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To improve the throughput performance of the Wireless Access in Vehicular Environment (WAVE) system, we propose a multi-channel MAC protocol that is able to adaptively adjust the intervals of Control Channel (CCH) and Service Channel (SCH) according to the probability distribution of the reservation time for service packet in CCH Interval. Numerical results show that our protocol can significantly improve the performance of WAVE system. Introduction WAVE system is a radio communications system intended to provide seamless, interoperable services to transportation. IEEE802.11p [1] defines a single channel MAC for WAVE vehicles. Recently, the IEEE1609.4 [2] is considered as a default multi-channel MAC standard for WAVE system, which defines the general framework for multi-channel management. This WAVE MAC adopts the split phrase mode [3] to coordinate multiple channels. Channel access time is divided into Synchronization Intervals (Sync Interval) with a length of 100ms, consisting of a CCH Interval and a SCH Interval, 50ms of each. During CCH Interval, all the devices have to monitor CCH, where safety frame, vehicle status frame, and WAVE service announcement (WSA), are delivered. When SCH Interval arrives, devices can optionally switch to SCHs, which are used for non-safety applications [4]. In a congested vehicular traffic condition, the limited length of CCH is unable to provide sufficient bandwidth to deliver a large amount of safety packets and control packets. On the other hand, if the vehicle density is sparse, the occasional transmission on the CCH channel will waste some of CCH interval, so that some large bandwidth consuming applications cannot obtain enough bandwidth resource on the SCHs. Hence the fixed length of CCH interval of WAVE MAC has certain limitations in improving the performance of WAVE system. This is confirmed by data in [5]. Q. Wang et al. [6] proposed a Variable CCH Interval (VCI) MAC protocol which can optimize the intervals based on the average time of reservation for service packet in CCH interval. In this paper we propose a dynamic CCH interval for vehicle environment, named Dynamic CCH Interval (DCI) MAC protocol in WAVE system, which is able to adaptively adjust the intervals of CCH and SCH according to the probability distribution of the reservation time for service packet in CCH Interval. An algorithm for our protocol is employed to calculate the duration of optimal CCH interval based on the traffic condition. The Proposed Protocol Multi-Channel Coordination Mechanism. We use a coordination mechanism that provides contention-free SCHs by the channel reservation on CCH. As show in Fig.1, CCH interval is divided into Safety Interval and WSA Interval. A new CCH interval begins from the Safety Interval, during which vehicles transmit safety information. It is known that Tsa is proportional to the number of vehicles in WAVE system and inverse proportion to the transmission data rate of CCH [6]. Proceedings of the 2nd International Conference On Systems Engineering and Modeling (ICSEM-13) Published by Atlantis Press, Paris, France. © the authors, 2013 0797 During the CCH Interval, service providers broadcast WSA packets, piggyback with the identity of the offered service and SCH ID. Other nodes that need the service can respond with an acknowledgment (ACK). Then, a positive ACK will indicate the access interval and the SCH ID to be used. After the CCH interval, nodes optionally switch to SCHs and transmit service data. The duration of CCH interval should be optimized to achieve the best case, i.e., the number of reservations made on CCH equals the number of packets transmitted on all SCHs.

Performance Analysis of A Multi-channel MAC with Dynamic CCH Interval in WAVE System