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3GPP 5G NR-V2X Sidelink Mode 2

Overview & Standards Provenance

5G New Radio Vehicle-to-Everything (NR-V2X) Sidelink Mode 2, standardized in 3GPP Release 16 and Release 17, enables direct peer-to-peer vehicular communication without cellular base station (gNodeB) assistance.

Operating primarily in the 5.9 GHz ITS band (3GPP Band n47), NR-V2X enhances sidelink capabilities with flexible numerologies, Two-Stage Sidelink Control Information (2-Stage SCI), autonomous sensing-based resource selection, re-evaluation lookahead, pre-emption, and closed-loop HARQ feedback over the Physical Sidelink Feedback Channel (PSFCH).

graph TD
    subgraph NrEnhancements ["5G NR-V2X Enhancements over LTE-V2X"]
        A[Flexible Numerologies mu in 0, 1, 2] -->|15, 30, 60 kHz SCS| B[Sub-millisecond Slots 0.25 - 1.0 ms]
        C[Two-Stage SCI] -->|PSCCH Polar + PSSCH LDPC| D[Decoupled Sensing & Addressing]
        E[Release 16 Re-evaluation T3] -->|Lookahead Sensing Check| F[Zero Conflict Late Reselection]
        G[Release 16 Pre-emption] -->|Priority Overrides| H[Safety-Critical Guaranteed Grants]
        I[PSFCH HARQ Feedback] -->|ACK / NACK Feedback| J[High-Reliability Sensor Fusion]
    end

3GPP TS 38.211 Section 4 defines multiple numerologies \(\mu\) for sidelink operations:

\[\Delta f = 2^\mu \times 15\,\text{kHz}, \quad T_{slot} = \frac{1.0\,\text{ms}}{2^\mu}\]
Numerology (\(\mu\)) Subcarrier Spacing Slot Duration Symbols / Slot Slots / 1 ms Subframe Primary Operational Profile
\(\mu = 0\) \(15\,\text{kHz}\) \(1.0\,\text{ms}\) 14 1 Macro coverage / low-density urban
\(\mu = 1\) \(30\,\text{kHz}\) \(0.5\,\text{ms}\) (\(500\,\mu\text{s}\)) 14 2 Standard Default for Band n47 @ 5.9 GHz
\(\mu = 2\) \(60\,\text{kHz}\) \(0.25\,\text{ms}\) (\(250\,\mu\text{s}\)) 14 4 High-speed highway & ultra-low latency

The channel bandwidth (10, 20, or 40 MHz) is partitioned into subchannels consisting of contiguous Resource Blocks (\(S_{sub} \in \{10, 12, 15, 20, 25\}\) RBs):

       5G NR-V2X SIDELINK TIME-FREQUENCY RESOURCE GRID (mu = 1, 30 kHz SCS)
    Frequency (RBs)
       ▲
 50 RB │ Subchannel 4 [ PSSCH (Data + SCI-2A) ] ... [ PSFCH (HARQ) ] [ Guard ]
 40 RB │ Subchannel 3 [ PSSCH (Data + SCI-2A) ] ... [ PSFCH (HARQ) ] [ Guard ]
 30 RB │ Subchannel 2 [ PSSCH (Data + SCI-2A) ] ... [ PSFCH (HARQ) ] [ Guard ]
 20 RB │ Subchannel 1 [ PSSCH (Data + SCI-2A) ] ... [ PSFCH (HARQ) ] [ Guard ]
 10 RB │ Subchannel 0 [ PSCCH (SCI-1A) | TB   ] ... [ PSFCH (HARQ) ] [ Guard ]
       └───────────────────────────────────────────────────────────────────────► Time (Slots)
                        Slot n (0.5 ms)                Slot n+1 (0.5 ms)

Unlike LTE-V2X (which uses a single SCI Format 1), NR-V2X decouples channel sensing from transmission control:

2.1 SCI Format 1-A (Stage 1 SCI)

  • Carrier Channel: Physical Sidelink Control Channel (PSCCH).
  • Modulation & Coding: QPSK, Polar coded with 24-bit CRC (CRC-24C).
  • Contents:
  • Priority: 3 bits (0 to 7, where 0 is highest priority).
  • Frequency Resource Assignment (RIV): Resource Indication Value.
  • Time Resource Assignment: Resource gap to future transmissions.
  • Resource Reservation Period (\(P_{rsvp}\)): 0 ms (aperiodic) or periodic interval (e.g. 100 ms).
  • MCS: 5 bits (0 to 28).
  • PSFCH Overhead Indicator: 1 bit indicating whether PSFCH symbols are present.

2.2 SCI Format 2-A (Stage 2 SCI)

  • Carrier Channel: Physical Sidelink Shared Channel (PSSCH).
  • Modulation & Coding: Polar coded with CRC-24C, multiplexed around DMRS.
  • Contents:
  • HARQ Process ID: 4 bits (0 to 15).
  • New Data Indicator (NDI): 1 bit.
  • Redundancy Version (RV): 2 bits (\(0, 2, 3, 1\)).
  • Source Layer-2 ID: 16 bits.
  • Destination Layer-2 ID: 16 bits (or 0xFFFF for broadcast).
  • Cast Type: Broadcast, Groupcast Option 1 (ACK/NACK), Unicast, or Groupcast Option 2 (NACK-only).
  • HARQ Feedback Enabled Indicator: 1 bit.

3. Sensing-Based Mode 2 Autonomous Resource Allocation

Defined in 3GPP TS 38.213 Section 16.6:

flowchart TD
    A["1. Sliding Sensing Window (T0 = 100 ms short / 1100 ms full)"] --> B["2. Selection Window [n + T1, n + T2]"]
    B --> C["Step 1: Exclude Half-Duplex Slots"]
    C --> D["Step 2: Project Sensed SCIs & RSRP Filter"]
    D --> E{"Remaining Candidates >= X%?"}
    E -->|"No: Increase Th_RSRP by 3 dB"| D
    E -->|"Yes"| F["Step 3: Average S-RSSI Sorting -> Lowest X% Pool S_B"]
    F --> G["Step 4: Uniform Random Draw"]
    G --> H["Step 5: Release 16 Re-evaluation Check at slot m - T3"]
    H -->|"Conflict Detected"| D
    H -->|"Safe"| I["Grant Allocated: Transmit at Slot m"]

3.1 Step 1: Candidate Enumeration & Half-Duplex Exclusion

All Candidate Single-Slot Resources within selection window \([n + T_1, n + T_2]\) are identified. Resources where the UE was transmitting during past sensing cycles are excluded due to half-duplex blindness.

3.2 Step 2: Sensed SCI Projection & RSRP Filtering

For each decoded SCI Format 1-A from neighboring UEs:

  • If the reservation overlaps candidate \(R_{x,y}\) in time and frequency:
  • If measured Reference Signal Received Power (\(\text{PSSCH-RSRP}\)) exceeds threshold \(\text{Th}_{RSRP}\), the candidate is excluded.

3.3 Step 3: \(X\%\) Condition & Backoff

If the remaining candidates are fewer than \(X\%\) of total resources (\(X \in \{20\%, 33\%, 50\%\}\)):

  • Threshold \(\text{Th}_{RSRP}\) is increased by \(+3\,\text{dB}\) and Step 2 is re-evaluated.

3.4 Step 4: S-RSSI Energy Ranking

Remaining candidates are sorted by their average past Sidelink Received Signal Strength Indicator (S-RSSI). The lowest \(X\%\) energy subset (\(S_B\)) is formed.

3.5 Step 5: Release 16 Re-evaluation Check (\(T_3\))

At slot \(m - T_3\) before scheduled transmission:

  • The UE evaluates new SCIs received between initial selection and \(m - T_3\).
  • If another UE claimed the slot with higher priority or earlier timestamp, the UE drops the grant and reselects, eliminating packet collisions.

3.6 Step 6: Release 16 Pre-emption

Higher-priority safety traffic (\(p_1 < p_2\)) forcibly claims contested slots, overriding lower-priority reservations during candidate filtering.

  • PSFCH Periodicity (\(N_{PSFCH}\)): Configurable as 0 (disabled), 1, 2, or 4 slots.
  • Timing Gap: Designates the first PSFCH slot with offset \(K_{PSFCH} \ge 2\) slots after PSSCH.
  • Feedback Modes:
  • Unicast: Positive ACK / Negative NACK.
  • Groupcast Option 2 (NACK-Only): Receiving UEs transmit energy on the shared PSFCH slot only if PSSCH decoding fails.
  • Redundancy Version Progression: Retransmissions cycle through:

$\(RV \in [0, 2, 3, 1]\)$

5. Physical Layer BLER Error Model (TS 38.214 Table 5.1.3.1-1)

Modulation and Coding Schemes (MCS 0..28) follow standard 3GPP curves calibrated against the UWICORE dataset under TDL-B / TDL-D vehicular fading:

  • PSCCH (Polar code): Demodulated with QPSK, error threshold at \(\text{SINR} \approx -4.0\,\text{dB}\).
  • PSSCH (NR LDPC): Steep logistic waterfall curve:

$\(\text{BLER}(\gamma) = \frac{1}{1 + \exp\left( \frac{\gamma - \gamma_{th}(MCS)}{0.55\,\text{dB}} \right)} \times \text{clamp}\left( \frac{L_{bytes}}{300}, 0.5, 3.0 \right)\)$

  • Dual Decoding Rule: Reception succeeds if and only if both PSCCH (SCI-1A), PSSCH (SCI-2A), and PSSCH Transport Block decode error-free.

6. Rust Configuration Example

use rustv2xsimulator::phy::nrv2x::{NrV2xConfig, NrV2xMode2Model, NrNumerology};

// 1. Standard 5.9 GHz ITS configuration (30 kHz SCS, 20 MHz bandwidth, 5 subchannels)
let mut cfg = NrV2xConfig::default();
cfg.numerology = NrNumerology::Mu1_30kHz;
cfg.bandwidth_mhz = 20.0;
cfg.subchannel_size_rbs = 10;
cfg.num_subchannels = 5;
cfg.p_rsvp_ms = 100; // 100 ms SPS reservation interval
cfg.pssch_mcs = 10;  // 16-QAM code rate 340/1024
cfg.enable_harq = true;

let mut model = NrV2xMode2Model::new(cfg, 42);