Cooperative Awareness
How the CAM Service Builds a Local Dynamic Map
Cooperative awareness solves a problem that on-board radar, lidar, and cameras cannot: knowing about vehicles that are occluded, around a corner, or beyond optical range. Instead of sensing the environment, every equipped station announces its own kinematic state. The application layer in each on-board unit assembles these announcements into a Cooperative Awareness Message that is handed down through the ITS facilities layer, the GeoNetworking and BTP transport, and finally onto the 5.9 GHz radio access layer. Because the messages are single-hop broadcasts with no acknowledgement, the design priority is fresh, frequent, low-latency delivery rather than guaranteed reception of any individual packet.
The generation rules are tied directly to vehicle dynamics. A new CAM is triggered whenever heading changes by more than 4 degrees, the station moves more than 4 meters, or speed changes by more than 0.5 m/s since the previous message, bounded by a 100 ms minimum and a 1 s maximum interval. This adaptive cadence means a car cornering at speed beacons near 10 Hz while a vehicle idling at a light relaxes toward 1 Hz, matching channel usage to safety relevance. A receiving station maintains a tracked object for roughly 1.1 seconds after the last CAM before the entry is aged out of its local dynamic map.
Channel load is the central RF design constraint. In dense urban intersections, hundreds of vehicles may share a single 10 MHz channel, so ETSI Decentralized Congestion Control (DCC) monitors the channel busy ratio and throttles message rate, transmit power, or data rate once occupancy passes about 62 percent. The link itself is engineered for the harsh mobile multipath of street canyons, where Doppler spread and rapidly changing fading drive the choice of robust low-order modulation over raw throughput.
Beacon Generation and Link Budget
Δheading > 4° OR Δposition > 4 m OR Δspeed > 0.5 m/s
bounded by 100 ms ≤ Tgen ≤ 1000 ms
Channel busy ratio (DCC input):
CBR = tbusy / twindow with throttling above CBR ≈ 0.62
Free-space path loss at 5.9 GHz:
FSPL(dB) = 20·log10(dkm) + 20·log10(fMHz) + 32.44
d = 300 m = 0.3 km, f = 5900 MHz → FSPL ≈ 97.4 dB
Where Tgen = CAM generation interval, CBR = channel busy ratio, dkm = range in kilometers, fMHz = frequency in megahertz. A 23 dBm EIRP beacon over 97.4 dB loss yields about −74 dBm at the receiver, well above a typical −90 dBm 6 Mbps sensitivity floor.
CAM Delivery: DSRC vs. C-V2X
| Parameter | ITS-G5 / DSRC | C-V2X PC5 Mode 4 | Notes |
|---|---|---|---|
| Physical layer | 802.11p / OCB OFDM | LTE/NR SC-FDM sidelink | Both in 5.855 to 5.925 GHz |
| Channel access | CSMA/CA contention | Sensing-based SPS | SPS reserves periodic slots |
| Channel width | 10 MHz | 10 or 20 MHz | Control channel at 5.9 GHz |
| CAM rate | 1 to 10 Hz | 1 to 10 Hz | Same ETSI facilities layer |
| Typical robust rate | 6 Mbps QPSK 1/2 | QPSK, MCS ≤ 11 | Range-optimized, not peak |
| Latency target | < 100 ms | < 100 ms | End-to-end safety budget |
| Payload | 200 to 800 bytes | 200 to 800 bytes | ASN.1 UPER encoded CAM |
Frequently Asked Questions
How often does a vehicle transmit Cooperative Awareness Messages?
ETSI EN 302 637-2 sets an adaptive rate of 1 to 10 Hz. A new CAM fires when heading changes more than 4°, position more than 4 m, or speed more than 0.5 m/s since the last message, bounded by a 100 ms minimum and 1 s maximum interval. A vehicle at highway speed beacons near 10 Hz; a parked one falls to 1 Hz. Decentralized Congestion Control further stretches the interval once the 5.9 GHz channel busy ratio exceeds about 62 percent.
What is the difference between Cooperative Awareness and a DENM event message?
A CAM is a periodic heartbeat every station broadcasts to advertise presence, position, speed, and heading, building the baseline local dynamic map. A Decentralized Environmental Notification Message (DENM) is event-driven, sent only for a hazard such as hard braking, ice, or a stationary vehicle. CAMs are single-hop and not relayed; DENMs can be geo-networked and repeated across a relevance area. Both share the same 5.9 GHz ITS-G5 or C-V2X PC5 link.
What data rate and channel does a CAM use on the 5.9 GHz ITS band?
ITS-G5 uses 10 MHz channels within 5.855 to 5.925 GHz, most often the control channel near 5.9 GHz. The OFDM PHY supports 3 to 27 Mbps, but safety beacons use the robust 6 Mbps QPSK 1/2 rate for range and reliability. A 200 to 800 byte CAM occupies only a few hundred microseconds of air time. C-V2X PC5 Mode 4 carries the identical CAM over SC-FDM sidelink resource pools with semi-persistent scheduling rather than CSMA/CA.