Stine

[Peter Horauer]

This already helps a lot! 

Maybe the AT%XCOUNTRYDATA documentation should be updated to be 70 stored entries as you confirmed.

Currently the Cellular modem is used. But thanks for also noting the NTN AT command speed up. I will keep this in mind!

Currently I am not using the GPS. So will set `AT%XSYSTEMMODE=1,1,0,3`.

I have also read about `AT%XBANDLOCK`. And I am planning to use this as well. As my product will be moving in europe and rarely in Turkey and Ukraine or other europe neighbouring states. I did some prelimary research and found that Bands  B1, B3, B8, B20 are used within europe.

One other documentation to firmware mismatch I found. Is in `AT%XSYSTEMMODE` as in documentation it is stated that: "Note: If <LTE_preference> is set to a non-zero value, <LTE_M_support> or <NB_IoT_support> or both must be set.".

But this is not completely true as when only CAT-M1 is enabled lte_preference cannot be set to 2 or 4 which does make sense but is not documented properly.

I have one more question. Does `AT%XFACTORYRESET=0` reset also the cached cells? Because I am testing with your `AT%FEACONF=0,3,1` setting after a reset with CAT-M1 only takes up to 6secs. I am testing with laboratory conditions.

[Peter Horauer]

I have now done some testing with the `AT%FEACONF=0,3,1` setting. With this the connection after every factory reset is very fast. With CAT-M1 I get connection times of 3-5sec and with NB-IoT I get connection times of 10-20sec.

Another question came up though. When exactly can limiting the modem to Bands speed up the connection time? I am setting `AT%XBANDLOCK=1,"10000000000010000101"` (bitmask for band 1,3,8,20 as these are mainly used in europe). With this limitation in my understanding the modem only take a fraction of the time in the band scan phase of the modem.

In my current tests the `AT%XBANDLOCK` command with settings from above does not seem to have any significant effect in reducing the connection time. Are my assumptions with `AT%XBANDLOCK` correct?