PMP 450m / 450b on CBRS: Repeated SM Disconnects with 16 Registered Subs on One AP

Products PMP
1

I’m troubleshooting a PMP 450m AP (3 GHz CBRS) with 16 registered SMs. The AP is running 24.1.1, and the subscribers are PMP 450b High Gain SMs on 20.3.1.2.

Sector details:

  • AP model: PMP 450m AP

  • Band: 3 GHz CBRS

  • Firmware: 24.1.1

  • Registered SMs: 16

  • CBRS enabled / authorized

Observed behavior:

  • Multiple customer disconnects on the sector

  • AP events show repeated SM drop/register activity across multiple subscribers

  • We have also seen repeated CBRS EIRP change events

  • Nearby affected subs are very close to the AP, so this does not appear to be a simple edge-of-cell issue

RF examples from two nearby 450b subs:

  • Jennings: DL RSSI mid -30s dBm, UL around -60 dBm, SNR generally in the 30 dB range, DL RSSI imbalance around 2–5 dB

  • Arterberry: usable RSSI, but more RF variation and DL RSSI imbalance around 7–10 dB

I reviewed Cambium material and found:

  • 450m on 3 GHz supports up to 4 subscribers simultaneously in MU-MIMO / 8 streams

  • high frame utilization on 450m does not necessarily mean overload

  • overload should be judged from overload statistics, not frame utilization alone

  • I also found discussion of the 5 ms / 30–40 MHz deep VC queue scaling issue, but that appears related to frame period/channel width rather than subscriber count

Main question:
Are there any known stability, scheduler, airtime, registration, or performance concerns specifically when a 3 GHz PMP 450m sector has more than 15 total registered 450b SMs, even if only a subset are active simultaneously?

Also, if not subscriber-count related, which counters would you consider most authoritative for proving sector stress on 450m:

  • Overload stats

  • Frame utilization

  • Scheduler stats

  • Session stats

  • Registration failures

  • Spatial utilization / sounding-related stats

Thanks.

2

Upgrade everything to 25.0.1. Make sure you’re using a valid NTP server on the AP and that the time is correct. Enable LTE coexistence mode on the AP, start with 5ms 75% downlink data, LTE frame config 2, LTE S frame conf 7, LTE cyclic prefix normal.

3

Thanks

any thoughts on growing more subs once we firmware upgrade and tweak the other items?

4

Assuming that there’s spectrum available, if you can move up to 30 or 40MHz channel width, that will allow you to load up quite a few more SM’s depending on what speed tiers you’re trying to deliver.

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5

We are at 40 Mhz channels now, looks like you are at 106 SMs so that is reassuring

On the Firmware upgrade, do the AP first then the SMs?

6

Typically with pretty much any radio manufacture’s firmware updates, you update the far radio first, so the SM’s in the case, and then the AP last.