Hierarchical code assignment algorithm and state-based CDMA protocol for UWSN The characteristics of underwater acoustic such as long propagation delay and low bit rate cause the MAC (Medium Access Control) protocols designed for radio channels to be either inapplicable, or have low efficiency for UWSN (Underwater Sensor Network). We propose a hierarchical and distributed code assignment algorithm based on divisive probability function which can avoid conflict between spread codes with high probability, and provide a state-based MAC protocol for UWSN. Without RTS/CTS (Request To Send/Clear To Send) handshaking, each node maintains a neighbor table including state field, and packages are forwarded according to the state of the receiver, so it can avoid the interference caused by transmitting to a sending node in a half-duplex system and improve channel utilization. The design of power control minimizes the near-far effect inherent in CDMA. Simulation results reveal that state-based CDMA protocol shows outstanding performance in throughput, delivery ratio and resource-consumption.