Vehicle onboard communication terminal and mobile communication system

A mobile communication system that establishes vehicle-to-vehicle communication and road-to-vehicle communication by using a carrier wave having an identical carrier frequency and employs Carrier Sense Multiple Access/Collision Avoidance method as an access method, the mobile communication system comprising:

• a plurality of vehicle onboard communication terminals;
• and
• a roadside device,
• wherein:
• the roadside device includes
• a roadside device transmission processing portion that performs a process of transmitting road-to-vehicle communication data, and
• a roadside device transmission control portion that detects usage of the carrier frequency, and when the carrier frequency is determined to be unused, operates so as to transmit the road-to-vehicle communication data;
• each of the vehicle onboard communication terminals includes
• a reception processing portion that receives the road-to-vehicle communication data transmitted from the roadside device and vehicle-to-vehicle communication data transmitted from a different vehicle onboard communication terminal,
• a transmission processing portion that performs a process transmitting the road-to-vehicle communication data and the vehicle-to-vehicle communication data,
• a transmission control portion that controls an operation of the transmission processing portion and has a regular transmission mode and a passive transmission mode as a mode of controlling the operation of the transmission processing portion, and
• a determination portion that determines based on determination data whether to place the transmission control portion in the passive transmission mode, the determination data determining whether a transmission of the vehicle-to-vehicle communication data from the vehicle onboard communication terminal is likely to obstruct the transmission of the road-to-vehicle communication data from the roadside device or from the different vehicle onboard communication terminal;
• the transmission control portion operates in the passive transmission mode when the determination portion has determined that the transmission control portion is to be placed in the passive transmission mode, and operates in the regular transmission mode when the determination portion has not determined that the transmission control portion is to be placed in the passive transmission mode;
• the transmission control portion detects the usage of the carrier frequency when the transmission control portion is operating in the regular transmission mode, and operates so as to successively transmit the road-to-vehicle communication data and the vehicle-to-vehicle communication data when the carrier frequency is determined to be unused;
• and
• the transmission control portion stops the transmission of the vehicle-to-vehicle communication data and receives data addressed to the vehicle onboard communication terminal from the roadside device when the transmission control portion is operating in the passive transmission mode, and operates so as to transmit the road-to-vehicle communication data when a response needs to be transmitted in response to the received data.