In weak uplink conditions, the UE does exactly what the standards tell it to do. It retries Msg1 with increasing PRACH transmit power until the eNB finally hears it.
In this real cell-edge capture, you can see:
Msg1 @ 20 dBm → no response
Msg1 @ 24 dBm → still no response
Msg1 @ 28 dBm → marginal
Msg1 @ 32 dBm → finally detected.
→ then Msg2, Msg3, Msg4 complete the access.
This is a classic reminder that uplink is usually the limiting link, not downlink.
You can have “good signal bars” and still fail to attach if UL is constrained.
Key takeaways:
Repeated Msg1 = uplink coverage problem, not core or NAS
Rising TX power tells you the UE is hitting its UL budget
Cell-edge issues are often PRACH-limited, not RSRP-limited
This behaviour is expected and standard-compliant.
No theory. Just real RACH behaviour under stress.
Field Testing - LTE RACH - When Msg1 Has to Shout to Be Heard.pdf (400.1 KB)
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