Why Your LTE Device Won’t Attach (Even With Signal)

4G LTE
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Before a CAT-M device can attach to an LTE cell, it must first decode:
:white_check_mark: MIB
:white_check_mark: SIB1
:white_check_mark: SIB2.

And for that to happen, the cell must meet minimum selection criteria defined in SIB1.

In this example, we extract:
cellSelectionInfo-v920 {q-QualMin-r9 = -22}

This tells the UE:
:right_arrow: “You may only select this cell if RSRQ ≥ –22 dB”

If the quality is worse the cell will be ignored, even if the signal is present.

Why it matters:
:right_arrow: You can often remain attached to a cell at lower quality
:right_arrow: But you cannot attach unless minimum criteria are met.

This is why real-world logs matter.

Field testing reveals these thresholds in action and helps tune for better reliability :white_check_mark:

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LinkedIn: :backhand_index_pointing_down:

1 Like
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Nice example. It’s a good reminder that attach issues are not always about missing coverage, sometimes the UE is simply respecting the minimum selection thresholds broadcast by the cell.

This is exactly why checking decoded SIB parameters in logs can save a lot of guesswork during field testing.