In this chapter, the probability of establishing a connectivity path between vehicles residing in dark areas of a roadway and a remote RSU is presented. In this context, this study examines the availability of intermediate vehicles serving as relay nodes. In addition, knowing that a packet undergoes multiple V2V hops in order to reach its final destination, the number of hops is characterized given the distance travelled by a packet on a per-hop basis. The conducted analyses were validated through extensive simulations. The results show that the probability of having an available path is directly proportional to the vehicular density, and inversely proportional to the distance between the source vehicle and the destination RSU. Next, the average end- to-end delivery delay was analysed after carefully examining the events that alter the network’s topology. A tight theoretical upper bound was established and validated
through simulations. Also, a Markov framework is established for the purpose of evaluating the per-vehicle and network throughputs. Results show the significant impact of the path availability for large separation distances between source and destination as well as the collision probability under high vehicular densities.
According to [61], the ITS can play a significant role in offloading the dense cellular infrastructure. As such, the availability and reliability of a connectivity path between an arbitrary vehicle and the closest RSU becomes remarkably important and worth further investigation.
In the study considered herein, the connectivity path is established using multiple V2V hops where a single V2V link is formed between a vehicle and the farthest one within its communication range. Note that, such a link may be vulnerable given the dynamic topology changes of a vehicular network. Therefore, optimization methods may be exploited in order to choose the highest reliability/lifetime connectivity path between vehicles and distant RSUs.
Another interesting research idea arises as multiple RSUs are deployed in tandem along a roadway, each of which providing its own set of services. A vehicle now may choose not to transmit its packets to the closest RSU, but instead, attempts to adopt the best promising connectivity path in terms of availability and delay. Machine learning techniques present themselves as strong candidates which may be utilized in order to optimize the selection of the best available connectivity path.
Chapter 4
A Vehicle’s Perspective of MAC
Schemes
4.1
Introduction
V2I Communication is the wireless exchange of critical, safety, and operational data between vehicles and highway infrastructure, intended primarily to avoid acci- dents and enable a wide range of other safety, mobility, and environmental benefits. V2I communications have been rapidly advancing and paved their way to becoming among the fundamental contributors to transportation intelligence. Over the past few years, the performance of V2I communication systems has received significant attention. Indeed, the literature encloses numerous seminal publications revolving around the mathematical modelling and performance analysis of such systems.
This chapter’s fundamental contributions can be summarized as follows:
1. The presentation of a modelling approach that differs from the existing work in the literature as it evaluates the performance of the V2I communication system as seen from the angle of any arbitrary vehicle residing within the coverage
range of a RSU. Observe that, by looking at the V2I scheme from the RSU’s perspective, several events may occur and complicate the theoretical modelling of the RSU’s buffer. For instance, and according to [62], vehicles’ Service Re- quests (SRs) queue into the RSU’s buffer until opportunities arise for them to be served. However, in that case, upon the departure of a vehicle from the RSU’s range, all of its associated queueing SRs at the RSU will be discarded and any of its SRs receiving service will be subject to service force-termination. Given the elevated complexity of capturing the dynamics of a V2I system from the RSU’s point of view, this study deviates to viewing the access scheme from the vehicle’s perspective.
2. The proposal of two Medium Access Control (MAC) schemes, namely: a) Ran- dom Vehicle Selection (RVS) and b) Least Residual residence Time (LRT). Under RVS, a RSU will grant access to a single vehicle being uniformly se- lected among all of the vehicles present within that RSU’s coverage range. The presentation of RVS herein has the purpose of clearly describing the channel assignment and access regulation mechanisms underlying a V2I communication system. In contrast, LRT implements vehicle prioritization based on the obser- vation that faster vehicles will reside within the RSU’s range for shorter periods of time than slower vehicles. Under such conditions, servicing the faster vehicles first is expected to increase bandwidth utilization efficiency and, hence, improve the system’s throughput performance.
3. The development of mathematical frameworks leading to the formulation of single-server queueing models to represent the V2I system’s operation and per- formance under both RVS and LRT. Together with their remarkable simplicity and the analytical tractability of their respective solutions, a distinguishing fea- ture of these models is their ability to capture the system’s dynamics from a
vehicle’s perspective with an accuracy that bypasses that of their much more complex existing counterparts. This is especially true since these models are built on top of a vehicular mobility model that accounts for fundamental macro- scopic parameters characterized in Vehicular Traffic Engineering and Theory. In particular, these models lead to the development of closed-form solutions for:
a) the per-vehicle throughput towards the time vehicles depart from the RSU’s
range as well as b) the packet service time and c) the system’s response time.
The remainder of this chapter is organized as follows. Section 4.2 surveys a se- lection of the existing publications and highlights the major contributions that dis- tinguish the work presented in this chapter from its existing counterparts. Section 4.3 lays out the traffic model adopted in this study. Section 4.4 presents a detailed description of two novel V2I MAC access methods. Section 4.5 is dedicated for the modelling and analysis of the vehicle’s OnBoard Unit Buffer’s (OBUB) queueing sys- tem under the two proposed MAC schemes. The performance evaluation results are reported in Section 4.6, and finally, concluding remarks are pointed out in Section 4.7.