An OFDM based MAC protocol for underwater acoustic networks

In this paper, we investigate OFDM based MAC protocols for underwater acoustic networks. Due to the severe multi-path effects of underwater acoustic channels, the guard time between OFDM blocks becomes significant, which greatly degrades the system rate. In addition, the highly dynamic nature of underwater acoustic channels makes it hard to choose the optimal transmitting power and modulation mode. Furthermore, the bandwidth of most current OFDM modems is only around tens of kHz, which is much less than the available bandwidth in a network with short range communication links. All these challenges make existing OFDM based MAC protocols for designed terrestrial wireless networks unsuitable. In this work, we propose a new MAC protocol called TDM with FDM over OFDM MAC protocol (TFO-MAC), which smoothly couples TDM with FDM/OFDM for the uplink traffic in a cellular like underwater acoustic network. In TFO-MAC, the acoustic channel is divided into multiple sub-channels in frequency via FDM technology, and OFDM modulation is used in every sub-channel. In addition, time is partitioned into slots and every node can use different channels in different slots. Powerful base stations in the network are responsible for the dynamic channel assignment, power optimization and modulation method selection for all nodes in every slot. We formulate the problem as a mixed integer programming problem and propose an efficient greedy algorithm to solve it. Simulation results show that TFO-MAC can achieve high throughput with good fairness.