Soft demodulation algorithm for underwater acoustic multichannel frequency hopping communication in inconsistent colored noise

To address the interference of colored noise, and its inconsistency in the underwater acoustic communication channel, this paper proposes a soft demodulation algorithm for the multichannel frequency hopping-multiple frequency shift keying (FH-MFSK) modulation scheme. Based on the criterion of maximum ratio combining (MRC), the proposed algorithm takes into account the inconsistency of noise intensity and spectral characteristics among channels, and expands MRC to a two-dimensional combining method that individually weights each frequency point of each array element. Meanwhile, two soft demodulation methods based on maximum a posteriori (MAP) probability are proposed in the channel of colored noise. The spectral structure of colored noise is estimated and exploited to calculate soft metrics for each bit. The accuracy of soft demodulation is improved, and the soft metrics of frequency points affected by intense noise are reduced, thereby reducing the bit error probability during decoding. Compared with the traditional soft demodulation algorithm, the proposed algorithm achieves a 2.7 dB performance gain in the simulation channel of single-channel colored noise and a 4.7 dB gain in the channel of multichannel inconsistent colored noise. Additionally, in the field test equipped with flank arrays, the proposed algorithm reduces the average bit error rate (BER) from 1.59 × 10⁻² to 0.