Vehicular networking is potentially an effective solution to the problems in today's transportation system. However, realizing efficient communications among vehicles with satisfactory reliability and latency requirements is challenging, especially when diverse applications are taken into consideration. In this paper, we investigate a cross-layer energy-efficient transmission design for a class of vehicular communication networks, in which two pairs of vehicle-to-vehicle links non-orthogonally

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... hare the available spectrum. Each link desires to deliver two types of messages that can support different delay-limited applications. The periodically-generated heartbeat messages should be transmitted subject to a reliability requirement, and the randomly-appeared sensing messages should be delivered with finite latency. We propose a power control strategy to achieve high energy efficiency, while ensuring the expected quality-of-service requirements, based on both channel state information in the physical layer and queue state information in the media access control layer. Simulation results show that our proposed method notably outperforms conventional methods. Index Terms Cross-layer design, Energy efficiency, Lyapunov optimization, Vehicular networks I. INTRODUCTION Road safety, traffic efficiency and energy consumption have become the major concerns in modern road transportation systems. Establishing an intelligent transportation system (ITS) using information and communication technologies (ICT) has been widely accepted as the key solution to these issues. Equipping vehicles with advanced sensing and computing devices enables human drivers and artificial intelligence (AI) driving engines to attain a better understanding of the complex driving environment. Using wireless communication to connect vehicles and roadside infrastructure further enhances the sensing range and accuracy of individual vehicles. For instance, a typical vehicular networking use case is illustrated in Fig. 1 . Under the concept of cooperative adaptive cruise control (CACC), the vehicles V S 1 and V D 1 form a platoon. The leading vehicle V S 1 can be driven by human or AI, and V D 1 intends to closely follow V S 1 . If V S 1 periodically sends its status (e.g., speed and location) to V D 1 , the following vehicle can attain accurate knowledge regarding V S 1 to direct its driving operations. When V S 1 detects certain objects of interest in front of it, it can also share such information with V D 1 to enhance the latter's sensing range. The coordinated sensing and maneuvering actions of the platoon allow the vehicles to safely drive with high speed and small intervehicle distance. Hence ITS has been deemed to be a core application scenario of 5G technologies [1] . With the support of strong communication capability, vehicles can even share sensing, computing, storage, and communication resources to realize the concept of vehicular cloud networks and fulfill advanced tasks far beyond individual vehicle's capability [2] . However, enabling high-performance communication in vehicular networks is challenging, due to the complex signal propagation environment. Different applications pose diverse quality-of-service (QoS) requirements on delay, accuracy and throughput etc. Efficient and reliable transmission designs are needed, especially when multiple users coexist and the available resources (e.g., bandwidth and battery) are limited.