## 1 Introduction  
With the increasing demand for faster data rates along with the integration of a wide range of devices into the network, it is necessary to explore new paradigms of wireless communication. Homogeneous physical layer communication, such as radio frequency (RF), laser and optical fiber, are morphing into a hybrid combination to improve the service provided to a large variety of devices [1; 2; 3]. A novel downlink satellite communication (SatCom) model was introduced to improve the system performance, in which free space optical (FSO) communication was adopted between a satellite and a high-altitude platform station (HAPS) node [4]. A unified system performance analysis of unmanned aerial vehicle (UAV)-assisted dual-hop FSO/FSO systems with the amplify-and-forward relaying protocol and intensity modulation/direct detection technique was presented [5]. Dual-hop hybrid FSO-RF relay systems combine the advantages of FSO and RF technologies to provide a superior performance. Multiple Input Multiple Output (MIMO) is widely used in RF wireless links in order to alleviate the effect of multi-path fading [6; 7]. Similarly, in order to combat the impact of atmospheric turbulence (AT) and pointing errors (PE) on the FSO link performance, MIMO is also employed in FSO systems [8]. A hybrid FSO/RF system is considered as a Vehicle to Everything (V2X). V2X plays an important role in vehicle connectivity and autonomous driving. However, due to the high frequency and short wavelength of millimeter wave (mmWave) signals, they are easily blocked by obstacles [9; 10]. Therefore, directional antennas are required for beam direction alignment to ensure stable communication quality. To mitigate the effect of path loss, MIMO can generate a high-gain directional beam using beamforming [11; 12]. However, obstacles such as buildings and roadside signs can block the line-of-sight (LoS) communication between vehicles and the base station (BS) in mmWave massive MIMO V2X. Therefore, joint beamforming of reconfigurable intelligent surface (RIS)-assisted mmWave massive MIMO V2X can effectively solve problems such as the presence of strong obstructions in the pathway [13; 14; 15]. Based on the above studies, a RIS-assisted V2X scheme was proposed to improve the communication range of urban V2X [16]. Since RIS can improve the quality of service (QoS) of wireless communication, a QoS-driven spectrum sharing scheme for RIS-assisted V2X was proposed [17]. The use of real-time software-controlled RIS was investigated to improve the reliability of V2X communication, which can effectively eliminate the multipath fading effect [18]. Considering the congestion under real road conditions, a visible light and RIS-assisted system was proposed to improve the coverage and communication performance of V2X [19]. Moreover, accurate direction-of-arrival (DOA) estimation is crucial in RIS-assisted communication systems. The one-bit RIS as a signal reflector was introduced to enhance signal transmission in non-line-of-sight (NLOS) situations and substantially simplify the physical hardware for DOA estimation. A non-iterative two-stage channel estimation framework was proposed for DOA estimation in point-to-point RIS-assisted mmWave MIMO systems [20]. To track DOA uncertainty, Bayesian beamforming for a mobile mmWave channel was proposed [21]. The beam tracking problem can be solved by incorporating the Bayesian approach with an expectation maximization (EM) algorithm. An improved genetic algorithm for RIS-assisted beamforming scheme was proposed [22]. The genetic algorithm greatly simplifies the physical hardware for DOA estimation. An atomic parametric-based DOA estimation method for RIS-assisted systems was proposed [23]. System architecture based on RIS-assisted autonomous vehicle localization was investigated, which enabled super-resolution DOA estimation for autonomous vehicles [24]. In addition, there are other studies on DOA estimation for RIS-assisted systems [25; 26; 27].