V2X Communications

Basic Safety Message (BSM)

Q: What data does a BSM contain?

SAE J2735 defines two BSM parts. Part I (mandatory, ~38 bytes): message count, temporary ID (random, rotated for privacy), GPS time, latitude/longitude (1/10 micro-degree precision), elevation, positional accuracy, speed (0.02 m/s resolution), heading (0.0125° resolution), steering wheel angle, longitudinal/lateral acceleration, yaw rate, brake system status, and vehicle size (width/length). Part II (optional, variable): path history (up to 23 past positions), path prediction, event flags (hard braking, ABS active, stability control, hazard lights), and exterior lights status. Total BSM size: 38-300+ bytes depending on Part II content.

Q: What RF technology carries BSMs?

Two competing radio technologies carry BSMs on the 5.850-5.925 GHz ITS band. DSRC (IEEE 802.11p/802.11bd): OFDM-based, 10 MHz channels, 3-27 Mbps, <2ms latency, ad-hoc mode (no infrastructure needed). Used in deployed systems since 2017. C-V2X (3GPP Release 14+ PC5 sidelink): based on LTE/NR sidelink, 10/20 MHz channels, same latency class, with network-assisted resource allocation available but not required. C-V2X offers ~3 dB link budget advantage in NLOS scenarios. Both technologies coexist in the 5.9 GHz band per FCC R&O 20-164. Interoperability requires dual-mode radios.

Q: How do BSMs prevent collisions?

BSMs enable cooperative collision avoidance by sharing real-time kinematic data between vehicles. Safety applications include: Intersection Movement Assist (IMA): warns when a vehicle is about to run a red light or stop sign. Forward Collision Warning (FCW): alerts when closing speed to a leading vehicle exceeds safe braking distance. Emergency Electronic Brake Light (EEBL): propagates hard-braking events beyond line-of-sight. Left Turn Assist (LTA): warns of oncoming traffic during left turns. These applications require BSM reception within 300ms and position accuracy within 1.5 meters (95th percentile). The 10 Hz update rate provides sufficient temporal resolution for threat assessment at highway speeds.

/BAY-sik SAYF-tee MES-ij/

A standardized V2X broadcast defined by SAE J2735 containing vehicle position, speed, heading, acceleration, and brake status. Transmitted 10 times per second on the 5.9 GHz ITS band using DSRC (IEEE 802.11p) or C-V2X (3GPP PC5 sidelink). BSMs enable cooperative awareness for collision avoidance applications including intersection assist, forward collision warning, and emergency brake light propagation. Received within 300 ms with 1.5-meter position accuracy.

Standard: SAE J2735

Rate: 10 Hz (100 ms)

Band: 5.850–5.925 GHz

Understanding the Basic Safety Message

The BSM is the cornerstone of vehicle safety communications. Every equipped vehicle continuously broadcasts its kinematic state, creating a real-time map of all nearby vehicles, even those hidden by buildings, trucks, or terrain. This cooperative awareness fundamentally transforms collision avoidance from a single-vehicle sensor problem (cameras, radar, lidar) into a networked information-sharing system.

The 10 Hz broadcast rate was carefully chosen to balance safety requirements against channel congestion. At highway speeds (120 km/h), a vehicle moves 3.3 meters between broadcasts, providing sufficient position update granularity for threat assessment. The message is short (38 to 300 bytes) and transmitted as a single OFDM burst, minimizing channel occupancy and latency.

BSM Specifications

BSM Part I (mandatory, 38 bytes):
Position: lat/lon (1/10 µ°), elevation
Speed: 0.02 m/s resolution
Heading: 0.0125° resolution
Accel: longitudinal + lateral
Brake status + vehicle size

Channel Requirements:
Frequency: 5.9 GHz (Ch. 172, 10 MHz BW)
TX power: 20 dBm (EIRP)
Range: 300–1000 m (LOS)
Latency: < 100 ms (air interface)

Channel Load (dense scenario):
1000 vehicles × 10 BSM/s × 300 bytes
= 24 Mbps aggregate (near channel limit)

DSRC vs. C-V2X for BSM

Parameter	DSRC (802.11p)	C-V2X (PC5)
Modulation	OFDM, BPSK to 64-QAM	SC-FDMA
Channel BW	10 MHz	10/20 MHz
Latency	< 2 ms	< 5 ms
Range (NLOS)	~200 m	~300 m (+3 dB)
Infrastructure	Not required	Optional (Mode 4)

Common Questions

Frequently Asked Questions

What data does a BSM contain?

Part I (mandatory, 38 bytes): position (lat/lon/elev), speed, heading, accel, brake status, vehicle size. Part II (optional): path history (23 points), event flags (hard braking, ABS), exterior lights. Total: 38 to 300+ bytes.

What RF technology carries BSMs?

DSRC (802.11p): OFDM, <2 ms latency, deployed since 2017. C-V2X (PC5 sidelink): LTE/NR-based, +3 dB NLOS advantage. Both on 5.9 GHz. Coexistence per FCC R&O 20-164. Dual-mode radios for interoperability.

How do BSMs prevent collisions?

Intersection assist (red-light warnings). Forward collision warning (closing speed). Emergency brake light (beyond line-of-sight). Left turn assist (oncoming traffic). Requires: <300 ms reception, 1.5 m position accuracy, 10 Hz updates.

Related Terms

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V2X Communications

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