Millimeter-Wave Vehicle-to-Infrastructure Communication: Challenges and Potential Solutions
Written by Ananya Chattopadhyay and Aniruddha Chandra
Smart transportation covers a broad spectrum: automatic toll collection, navigational aids, smart parking, driver assistance, emergency warning, collision avoidance and autonomous driving. Using high-data-rate millimetre wave (mmWave) band for vehicle-to-infrastructure (V2I) communication can substantially accelerate the growth of smart transportation projects. However, V2I link designers need to solve some new challenges such as beamforming, beam alignment, tracking, and coverage/ connectivity issues, before fully utilizing the mmWave potential.
V2I: Current State and Future
In April 2022, DEKRA and Vodafone, in association with the 5G automotive association (5GAA), demonstrated cellular vehicular communications standard (C-V2X) platform designed to connect road users directly with transport authorities at a European telecommunications standards institute (ETSI) event at Klettwitz, Germany. In fact, over the last few years, the government and industries have dramatically increased their investments to improve traffic management. The vehicle-to-everything (V2X) market, valued at 22 billion USD in 2016, is expected to reach 99.55 billion USD by 2025 at a CAGR of 38.2% .
For decades, IEEE 802.11p-based dedicated short range communication (DSRC) , operated in 5.9 GHz band, has been the only V2X technology capable to deliver ~5 Mbps data rate over <1000 meters range . For high-mobility environments with the presence of obstructions, DSRC suffers from limited coverage, low data rate, and unbounded channel access delay. The 3rd generation partnership project (3GPP) has been developing C-V2X to satisfy high-quality multimedia service. In parallel, 3GPP release 14 demonstrates long term evolution-advanced (LTE-A) in vehicular networks (~100 Mbps) without support from the cellular network infrastructure , which incurs lower communication latency than DSRC.
The development of fully autonomous vehicles will further push the data rate demands of the existing V2I links. Thus, exploring the less-congested millimeter wave (mmWave) band, ranging between 30 to 300 GHz, is a promising solution. The mmWave technology is being researched extensively by 3GPP for implementing 5G New Radio (NR), whereas IEEE is coming up with IEEE 802.11ad in the unlicensed 60 GHz band with 2.16 GHz bandwidth and ~7 Gbps data rate. Recently, the Federal Communications Commission (FCC) has authorized 28, 37, and 39 GHz bands for licensed users and 64–71 GHz for unlicensed users, facilitating the use of mmWave for smart transport applications.
V2I: Network Design and Components
Figure 1. Future wireless V2I components in an urban settlement
Future urban settlements are envisaged to deploy a plethora of wireless V2I communication equipment. Depending on their height and density, they may be categorized into four layers, quite similar to a rainforest, as shown in Figure 1. In a rainforest, the lowest layer is the forest floor, mainly consisting of grass. In an urban jungle of wireless infrastructure, the same role will be played by various embedded road sensors that can perform automatic pothole detection, road condition monitoring, vehicle detection, and detecting any pedestrian trying to jaywalk. Another dense layer is formed by trees of similar heights, known as the canopy. The corresponding layer in the urban scenario is formed by overhead traffic signals, smart street lights, billboards etc., which are plenty in number and can be equipped with mmWave equipment. The layer between the canopy and floor is the understory layer, and this layer is formed by other road-side units (RSUs) such as sideways traffic lights and other road signs. Finally, the emergent layer constitutes occasional overshooting entities, such as a C-V2X cell tower.
V2I Communication via mmWave: Challenges and Potential Solutions
The growing popularity of mmWave communication can be accounted for several advantages: a huge available spectrum, high reliability, low latency, high security, high resistance to interference and jamming, and a short wavelength resulting in an easy configuration of massive antenna arrays. However, the signals at mmWave frequencies are more prone to atmospheric attenuation loss (up to 15 dB/km in the 60 GHz band ), absorption loss, penetration loss, blockage  and mobility issues with frequent channel fluctuations. Through extensive research, potential solutions to these challenges can be resolved to start a new era  of smart cities.
Beamforming and Alignment
Due to the extremely short wavelengths of mmWave signals, efficient high-gain array antennas are designed to produce narrow directional beams which overcome the path loss and shadowing effects. The highly directional wireless links, and simultaneous tracking to all the vehicles for multi-lane highways demand precise beam alignments and frequent fast switching of beams impose beam-tracking overhead to the system. Prior knowledge of the vehicle's speed and propagation environment with some predictive techniques can help to reduce the beam alignment overhead, decrease the communication latency, and resist deafness due to beam misalignment.
Coverage and Connectivity
To achieve long-lived connectivity on move, fast beam sweeping according to the vehicular movement, range extension by designing antenna geometry, multiple low-cost road-side-unit (RSU) deployment across the road, introducing multiple-input multiple-output (MIMO) technology and designing advanced transceivers are attractive solutions.
The time-varying nature of the mmWave channel and high relative mobility between the vehicles affect the frame transmission, which eventually degrades the signal detection performance. To characterize the channel impulse response in high mobility scenarios, reliable channel prediction with geometry and non-geometry-based stochastic models, ray tracing approaches, graph-based models and angle-of-arrival measurement methods for different scenarios can be employed.
Figure 2. Challenges in mmWave V2I Communication
The propagation characteristics of mmWave in vehicular environments impose high susceptibility to blockage. The assigned direct line-of-sight (LOS) beams may be blocked by pedestrian users, trees or cars. The network performance significantly depends on the surrounding environment, solid materials like walls or trees, the number of moving vehicles, mobility of reflectors and obstacles, the position of RSU or the position of communicating device. Therefore, proper placement of the RSU antenna and beam allocation according to the traffic model should be analyzed to get an uninterrupted service.
With the rapid growth of information globalization, such as mobile cloud, ultra-high-definition (UHD) 3D video, virtual/augmented realities, and internet-of-things (IoT), the vehicular industry is also looking for some innovation services by forming a ‘smarter’ transport network. For high speed V2I communication, mmWave spectrum is a promising candidate that can partially address the data rate and end-to-end delay challenges of the existing wireless technologies. However, link establishment, coverage/connectivity, beam alignment and simultaneous tracking overhead are significant issues for system designers which need further research.
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This article was edited by Vladimir Orlic
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