Throughput and Energy Efficiency of Cooperative ARQ Strategies for VANETs Based on Hybrid Vehicle Communication Mode

Vehicular ad-hoc networks (VANETs) are promising research areas which mainly include three communication modes: vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I) and hybrid vehicle communication (HVC). But most of the current research on HVC mode in which vehicle nodes and infrastructures coexist only focuses on the analysis of the optimal single-type relay selection schemes. Inspired by this, in order to design an optimal multi-relay selection scheme which can select different types of relays simultaneously, and to compare it with single-type schemes, this paper firstly introduces a more practical network scenario by proposing a four-node system model which considers communication links between relays and building different channel models for different types of links. Four optimal relay selection protocols, which are named as Non-OR, SOR-AP, SOR-V and DOR-APV respectively, are then designed from the perspective of the different types and numbers of selected relays. The exact expressions of outage probability for each protocol are calculated based on the automatic repeat request (ARQ) protocol to evaluate the link interruption of the four protocols. Furthermore, this paper establishes a generalized 2-dimensional discrete time Markov chain (DTMC) model and analyzes the one-step state transition probability for each protocol. The closed-form expressions of system throughput and energy efficiency are derived by calculating the steady state distribution of the DTMC. Finally, the simulation results summarize the suitable network scenarios for which the four proposed protocols are recommended by comparing their system performance, so as to provide some suggestions for the future design and optimization of vehicle mobile networks.