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Different from the mechanism of medium access control (MAC) for the land-based sensor networks, the underwater acoustic sensor network faces problems such as high propagation delay, spatial-temporal uncertainty and high energy consumption. Orthogonal frequency division multiple access (OFDMA) is a channel access mechanism based on OFDM multi-carriers modulation technology. In OFDMA, multiple users can make use of the orthogonality of the subchannels to simultaneously access channel for parallel transmission. For underwater multi-hop network scenario, this paper proposes a MAC protocol of the joint relay selection and power allocation (JRSPA-MAC) based on OFDMA mechanism. The proposed protocol is divided into flowing phases: Firstly, the node determines the hop distance from the sink nodes and sends the request when there are packets to be sent. Then forwarding nodes election algorithm is proposed based on the channel condition and the delay information. On this basis, the sending nodes autonomously use the particle swarm optimization (PSO) algorithm to allocate its subchannels' power. Secondly, taking advantage of the high underwater acoustic propagation delay, the receiving node reasonably schedule the sending time, makes sure the parallel communication of multiple pairs of nodes in one handshake period. Finally, the destination nodes and the forwarding nodes acknowledge the received packets and completes this data transmission. The simulation results show that compared with the hand-shake based MAC protocols, the JRSPA-MAC reduces the probability of packet loss caused by data conflicts between nodes. In case of low power consumption, the network throughput are effectively improved, and the average end-to-end delay of the network is reduced.

A Joint Relay Selection and Power Allocation MAC Protocol for Underwater Acoustic Sensor Network