scope:

ITS road safety and traffic efficiency systems include both vehicle to vehicle communications and related vehicle to roadside communication in highly dynamic vehicular ad hoc networks. These systems (ITS stations) are based on a set of protocols and parameters called ITS-G5 as specified in the European profile standard on the physical and medium access layer of 5 GHz ITS [2].

Many applications and services in ITS rely on the cooperative behavior of the vehicles and roadsides units which form a vehicular ad hoc network (VANET). The VANET enable the time critical road safety applications where fast information exchange is necessary to timely warn and support the driver. Special care should be taken to ensure proper functioning of the VANET and this includes decentralized congestion control (DCC) for the channels of ITS-G5.

DCC is a cross layer function, i.e. it has functions located on several layers of the ITS station reference architecture. Therefore the present document defines which DCC components are located on which layer of the ITS station communication architecture [5]. Furthermore the present document specifies the DCC mechanisms on the access layer (DCC_access) including transmit power control (TPC) per packet, transmit rate control (TRC) and transmit datarate control (TDC). The latter two control functions modify the average transmit power by modifying the duty cycle of the ITS station, i.e. the fraction of time that the ITS station is in "transmit" state. Additionally, DCC sensitivity control (DSC) adapts the clear channel assessment to resolve local channel congestion. Packets with higher priority are handled less restrictive introducing a transmit queueing concept and transmit access control (TAC).

The DCC mechanisms rely on knowledge about the channel. The channel state information is gained using channel probing. Channel probing measures are defined that enable the DCC methods TPC, TRC and TDC. The measures are on receive signal level thresholds or preamble information of detected packets.

The present document does not define the mechanisms at other layers than the access layer nor defines management aspects. These other mechanisms and the management aspects are necessary in order to make DCC work properly. The present document is primarily intended for trial use and may need to be updated after validation in field trials and/or other projects.