A novel CM-DSSS modulation scheme for robust communication over the underwater acoustic channel

Abstract

Direct-sequence spread spectrum (DSSS) is a candidate technique for long-range underwater acoustic communication (UAC). However, its data rate is quite low. A coded M-ary DSSS (CM-DSSS) modulation scheme that adopts a pseudo-noise sequence (PNS) family spreading spectrum together with dual-K coding is proposed to increase the transmission efficiency and robustness. A UAC system that uses CM-DSSS modulation is designed. The system shares the advantages of the correlating receiver of the DSSS, and more coding gain based on Viterbi soft-decision algorithm (VSDA). The multipath diversity gain can be further obtained by passive-phase conjugation (PPC) processing. The process of estimating the maximum likelihood path (MLP) of dual-K decoder using VSDA is described, and the soft-decision of decoder calculating is much simpler than that of the other scheme. The bit error rate (BER) performance in the additive white Gaussian noise (AWGN) channel is compared by computer simulation with different modulation parameters such as symbol size, varying PNS family sizes, and PNS length, and the optimal parameter design problem of the modulation scheme is discussed. Simulation results indicate that the dual-K coding is very effective for performance improvement. The performance of the proposed CM-DSSS modulation scheme has been verified by means of offline processing of data acquired during shallow sea trials near Dalian Tiger Beach Ocean Park and deep sea trials far away from the Sanya of China. Results show that the scheme has a low bit error rate (BER) and can robustly communicate over the different underwater acoustics channels.