Skill: 5G NR MAC Layer & Radio Resource Scheduling (TS 38.321 v19.1.0)

1. Answer strictly from TS 38.321 v19.1.0 and TS 38.213 v19.x (authoritative).
2. Cite clause numbers (e.g. "§5.4.3.1") when referencing normative behaviour.
3. Distinguish shall (normative) from should/may (non-normative).
4. Map spec language to implementation patterns when asked for code advice.
5. Flag NTN-specific procedures and parameter ranges where relevant.
6. Cross-reference TS 38.213 for PDCCH/DCI/PUCCH, TS 38.214 for PDSCH/PUSCH, TS 38.211 for physical channels, TS 38.331 for RRC configuration.

Primary Sources

1. MAC Architecture (TS 38.321 §4)

Channel Mapping

Logical Channel Types

MAC Sublayer Functions (§4.4)

• Mapping between logical and transport channels
• Multiplexing/demultiplexing of MAC SDUs into/from MAC PDUs
• Scheduling information reporting (BSR, PHR)
• Error correction via HARQ
• Priority handling (LCP) between logical channels
• Padding
• Random access
• Discontinuous reception (DRX)
• Timing Advance
• Beam failure recovery (Rel-16+)

2. MAC PDU Structure (§6.1)

DL-SCH / UL-SCH MAC PDU

[ MAC subheader | MAC SDU or MAC CE ] × N
[ Padding MAC subheader | Padding ]   (optional)

Subheader formats:

Fixed-size CE or short SDU:
[ R | F=0 | LCID(6b) ]  — 1 byte

Variable-size CE or long SDU (L ≤ 255 bytes):
[ R | F=0 | LCID(6b) | L(8b) ]  — 2 bytes

Variable-size CE or long SDU (L > 255 bytes):
[ R | F=1 | LCID(6b) | L(16b) ] — 3 bytes

LCID values (uplink, key):

MAC CE Ordering in PDU (§6.1.2)

• DL: MAC CEs (with subheaders) precede MAC SDUs; padding last
• UL: MAC SDUs (high priority first) precede MAC CEs; padding last
  • Exception: BSR and PHR CEs may precede data when they justify inclusion

3. RNTI Types and Usage (TS 38.213 §16)

4. References

For detailed topic coverage, read the relevant reference file:

5. NTN-Specific MAC Overview (TS 38.321 §5.4, 3GPP Rel-17/18/19)

NTN introduces unique challenges due to large propagation delays. Key MAC adaptations:

HARQ Disable (GEO, §5.4.2.2.1)

• HARQ is disabled on a per-TB basis via RRC: harq-DisableNTN
• When disabled: NDI always toggled → every TB treated as new data
• Retransmission handled at RLC (AM) or application layer
• HARQ feedback (HARQ-ACK) still reported to avoid UE ambiguity

UE Pre-Compensation (Rel-17, §5.4.5.1a)

• UE computes uplink TA using GNSS position + satellite ephemeris from SIB19
• Pre-compensation = 2 × (propagation delay to reference point) / T_s
• gNB can additionally send TA command (MAC CE or RAR) for residual correction
• ta-CommandProhibitTimer prevents processing outdated TA during fast handover