A delay-tolerant OFDMA-based MAC protocol for underwater acoustic sensor networks

In underwater acoustic sensor networks, the solution to augment sensor nodes´ life time is crucial due to the difficulties in retrieving discharged sensor nodes. Thus, many energy efficient MAC protocols have employed mainly logical ways, such as sleep mode, hand-shaking signaling between nodes, and scheduling of transmission time, rarely using the physical characteristics of underwater acoustic channels. In this paper, we propose a novel energy efficient MAC protocol named NOGO-MAC (NOde Grouped Ofdma MAC) which is based on orthogonal frequency division multiple access (OFDMA) and exploits the physical characteristic that propagation loss of acoustic wave depends on the distance more heavily at high frequency than at low frequency. In the proposed scheme, sensor nodes are grouped according to the distance to sink node. Then, each group uses a different frequency band in such a way that sensor nodes which are closer to the sink node use higher frequency band and farther ones use lower frequency band. The proposed scheme not only enables all sensor nodes to maintain the signal-to-noise ratio above a certain required level (Accepted Minimum SNR : AMS), but also reduces overall transmission power consumption. In addition, an adaptive sub-channel allocation is employed in order to improve data transmission rate. Numerical results show that NOGO-MAC is able to reduce the average transmission power by at least 3 dB up to 7 dB comparing non-grouping methods under bandwidth changing from 15 kHz to 35 kHz. And the more sensor nodes, the higher system average throughput can be achieved. The saturation system average throughput is about 8 kbps which is achieved with 800 sensor nodes density.