**1. Introduction**

Vehicular ad-hoc networks (VANETs) can provide multimedia communication between vehicles with the aim of providing efficient and safe transportation [1]. Vehicles with different sensors can exchange and share information for safely breaking, localization and obstacle avoiding. Moreover, the sharing of traffic accident's live video can improve the rescue efficiency and alleviate traffic jams. However, video transmission has been considered as a challenging task for VANETs, because video transmission over VANETs can significantly increase bandwidth [2]. This work focuses on the development of video codec that supports real-time video transmission over VANETs for road safety applications.

The demanding challenges of VANETs are bandwidth limitations and opportunities, connectivity, mobility, and high loss rates [3]. Because of the resource-demanding nature of video data in road safety applications, bandwidth limitations is the bottleneck for real-time video transmission over VANETs [4,5]. Moreover, due to the limited vehicle node's battery lifetime, video delivery over VANETs remains extremely challenging. Essentially, the low-complexity video encoder can accelerate the video transmission in real-time, and achieve low delay for video streaming. Hence, in order to transmit real-time video with bandwidth constraint, it is vital to develop an efficient video encoder. A video encoder with high encoding efficiency and low encoding complexity is the core requirement of VANETs [6].

Concurrently, the main video codecs are High Efficiency Video Coding (HEVC, or H.265) that are developed by the Joint Collaborative Team on Video Coding (JCT-VC) Group, and AV1 that is developed by Alliance for Open Media (AOMedia) [7]. Nevertheless, video codec is faced with several challenges. AV1 is a newer codec, royalty free and open sourced. However, the hardware implementation of AV1 encoder will take a long time. H.265/HEVC is the state-of-the-art standardized video codecs [8]. Compared with H.265/HEVC, AV1 increases the complexity significantly without achieving an increase in coding efficiency. When supporting the most available (resource-limited/mobile) devices or having a need for real-time, low latency encoding, it would be better to stick to H.265/HEVC. However, the encoding complexity of H.265/HEVC encoder increases dramatically due to its recursive quadtree representation [9]. Although previous excellent works have been proposed for reducing H.265/HEVC encoder complexity [10–14], most of them balance the encoding complexity and encoding efficiency unsuccessfully.

To address this issue, spatial and temporal information is widely used to reduce the computation redundancy of the H.265/HEVC encoder. However, the spatiotemporal correlation between the coding unit and the neighboring coding unit is not better used. To the author's best knowledge, complexity reduction in real-time coding with the available computational power at the VANET nodes has not been well studied, especially from the viewpoint of hardware implementation. The key contributions of this work are summarized as follows:


The rest of paper is organized as follows. Related works are reviewed in Section 2. Section 3 discusses background details. In Section 4, the fast CU decision algorithm is presented. Simulation results are discussed in Section 5. Section 6 concludes this work.
