3GPP C-V2X PC5 Sidelink Mode 4

Overview & Standards Provenance

Cellular Vehicle-to-Everything (C-V2X) PC5 Sidelink Mode 4, standardized in 3GPP Release 14/15 (LTE-V2X), allows direct peer-to-peer vehicular communications in the 5.9 GHz ITS band (3GPP Band 47) without requiring cellular tower (eNodeB) coverage or SIM-card subscription.

In contrast to the asynchronous CSMA/CA carrier sense mechanism of IEEE 802.11p, C-V2X Mode 4 organizes communication into a synchronized 2D Time-Frequency Resource Grid using Sensing-Based Semi-Persistent Scheduling (SB-SPS).

1. Time-Frequency Sidelink Resource Grid

                         C-V2X PC5 MODE 4 RESOURCE GRID
      Frequency (RBs)
        ▲
   50 RB│ Sub-channel 4 [ PSSCH (Data) ] [ PSSCH (Data) ] ...
   40 RB│ Sub-channel 3 [ PSSCH (Data) ] [ PSSCH (Data) ] ...
   30 RB│ Sub-channel 2 [ PSSCH (Data) ] [ PSSCH (Data) ] ...
   20 RB│ Sub-channel 1 [ PSSCH (Data) ] [ PSSCH (Data) ] ...
   10 RB│ Sub-channel 0 [PSCCH (SCI)|TB] [PSCCH (SCI)|TB] ...
        └───────────────────────────────────────────────────► Time (TTI)
                         Subframe t (1ms)  Subframe t+1 (1ms)

1.1 Channel Bandwidth Options

RustV2XSimulator supports standard 10 MHz and wideband 20 MHz channel configurations:

Parameter	Standard 10 MHz Channel	Wideband 20 MHz Channel
Total Resource Blocks (RBs)	50 RBs (180 kHz / RB)	100 RBs
Sub-channel Size	10 RBs (1.8 MHz)	10 RBs (1.8 MHz)
Active Sub-channels per Subframe	5 sub-channels	10 sub-channels
Subframe Duration (TTI)	1.0 ms	1.0 ms
Candidate Resources per 100 ms Window	500 CSRs	1,000 CSRs

2. Physical Channels & Dual Decoding

Sidelink transmissions multiplex two physical channels:

PSCCH (Physical Sidelink Control Channel):
Occupies 2 Resource Blocks.
Carries Sidelink Control Information (SCI Format 1), which announces transmitter priority (PPPP), Resource Reservation Interval (\(P_{\text{rsvp}}\)), sub-channel index, and MCS.
Demodulated using robust QPSK rate 1/2 with an error transition around \(\text{SINR} \approx -2.5\,\text{dB}\).
PSSCH (Physical Sidelink Shared Channel):
Occupies the remaining Resource Blocks in the allocated sub-channel.
Carries the actual transport block payload (ETSI CAM, DENM, or custom message).
Demodulated using Turbo coding (MCS 0..28, default MCS 7: QPSK 0.58) with an error transition around \(\text{SINR} \approx +3.5\,\text{dB}\).

[!IMPORTANT] Dual Decoding Condition: In compliance with 3GPP Rel-14, an incoming frame is successfully delivered if and only if both the PSCCH (SCI) and PSSCH (TB) decode without error. If the SCI fails, the receiver cannot locate or demodulate the accompanying data block.

3. Sensing-Based Semi-Persistent Scheduling (SB-SPS)

The core autonomous MAC protocol is defined in 3GPP TS 36.213 Section 14.1.1.6:

flowchart TD
    A["1. Sensing Window (1000 ms)"] -->|"Track SCIs & S-RSSI over past 1000 subframes"| B["2. Selection Window [t + T1, t + T2]"]
    B --> C{"Step 1: Exclusion"}
    C -->|"Exclude Half-Duplex Subframes"| D["Step 1b: RSRP Collision Check"]
    D -->|"Exclude if Reserved SCI and RSRP >= Th_RSRP"| E{"Remaining >= 20%?"}
    E -->|"No: Increase Th_RSRP by 3 dB"| D
    E -->|"Yes"| F["Step 2: 20% Lowest Energy Pool S_B"]
    F -->|"Average S-RSSI sorting"| G["Step 3: Uniform Random Draw"]
    G --> H["Grant Allocated: Reserve for C_resel in [5, 15] periods"]

3.1 Step 1: Resource Exclusion

When a packet needs scheduling at subframe \(t\), all Candidate Single-Subframe Resources (CSRs) within the selection window \([t + T_1, t + T_2]\) (typically \([t + 4\,\text{ms}, t + 100\,\text{ms}]\)) are examined (Set \(S_A\)):

Half-Duplex Constraint: A vehicle cannot receive while transmitting. If the node transmitted during subframe \((x - j \cdot P_{\text{rsvp}})\) in its 1,000 ms sensing history, resource \(x\) is excluded.
RSRP Thresholding: If an SCI from another vehicle reserved resource \(x\), and its measured Reference Signal Received Power (\(\text{PSSCH-RSRP}\)) exceeds the threshold \(\text{Th}_{\text{RSRP}}\) (default \(-110\,\text{dBm}\)), the resource is excluded.

3.2 Step 2: 20% Condition & Threshold Backoff

If the remaining candidates in \(S_A\) are fewer than \(20\%\) of the total available resources, the exclusion threshold \(\text{Th}_{\text{RSRP}}\) is increased by \(+3\,\text{dB}\), and Step 1 is re-evaluated until at least \(20\%\) remain.

3.3 Step 3: Energy Ranking & Candidate Selection

The remaining candidate resources are sorted by their average past Sidelink Received Signal Strength Indicator (S-RSSI). The lowest \(20\%\) energy subset (\(S_B\)) is formed.

3.4 Step 4: Random Draw & Reselection Counter

A single resource is drawn uniformly at random from \(S_B\). A Reselection Counter (\(C_{\text{resel}}\)) is drawn:

\[C_{\text{resel}} \sim \mathcal{U}[5, 15]\]

The resource is used periodically every \(P_{\text{rsvp}} = 100\,\text{ms}\). With each transmission, \(C_{\text{resel}}\) decrements by 1. When it reaches 0:

With probability \(P_{\text{keep}} \in [0.2, 0.8]\) (default 0.4), the vehicle keeps the resource and resets \(C_{\text{resel}}\).
Otherwise, a new resource is reselected.

4. Interaction with Adaptive ETSI CAM Generation

A frequent question in European C-ITS deployment is: How does periodic SPS reservation interact with aperiodic, kinematic-driven CAMs (ETSI EN 302 637-2)?

RustV2XSimulator implements seamless co-existence:

Nominal CAMs (Periodic Driving): When vehicles cruise at steady speeds, kinematic trigger intervals stabilize around \(100\,\text{ms}\) (\(10\,\text{Hz}\)). The scheduler reuses the active SPS reservation, avoiding channel contention and achieving near-zero packet delay.
Kinematic Triggers (Turning / Braking): When an emergency braking event or sharp curve triggers a CAM earlier than expected (\(< 100\,\text{ms}\)), the scheduler checks if the upcoming reserved SPS slot falls within the packet latency tolerance (\(T_2 \le 100\,\text{ms}\)).
If the reserved slot is within tolerance, it is utilized.
If no grant exists or the deadline would be violated, an immediate autonomous reselection is triggered, preventing stale information without creating packet collisions.

5. Usage in Rust Code

Instantiating and running C-V2X Mode 4 within RustV2XSimulator:

use rustv2xsimulator::phy::cv2x::config::Cv2xConfig;
use rustv2xsimulator::phy::technology::Cv2xMode4Model;

// Configure standard 10 MHz channel
let mut cv2x_cfg = Cv2xConfig::default();
cv2x_cfg.bandwidth_mhz = 10.0; // 50 RBs, 5 sub-channels
cv2x_cfg.p_rsvp_ms = 100;     // 100 ms SPS reservation interval
cv2x_cfg.tx_power_dbm = 23.0; // 200 mW

let mut model = Cv2xMode4Model::new(cv2x_cfg, 42);