C-V2X (Sidelink Details)
Understanding C-V2X Sidelink Channels & Structure
To implement C-V2X systems, hardware and protocol engineers must understand the underlying structural details of the 3GPP sidelink specifications. Sidelink communications use Single-Carrier Frequency Division Multiple Access (SC-FDMA) in LTE-V2X and Orthogonal Frequency Division Multiple Access (OFDMA) in 5G-NR V2X to combat multi-path fading in highly dynamic automotive environments. The physical layer divides the 5.9 GHz ITS channel into 10 MHz or 20 MHz bands, which are structured into 1-millisecond slots (sidelink slots) in the time domain, and resource blocks (RBs) in the frequency domain.
Each transmission consists of a control portion and a data portion. The control information is transmitted via the Physical Sidelink Control Channel (PSCCH) as a Sidelink Control Information (SCI) message, specifying the modulation and coding scheme (MCS), frequency allocation, and retransmission parameters. The corresponding payload (such as a Cooperative Awareness Message, or CAM) is carried by the Physical Sidelink Shared Channel (PSSCH). Demodulation Reference Signals (DMRS) are densely embedded within the slot structure to enable accurate channel estimation under high Doppler shifts (up to 500 km/h relative speed).
A defining characteristic of C-V2X is its resource allocation mode. In Mode 3 (LTE) or Mode 1 (5G), resource allocation is controlled dynamically by a base station (gNodeB). This is ideal when network coverage is available. In Mode 4 (LTE) or Mode 2 (5G), resource allocation is completely autonomous. Vehicles use a sensing-based semi-persistent scheduling (SPS) mechanism. Over a sliding window, a vehicle decodes the SCI messages from neighboring cars and measures the received signal strength indicator (RSSI) of all resource blocks. Resources with RSSI values exceeding a specific threshold are marked as occupied. The vehicle then randomly selects its transmission slot from the remaining free resources, reserving it for a set number of cycles before re-evaluating. This minimizes collision probabilities without requiring a central coordinator.
Key Equations
Δf = 15 × 2μ kHz (Typically μ = 0, 1, 2 for 15, 30, 60 kHz)
Sidelink Slot Duration:
Tslot = 1 / 2μ ms (Slot length halves as subcarrier spacing doubles)
Doppler Shift (fD) at Relative Velocity (v):
fD = [ v × fc ] / c
For v = 139 m/s (500 km/h) and fc = 5.9 GHz, fD ≈ 2.73 kHz
Comparison of Sidelink Resource Allocation Modes
| Feature | Mode 3 (LTE) / Mode 1 (5G-NR) | Mode 4 (LTE) / Mode 2 (5G-NR) |
|---|---|---|
| Resource Allocation | Centrally scheduled by cellular base station (gNodeB). | Autonomously scheduled by individual vehicles. |
| Network Coverage Required | Yes (requires active connection to cellular tower). | No (works out-of-coverage, in tunnels, or remote areas). |
| Scheduling Algorithm | Dynamic allocation or network-configured grant. | Sensing-based Semi-Persistent Scheduling (SPS). |
| Primary Channel Interfaces | Uu control link + PC5 sidelink. | PC5 sidelink only. |
| Typical Application | Urban fleet coordination, managed intersections. | Critical safety alerts, rural highways, fallback mode. |
Frequently Asked Questions
What are PSCCH and PSSCH, and how do they interact in C-V2X?
The Physical Sidelink Control Channel (PSCCH) and Physical Sidelink Shared Channel (PSSCH) are the primary physical channels on the PC5 interface. The PSCCH carries the Sidelink Control Information (SCI) packet, which acts as the header for the transmission. The SCI specifies the exact subchannels, MCS, and timing offsets for the payload. The PSSCH carries the actual data payload. A receiving device must first decode the PSCCH to extract the SCI, which then tells it where and how to demodulate the accompanying PSSCH payload in the same slot.
How does 5G-NR V2X improve upon LTE-V2X at the physical layer?
5G-NR V2X (3GPP Release 16) introduces several enhancements over LTE-V2X (Release 14). It transitions from SC-FDMA to flexible OFDMA, allowing adjustable subcarrier spacing (15, 30, or 60 kHz) to optimize performance against Doppler spread. It supports higher-order modulation (up to 64-QAM) and feedback-based unicast/multicast communications via a Physical Sidelink Feedback Channel (PSFCH), enabling Hybrid Automatic Repeat Request (HARQ) retransmissions. These features increase throughput and reliability for cooperative maneuver planning and raw sensor sharing.
What is the function of the Demodulation Reference Signal (DMRS) in C-V2X sidelink slots?
The DMRS is a reference signal embedded in specific resource elements of the sidelink slot. Because vehicles move at high relative velocities, the wireless channel changes rapidly (high Doppler spread), causing phase and amplitude distortion. To track these changes, C-V2X uses a dense DMRS pattern (up to 4 symbols per slot in high-speed configurations). The receiver compares the received DMRS symbols with their known reference values to estimate the channel response, allowing accurate demodulation of the data payload in the rest of the slot.