Capacity Criterion-Based Bit and Power Loading for Shallow Water Acoustic OFDM System with Limited Feedback

In orthogonal frequency division multiplexing (OFDM) underwater acoustic (UWA) communications, some subcarriers may be subject to deep fading. If the channel state information (CSI) is available at the transmitter, adaptive transmission techniques can be applied to mitigate the deep fading effect and increase the system's overall performance. Therefore, it is more valuable to analyze the UWA channel with limited CSI feedback. In this paper, we adopt ambient noise in the OFDM UWA communication system for the first time and calculate its power for each subcarrier. We explain the properties of UWA channel and investigate the optimal power loading strategy for the UWA channel with different levels of transmitter's CSI, which is combined with bit loading for a given system target bit-error rate (BER). The optimum result is derived by maximum system capacity with the constraints of the system's total transmission power and target BER. The Lloyd algorithm is employed to quantize the CSI at the receiver and construct the codebook, which is also known to the transmitter. Simulation results compare the performance difference between a few bits of feedback, perfect feedback and non-feedback, which show that a few bits of feedback can significantly improve the system performance.