The Impact of Multipath in Tunnels on Different Wireless Links Configurations—Model and Experimental Verification

This research investigates the challenges of wireless communication in tunnel environments, where signal reflections from walls cause multipath propagation, degrading signal power and phase, and thus transmission reliability. Factors such as antenna placement, signal polarization, tunnel geometry (straight or curved), and dynamic obstructions like pedestrians or vehicles further complicate propagation dynamics. The main goal of this work is to model and validate the impact of tunnel environments on wireless signal propagation using a versatile ray-tracing approach. To address these issues, a robust and adaptive Ray tracing model is developed and validated through extensive real-world experiments. The study evaluates the root mean square delay spread, a key metric for assessing communication link performance and determining the maximum reliable data rate. Results show that tunnel geometry significantly influences optimal polarization, with horizontal polarization recommended for tall, narrow tunnels. Pedestrian obstructions increase delay spread by approximately 50% compared to unobstructed conditions. Additionally, sharp 90-degree corners yield delay spread values four times higher than chamfered corners, accompanied by significant frequency-selective fading. These experimentally validated findings provide essential insights into wireless signal behavior in confined spaces, supporting the development of more reliable tunnel communication systems, with direct relevance to future 6G and IoT applications.