5G NR RLC Layer Failures: Root Causes and Solutions

Here are some common 5G NR test cases at the RLC layer, along with typical failures, their root causes, and suggested solutions.

Failure Case #1: RLC PDU Loss or Discard Rate High

This scenario describes a situation where the RLC layer is experiencing a high rate of packet loss or discarding PDUs (Protocol Data Units).

Root Causes:

• Buffer overflow due to high traffic load. The RLC layer simply can’t keep up with the incoming data.
• Poor flow control. Mechanisms to regulate the data flow aren’t working effectively.
• Inadequate link adaptation strategies. The modulation and coding scheme (MCS) may be too aggressive for the current channel conditions.

Solutions/Rectifications:

• Optimize buffer management and flow control settings to prevent buffer overflows. Tune the parameters to match the expected traffic patterns.
• Adjust link adaptation algorithms to dynamically change RLC modes (e.g., AM, UM, TM) based on the traffic type. Switch to a more robust RLC mode under poor channel conditions. (AM - Acknowledged Mode, UM - Unacknowledged Mode, TM - Transparent Mode)
• Use efficient segmentation and reassembly algorithms to handle different PDU sizes. Minimize overhead and maximize throughput.

Failure Case #2: RLC Retransmission Failure

This occurs when the RLC layer is unable to successfully retransmit lost PDUs.

Root Causes:

• Persistent poor channel conditions. If the channel is consistently bad, retransmissions are likely to fail repeatedly.
• Incorrect RLC timer settings. Timers related to retransmission (e.g., how long to wait for an ACK) might be configured inappropriately.
• Errors in segmentation and reassembly procedures. Issues in how PDUs are split and reassembled can lead to corrupted or incomplete data, triggering retransmissions that may ultimately fail.

Solutions/Rectifications:

• Fine-tune retransmission timers (e.g., T-Reordering, T-Status-Prohibit) to improve retransmission efficiency. Optimizing these timers can prevent unnecessary retransmissions or ensure timely retransmission attempts.
• Implement redundancy techniques or use advanced error correction schemes. Adding redundancy or employing more robust error correction can improve the chances of successful delivery, even under challenging channel conditions.
• Optimize retransmission limits based on the quality of service (QoS) class. Different QoS classes might warrant different retransmission limits. For instance, delay-sensitive services might benefit from fewer retransmissions to minimize latency.

Failure Case #3: RLC Re-establishment Failure

This happens when the RLC layer fails to re-establish its connection after an interruption.

Root Causes:

• Failure in upper layer procedures like reconfiguration or handover. Issues during network changes can disrupt the RLC layer.
• Poor radio conditions. A sudden and drastic drop in signal strength can cause the RLC connection to be lost.
• Protocol misalignment between UE and gNB. Inconsistencies in the RLC configuration or state between the user equipment (UE) and the base station (gNB) can lead to re-establishment failures.

Solutions/Rectifications:

• Improve handling of radio link failure and recovery procedures. Ensure robust procedures are in place to detect and recover from radio link failures.
• Ensure proper signaling coordination between layers for state transitions. Smooth communication and synchronization between layers are crucial for successful state transitions, such as during handover or reconfiguration.
• Optimize handover and reconfiguration parameters to reduce failure rates. Fine-tuning these parameters can minimize the chances of disruption during network changes.