Wireless access vehicle environment (WAVE) architecture of intelligent transportation system has been standardized in IEEE 802.11p standard and it is going lobe widely deployed in many roadway environments in order to provide convenient services. However, the IEEE 802.11p contention-based medium access control protocol would significantly downgrade transmission efficiency between road side unit (RSU) and on-board units (OBUs) because the number of OBUs served by a RSU could be large and the transmission rate between RSU and an OBU may be different from the others. This paper processes a simple vehicular grouping access (VGA) strategy to reduce access latency and opportunities of collisions on a service channel (SCH) by equally distribute vehicles onto multiple SCHs. Since the RSU periodically broadcast the WAVE service advertisement (WSA) frames on the control channel (CCH), the proposed VGA strategy makes an OBU determine to join an appropriate SCH according to the RSSI threshold information of each SCH, which is carried in WSA frame, and the signal strength indicator (RSSI) while receiving WSA frame. Using appropriate RSSI thresholds, the system could equally allocate OBUs to all available SCHs. This paper futher investigates the appropriate thresholds for three popular roadway scenarios to achieve the goal of load balancing. Simulation results demonstrate that the proposed VGA strategy not only improves the channel capacity but also provides useful information on roadway planning in IEEE 802.11p wireless vehicular networks.
