Direct UE-to-UE over PC5: Mode 1 gNB-scheduled, Mode 2 autonomous sensing, SL HARQ & V2X
Sidelink supports two resource allocation modes: Mode 1 (gNB schedules SL resources via DCI 3_0/5_0) and Mode 2 (UE autonomously selects resources using sensing). The SL MAC entity is separate from the Uu MAC.
Compare how resources are allocated: gNB-coordinated (Mode 1) vs autonomous sensing-based (Mode 2).
UE sends SL-BSR → gNB grants via DCI 3_0 → UE transmits. Low collision, needs RRC_CONNECTED + coverage.
UE senses → excludes occupied → randomly selects. Works out-of-coverage, higher collision risk.
Type 1: RRC periodic (no DCI). Type 2: DCI-activated periodic. Ideal for periodic CAM/BSM (100 ms).
| Aspect | Mode 1 | Mode 2 |
|---|---|---|
| Scheduler | gNB (DCI 3_0/5_0) | UE autonomous |
| Coverage required | Yes (RRC_CONNECTED) | No (works OOC) |
| Collision risk | Low (coordinated) | Higher (distributed) |
| Latency | ~4-10 ms (SR+grant) | ~0 ms (immediate) |
| Complexity | Low (UE side) | High (sensing algo) |
Mode 2 uses a 4-step sensing algorithm: (1) monitor resource pool for 100-1100 ms, (2) decode SCI and measure RSRP, (3) exclude occupied resources (keep ≥20% candidates), (4) randomly select from remaining.
Watch the UE scan the resource pool. Red = occupied (RSRP above threshold), yellow = excluded, green = available candidate.
UE reselects resources when: counter expires, packet delay budget unmet, preemption by higher priority, or CBR exceeds threshold. Rel-16 adds re-evaluation check — UE re-senses before each TX to detect hidden nodes.
Step through the reselection flow: counter expiry → re-sense → exclude → select → reserve. Watch preemption in action.
| Trigger | Action | Priority |
|---|---|---|
| Reselection counter = 0 | Full reselection (sense + select) | Normal |
| Packet delay exceeded | Immediate reselection | High |
| Preemption (Rel-16) | Release resource, reselect | Critical |
| CBR > threshold | Reduce TX power/rate + reselect | Congestion |
SL HARQ operates independently from Uu — feedback on PSFCH. Unicast uses full ACK/NACK. Groupcast supports NACK-only (saves resources) or ACK/NACK. Broadcast has no HARQ.
Full HARQ ACK/NACK. Max retx configurable (sl-MaxTransNumber). PC5-RRC established between UEs.
Receivers silent on ACK, send NACK only on failure. Saves PSFCH resources. Retx if ANY NACK.
No HARQ feedback. Fire-and-forget. Used for safety messages (CAM/BSM/DENM).
| Scenario | Mode | Key Consideration |
|---|---|---|
| In-coverage V2V (urban) | Mode 1 | gNB optimizes, low collision |
| Out-of-coverage (rural) | Mode 2 | No gNB, sensing-only |
| Public safety (disaster) | Mode 2 | Must work without network |
| High density (>100 veh/km) | Mode 1 pref. | CBR congestion control critical |
| Periodic CAM (100 ms) | SL-CG or Mode 2 reservation | Match periodicity |
| Aspect | LTE SL | 5G NR SL |
|---|---|---|
| HARQ | No (broadcast only) | Yes (unicast, groupcast) |
| Unicast | No | Yes (PC5-RRC) |
| QoS | PPPP (8 levels) | PQI (5QI-like) |
| CA | No | Yes (Rel-17) |
| Re-evaluation | No | Yes (Rel-16) |
| Preemption | No | Yes (priority-based) |
Vehicles share raw sensor data (camera, LiDAR) over SL. AI fuses multi-vehicle views for extended sensing range.
Multi-hop SL: UE→UE→UE→gNB. Self-organizing mesh extends coverage beyond single-hop range.
gNB jointly optimizes Uu and SL. AI decides per-packet: send via network or direct? Single MAC entity.
Compare sidelink capabilities across generations.
| Dimension | 4G (LTE SL) | 5G NR SL | 6G (Target) |
|---|---|---|---|
| HARQ | None | Unicast + Groupcast | Full mesh HARQ |
| Unicast | No | Yes | Multi-hop relay |
| Cooperative | No | Basic | Joint perception |
| Duplex | Half only | Full capable | Full + ISAC |
| Topology | Single hop | Single + relay | Mesh (multi-hop) |