Abstract
Connected and fully automated vehicles are expected to revolutionize our mobility in the near future on a global scale, by significantly improving road safety, traffic efficiency, and traveling experience. Enhanced vehicular applications, such as cooperative sensing and maneuvering or vehicle platooning, heavily rely on direct connectivity among vehicles, which is enabled by sidelink communications. In order to set the ground for the core contribution of this paper, we first analyze the main streams of the cellular-vehicle-to-everything (C-V2X) technology evolution within the Third Generation Partnership Project (3GPP), with focus on the sidelink air interface. Then, we provide a comprehensive survey of the related literature, which is classified and critically dissected, considering both the Long-Term Evolution-based solutions and the 5G New Radio-based latest advancements that promise substantial improvements in terms of latency and reliability. The wide literature review is used as a basis to finally identify further challenges and perspectives, which may shape the C-V2X sidelink developments in the next-generation vehicles beyond 5G.
Highlights
We are entering a new mobility era, characterized by an increasing number of vehicles which are connected to the Internet and to neighboring road elements, such as other vehicles, pedestrians, traffic lights, or roadside units
These three combined features contribute to a link budget improvement, which directly translates into extended coverage at fixed transmit power or enhanced reliability at fixed range. 5GV2X sidelink transmissions use cyclic prefix orthogonal frequency-division multiplexing (CP-OFDM), which ranks best on the performance indicators that matter most, namely compatibility with multi-antenna technologies, high spectral and throughput efficiency, and low implementation complexity
Inefficiencies with packets of variable size and proposed modification Using sub-reservations reduces bursts of errors and allows improved decentralized congestion control (DCC) Long bursts of errors avoided by limiting the number of times a resource can be maintained Using two resources alternately reduces the probability of consecutive collisions Blind random allocation of retransmissions over secondary subchannels can improve packet reception ratio (PRR) Mitigated impact of attacks and persistent collisions through jittering reservations semi-persistent scheduling (SPS) periodicity set based on collision estimation and coordination, with dedicated short range communication (DSRC) as backup Emphasis on more recent values lets vehicles better estimate future resources status Continuous sensing and cooperative learning; Q-learning based power control
Summary
We are entering a new mobility era, characterized by an increasing number of vehicles which are connected to the Internet and to neighboring road elements, such as other vehicles, pedestrians, traffic lights, or roadside units. Wireless communications will be the key to boost their perception capability of the surrounding environment and to enable exchanging manoeuvring intentions This will allow the vehicles to cooperatively drive and build an extended (even in non-line of sight) horizon, so to promptly and accurately detect the presence of nearby cars, objects and other road users, and to react . The first set of radio standards enabling V2X communications was based on the IEEE 802.11 technology [1], and was referred to as dedicated short range communication (DSRC)/wireless access in vehicular environment (WAVE) This technology has long been considered, in both Europe and the US, as the only key enabler for V2X connectivity.
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