Multi-frequency DPSK modulation for long-range underwater acoustic communication

Long-range shallow water channels are characterized by severe channel fading and low available signal to noise ratio (SNR). In classical underwater acoustic communication systems for above channels, incoherent MFSK modulation, or coherent modulation based on spatial diversity and adaptive equalizations are employed. Although being robustness to fading channels, the incoherent modulation cannot solve efficiently the conflict demands between the high signaling rate and reliability. Large receiver arrays may be restricted to be used for some size limited receiver platforms. A novel multi-frequency NDPSK (MFSK-NDPSK) modulation approach for long-range underwater acoustic communications is presented in this paper which is well suited in shallow water channels. The proposed approach is an efficient combination of MFSK and NDPSK modulation. Signals are NDPSK modulated with multi-carrier frequencies, and the carrier frequencies of adjacent symbols are located in different subbands. Information is transmitted by both frequencies and phases of signals. The theoretical analysis and simulation results are given, and also the experiments in lake were conducted. The results demonstrate that compared with single-carrier modulation, the MFSK-NDPSK modulation can increase data rate in same communication range, or increase the communication range while keeping the same data rate due to input SNR can be increased to improve the performance of decreasing inter-symbol interference and increasing reliability of communication systems.