Time-Reversal Cross-Layer Opportunistic Routing Protocol for Reliable Internet of Underwater Things

In the complex, time-varying Internet of Underwater Things (IoUT), traditional hierarchical routing struggles to balance reliability, low delay, and energy efficiency due to measurement delays, routing voids, and frequent transmission collisions. To address these challenges, we propose a high-reliability, time-reversal (TR)-based cross-layer opportunistic routing (HRT-TCOR) protocol that tightly integrates time-reversal, reservation-based multiple access control (MAC), and opportunistic forwarding to collaboratively optimize the physical, data link, and network layers. First, lightweight beacons enable void sensing and preselection of candidate-forwarding sets (CFSs). Next, a hybrid sender-receiver cooperation strategy forms the CFS, while a two-tier suppression mechanism eliminates redundant transmissions. Finally, pipelined handshakes merge reception acknowledgments (ACKs) with channel reservations to eliminate interhop idle gaps and ensure seamless end-to-end reliable delivery. Simulations show that, under both good and poor channel conditions, HRT-TCOR consistently achieves the highest packet delivery ratio (PDR), a lower delay, and competitive energy efficiency compared to other protocols.