Layer 2 (L2) Issues in LTE and Troubleshooting?

 

Layer-Wise Troubleshooting Approach:

Step 1: Physical Layer (L1) - Check Signal Strength

• Use LTE_PHY_Connected_Mode_Intra_Freq_Meas to check:
• RSRP (Should be > -100 dBm)
• RSRQ (Should be > -10 dB)
• SINR (Should be > 5 dB)
• Action: If weak signal, move to a better location, retry connection.

Step 2: MAC Layer - Debug RACH, HARQ, & Scheduling Issues

(a) RACH Failures

• Use LTE_MAC_Rach_Trigger and LTE_MAC_Rach_Attempt to verify:
• Preamble Sent but no response → Increase power ramping
• Multiple Attempts but failure → Check contention resolution failure
• Action: Adjust RACH power control, increase RACH attempts.

(b) HARQ Failures

• Use LTE_MAC_HARQ_DL_ACK_NACK to verify:
• NACK received frequently → Check interference
• No ACK/NACK response → HARQ timing issue
• Action: Check BLER, adjust retransmission timers.

(c) Scheduling & Buffering Issues

• Use LTE_MAC_UL_Buffer_Status_Report to check:
• Large buffer queue → Congestion issue
• No scheduling grant → eNB resource allocation failure
• Action: Increase buffer size, check network congestion.

Step 3: RLC Layer - Debug Retransmissions

• Use LTE_RLC_UL_AM_PDU and LTE_RLC_DL_AM_PDU to verify:
• High retransmission rate → Poor HARQ performance
• Sequence number mismatch → Data loss
• Action: Adjust RLC reordering window, reduce retransmission attempts.

Step 4: PDCP Layer - Debug Integrity & Encryption Issues

• Use LTE_PDCP_UL_Data_PDU and LTE_PDCP_DL_Data_PDU to check:
• Missing sequence numbers → Packet loss
• Integrity check failures → Encryption mismatch
• Action: Verify security keys, adjust PDCP sequence number window.

 Step 5: DRX & Power Control Issues

• Use LTE_MAC_DRX_Events to verify:
• UE not waking up on time → DRX misconfiguration
• High latency in VoLTE calls → TTI Bundling issue
• Action: Adjust DRX timers, enable TTI bundling for VoLTE.