SPS lifecycle, CG Type 1/2, K-repetition, PDCCH overhead savings & 6G AI-driven adaptation
For predictable periodic traffic like Voice over NR (VoNR, 20 ms) or URLLC telemetry, dynamic scheduling wastes enormous PDCCH resources — one DCI per packet. SPS and CG pre-allocate periodic resources with a single activation DCI, saving up to 99.9% PDCCH overhead.
Every packet needs a DCI. 50 VoNR UEs = 2,500 CCEs/sec on PDCCH. Limits cell capacity for other users.
One activation DCI → periodic PDSCH without new DCI. 95%+ PDCCH savings for periodic DL traffic.
One activation DCI (or none for Type 1) → periodic PUSCH. UE transmits without waiting for grants.
SPS pre-allocates periodic PDSCH resources. The gNB configures periodicity via RRC, activates with a single DCI (CS-RNTI), and the UE receives PDSCH on every occasion without a new DCI. Retransmissions use dynamic grants (C-RNTI).
Watch the complete SPS lifecycle: RRC config → activation DCI → periodic PDSCH (no DCI) → deactivation DCI.
| Phase | Trigger | What Happens |
|---|---|---|
| RRC Config | RRC Reconfiguration | CS-RNTI, periodicity, PUCCH resource configured |
| Activation | DCI with CS-RNTI | UE validates DCI → starts periodic PDSCH |
| Periodic PDSCH | Every period (no DCI) | UE receives and decodes → sends HARQ-ACK on PUCCH |
| Retransmission | Dynamic DCI (C-RNTI) | If NACK, gNB sends reTX via dynamic grant |
| Deactivation | DCI with CS-RNTI | UE stops periodic PDSCH |
Compare the activation flows: Type 1 needs only RRC (immediate), Type 2 needs RRC + activation DCI (flexible MCS).
All parameters in RRC. Active immediately after config. No DCI needed. Best for handover — CG ready before data arrives.
Periodicity in RRC. MCS/RBs via activation DCI. Can be deactivated. More flexible — gNB chooses MCS per activation.
Per BWP (Rel-16). Different periodicities for VoNR (20 ms), URLLC (2 ms), IoT (100 ms). UE selects based on LC→LCG→CG mapping.
K-repetition transmits each CG packet K times with cycling RV values (0→2→3→1). gNB combines all K transmissions for higher decoding probability. Essential for URLLC (10⁻⁵ BLER target).
Adjust K and watch the repetition pattern. Higher K = more reliability but more resource usage. gNB can early-terminate if decoded before all K.
Adjust the number of VoNR UEs and see the PDCCH CCE consumption. Dynamic scheduling saturates PDCCH quickly; SPS/CG barely uses it.
| Metric | Dynamic | SPS / CG |
|---|---|---|
| DCIs per UE/sec | 50 | ~0.05 |
| CCEs per UE/sec | 150 | ~0.15 |
| Grant latency per packet | 1–2 ms | 0 ms |
| PDCCH utilization (20 UEs) | 60% | < 1% |
CG period: 20 ms (matches AMR-WB codec). TB: 40-60 bytes. MCS: 4 (QPSK). 24-32 RBs. During silence: SID every 160 ms or deactivate.
CG period: 0.5-2 ms. K-rep: 4-8. Priority: 1 (high). MCS: 6. 32-64 bytes. Latency: ~1 ms. Reliability: 10⁻⁵ BLER.
SL CG Type 1. Period: 100 ms (CAM). Works out-of-coverage. No gNB needed. Deterministic timing for safety.
CG Type 2 (on-demand). Period: 100 ms. MCS: 10 (16-QAM). 48 RBs. Activated when data arrives.
| Scenario | Period | Type | K-rep | MCS | Priority |
|---|---|---|---|---|---|
| VoNR voice | 20 ms | CG-1 | 1 | 4 | 0 (normal) |
| VoNR SID | 160 ms | CG-1 | 1 | 2 | 0 |
| URLLC control | 2 ms | CG-1 | 4 | 6 | 1 (high) |
| IoT telemetry | 100 ms | CG-2 | 1 | 10 | 0 |
| V2X CAM | 100 ms | SL-1 | 2 | 8 | 0 |
| Aspect | LTE (SPS only) | 5G NR (SPS + CG) |
|---|---|---|
| Direction | SPS (DL + UL shared) | SPS (DL) + CG (UL) separate |
| Types | 1 type | CG Type 1 + Type 2 |
| Max configs | 1 | 8 SPS + 4 CG per BWP |
| Min period | 10 ms | 2 symbols |
| K-repetition | None | Up to K=8 |
AI monitors traffic and adjusts CG period in real-time: 20 ms (talk) → 160 ms (silence) → 5 ms (burst). 40-60% resource savings.
AI predicts data arrival → activates CG before data arrives → zero activation latency. Deactivates when burst ends.
CG on cell + WiFi + satellite. AI selects best link per packet. 99.9999% reliability via multi-link diversity.
SPS carries both data AND radar sensing signals. Deterministic timing for range/Doppler. Zero extra overhead.
Compare SPS/CG capabilities across generations: configs, min period, K-repetition, and AI features.
| Dimension | 4G (LTE) | 5G NR | 6G (Target) |
|---|---|---|---|
| DL | SPS | SPS (8 configs) | AI-adaptive SPS |
| UL | SPS (shared) | CG Type 1/2 (4 configs) | Predictive CG |
| Min period | 10 ms | 2 symbols | < 1 symbol |
| K-repetition | None | Up to 8 | Up to 32 |
| Period adaptation | Manual | Manual | AI-driven |
| Sensing | None | None | SPS for ISAC |