Abstract
Recently, the interest regarding Vehicular Ad-hoc Networks (VANETs) has been rising in industrial and academic fields in view of its crucial role in Intelligent Transportation Systems (ITS) to support safety-related applications that aim to reduce heavy casualty tolls that are caused by vehicle crashes, and provide infotainment-related applications that ensure comfort and efficient journeys for drivers and passengers. In order to support various safety and infotainment applications, Wireless Access in Vehicular Environment (WAVE), which is a standard designed specifically for VANETs, stipulates the multichannel operations, where the overall bandwidth is subdivided into seven channels, namely, one Control Channel (CCH) and six Service Channels (SCHs). In spite of great potential, the distinguished characteristics of VANETs such as intermittent connectivity, time-varying vehicular traffic environment, and fast topology change have presented significant challenges to the design of an efficient Medium Access Control (MAC) scheme and performance analysis for VANETs.The IEEE 802.11 standards are originally designed for small-scale, single-channel-based indoor applications. Their performances in outdoor vehicular environment, where a large number of fast-moving vehicles simultaneously contend for channel access according to the multichannel operations, are still unclear. In the literature, existing analytical models proposed for performance analysis and evaluation of VANETs only consider single channel environment with an ideal channel condition. In fact, the regulations of multichannel access and realistic channel conditions have significant impacts on the performance of VANETs.
In this thesis, in order to study the performance of multichannel VANETs, a unified analytical framework is established. Specifically, a multiserver queueing model, which plays the role of a bridge connecting the uplink (i.e., CCH) and downlink (i.e., SCHs) communications is developed for accurately capturing the dynamics of the multichanneloperations. Based on the developed framework, we show that the saturated throughput of SCHs decreases rapidly as the intensity of channel contention increases. In order to keep the throughput at the maximum level regardless of the channel contention intensity while maintaining the system stability, we propose a centralized coordination mechanism. Extensive simulations are carried out to validate the accuracy of the developed analytical framework and the effectiveness of the proposed centralized coordination mechanism.
For designing VANET MAC scheme, one crucial objective is to improve the reliability of safety applications. Safety applications rely on safety messages broadcast on the CCH, which include the information of sending vehicles. However, fast topology change and high node mobility of VANETs increase the challenges in the design of a MAC scheme to provide reliable broadcasting of safety messages. Another primary objective of MAC scheme design is to efficiently support infotainment applications with large throughput and fairly short delivery delay of service data messages. Users in moving vehicles can request infotainment services (e.g., downloading an MP3 file) to a nearby RSU on the CCH, and then receive the requested service data messages (e.g., an MP3 file) on one of the SCHs as the vehicles pass through the coverage range of the RSU, which is referred to as Drive-thru Internet system. However, together with the environmental dynamics, the complexity of coordinating multiple channels for service data message transfer makes it difficult to fully utilize the SCH bandwidths.
For the purpose of efficiently supporting both safety and infotainment applications, inspired by the analytical framework, we propose a Coordinated, Adaptive and Reliable multichannel MAC scheme for VANETs (VCAR-MAC). In VCAR-MAC, a novel Time Division Multiple Access (TDMA)-based scheme is used to identify every vehicle quickly so that a time slot on the CCH can be allocated efficiently for reliable safety message transmission. On the SCHs, VCAR-MAC provides a multi-SCH coordination scheme, which adaptively adjusts service request period in order to fully utilize the channel bandwidth. Further, a dynamic Contention Window (CW) mechanism is proposed, in which the initial CW size is adaptively optimized in tune with current vehicular traffic environment to maximize the number of successful SCH reservations, thereby maximizing the service data message throughput. It has been proven via mathematical analysis and simulation experiment that VCAR-MAC can significantly improve data throughput and reduce message delivery delay for both safety and infotainment applications.
| Date of Award | 5 Sept 2018 |
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| Original language | English |
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| Supervisor | Chung Sing Victor LEE (Supervisor) |