LTE cell selection behavior - frequently loosing strong cells

jrypacek · Tue Jan 09, 2024 3:04 pm

Hi all,

I've got wAP ac LTE kit and placed it at nice spot where I have full signal with cell phone under the same mobile operator, but I've found an issue which I don't understand.
It's frequently changing excellent cell for weak one. I'm not sure why it is happening as there are other cells with great signal. Can it be related to bands? How can I debug it?

EDIT: I was able to "fix" it using lock to particular frequency (earfcn). Now it stays on cells with strong signal.

/interface lte at-chat lte1 input="AT*Cell=1,3,,6400,"

- - -

Usual monitor ouptut. It doesn't show cell 403 on B20 while I'm connected to it.

PHY-CELLID  BAND  EARFCN  RSRP     RSRQ     AGE
       162  B20     6400  -82dBm   -13dB    1s 
       183  B20     6400  -88dBm   -19.5dB  1s 
       450  B20     6400  -90dBm   -18.5dB  1s 
       403  B3      1849  -107dBm  -19.5dB  1s 
       162  B3      1849  -109dBm  -19.5dB  1s 
       250  B3      1849  -121dBm  -19.5dB  4s

Usual connection detail to cell with excellent signal.

           pin-status: ok
  registration-status: registered
        functionality: full
         manufacturer: "MikroTik"
                model: "R11e-LTE"
             revision: MikroTik_CP_2.160.000_v021
     current-operator: Vodafone CZ
                  lac: 37030
       current-cellid: 1073677
               enb-id: 4194
            sector-id: 13
           phy-cellid: 403
    access-technology: LTE
       session-uptime: 27m25s
:
               earfcn: 6400 (band 20, bandwidth 10Mhz)
                 rsrp: -78dBm
                 rsrq: -10dB
                 sinr: 7dB

Usual connection detail to cell with weak signal.

     current-operator: Vodafone CZ
                  lac: 37030
       current-cellid: 650246
               enb-id: 2540
            sector-id: 6
           phy-cellid: 162
    access-technology: LTE
       session-uptime: 17m5s
:
               earfcn: 1849 (band 3, bandwidth 20Mhz)
                 rsrp: -110dBm
                 rsrq: -19.5dB
                 sinr: -10dB

Screenshot_20240109_134507_MikroTik (1).png

mkx · Wed Jan 10, 2024 7:06 pm

It's frequently changing excellent cell for weak one. I'm not sure why it is happening as there are other cells with great signal. Can it be related to bands?

It's MNO steering users towards bands with higher bandwidth. One better cell belongs to B20 which operates at 800MHz and has thus lower attenuation when passing air snd obstacles. The worse cell belongs to B3 whuch operates at 1800MHz and has thus higher attenuation. Higher attenuation most often correlates with lower signal strength but Tx power and chanbel bandwidth also play some role in it.

Alas, B3 cells have 20MHz bandwidth while B20 only have 10MHz of channel bandwidth. And cell thtoughput is proportional to channel bandwidth, so B3 cell has 2x capacity of B20 cell. At the same time it's likely that B20 cell covers more users (due to lower attenuation and smaller bandwidth), meaning that in average users of B20 cell get less air time than users of B3 cell.

Which, all in all, means that most of time it's better to conect to cell with lower signal level as it likely still provides better service. But the line between "better service" and "no service" can be very thin and the steering parameters have to be fine tuned by MNO (by the looks of it your MNO did it poorly). You can't do anything about it apart from band lock.

jrypacek · Wed Jan 10, 2024 7:47 pm

Thanks, that makes perfect sense.

I've also found that wAP has very bad and non directional LTE antenna, probably with low gain on 800 MHz, and it's performance (SINR and throughput) is much worse than cell phone on the same B20. I'll try to buy external antenna / SXT as this is really disappointment...

EDIT: I tested cells on B30 with much worse signal and it seems I really get higher throughput. So the real issue is that the modem is switching cells / bands frequently, so I need to lock frequency to get stable connection and I need to decide which one is really better.

LTE cell selection behavior - frequently loosing strong cells

LTE cell selection behavior - frequently loosing strong cells

Re: LTE cell selection behavior - frequently loosing strong cells

Re: LTE cell selection behavior - frequently loosing strong cells

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