Experimental assessment of a multiple sequence direct sequence spread spectrum (MS-DSSS) underwater acoustic communication scheme

This paper describes analysis of data acquired during lake trials of multiple sequence direct sequence spread spectrum (MS-DSSS) system carried out in very shallow water (20-50 m) areas with uneven bottom. The transmitted signals, which consist of frames of multiple spreading sequences, are modulated using coherent phase shift keying. The use of multiple pseudo noise sequences (PN) in a single frame allows one to achieve robust communication with a relatively simple receiver structure as compared to traditional single spreading sequence approach. This long code multiple sequence approach also helps in realizing low probability of intercept (LPI) communication. On the receiver side, a rake type receiver collects the energy present in multiple propagation paths after synchronization and Doppler compensation. Channel estimation necessary for the correct working of rake receiver is done together with a Doppler tracking that allows an adaptation to the instantaneous channel variations. The analysis of the trial data shows that the developed communication algorithm can overcome multipath propagation and Doppler shifts typically present in shallow water channels. The results of the lake trials show that the proposed system gives good performances up to 75 bits per second to a horizontal range of more than four kilometer in a complex shallow water channel. The post processing of the data also shows that we can further enhanced data rate if CDMA approach is employed to transmit parallel data using more PN sequences simultaneously.