# Research on Vehicle-to-Vehicle MIMO Wireless Channels in Various Tunnels
## 1 Introduction
### Prior Works  
It is well known that when we investigate the propagation characteristics of V2V channels, it is necessary to consider different communication scenarios between a transmitter and a receiver, such as streets, tunnels, highways, and so on. To achieve this research goal, we are required to model effective channels to describe the scatterer distributions in V2V communication scenarios [13]. Since the traffic infrastructure throughout the world has been rapidly developed in recent years, wireless communication scenarios behave in a more complicated manner than before, especially the tunnel communications [14; 15]. It is worth mentioning that channel models used for wireless communication analysis are typically fundamental in terms of facilitating theoretical analyses and making simulations tractable; therefore, it is important to propose 3D models to describe the V2V channels in tunnel scenarios. Meanwhile, multiple-input multiple-output (MIMO)-assisted vehicle-to-vehicle (V2V) communications, which aim to minimize the number of traffic fatalities, injuries, and accidents while fostering the improvement of new applications, ought to become hotspot scenarios in 6G networks [16; 17]. To efficiently study the performance of realistic V2V wireless communication systems, and meanwhile compare different 6G communication proposals, it is essential to gain insights into the underlying propagation characteristics in V2V wireless networks [18]. In the existing literature, there have been many research works concerning MIMO V2V channel modeling. To be specific, the authors proposed a geometry-based MIMO V2V scattering channel model in [19], which used the semi-circular model to describe the scatterer distributions in tunnel communication scenarios. In this study, [PERSON] et al. derived the correlation property functions to investigate the propagation characteristics of the MIMO V2V tunnel channels in different space domains. In [20], [PERSON] et al. developed a geometry-based statistical channel model for MIMO V2V communications in tunnel scenarios. In this work, the authors assumed that the propagation waves emerging from the transmitter in wireless channels experienced a single interaction before arriving at the receiver. The authors in [5; 21] proposed a twin-cluster channel model to depict the scattering environments in V2V tunnel scenarios. In these research works, some underlying V2V propagation characteristics, such as local correlation properties of the propagation links experiencing the line-of-sight (LoS) and non-LoS (NLoS) paths in the time, space, and frequency domains, were derived and discussed in detail. Furthermore, the authors in [22] studied the high dynamic properties of V2V channels in both the time and frequency domains, which derived the channel impulse response (CIR) to characterize the high dynamic physical properties of the V2V channel model. The authors in [23] proposed a cluster-based non-stationary MIMO channel model to investigate the time variation of the wireless channels in different motion environments by considering the small-scale time-varying parameters, such as the angle of departure (AoD), the angle of arrival (AoA), and the propagation path lengths. In [24], the authors developed a non-wide-sense stationary channel model for MIMO communications in high dynamic vehicular communication systems, which derived the real-time model parameters, i.e., angular parameters and distances from the transmitter/receiver to the cluster, to investigate the non-stationary properties of the channel models. It is worth mentioning that the interfering objects in the aforementioned channel models, which mainly mimic moving vehicles, buildings, and other features, are generally randomly situated in complicated V2V communication environments, such as tunnel scenarios.