A New Media Access Control Protocol For VANET: Priority R-ALOHA (PR-ALOHA)

More practical applications of Media Access Control (MAC) protocols arise as the world turns increasingly wireless. Low delay, high throughput and reliable communication are essential requirements for standard performance in safety applications (e.g., lane changes warning, pre-crash warning and electronic brake lights). In particular, multi-priority protocols are important in Vehicular Ad Hoc Networks (VANETs), specifically in Inter-Vehicle Communication (IVC) where safety messages are given higher priority and transmitted faster than normal messages. The R-ALOHA protocol is considered one of the few promising protocols for VANETs because it is simple to implement and suitable for medium access control in Ad Hoc wireless networks. However, R-ALOHA lacks the property of prioritizing the different messages. In this dissertation, a new two-level priority MAC protocol called Priority R-ALOHA (PR-ALOHA) is presented to overcome the lack of priority problem in R-ALOHA. The two levels are low priority and high priority where priority is introduced by reserving specific time slots in the frame exclusively for high priority messages. This effectively increases the number of slots that a high priority message may compete for and thus decreases its delay. A two-dimensional Markov model coupled with Monte Carlo simulation is introduced to investigate the dynamic behavior of PR-ALOHA in steady and transient states. Modeling and simulation results of PR-ALOHA show that PR-ALOHA improves the performance of high priority traffic with limited effect on normal network traffic. Then, a dynamic slot allocation algorithm is introduced to PR-ALOH to optimize slot usage. Finally, a mobility model is introduced to emulate the behavior of the vehicles on the road where the performance of the PR-ALOHA with variable parameters, such as the length of the highway, the vehicle transmission range and the number of vehicles on the road have been investigated. Based on the findings of this dissertation, PR-ALOHA combined with dynamic slot allocation and mobility has a potential in applications like IVC where it can prevent car accidents through faster channel access and rapid transfer of warning messages to surrounding vehicles

Link-Layer Cooperative Communication in Vehicular Networks

Vehicular ad hoc networks (VANETs) are a special kind of communication networks and possess unique characteristics as compared with general mobile ad hoc networks (MANETs), where vehicles communicate with each other or with stationary road side units. Hence, directly applying the existing communication protocols designed for MANETs may not be reliable and efficient in VANETs. Thus, this thesis presents link-layer cooperative frameworks to improve transmission reliability and network throughput over distributed TDMA MAC protocols for VANETs. We present a link-layer node cooperation scheme for VANETs, referred to as Cooperative ADHOC MAC (CAH-MAC). In CAH-MAC, neighboring nodes cooperate to utilize unused time slots to retransmit failed packets. Throughput improvement is achieved by using idle time slots that are wasted in the absence of node cooperation. In addition, as a packet is retransmitted earlier by a relay node, transmission delay and packet dropping rate are reduced. We study the effects of a dynamic networking environment on the performance of CAH-MAC. It is observed that, system performance degrades due to cooperation collisions. To tackle this challenge, we present an enhanced CAH-MAC (eCAH-MAC) scheme. In eCAH-MAC, using different types of packet and by delaying or suspending some relay transmissions, cooperation collisions can be avoided and cooperation opportunities can be efficiently utilize without disrupting the normal operations of the distributed TDMA MAC. We propose a node cooperation based makeup strategy for vehicular networks, referred to as cooperative relay broadcasting (CRB), such that neighboring nodes proactively rebroadcast the packet from a source node. An optimization framework is developed to provide an upper bound on the CRB performance with accurate channel information. Further, we propose a channel prediction scheme based on a two-state first-order Markov chain, to select the best relaying node for CRB. As packets are repeatedly broadcasted by the neighboring nodes before they expire, the proposed CRB framework provides a more reliable broadcast service as compared with existing approaches. The proposed node cooperation frameworks enhance the performance of distributed TDMA MAC and make it more robust to tackle VANET's dynamic networking conditions