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1.Introduction In 5G baseband / protocol validation, understanding MAC PDU decoding is critical — especially when debugging uplink scheduling issues, BSR anomalies, TA problems, or HARQ behavior in logs. This article provides: NR MAC PDU structure (DL & UL) Subheader parsing logic LCID types and mapping MAC SDU vs MAC CE types UE log decoding walkthrough Comparison with LTE MAC PDU Real-time debugging perspective MAC Layer in 5G NR The MAC layer sits between: RLC (above) PH

1. Introduction Medium Access Control (MAC) scheduling is one of the most critical real-time functions in a 5G NR gNB. It directly determines throughput, latency, fairness, spectral efficiency, and QoS compliance. Unlike LTE, 5G MAC scheduling operates in a much more complex design space due to: Flexible numerology (multiple SCS) Mini-slots and slot aggregation Beam-based transmissions Massive MIMO QoS flows (5QI-driven scheduling) URLLC pre-emption and puncturing Dynamic TDD