UE behavior when RACH is aborted during RRC Connection Attempt

Hello experts,

If a UE is powered on and scans Band-X based on its priority configuration, but the RRC connection attempt with the eNB fails due to RACH being aborted, what should be the expected UE behavior?

Should the UE attempt to connect on another band, or will it temporarily block that PLMN?

UE behavior when RACH is aborted during RRC Connection Attempt

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During the cell search phase, the UE collects information from all available cells. However, in the cell selection phase - before sending the RACH - it reads SIB1 to obtain the available PLMN IDs.

Therefore, I assume that before attempting to connect to another network, the UE will first try other inter-frequency options within its home PLMN.

Please note that this is my assumption, and different scenarios may lead to varying behaviors.

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Actually, the UE keeps attempting to connect to the same cell multiple times to establish the RRC connection and proceed with the attachment to the core network.

However, due to repeated RACH failures leading to RRC establishment failures, the UE is unable to send the attach request. As a result, this process keeps restarting, causing the UE to get stuck on that cell since it is considered the best available option.

In this case, further debugging is needed to identify the root cause of the RACH failure.

Based on the findings, appropriate steps can be taken to resolve the issue, such as addressing PCI conflicts, RSI issues, hardware malfunctions, or other potential causes.

Alternatively, you can lower the selection priority of the best cell in the SIB to avoid it.

The reasons for RACH failure can include factors such as RAR window expiry, Msg4 failure, or the base station not receiving Msg1.

My question is: Should the UE remain stuck on the same cell and band, or should it attempt to select another cell, band, or PLMN?

Is there a specific criterion defined in the specifications for this scenario?

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I don’t recall if there is any penalty mechanism in the RACH case to lower the priority or RxLevMin.

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As far as I know, no such criteria are defined. Perhaps a 3GPP specification expert could confirm this.

If a cell is ranked and designed as the best cell for the RACH procedure in a specific area or for certain UEs but experiences a 100% failure rate, there should be a mechanism to detect it; otherwise, identifying such problematic cells would be challenging.

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4G or 5G?

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