nRF9151: GCI searches returning limited results

[JordanYates]

I have an application that is attempting to use multi-cell positioning based on the results of %NCELLMEAS.
The application is following the flow of the NCS positioning module (EXTENDED_LIGHT -> GCI_DEFAULT -> GCI_EXTENDED_LIGHT).
I am observing that most of the time, the GCI scans are failing to find any cells apart from the currently serving cell.
Differing from other similar questions here, the application is in RRC idle for all of the scan requests.
The initial scan results in multiple neighbor cells found and measured, so it is not an issue of no other towers being nearby (I am ~10km from a city center)

[00:17:12.676,361] <err> modem_monitor:    State: Active
[00:17:23.825,866] <err> modem_monitor:    State: Idle
... Radio in RRC Idle
[00:18:00.633,056] <wrn> task_network_scan: Cell Scan: Mode 2 GCI_Count 0
[00:18:01.463,928] <inf> task_network_scan: Serving Cell Valid: Yes, Neighbour Cells: 3
[00:18:01.463,958] <inf> task_network_scan: Serving ECI: 147045644 (-106 dBm)
[00:18:01.463,958] <inf> task_network_scan: Neighbour: 337 (-118 dBm)
[00:18:01.463,989] <inf> task_network_scan: Neighbour: 238 (-120 dBm)
[00:18:01.463,989] <inf> task_network_scan: Neighbour: 70 (-129 dBm)
[00:18:01.464,141] <wrn> task_network_scan: Cell Scan: Mode 4 GCI_count 7
[00:18:01.836,669] <inf> task_network_scan: Global Cells: 1
[00:18:01.836,669] <inf> task_network_scan: Global ECI: 147045644 (-107 dBm)
[00:18:01.836,791] <wrn> task_network_scan: Cell Scan: Mode 5 GCI_count 7
[00:18:09.843,841] <inf> task_network_scan: Global Cells: 1
[00:18:09.843,872] <inf> task_network_scan: Global ECI: 147045644 (-107 dBm)
... Uplink here, transition to RRC Active
[00:18:12.688,781] <err> modem_monitor:    State: Active
[00:18:25.178,192] <err> modem_monitor:    State: Idle

While the lack of GCI cells is not a huge issue here with the multiple neighbors reported, the application also operates in regions where the network does not provide neighbor information, so the performance of the GCI scanning is critical.
Occasionally the `GCI_DEFAULT` search does find a second cell, but I am yet to see `GCI_EXTENDED_LIGHT` return useful information, despite searching for ~7.5 seconds each run.

I feel like I must be doing something wrong, but I have no idea what it could be.

           Model: nRF9151-LACA
        Firmware: mfw_nrf91x1_2.0.3
            Mode: LTE-M + GNSS (Prefer: No preference)

[Syed Maysum Abbas Zaidi]

Hi,

Thanks for a detailed write up. Can you tell the NCS version you are using and to understand why the GCI search is only returning the serving cell we need to check what the modem actually did during the search so it would be great if you could share the modem trace.

Best Regards,
Syed Maysum

[JordanYates]

Attached is the modem trace file from boot across a few iterations of the cell searches.
The global search found a second cell more often than last week, but was still measurably worse than the EXTENDED_LIGHT search for all runs.
GCI_EXTENDED_LIGHT also never found any extra cells over GCI_DEFAULT, and returned fewer result on at least one run.

[00:04:00.000,122] <inf> task_network_scan: Search Type: 2 GCI_Count: 0
[00:04:00.663,726] <inf> task_network_scan: Serving Cell Valid: Yes, Neighbour Cells: 4
[00:04:00.663,726] <inf> task_network_scan: ID: 147045644 RSRP: 108 dBm
[00:04:00.663,757] <inf> task_network_scan: Neighbour ID: 372 RSRP: 108 dBm
[00:04:00.663,757] <inf> task_network_scan: Neighbour ID: 70 RSRP: 118 dBm
[00:04:00.663,787] <inf> task_network_scan: Neighbour ID: 302 RSRP: 116 dBm
[00:04:00.663,787] <inf> task_network_scan: Neighbour ID: 238 RSRP: 121 dBm
[00:04:00.663,940] <inf> task_network_scan: Search Type: 4 GCI_Count: 11
[00:04:00.967,102] <inf> task_network_scan: Global Cells: 2
[00:04:00.967,102] <inf> task_network_scan: Global ID: 147249163 RSRP: 108 dBm
[00:04:00.967,132] <inf> task_network_scan: Global ID: 147045644 RSRP: 109 dBm
[00:04:00.967,285] <inf> task_network_scan: Search Type: 5 GCI_Count: 11
[00:04:07.806,121] <inf> task_network_scan: Global Cells: 1
[00:04:07.806,152] <inf> task_network_scan: Global ID: 147045644 RSRP: 105 dBm

I updated the modem firmware from 2.0.3 to 2.0.4 before capturing the trace.

1784505677_nrf_modem_trace.zip

[Syed Maysum Abbas Zaidi]

Hi,

Thanks for providing the trace. I will analyze the trace and will get back to you by tomorrow. Appreciate your patience. 

Best Regards,
Syed Maysum