Vehicle-To-Anything: The Trend of Internet of Vehicles in Future Smart Cities

Abstract

This chapter includes five parts—the concept of vehicle-to-anything (V2X), introduction of visible light communication (VLC), free-space optical communication (FSO), and terahertz (THz). The first part will present the concept of V2X. V2X is the basis and fundamental technology of future smart cars, autonomous driving, and smart transportation systems. Vehicle-to-network (V2N), vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I), and vehicle-to-people (V2P) are included in V2X. V2X will lead to a high degree of interconnection of vehicles. The concept of VLC is presented in the second part. Intelligent reflecting surface (IRS) for nano-optics and FSO communication is introduced in the third part. At the same time, IRS keeps pace with the phase in communication links. Prospects of THz in glamorous cities are introduced in the fourth part. These new technologies will lead to trends in the future. A comparison of optical communication technology and applications in V2X is described in the fifth part.

1. Part I: V2X Introduction

1.1 What is V2X?

V2X refers to the realization of a full range of network connections among V2V, V2P, V2I, and V2N with the help of new advances in information and communication to improve the level of intelligence and autonomous driving capabilities of the vehicle. Figure 1 shows the components of V2X. On one hand, V2X will improve traffic efficiency; on the other hand, it will provide users with intelligent, comfortable, safe, energy-saving, and efficient integrated services. V2X will establish a new direction for the development of automotive technology by integrating global positioning systems, wireless communication, and remote sensing technologies [1]. At the same time, V2X will realize the compatibility of manual driving and automatic driving. In the automatic driving mode, it is possible for the automatic vehicle to randomly select the driving route with the best road conditions through the analysis of real-time traffic information. This driving mode can alleviate traffic jams. In addition, through the use of onboard sensors and cameras, vehicle can perceive the surrounding environment and make rapid adjustments to achieve “zero traffic accidents.” For example, if a pedestrian suddenly appears, the car will automatically slow down to a safe speed or stop [2].

Figure 1.

The components of V2X.

The earliest application of V2X was shown on a Cadillac by General Motors in 2006. Since then, other auto-product suppliers have begun to study this technology. However, the application of V2X was put on the agenda because of two traffic accidents that originated in the United States.V2X is the key technology of the future intelligent transportation system (ITS). V2X makes communication between vehicles and base stations easier. A series of messages, such as real-time road conditions, traffic signals, and pedestrian information, can be obtained. These messages can improve driving safety, reduce congestion, and improve traffic efficiency. The purpose of V2X is to reduce accidents, alleviate traffic congestion, reduce environmental pollution, and provide additional information services.

1.2 Motivation

If a vehicle can be illustrated as the driver’s second pair of “eyes,” it can theoretically reduce the occurrence of traffic accidents caused by driver’s distraction or low visibility. V2X is a clever technology that turns the vehicle into the driver’s eyes. V2X can see animals that suddenly run on the road before the driver notices. Generally, V2X uses neighbor cars to see traffic signal indicators and remind the driver while the driver can be hard to notice. Compared with cameras or Lidar commonly used in autonomous driving [3], V2X has the ability to break through visual blind spots and cross obstructions to obtain traffic information. At the same time, V2X shares real-time driving status with other vehicles or facilities and decides the driving state of the vehicle immediately through study and judgment algorithms information. In addition, V2X is free subject to extreme weather conditions, such as rain, fog, and strong light exposure. Therefore, V2X is being developed in transportation, especially in the field of autonomous driving.

1.3 V2X development status

In 2015, US launched the ITS five-year plan with the theme “change the way in which society moves forward.” The main technical goals of planning are to “To realize application of connected vehicles” and “To accelerate autonomous driving.” Six categories of projects are defined in the plan—accelerated deployment, connected vehicles, autonomous driving, emerging capabilities, interoperability, and enterprise data. Connected vehicles, autonomous driving, and emerging capabilities are the three paths of technological development, while interoperability and enterprise data are the cornerstones of ITS development. To promote the further development of V2V and to reverse the subsequent legislative decisions of US, US Department of Transportation has led the “Safety Pilot Demonstration Deployment” project based on V2V and V2I. On the basis of the test and verification of the “Safety Pilot Demonstration Deployment” project, in 2014, the US National Highway Traffic Safety Administration announced the draft of the Vehicle-to-Vehicle Communication Advance Law, and launched the NPRM process in 2016 to enforce the light-duty vehicle V2V Communication, the main content includes:

1. Proposed mandatory V2V communication based on IEEE 802.11p

2. Specified the content of the BSM message

3. Specified V2V communication performance requirements

4. Specified privacy and security requirements

5. Designated equipment authorization system

The period from 2022 to 2025 is the deployment and development period of C-V2X industrialization. After 2025, the rapid development of the C-V2X industry will gradually achieve national coverage of C-V2X, and China will build a nationwide multi-level data platform, achieve cross-industry data interconnection, and provide diversified travel services.

1.4 V2N

V2N refers to the connection of vehicle devices with the network. Network exchanges data with the vehicle, stores and processes the acquired data, and provides various application services required by the vehicle. V2N communication is mainly used in vehicle navigation, remote vehicle monitoring, emergency rescue, and infotainment services.

1.5 V2V

V2V refers to communication between vehicles through onboard terminals. The vehicle-mounted terminal can obtain information, such as the speed, location, and driving conditions of surrounding vehicles in real time. Vehicles can also form an interactive platform to exchange information, such as pictures and videos in real time. V2V communication is mainly used to avoid or reduce traffic accidents, vehicle supervision, and management [4].

V2V enables sensors to communicate with neighboring vehicles, and it is more accurate and energy-efficient than any onboard surround sensing system. If we study further, we will come up with autonomous driving not a solution for the transition of automated vehicles from A to B, but a network protocol that optimizes traffic parameters and allows all commuters to reach their destination quickly and safely. Wireless upgrade is another basic autonomous driving function enabled by V2N. Since autonomous driving is a life-critical application, it must be kept up to the latest status.

1.6 V2I

V2I refers to the communication between vehicle-mounted equipment and roadside infrastructures, such as traffic lights, traffic cameras, and roadside units. The roadside infrastructure can also obtain information about vehicles in nearby areas and release various real-time information. V2I communication is mainly used in real-time information services, vehicle monitoring and management, and non-stop toll collection. V2I sensors collect information about traffic, traffic light status, radar equipment, cameras, and other road signals work as shared nodes to maximize infrastructure throughput. Even object-list lane markings or road barriers will one day become “smart” and become V2I communicators. For autonomous driving, information is critical because the vehicle may rely on stationary object data specifically for certain road events. Vehicles approaching the work area can notify and slow down. The parking lot can announce the availability of the previous passenger the moment they leave the scene [4].

In addition, vehicle can collect traffic data flow and help the driver choose the best route. Due to real-time traffic updates, V2I can reduce fuel consumption. Pre-filtering through V2I and autonomous driving can increase density, which will quadruple the current infrastructure capacity, keep road accidents at zero and increase traffic speed.

1.7 V2P

V2P means vulnerable traffic groups, including pedestrians and cyclists, use user equipment to communicate with vehicle-mounted devices. V2P communication is mainly used to avoid or reduce traffic accidents. By organically linking “people, vehicles, infrastructure, network” and other elements, V2X can not only support vehicles to obtain more information than bicycles perceive, promote the innovation and application of autonomous driving, but also help build a more intelligent environment. The transportation system promotes the development of new models and business of automobiles and transportation services. V2P is of great significance for improving traffic efficiency, saving resources, reducing pollution, reducing accident rates, and improving traffic management [5]. Figure 2 shows the typical V2X scenario as follows.

Figure 2.

Typical V2X scenario.

2. Part II: Visible light communication

2.1 Introduction of VLC

VLC refers to a type of communication that transmits data by modulating light waves in the visible spectrum (wavelength range from 380 nm to 750 nm). VLC is an emerging technology that realizes data communication by modulating the light intensity information emitted by light-emitting diode. Generally speaking, a system using visible light can be called VLC. VLC transmits data in a subtle way without affecting the normal lighting environment [6]. VLC is an optical communication technology, which uses optical transmission for information transmission. The spectrum is the physical threshold of light transmission, as shown in Figure 3 is the spectrum of VLC [7].

Figure 3.

The frequency band of visible light in the electro-magnetic spectrum [7].

Visible light usually uses LED as a communication medium. LED equipped electroluminescence and semiconductor to generate light, which is made by conducting materials. Due to high energy efficiency, durability, and low cost, LED sales have doubled. LED has been widely used in various devices, such as smartphones, vehicles, video screens, and signs. The universe of VLC has brought many benefits to the industry. LED bulbs have become the main medium for visible light communication.

2.2 Development of VLC in transportation system

So far, ITS relies on RF. However, the last decade has seen a major shift in lighting technology. With the major breakthrough of optical communication technology and the wide application of LED in indoor or outdoor lighting stimulation, VLC has become a feasible communication technology, making Vehicular VLC (V-VLC) possible in ITS [8].

A major advantage of VLC is the use of existing infrastructure to provide communication services. Data and energy can be transmitted simultaneously through LED. That is, energy transmitted does not increase the cost [9].

One advantage of visible light over radio frequencies is the size of the frequency spectrum. The allocation of frequencies in the radio frequency band of the electromagnetic spectrum is greatly limited, regulated by each country, and coordinated through international telecommunications agencies. Light, however, is a different material. The spectrum of visible light is completely free, and it will lead to different commercials and academic possibilities [9].

Because of its propagation properties, light has a security advantage over radio waves. RF for vehicle-mounted networks has nondirectional propagation, relatively long communication distances, and it can penetrate objects. RF has been well studied over the past few decades and the technology is quite mature. But due to the potential security attacks, such as jamming eavesdropping, and man-in-the-middle attacks, it will raise concerns about their use in security-critical on-board networking applications. Light, on the other hand, does not follow this behavior. Light has high directivity, and its physical properties provide a more secure environment for communication systems [10].

Finally, one of the main advantages of light is the wave’s high frequency, which allows for very high data rate communication. A large amount of bandwidth available in the visible spectrum allows for huge potential data rates. Currently, in terms of Wi-Fi, the highest data rates achieved in standard Wireless Gigabit are close to 1 Gbps [11]. Due to the high frequency of light waves, VLC searches have yielded impressive results, which speed up to 100 Gbps [12].

2.3 VLC modulation

VLC contains an irradiance modulation with direct detection (IM/DD) to communicate data faster than the persistence of human eyes by modulating LED intensity. Compared with traditional RF, VLC has superior speed and efficiency, security, and low cost. VLC fulfills its dual purpose of lighting and high-speed data communication. According to the characteristics of different modulation schemes, VLC modulation in visible light communication is divided into single carrier modulation multi-carrier modulation and Color Gamut-based Modulation.

2.3.1 Single carrier modulation

Single carrier modulation is the transmission of all data signals using a single signal carrier. Single carrier avoids the problem that the ratio of maximum instantaneous electric power to average electric power of a multi-carrier system at the same time of each phase is very large. This technology is more mature and the system has higher stability. For the best point-to-multipoint communication system, the single carrier modulation can make the frequency and time synchronization design easier, and improve the stability of the system. A single carrier modulation system provides a point-to-multipoint wireless communication solution with high efficiency, high flexibility, and high stability.

Common single-carrier modulation schemes include on–off Keying (OOK) and Pulse Position Modulation (PPM). OOK is a simple amplitude shift keying modulation. Because its modulation is simple and easy to implement, it is widely used in low- and medium-speed data rate demand application scenarios. Although the latest research has realized data transmission at 1250 Mbits over a distance of 1 m [13], transmission cannot be promoted due to the limitation of transmission distance.

PPM has been developed as an alternative communication technology to improve the anti-interference capability of information transmission. PPM is a good way of modulation [14]. Considering the bit error rate performance, bandwidth requirements, optical power, and optical implementation complexity, PPM modulation is a viable candidate for VLC communication. Compared to OOK, pulse position modulation has low noise interference because the amplitude and width of the pulse are constant during modulation. In pulse position modulation, noise removal and separation are very easy. Due to the constant pulse amplitude and width, the power consumption is also very low compared with other modulation methods.

In the past few decades, PPM modulation technology has received extensive attention, and research has been extended to various forms, such as differential pulse position modulation (DPPM), digital pulse interval modulation, multi-pulse position modulation, overlapping pulse position modulation (OPPM), and pulse rate modulation. DPPM is a simple improvement of PPM modulation. As long as deleting all the “0” time slots behind the “1” time of the PPM modulation, we can get the corresponding DPPM signal. Compared to PPM, DPPM symbols do not have strict symbol synchronization requirements, and more importantly, they can provide higher power utilization and bandwidth utilization. However, the bit error rate in DPPM is higher than that in PPM [15]. The main disadvantage of this scheme is that the pulse width is very short, and the high order M-element PPM modulation VLC can improve the power and bandwidth efficiency [16]. PPM modulation index can improve the power of the system from 1 dB to 2.5 dB by reducing the average bit error rate (BER) [16]. OPPM signal modulation schemes offer key advantages over other existing PPM schemes, such as greater sensitivity and smaller bandwidth expansion [17]. A priority decoding OPPM error correction scheme is proposed, which can significantly improve the system’s BER without affecting the system bandwidth [14].

But PPM still has some disadvantages, for example, synchronization between transmitter and receiver, which is not always possible; we need dedicated channels and like pulse-amplitude modulation, transmission requires high bandwidth and this modulation requires special equipment. In addition, single carrier modulation is commonly subject to inter-symbol interference (ISI) during high-speed data transmission, which means that new modulation techniques are required.

2.3.2 Multi-carrier modulation

With the increase in the VLC network data rate, MCM was developed to solve ISI during high-speed data transmission. MCM divides the transmitted data stream into several different components through different sub-channels. Under ideal propagation conditions, subchannels are usually orthogonal, and the number of substrates is chosen so that the bandwidth of each subchannel is lower than the coherent bandwidth of the channel, thus making it relatively immune to flat fading. Compared with incoherent modulation, MCM has lower energy efficiency and higher bandwidth efficiency. Common multi-carrier modulation includes subcarrier intensity modulation (SIM) and orthogonal frequency division multiplexing (OFDM), which have the advantages of high spectral efficiency and recovery ability against channel damage. However, SIM modulation is mostly used in the study of FSO, and here we discuss more OFDM [18].

OFDM modulation solves the multi-user problem by dividing the parallel data stream into different narrowband channels at different frequencies. However, most VLC systems use IM/DD, which requires that the electrical signal must be a real positive signal, so baseband OFDM cannot be directly applied. The improved schemes for OFDM, VLC include direct current (DC) bias optical OFDM (DCO-OFDM) and asymmetric limiting optical OFDM (ACO-OFDM). In the DCO-OFDM system, a DC offset is added to the normal OFDM symbol to reduce signal distortion and noise caused by negative limiting. In ACO-OFDM, only odd-indexed subcarriers are modulated, and the negative signal is clipped to zero during transmission. Compared with ACO-OFDM, DCO-OFDM has a lower power efficiency, but higher spectral efficiency. With the increase of the modulation order, BER performance of ACO-OFDM is about 4.5 dB better than that of DCO-OFDM, reaching 10⁻³ [19]. In the case of small bias, BER of ACO-OFDM with 16 quadrature amplitude modulation (QAM) format is lower than that of ADO-OFDM with 4 QAM format.

2.3.3 Color-shift keying based on color gamut

Color-shift keying (CSK) is a visible light communication intensity modulation scheme proposed in IEEE 802.15.7, which sends signals through the color intensity emitted by red, green, and blue (RGB) light-emitting diodes. CSK signal points can be represented by an intensive combination of RGB colors corresponding to the transmitted data. The flicker of the light source is reduced by keeping the total emission intensity constant. Due to its unique advantages in preventing scintillation effect and light intensity fluctuation, the research of CSK in vehicle-mounted VLC has attracted more and more attention in recent years.

RGB LED consists of three LEDs in a package and produces white light through a combination of red, green, and blue outputs. Although more costly, RGB LED light can produce any perceived lighting color and can increase VLC data throughput by acting as a separate communication band. The perceived lighting color can be modified while achieving higher spectral efficiency.

3. Part III: Free-space optical communication

3.1 FSO introduction

FSO is known as an optical wireless system, which uses the atmosphere between the transmitter and receiver as the propagation medium, and FSO communication link is a line of sight (LOS). It can be used on various platforms, such as satellites, ships, airplanes, and other stationary or moving space and atmosphere. Due to its unregulated spectrum, inherent security, high data rate, and wider bandwidth, FSO is considered to be a supplement to radio frequency communication [20]. Although optical communication has advantages that traditional communication links cannot match, because its propagation medium cannot be controlled or adjusted, optical communication systems will be affected by some atmospheric phenomena. The main challenges of FSO are narrow beamwidth, transmission signal scattering, and scintillation. Due to the influence of atmospheric turbulence, the received signal intensity will fluctuate, that is, flicker [21]. FSO works according to the principle of sight. For continuous data transmission, LOS generated by the light beam should be straight. FSO is a combination of wireless technology and optical technology. The main factor that needs to be considered is the optimal bandwidth of the light beam used for communication and transmission of information signal data, such as audio and video. Free-space technology is an older technology used for lower data communications. Due to the limited bandwidth, RF is limited. Using lighting to transmit data, Li-Fi technology was proposed in optical communication [22]. The wireless optical communication technology was developed by the National Aeronautics and Space Administration and used for military purposes with high-speed communication links [23].

In recent years, FSO have received extensive attention in terms of ground-to-satellite transmission links and last-mile applications due to their high capacity and easy implementation. However, atmospheric turbulence can cause random fluctuations in the amplitude and phase of the received signal, which limits the application of FSO links. Multiple-input multiple-output (MIMO), adaptive optics, and fiber laser phased array (FLPA) are important ways to suppress atmospheric turbulence [24].

3.2 Performance of FSO

The performance analysis of FSO should be considered from external and internal parameters. The specifications and ratings of the components used, including operating frequency, divergence, power consumption, and transmission angle are all internal parameters. The ability of the lens and the error rate are all at the receiver end. External parameters include environmental factors, such as alignment, atmospheric attenuation, weather conditions, and scintillation.

FSO communication depends on weather conditions. If weather conditions are cloudy or visibility is lower, the formed communication link would not be sufficient for effective communication, whereas the performance of FSO relies on the weather conditions. In the FSO system, the transmitter would produce a narrow beam of light, and the narrow beam of light is straight. At the same time, the receiver should receive the narrow beam of light from the strong communication link on a straight line [25]. In the optical communication system, a straight beam with a diameter of 5–8 cm passes through and spreads to 1–5 m within 1 km.

3.3 FSO efficiency

FSO technology is changing rapidly day by day. This technology would increase and maximize signal bandwidth, at the same time, this technology transmitting data would be at high speeds. FSO technology is similar to fiber optical communication. The only conversion is signal path flow. That is to say, wireless communication between the transmitter and receiver, without cables, so it reduces costs and can be more efficient [26]. The efficiency of FSO mainly depends on the external aspects or the medium aspects between the transmitter and the receiver. Data transmission is lossless and high-speed if the transmission medium has strong visibility. The data transmission speed of the LED can reach 100 Mbps, and various experiments have been carried out to increase the data rate.

A fiber laser phased array transmitter into a FSO communication system and compared BER and optical transmit power of the two systems, which used single-aperture transmitter and FLPA transmitter. Experiments show that the power budget gap is about 8–10 dBm [24]. This shows that the FLPA transmitter provides a higher power budget. A new type of FSO switch capable of multicasting, the cost analysis of this switch shows that even if the cost of T-SE is 1.2 to 3.5 of micro-electro-mechanical system mirroring, its cost is lower than that of AD-based switches [27]. In Ref. [16], this paper uses avalanche photodiode (APD) and the positive intrinsic negative (PIN) receivers, respectively, and considers a single input multi output system with strong gas turbulence defined by M-ary PPM modulation and gamma-gamma distribution. Then a comprehensive comparative evaluation of the two situations is carried out. The experimental results show that the performance of the system can be improved by increasing the strength. In addition, we compared the main parameters of FSO and RF in order to distinguish the differences between them more intuitively, as shown in Table 1.

Parameters	FSO	RF
Light source characteristics	Laser communication and non-laser communication	Radio frequency identification
Modulation rate	high	low
Anti-interference ability	low	high
Power	2.00E-03(J/Mb)	2.31E-02 (J/Mb)
Power loss	5–15 db/km	108 dB/km
Output power	5–500 mWatt	50 mWatt
Range	4 km	4 km
Data rate	10 Gbps	100 Mbps
Capacity	Not Allowed	Allowed
Advantage	Unlicensed band	NLOS
Security	high	low
Limitation	environment	spectrum
Spectrum range	0.8–1.2 THz	2–6 GHz
Wavelength	850 nm–1550 nm	22–2500 m

Table 1.

The performance comparison between FSO and RF.

3.4 What is IRS?

As a new invention, IRS can be called smart wall, smart reflective light, passive smart mirror, smart reflective surface, and large smart surface. IRS is composed of a large number of passive, low-cost components. It is a low-carbon and environmentally friendly smart component that can effectively control the phase, frequency, amplitude, and even polarization of the collision signal, IRS will build a real-time and reconfigurable propagation environment. The signal coverage of IRS is small, easy to deploy, and will not interfere with each other. By increasing the number of reflective elements, the quality of the received signal can be significantly improved. IRS does not require a power supply, complex algorithms, and hardware. IRS is easy to integrate into current wireless communication systems. These advantages make IRS a promising candidate for future wireless communication systems. IRS can greatly adjust the signal reflection to change the wireless channel to enhance communication performance. IRS is used to realize the intelligent and reconfigurable wireless channel propagation environment of the B5G/6G wireless communication system. Generally speaking, IRS is a plane composed of a large number of passive reflection units, and each passive reflection unit can independently produce a controllable amplitude and/or phase change of the incident signal. By densely deploying IRS units in the wireless network, the reflection of the IRS array is cleverly coordinated. The signal propagation between the transmitter and the receiver can be flexibly reconfigured to achieve the required realization and distribution, which provides a new means to fundamentally solve the problem of wireless channel fading damage and interference. It is possible to achieve a leap in wireless communication capacity and reliability.

3.4.1 Features of IRS

1. Passive

   IRS is composed of a large number of low-cost passive reflective components, which are only used to reflect signals and do not need to transmit signals. Therefore, IRS is almost passive and ideally does not require any dedicated energy.

2. Programmable control

   IRS can control the scattering, reflection, and refraction characteristics of radio waves through the program, thereby overcoming the negative effects of natural wireless propagation. Therefore, IRS-assisted wireless communication can intelligently control the wave-front, such as phase, amplitude, frequency, and even polarization, which can hit the signal without complicated decoding, encoding, and radio frequency processing operations.

3. Good compatibility

   IRS can be integrated into the existing communication network protocol only by changing the network, without changing the hardware facilities and software of their equipment. At the same time, the IRS has a full-band response, and it can ideally work at any operating frequency.

4. Easy to deploy

   IRS is characterized by small size, lightweight, conformal geometry, and thinner than the wavelength, so it is easier to install and disassemble. Therefore, IRS can be easily deployed on exterior walls of buildings, billboards, ceilings of factories and indoor spaces, and people’s clothes.

3.4.2 Application of IRS in V2X

In the future, IRS will be everywhere. IRS can be deployed on outdoor walls, drones, and transportation equipment of smart buildings in smart cities. Self-driving vehicles can use IRS as an intermediate medium to realize free wireless optical transmission, quickly, accurately, and accurately convey various information to the vehicle, and realize V2X. Centralizing vehicles into the Internet of Things makes the vehicle and the Internet of Things closely connected. The lightweight, convenient, and flexible deployment characteristics of IRS enable IRS to play a big role in V2X.

Another promising direction is IRS-assisted RF sensing and positioning. The large aperture size of the IRS and its ability to shape the propagation environment can significantly enhance RF sensing capabilities. The channel can be changed to provide favorable conditions for RF induction, and then it can be monitored with high precision. Encouraging results were reported in Ref. [28], and these results may have applications in energy-saving monitoring, assisted living, and remote health monitoring. However, the issue of optimizing the configuration of the IRS to enhance RF sensing remains to be studied. The effective combination of radio frequency technology and IRS can also be applied in the future V2X. Vehicles can use sensors to sense the signals of surrounding vehicles and traffic signs and provide evidence of effective traffic information for real-time V2X decision analysis. The combination of RF and IRS makes the monitoring data correct.

4. Part IV: Terahertz technology

4.1 Introduction to THz

In the past few years, wireless data traffic has seen unprecedented growth. On one hand, from 2016 to 2021, mobile data traffic is expected to increase seven times. On the other hand, video traffic is expected to triple in the same period [29]. In fact, by 2022, wireless and mobile device traffic is expected to account for 71% of total traffic. In fact, by 2030, wireless data rates will be sufficient to match wired broadband Competition [30]. The growth of use of wireless communication has led the researcher to explore appropriate radio spectrum ranges to satisfy the growing needs of individuals. For this reason, THz frequency band (0.1–10 THz) has begun to attract attention. Seamless data transmission, unlimited bandwidth, microsecond delay, and ultra-high-speed downloading of THz will completely lead the innovation of communication and change the way of communication and access information.

The term terahertz was first used in the field of microwave science in the 1970s to describe the spectral frequency of interferometers, the coverage of diode detectors, and water laser resonance [31, 32]. In 2000, terahertz was called a sub-millimeter wave, and the frequency range was between 100 GHz and 10 THz. However, the dividing line between sub-millimeter wave and far infrared was not clearly identified [33, 34]. The concept of ultra-wideband communication using THz for no line of sight signal components was first proposed as a powerful solution for extremely high data rates [35]. Since then, THz technology, especially communication technology, has captured the enthusiasm of the research community.

In fact, the rise of terahertz wireless communication started as early as 2000 when the 120 GHz wireless link produced by photonic technology started [36]. The 120 GHz signal is the first commercial terahertz communication system, and its allocated bandwidth is 18 GHz. Data rates of 10 gbps and 20 gbps are achieved through OOK or QPSK modulation, respectively [37, 38]. The terahertz frequency band guarantees a wide range of throughput, and theoretically can be extended to several terahertz to reach terabits per second (Tbps) [39]. This potential associated with terahertz technology has attracted a wider research community. In fact, the joint efforts of active research teams are producing new designs, materials, and manufacturing methods, providing unlimited opportunities for the development of terahertz. The potential benefits of the THz band are discussed [33]. THz can be applied to terahertz imaging and tomography [34]. THz wave is an electromagnetic wave between microwave and infrared, with a wavelength of 0.03–3 mm and a frequency of 0.1–10 THz. THz waves not only have the same straightness as light waves but also have similar penetrating and absorptive properties to radio waves.

4.2 Characteristics of THz radiation

1. Transient: The typical pulse width of a THz pulse is in the order of picoseconds.

2. Broadband: THz pulse source usually only contains several periods of electromagnetic oscillation, and the frequency band of a single pulse can cover the range from GHz to tens of THz.

3. Coherence: The coherence of THz comes from its generation mechanism.

4. Low energy: The energy of THz photons is only Millielectron volts. Compared with X-rays, it will not damage the detected substance due to ionization.

5. Penetration: THz radiation has a strong penetrating power for many nonpolar materials, such as dielectric materials and plastics, cartons, and other packaging materials. It can be used for quality inspection of packaged items or for a safety inspection.

Most polar molecules, such as water molecules and ammonia molecules, have strong absorption of THz radiation. The spectral characteristics of THz can be analyzed to study material composition or perform product quality control.

4.3 Problems with THz

Because most biological tissues are rich in water, water absorbs THz radiation very strongly, it greatly reduces the sensitivity of imaging of biological samples, and THz cannot make a clear image for samples with a lot of water, especially thick samples. This severely limits the application of THz imaging in biomedicine.

At present, the average energy of THz waves generated by most femtosecond lasers is only on the order of Nanowatts and can reach a signal-to-noise ratio of 100,000 or higher for single-point detection, but the signal-to-noise ratio of real-time two-dimensional imaging is very low. To obtain a high signal-to-noise ratio for imaging, a higher energy source is required.

4.4 Future research direction of THz

4.4.1 Terahertz ultra-massive MIMO

THz ultra-large MIMO frequency band can meet the needs of high data rates, but under the premise of providing a huge bandwidth, this band suffers a huge atmospheric loss. Therefore, high-gain directional antennas should be used for communication over a distance of more than a few meters. In the terahertz band, antennas are installed in the same space in a small and dense manner. Ultra Massive MIMO (UM-MIMO) channel was proposed [40, 41], the concept of UM-MIMO relies on the use of ultra-dense frequency-tunable plasma nano-antenna arrays, UM-MIMO was used for both transmitting and receiving, thereby increasing the communication distance and ultimately increasing the achievable data rate at terahertz frequencies [42]. In fact, when ensuring a two-dimensional or planar antenna array instead of a one-dimensional or linear array, the radiated signal can be adjusted in elevation and azimuth directions. This results in 3D or full-dimensional MIMO. The performance of UM-MIMO technology depends on two indicators, namely the prospect of plasmonic nano-antennas and the characteristics of the terahertz channel. Another important aspect is dynamic resource allocation, which can make full use of the UM-MIMO system and obtain maximum benefits through adaptive design schemes [43].

4.4.2 Terahertz virtual reality perception through cellular networks

Facing the technical barriers of 5G communication, THz is expected to have breakthroughs in reliability and low latency. Currently, video requires extremely high bandwidth. Therefore, the terahertz frequency band is sought as a technological supplement, and THz will provide high capacity and dense coverage to meet user needs. The terahertz cellular network will enable interactive, high dynamic range video with higher resolution and higher frame rate, which actually requires 10 times the bit rate required for 4 K video. Terahertz transmission will help solve any interference problems and provide additional data to support various instructions in video transmission. In addition, the terahertz band will become an enabler of 6-degree-of-freedom (6DoF) video, providing users with the ability to move inside and interact with the environment. The results of the literature [44] absorbing the impact on the terahertz link greatly limits the communication range of small base stations. This impact can be mitigated by the densification of the network. Therefore, the terahertz can provide a rate of up to 16.4 Gbps with a delay threshold of 30 ms.

4.4.3 The application of THz technology in unmanned driving

At present, 5G has been put into use worldwide, and the B5G system will be a supplement to the current 5G. Due to the low latency and high reliability of THz technology, THz can be applied to driverless vehicles. The main goals of the current B5G system are as follows:

1. Extremely high data rates of each device (from tens of Gbps to Tbps)

2. A large number of connected devices

3. Ultra-large data rate per region

4. Ultra-reliable transmission, supporting various key applications, such as V2V communication, industrial control, and medical care

5. Part V: Prospect of V2X

5.1 Comparison of optical communication technology

The application of visible light, free optical communication and THz technology, and related technologies in V2X was described above. The following focuses on comparing related technologies.

5.1.1 Comparison of VLC and THz

Communication through visible light is a promising energy-sensing technology, attracting people from industry and academia to study its potential applications in different fields. VLC carries information by modulating light in the visible spectrum (390–750 nm) [45]. Recent advances in LED lighting have enabled unprecedented energy efficiency and lamp life because LEDs can be pulsed at very high speeds without significant impact on lighting output and the human eye. LED also has several attractive features, including low power consumption, small size, long life, low cost, and low heat radiation. Therefore, VLC can support many important services and applications, such as indoor positioning, human-computer interaction, device-to-device communication, vehicle networks, traffic lights, and advertising display [46]. Despite the advantages associated with deploying VLC communications, several challenges exist that may hinder the effectiveness of wireless communication links. To achieve high data rates in a VLC link, LoS channel should first be assumed, in which both the transmitter and receiver should be aligned with the field of view (FOV) to maximize channel gain. However, due to the continuous change of the movement and direction of the receiver, the field of view of the receiver may not always be aligned with the transmitter. This misalignment leads to a significant drop in received optical power [7]. When an object or man obstructs the line of sight, the optical power will drop significantly, resulting in a severe drop in the data rate. Similar to infrared waves, ambient light interference will significantly reduce the signal-to-noise ratio (SNR) of the received signal and reduce the communication quality [45]. The current research on visible light networks also reveals downstream traffic but does not consider how the uplink runs. Since the directional beam to the receiver should be maintained in the VLC uplink communication, when the mobile device is constantly moving/rotating, a significant throughput drop may occur. Therefore, other wireless technologies should be used to transmit uplink data [46]. Contrary to the VLC system, the THz band allows NLOS to propagate when LoS is not available as a supplement [47]. In this case, NLOS propagation can reflect the beam to the receiver by strategically installing dielectric mirrors. Due to the low reflection loss of the dielectric mirror, the resulting path loss is sufficient. In fact, for a distance of up to 1 meter and a transmission power of 1 watt, only the NLOS component in the terahertz link has a capacity of about 100 Gbps [48]. In addition, the terahertz frequency band is considered a candidate frequency band for uplink communication, which is a capability lacking in VLC communication. Another specific application where terahertz has become a valuable solution is the need to turn off the lights when looking for network services. Due to the limitation of the positive signal and the real signal, the VLC system will suffer a loss of spectrum efficiency. In fact, compared with the traditional bipolar system, using the unipolar OFDM system to impose Hermitian symmetry will cause a performance loss of 3 dB [49]. Both THz and VLC can be used as communication technologies to realize V2X in the future. Realize a technological breakthrough in V2X.

5.1.2 Comparison of VLC and FSO

VLC has become an attractive alternative to indoor RF communication to meet the growing demand for massive data services. In addition to providing a huge and unlicensed bandwidth to cope with the crowded radio spectrum, VLC has various other advantages, such as ease of use, no radiation, and no electromagnetic interference. On one hand, FSO is a line-of-sight, which has attracted great attention as a high-bandwidth last-mile transmission technology. On the other hand, FSO is a reasonable alternative to optical fiber because it requires less initial deployment [50] and it can be installed in locations where wired connection deployment is challenging.

The indoor VLC must be connected to the base station to achieve the purpose of communication. The most economical solution for connecting an indoor VLC to an outdoor base station is to use a power cord. In this case, various studies have been proposed [51, 52], involving the integration of VLC and power line communication (PLC) as a backbone network. However, PLC channels suffer from multiple damages—deep notches, high attenuation, and colored background noise that limits the data rate. To provide better data rates and improve system performance, VLC should be supplemented by high-bandwidth FSO links to achieve high-data-rate indoor multimedia services [53]. The direct FSO/VLC heterogeneous interconnection with data aggregation and distribution has been proved through experiments.

The combination of visible light and free optical communication technology can be used on V2X. Visible lights can be used on traffic signs, such as traffic lights. The combination of VLC and FSO can achieve V2I. The combination of optical communication and smart vehicles will provide better services. The combination of optical communication and smart vehicles will provide better services in the future smart vehicles.

5.1.3 Comparison of FSO and THz

FSO technology is an excellent candidate for high-performance secure communication due to its safety, anti-interference, high beam directivity, flexibility, and energy efficiency. However, to date, the large-scale deployment of FSO communication systems has been affected by availability and reliability issues due to flicker on sunny days, low visibility on foggy days, Mie scattering effects, and high sensitivity to beam drift effects [54]. Due to the high directivity of the beam, FSO links are more difficult to intercept than RF systems. Nevertheless, Eve can still apply beam splitting attacks on the transmitting end, and blocking attacks or beam divergence attacks on the receiving end. Judging from the number of recent papers related to physical layer security (PLS). PLS research on FSO communication systems seems to be gaining momentum, such as [55, 56]. Unfortunately, almost all PLS papers related to FSO links use the eavesdropping channel method and direct detection introduced by Wyner [57]. Fog is the most unfavorable factor affecting FSO link reliability. In contrast, terahertz signals are less affected by these problems but are affected by other weather conditions, such as rain and snow. This shows that the two transmission media (FSO and THz) can operate in a complementary manner, depending on the prevailing weather and atmospheric conditions.

The above is the comparison of related technologies. In the future research direction, these several technologies can support the related research of V2X. The combination of pairwise or the combination of several technologies can play a very important role in future research.

5.2 Application scenarios of VLC

V-VLC is a fairly novel technology, although experimental studies in real driving scenarios have shown the feasibility of this technology in the application of vehicle networking. However, the current research on V-VLC mainly focuses on the understanding and characterization of the V-VLC channel, as well as the development of the V-VLC prototype. Although in the aspect of the physical layer, V-VLC still has many unsolved problems, especially regarding the performance and channel model of V-VLC in transportation system channels. But now the research will also focus on higher layer protocols V-VLC and IEEE 802.11p C-V2X and other different communication technologies that can make up for each other’s shortcomings and improve the overall performance of applications [8].

Vehicular networking applications V-VLC can be used alone or implemented as part of a heterogeneous vehicular networking system V-VLC can benefit these specific applications as follows:

Cooperative sensing: The V-VLC can share perceptual data with nearby vehicles via onboard cameras, or collect sensor data to sense larger driving situations. Using headlights and taillights, high-throughput data can be transmitted to both front and rear vehicles, respectively, to facilitate cooperative awareness and cooperative awareness applications.

Information query: V-VLC can be used for information query and publishing within the range. These applications achieve this by utilizing VLC communication based on V2I and I2V to query and publish highly scalable and propagated information without strong latency and reliability requirements. In this way, V-VLC can be used to transmit information in part of the network without LED traffic lights, traffic signs, or road lighting coverage.

Intersection assistance: Intersection assistance applications, such as intersection collision avoidance, improve intersection safety by providing coordination and warning means between vehicles rather than traditional methods, such as traffic lights. When vehicles face each other at an intersection, head-to-head V-VLC links can be used to communicate with vehicles on the opposite side of the intersection. In addition, LED-based traffic lights or other infrastructure elements can facilitate communication.

Collision avoidance: To avoid rear-end collision in intelligent transportation systems, microwave radar and short-range radio communication are proposed. However, these technologies are affected by radio frequency competition and changing weather conditions, and cannot achieve fully autonomous collision avoidance and safety queuing. VLC has the advantages of personal safety, unreasonable frequency allocation, large transmission capacity, and mature white LED light source. It can supplement the existing automatic driving system to achieve higher safety and driving efficiency, especially in the automotive lighting system and traffic light scenes [58].

Visible light localization system: Visible light system benefits from the ideal characteristics of visible light and its spectrum. Compared with traditional RF communication systems, visible spectrum has huge free bandwidth, facilitating high-speed data transmission and reducing the cost of operators. LED-based VLP systems can be easily integrated into existing lighting infrastructure (street light parking lights and traffic lights) for localized purposes, often without the need for rewiring beyond their basic lighting functions. In general, VLP systems can be used appropriately in any application that uses LEDs [59].

5.3 Applications of FSO in V2X

FSO can be used in future V2V systems by using highly collimated beams for enhanced vehicle-to-everything applications. A low-rate control link with multiple Gbps-assisted FSO links running in parallel is proposed [60]. Previous a control link is used to exchange sensor data about vehicle attitude dynamics to perform FSO beam tracking and provide an ultra-reliable high data rate connection on the latter FSO link. The joint contribution of local and distributed processing guarantees continuous and precise pointing to fully support autonomous driving applications.

To counteract the adverse effects of the limited sampling frequency of onboard sensors and control link delays, the evolution of vehicle kinematics can be estimated by simultaneously predicting and fusing multiple inertial measurement unit (IMU) data, augmented by real-time information on vehicle position.

5.4 Application of THz in V2X

When it comes to vehicle networks, there are several additional reasons to explore higher frequency bands that can support multiple Gbps and Tbps links. Firstly, when transmitting at such a high data rate, even if a user is mobile, from a data point of view, the link actually seems to be static because the transmission is almost instantaneous. In short, although the systems change over time, they do so much slower than the actual data rate. Therefore, during the transmission of a given frame, the system appears to be static. In addition, even if the user’s connection is intermittent, the amount of information that can be transmitted per connection may be huge (1 Tb/ s). In addition, by moving to a higher carrier frequency, the influence of the Doppler effect can be reduced. Although this may not be a problem for automotive networks, it is very important for wireless data transmission between or between aircraft flying at high speeds. Therefore, there are inherent characteristics that prompt the exploration of vehicle networks in the terahertz frequency band [61]. This is undoubtedly the future trend for the realization of V2X. In the future, THz technology can shine in V2X.

Based on the results left over from the millimeter-wave band, the main attributes of terahertz communication are expected to include the following:

1. High frequency provides very large available bandwidth, therefore, potentially high data rate.

2. In response to high path loss, directional antennas will be mandatory. Highly directional antennas result in narrow beamwidths and very limited interference. Therefore, a very high data rate can be expected for each area.

3. If effective beam search and alignment mechanisms are in place, high rates can also lead to low delays.

THz will shine in unmanned driving. THz in V2X can be a direction and trend in the future, connecting everything. THz will deliver information quickly and efficiently in terms of high speed and high accuracy.

6. Part VI: Conclusion

V2X is the key technology of the Internet of Vehicles. The Internet of Vehicles in the true sense consists of the network platform, the vehicle, and the driving environment. This chapter focuses on the investigation and review of the important research results and forward-looking technologies of optical wireless communication in the application of V2X. VLC communication technology maximizes the use of existing traffic infrastructure to build a multi-user communication network structure for people-vehicle-traffic lights; the unique high-speed communication speed of THz and FSO communication technology provides strong communication speed support for V2X; IRS equipment research provides the possibility for long-distance NLOS communication. In addition, these aforementioned technologies and their key features are summarized, and their emerging future research and engineering directions are given. It is anticipated that, in building a smart city, optic-/THz-based technology will play an important role in a future highly developed V2X networking era.