Scheduling in 5G NR & 6G

#1Why Scheduling Matters

The gNB scheduler is the brain of the radio resource manager — every slot it decides which UEs get resources, how much bandwidth, and what QoS:

#2Dynamic Scheduling — Per-Slot Grants

The baseline mode: gNB sends a DCI grant every slot to allocate resources to a specific UE:

#3Scheduling Algorithms

The algorithm decides which UE gets resources in each slot. The three main options:

#4Semi-Persistent Scheduling (SPS)

SPS pre-allocates periodic resources without per-slot DCI — reducing PDCCH overhead for predictable traffic (VoIP, video, IoT):

#5Configured Grants (CG) — For URLLC

CG extends SPS to uplink for zero-latency URLLC: UE transmits immediately on pre-configured resources:

#6Uplink Grant Handling — UE-Side Processing

When the gNB sends UL grants, the UE must process them correctly — handling N2 timing, overlapping grants, cancellation (DCI 2_4), and PUSCH priority:

N2 — UE Processing Capability

Constraint: K2 ≥ N2 — gNB must not schedule earlier than UE can process.

Overlapping Grants & Cancellation

#7Time-Domain — Mini-Slots & Preemption

Mini-slots (2, 4, or 7 symbols) enable URLLC to preempt ongoing eMBB transmissions:

  Full Slot (14 symbols) vs Mini-Slot (2/4/7 symbols):

  ┌──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┐
  │ 0│ 1│ 2│ 3│ 4│ 5│ 6│ 7│ 8│ 9│10│11│12│13│  Full slot
  └──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┘

  URLLC arrives at symbol 5:
  ┌──┬──┬──┬──┬──┬██┬██┬──┬──┬──┬──┬──┬──┬──┐
  │  │  │  │  │██│██│  │  │  │  │  │  │  │  │  URLLC preempts 5-6
  └──┴──┴──┴──┴──┴██┴██┴──┴──┴──┴──┴──┴──┴──┘

#8Frequency-Domain — PRB Allocation

#9QoS-Aware Scheduling

Scheduler uses 5QI (QoS identifier) to weight PF metric per flow:

  QoS-Weighted PF:
  PF_metric(i) = w(5QI) × R_instant(i,j) / R_avg(i)
  → URLLC packets preempt eMBB when deadline approaches

#10Practical Field Considerations

#116G Scheduling Evolution

The research community and 3GPP are defining 6G scheduling paradigms (standardization ~2029, commercial ~2032):

#12Further Reading

3GPP Specifications

• TS 38.321 — NR MAC Protocol (scheduling, SPS, CG, BSR, HARQ)
• TS 38.214 — NR Physical Layer Procedures for Data (PRB allocation, TBS)
• TS 38.213 — NR Physical Layer Procedures (PDCCH, DCI, SPS activation)
• TS 38.821 — NR NTN (Release 17 satellite scheduling)

Companion Resources

Link Adaptation → Power Control → ☆ AI Scheduling → ☆ ISAC →