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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