Saturation Throughput Analysis of the Slotted BiC-MAC Protocol for Underwater Acoustic Networks Unlike most existing underwater medium access control (MAC) protocols that use unidirectional data exchange, the recently proposed BiC-MAC protocol allows each sender-receiver pair to exchange multiple rounds of bidirectional-concurrent data transmissions. Via simulations, it was shown that BiC-MAC greatly improves channel utilization. In this paper, we propose a novel analytical framework based on an absorbing Markov chain to analyze the single-hop saturation throughput for slotted BiC-MAC, under error-prone channel conditions. Motivated by the insight that time-slotting loses its effects when inter-nodal delay is longer than packet transmission time, the presented results can serve as a close approximation for the original, unslotted BiC-MAC protocol. We model the protocol behavior for a sender-receiver pair that attempts to bidirectionally exchange their backlogged batch of packets. To compute saturation throughput from batch service time, we systematically derive both the state transition probabilities and the expected time spent in each Markov chain. Via comparison against the simulation results of unslotted and slotted BiC-MAC in both small and large topologies with channel errors, we show that the model approximates unslotted BiC-MAC reasonably well. We also present another approach that uses the actual inter-nodal delay information and offers even closer approximation to the throughput of unslotted BiC-MAC.