Vehicular ad hoc networks (VANETs) are an emerging type of mobile ad hoc networks (MANETs) with robust applications in intelligent traffic management systems. VANET has drawn significant attention from the wireless communication research community and has become one of the most prominent research fields in intelligent transportation system (ITS) because of the potential to provide road safety and precautionary measures for the drivers and passengers. In this survey, we discussed the basic overview of the VANET from the architecture, communication methods, standards, characteristics, and VANET security services. Second, we presented the threats and attacks and the recent state-of-the-art methods of the VANET security services. Then, we comprehensively reviewed the authentication schemes that can protect vehicular networks from malicious nodes and fake messages. Third, we discussed the latest simulation tools and the performance of the authentication schemes in terms of simulation tools, which was followed by the VANET applications. Lastly, we identified the open research challenges and gave future research directions. In sum, this survey fills the gap of existing surveys and summarizes the latest research development. All the security attacks in VANETs and their related countermeasures are discussed with respect to ensuring secure communication. The authentication schemes and comprehensive applications were introduced and analyzed in detail. In addition, open research challenges and future research directions were issued.

A novel compact multiple-input multiple-output (MIMO) antenna is proposed with wideband gain enhancement. It consists of two identical antenna elements spacing about half wavelength. Firstly, a 1-to-2 Y-shaped power divider with wideband anti-phase outputs is designed to feed two modified Vivaldi radiators. The currents on the two radiating surfaces keep the same direction in a wide band to increase radiation without being cancelled. Secondly, the square-ring unit cell is analyzed to form metasurface, placed in the front of the radiator. It can guide the forward electromagnetic radiation while reducing the backward radiation. Thirdly, a U-shaped slot is etched between the two modified Vivaldi radiators. It is approximated as an effective electrically small dipole radiator. Such arrangement further effectively enhances the radiation performance in the direction of the main lobe and destructively interferes with the side lobe radiation. These features make the antenna far-field radiation pattern re-shaped. Thereby, the gain is improved in a wide bandwidth. Meanwhile, the MIMO antenna diversity performance is analyzed with low mutual coupling. Wideband gain enhancement contributes to improving the detection capability of the frequency modulated continuous wave (FMCW) radar sensor. As a proof of concept, the proposed MIMO antenna prototype is fabricated. The measured impedance bandwidth ranges from 11.5 to 21.3 GHz (59.76% FBW) with isolation of ≥ 24.92 dB. The gain is up to 10.6 dBi, the radiation efficiency is 88.01-90.02%, and the envelope correlation coefficient (ECC) is ≤ 0.00122. Integrated with the transceiver chip fabricated on the 65-nm CMOS process, the proposed MIMO antenna is applied in the FMCW radar sensor. The test system is built up, and the experiments on different breathing conditions are conducted for different human subjects. The proposed MIMO antenna could reduce measurement error, thereby improving measurement accuracy for human respiratory monitoring.

A compact dual-polarized multiple-input-multiple- output (MIMO) Vivaldi antenna with a wide bandwidth, high gain, and high isolation is proposed for wireless communications in intelligent Internet of Vehicles (IoV). It consists of four identical orthogonally-crossed Vivaldi radiators, crossed-fishbone-shaped slot, metasurface lens, parasitic stubs, electromagnetic (EM) wave reflector, and three-dimensional (3D) printed radome with dielectric resonator (DR) array. These functional modules jointly contribute to optimized antenna performance. Four identical antenna elements are rotationally symmetrical. To reduce the inter-element mutual coupling, the crossed-fishbone-shaped slot is etched to reduce the surface current flowing in adjacent radiators. To raise the gain, the metasurface lens, parasitic stubs, and DR array are loaded to guide the EM wave in the frontward radiation. An EM wave reflector located underneath the Vivaldi radiators could increase the boresight gain and reduce the influence of the vehicle body on the antenna performance. Diversity performance analysis indicates the proposed antenna could offer better MIMO capability. Besides, the impact of weather conditions on the antenna is analyzed. As a proof of concept, an expanded 8-element antenna array prototype is designed, fabricated, and tested. A good agreement is observed between measured and simulated results. It achieves an -10-dB impedance bandwidth of 7.55-22.85 GHz (100.66% FBW) with port isolation of higher than 18.05 dB. The realized gain is 6.98-11.54 dBi, and the radiation efficiency is 85.05-90.27%. The proposed antenna could be used in wireless communications between vehicles and the satellite/infrastructure.

A gain-enhanced wideband multiple-input multiple-output (MIMO) antenna with dual polarization and high isolation is proposed for microbase station application. It includes two mutually orthogonal bowtie dipole radiators, four bowtie parasitic elements with the switch function of ON-/OFF-state, four vertical stubs, and a cavity, which are fed by a pair of 50 $\Omega $ coaxial cables. Shifting between the dual-band and single-band is realized by the switch. Four resonant frequencies are generated by cooperative operation of the radiators and cavity, and thus two wide bands are formed. C-shaped slots on radiators and switch slots on parasitic elements are analyzed for estimating the impact on resonant frequency and bandwidth, and the vertical stubs improve the impedance matching. The proposed wideband antenna achieves enhanced diversity performance in the MIMO antenna system. As a proof of concept, a prototype of the expanded four-element MIMO antenna is fabricated. Measured results match well with simulated data. When the switch is ON-state, it operates in 1.58–2.77 and 4.71–6.18 GHz with gains of 9.4–11.2 and 9.1–10.9 dBi, and radiation efficiencies of ≥87.5% and ≥87%, respectively. When the switch is OFF-state, it operates in 1.56–2.76 GHz with a gain of 8.9–10.1 dBi and radiation efficiency of ≥88%. Element isolation is ≥35.3 dB, and envelope correlation coefficient (ECC) is ≤0.002. The experiment of the established $2 \times 2$ MIMO system demonstrates that it can operate with a lower bit error rate (BER), and the design shows enormous potential to be applied as massive MIMO microbase station antennas.

A novel compact multiple-input-multiple-output (MIMO) antenna with a wide impedance bandwidth and high port isolation is proposed for vehicular base stations. The design consists of four identical vertically-polarized monopole-based antenna elements, which are symmetrical about the central axis and are placed 90 degrees apart from each other. The wideband and decoupling features are theoretically analyzed and experimentally demonstrated. Four resonant frequencies are created by the F-shaped radiator, independent reflector and cross-π-shaped structure, which increase the MIMO antenna bandwidth. Parasitic stubs further improve the impedance matching in the high-frequency band. The cross-π-shaped structure acts as a decoupling function and reduces the mutual coupling, which is realized by using two metal strips shorted to the ground plane, perpendicular to each other. The etched slots further improve port isolation. The effects of imitated perfect vehicle metallic surface are also analyzed. The proposed MIMO antenna prototype is fabricated and measured to verify the concept of design. The operating frequency band is from 1.95 GHz to 6.25 GHz with a reflection coefficient of better than −10 dB, where the realized gain is higher than 3.3 dBi, and the maximum value is 6.16 dBi at 2.6 GHz. High isolation of >16.5 dB and low envelop correlation coefficient of <0.028 among designed MIMO antenna elements are also achieved.

A high-isolation Fabry–Pérot (FP) antenna array with wide bandwidth and high gain is proposed for Ku -band vehicle satellite communications. Each antenna element consists of a pair of tandem circular parasitic patches (TCPPs), a radial gradient partially reflective surface (PRS), and a reflector, respectively, separated from the ground plane with a double-dumbbell-shaped slot (DDSS) by air layer and foam dielectric. The slot radiator steers the TCPPs to create multiple resonant frequencies and then the impedance bandwidth is widened. The impedance matching could be improved by adjusting four circular slot diameters of the DDSS. The gain is raised by TCPPs and radial gradient PRS, which function to guide the electromagnetic field and uniformize the aperture near-field, successively. The reflector with several resonator elements is used to reduce the backside radiation. As proof of concept, the FP antenna array prototype with a height of $0.613\lambda _{0}$ has been fabricated and measurements confirming simulations are provided. Experimental results show that the impedance bandwidth ranges from 13.48 to 16.95 GHz [22.8% fractional bandwidth (FBW)], and the realized gain is up to 21.59 dBi. A decoupling metallic strip could achieve over 40.8 dB isolation. The proposed antenna element is easily extended for the multiple-input multiple-output (MIMO) and beamforming antennas, further improving the directivity.