Expert knowledge of the 5G NR MAC layer and radio resource scheduling based on 3GPP TS 38.321 v19.1.0 (Release 19). Use this skill when the user asks about MAC sublayer architecture, logical/transport/physical channel mapping, downlink/uplink scheduling and DCI formats, HARQ process management and timing, logical channel prioritization and MAC PDU construction, Buffer Status Reports (BSR), Power Headroom Reports (PHR), Scheduling Requests (SR), random access procedures (4-step and 2-step RACH), Timing Advance commands, configured scheduling (CS-RNTI), semi-persistent scheduling, or NTN-specific MAC adaptations (HARQ disable, extended timers, pre-compensation). Also covers related PHY control procedures in TS 38.213 (PDCCH monitoring, search spaces, DCI formats, PUCCH, PRACH).
40:T1aa4,
Use this skill when the user asks about:
| Spec | Layer | Version |
|---|---|---|
| TS 38.321 | NR MAC | v19.1.0 (Release 19, 2026-02) |
| TS 38.213 | PHY — Control Procedures | v19.x |
| TS 38.214 | PHY — Data Channels | v19.x |
| TS 38.211 | Physical Channels | v19.x |
| TS 38.331 | RRC | v19.x |
| Logical Channel | Transport Channel | Physical Channel |
|---|---|---|
| BCCH | BCH | PBCH |
| BCCH | DL-SCH | PDSCH |
| PCCH | PCH | PDSCH |
| CCCH, DCCH, DTCH, MCCH, MTCH | DL-SCH | PDSCH |
| CCCH, DCCH, DTCH | UL-SCH | PUSCH |
| — | RACH | PRACH |
| SCCH, STCH (sidelink) | SL-SCH | PSSCH/PSCCH |
| Channel | Direction | Carried Data |
|---|---|---|
| BCCH | DL | Broadcast control (SIBs, MIB) |
| PCCH | DL | Paging |
| CCCH | DL+UL | Common control (RRC setup, re-establishment) |
| DCCH | DL+UL | Dedicated control (RRC Reconfiguration, etc.) |
| DTCH | DL+UL | Dedicated traffic (user data) |
| MCCH | DL | Multicast/broadcast control (MBS) |
| MTCH | DL | Multicast/broadcast traffic (MBS) |
| SCCH | Sidelink | Sidelink control |
| STCH | Sidelink | Sidelink traffic |
[ 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):
| LCID | Meaning |
|---|---|
| 0–32 | Logical channel ID (for MAC SDU) |
| 59 | Long Truncated BSR |
| 60 | Short Truncated BSR |
| 61 | Long BSR |
| 62 | Short BSR |
| 63 | CCCH 48-bit / Padding |
| RNTI | Purpose |
|---|---|
| C-RNTI | Cell-level unique UE identifier; used for dynamic scheduling |
| CS-RNTI | Configured scheduling RNTI; activates/deactivates semi-persistent grants |
| TC-RNTI | Temporary C-RNTI during random access |
| P-RNTI | Paging |
| SI-RNTI | System information |
| RA-RNTI | Random Access Response |
| MCS-C-RNTI | Alternate MCS table (low spectral efficiency) |
| SL-RNTI | Sidelink scheduling |
| INT-RNTI | Interruption indicator |
| SFI-RNTI | Slot format indication (for TDD) |
| SP-CSI-RNTI | Semi-persistent CSI on PUSCH |
For detailed topic coverage, read the relevant reference file:
| Topic | File | When to Read |
|---|---|---|
| DL/UL Scheduling, DCI formats, PDCCH, LCP, configured scheduling | references/scheduling.md | Scheduling grants, DCI field decoding, MAC PDU assembly with LCP |
| HARQ process model, NDI, RV, timing, NTN HARQ | references/harq.md | HARQ operation, timing diagrams, NTN HARQ disable |
| Random access (4-step, 2-step RACH, RAR, contention resolution) | references/random-access.md | RACH procedures, MSG1–4, MSGA/MSGB, beam failure recovery |
| BSR, PHR, Scheduling Request, Timing Advance, DRX | references/bsr-phr-ta.md | Buffer reporting, power headroom, SR failure, TA commands, DRX |
NTN introduces unique challenges due to large propagation delays. Key MAC adaptations:
| Aspect | Terrestrial | NTN LEO (~600 km) | NTN GEO (~35786 km) |
|---|---|---|---|
| One-way propagation delay | ~0.1 ms | ~2–10 ms | ~270 ms |
| HARQ RTT | ~4–8 ms | ~16–25 ms | ~560 ms |
| HARQ mode | Enabled | Enabled (extended timers) | Disabled (GEO) |
| HARQ process count | 8–16 | Up to 32 (Rel-17) | N/A (disabled) |
| t-Reassembly | ~35 ms | ≥ 25 ms | ≥ 600 ms |
| Timing Advance | gNB-commanded | UE pre-compensates via GNSS + ephemeris (SIB19) | Same |
| SR / BSR timing | Normal PUSCH timing | Extended k2 (PUSCH delay offset) | Extended k2 |
| Random access | Standard | Extended RAR window, RA preamble guard period | Extended |
harq-DisableNTNta-CommandProhibitTimer prevents processing outdated TA during fast handover