Capacity criterion-based power loading for underwater acoustic OFDM system with limited feedback

In the orthogonal frequency division multiplexing (OFDM) underwater acoustic (UWA) communications, some subcarriers may be subject to a deep fading. If the channel state information (CSI) is available at the transmitter, adaptive transmission techniques can be applied to mitigate deep fading effect and increase the overall system performance. Since most practical systems only obtain partial CSI at the transmitter, meanwhile, the concept of limited feedback was not been used for UWA channels and in particular not for shallow water in literature. Therefore, it´s more valuable to analyze the underwater acoustic channel with limited CSI feedback. In this paper, it is for the first time combines ambient noise and UWA channel and calculates the ambient noise power in each subcarrier, which makes it extremely challenge. We explain the properties of UWA channel and investigate the optimal power loading strategy for UWA channel with different levels of transmitter CSI,which is combined with bit loading for a given system target bit-error rate (BER). The optimum results of the problem are derived by maximum system capacity with system total transmit power and target BER constraint. The Lloyd Algorithm is employed to quantize the CSI at receiver and construct the codebook, whose copy is also known to the transmitter. Simulation results compare the performances among different bits feedback, perfect feedback and non-feedback, which show that a few bits feedback can significantly improve the system performance.