A joint power allocation and bit loading algorithm for M-QAM-OFDM underwater acoustic communication system

In this paper, we have proposed a joint power allocation and bit loading algorithm for underwater acoustic (UWA) OFDM system. We first derive the UWA OFDM model, which integrates both the large-scale and small-scale effects of UWA channel. Due to the poor quality of the UWA channel, most practical systems admit input signals from small finite discrete constellations. We then investigate the optimal power allocation to maximize the mutual information based on our model. Furthermore, a two-step joint power allocation and bit loading algorithm was proposed to maximize the transmission rate. Specifically, we resort to mercury/waterfilling algorithm to initially allocate the transmission power and bits among all subcarriers. Then all allocated power is rescaled to meet the power and discrete constellation constraints.