RIS Cooperative Multi-Relay Assisted Vehicular MIMO Networks

In this paper, we propose a downlink vehicular Multiple-Input Multiple-Output (MIMO) wireless communication system that differs from most existing studies. The considered system employs an Reconfigurable Intelligent Surface (RIS)-assisted radio-frequency (RF) source as the transmitter, while the receiving vehicle is equipped with multiple antennas. Furthermore, additional RIS and multiple relay nodes are deployed between the base station and the target vehicle to enhance communication. All relays operate under the amplify-and-forward (AF) protocol, including both fixed-gain relaying and channel state information (CSI)-assisted relaying schemes. Each relaying scheme is further analyzed under three relay selection strategies: fixed relay selection (FRS), partial relay selection (PRS), and opportunistic relay selection (ORS). To evaluate the effects of different relaying schemes, selection strategies, and system parameters on outage performance, we derive closed-form approximate expressions for the outage probability under each strategy. All analytical results are verified through Monte Carlo simulations. Numerical and simulation results indicate that system parameters significantly affect outage performance in fixed-gain relaying, whereas in CSI-assisted relaying, outage performance is primarily determined by the relay-to-vehicle link. Additionally, comparative analysis demonstrates that fixed-gain relaying achieves better outage performance, with the ORS strategy exhibiting substantial advantages over FRS and PRS.