Any way to force RM-520N-GL onto one band as PCC while still using others as SCC?

Is there a way to lock the RM-520N-GL onto one band for its primary 5G (or NSA LTE anchor) connection, without losing carrier-aggregation altogether?

I’ve found that forcing a desired ARFCN/PCI, via commands like
AT+QNWLOCK=“common/5g”,224,126510,15,71
or
AT+QNWLOCK=“common/4g”,1,66786,458

does work, but this causes the module to use ONLY that band, with no aggregation across available secondary bands - these will still be listed in +QCAINFO: “SCC”… status lines and elsewhere, but performance tests indicate that only the PCC is ever used for data. The same thing happens in both 5G-SA and 5G-NSA mode. Removing the band-lock immediately resolves this, but of course the bands then bounce back and forth again randomly between primary & secondary.

I also tried setting a band/frequency preference using other documented commands,
AT+QNWCFG=“nr5g_pref_freq_list”,1,126510:15 # (listing only desired PCC)
AT+QNWCFG=“nr5g_pref_freq_list”,4,126510:15:501390:30:521310:30:396250:15
AT+QNWPREFCFG=“lte_band_priority”,66:4:2:12:71 # prefer B66 for NSA LTE anchor

But these had no apparent effect. They’re accepted, a readback of the config shows what was set, and CA continues to work, but the PCC band choice will still change at random.

Is there another way? Could a firmware upgrade allow these features to work better? I’m on RM520NGLAAR01A06M4G_01.200.01.200, which shipped with my module in late 2022 or early 2023. Not sure how far behind that is.

This module is in a location with heavy tree cover blocking line-of-sight to all nearby towers, so low-band connections like n71 600MHz work well, while high-band (e.g. n41 2.6GHz) works for downlink only, but very poorly for uplink, due to the module having insufficient transmit power to overcome high attenuation at those frequencies.

Since 5G SA transmits exclusively on the primary (PCC) band, aggregating SCC’s only for downlink, PCC=n71 (15MHz FDD) and SCC=n41 (100MHz TDD) works very well, while PCC=n41 & SCC=n71 gives terrible performance, due to excessive reverse-path loss on 2.6GHz. Unfortunately, the network isn’t smart enough to realize this, and will randomly reverse PCC/SCC assignments every few days.

Using NSA mode, where uplink is split between one 4G and 5G carrier, and dropping n41 altogether makes for less variation, but even there, certain LTE bands work better than others, and there seems to be no way to stay on a single band as 4G PCC, without losing all CA capability.

Has anyone had more success with this?

I’m on T-Mobile US (310 260), in case that makes a difference.

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I am also in the same situation, with T-Mobile. I have tried the band priority commands with no luck either and need to lock n71 as PCC and n41 as SCC in SA mode.

Would appreciate any assistance with this.

Yes this also happens to me, n25 is weaker and slower than n71 for me but the modem will randomly decide to make n25 the PCC.

This seems like a bug to me.

I also would like to know the answer to this question.

I just disable n25 altogether… at least in my location, from past testing there seems to be no benefit from having it in the mix, especially with an x62-based modem that can only CA two bands together in SA mode. So,
AT+QNWPREFCFG=“nr5g_band”,41:71

Lately I’ve been running more in NSA mode, with B66 (aka B4) as the only allowed LTE band, and n71+n41 on the 5G side:

AT+QNWPREFCFG=“nr5g_disable_mode”,1
AT+QNWPREFCFG=“nsa_nr5g_band”,41:71
AT+QNWPREFCFG=“lte_band”,4:66

This increases latency a bit, but usually results in two solid uplink paths (n71 appears to always end up as NSA 5G PCC when it’s present at all), plus access to more downlink spectrum overall (potentially 20MHz LTE + 20MHz n71 + 100MHz n41) than in SA, with its limit of two channels and 120MHz of spectrum.

That 120MHz modem limit actually helps in keeping a lock on n71… back when T-mobile still used part of their band 41 for LTE, I’d sometimes end up with n41(100MHz) + n41(20MHz) on the 5G side whenever n41 was allowed at all, resulting in poor upload performance, but now that they’ve widened the second n41 carrier to 90Mhz, the x62 is incapable of camping on both that and the 100Mhz one, making n71 much more “sticky”… though it still occasionally goes away on its own :frowning:

Keeping B2 in the LTE list can result in getting stuck on B66+B2+n41, or worse B2+B66+n41 (transmitting on B2), with poor upload performance. Despite being a few dB down from B2 on receive, B66 benefits from its weirdly wide 400MHz duplex gap (2100 down/1700 up)… the 200MHz lower transmit frequency compared to B2 1900MHz makes a surprisingly large difference. And LTE B12/B71 (latter’s retired on my towers) get through OK, but are only 5MHz each here, and often quite congested.

More direct control over PCC assignment would certainly be nice, but I’m still not completely clear on how much of that’s under the UE’s control, and how much is set by the tower. Changing QNWPREFCFG band lists back and forth can help get “unstuck” from an undesirable CA combo without ever losing the link, though latency goes high (200+ms) for a few seconds after. I’ve thought of writing a script query modem stats every few seconds, and automatically flip the allowed-band lists around (say, dropping n41, then adding it back) upon noticing a bad CA mix.

I recently found out about the command AT+QNWCFG=“lte_band_priority” and it has been a real game changer for NSA at my location.

My issue has always been in NSA mode my gateway would randomly select B66+B2+n71 which is okay but has bad latency and the speed is not great, when it selects B2+B66+n71 with B2 as PCC instead of B66 my latency and speed is excellent. I didn’t know you could set LTE band priority but once I found out it’s been a game changer as I mentioned.

AT+QNWCFG=“lte_band_priority”,2:4:66 is what I’ve got it set to and it fixed my NSA issues!