Multicarrier Spread Spectrum for Covert Acoustic Communications

A multicarrier modulation scheme is presented to achieve the objective of clandestine acoustic communications. The modulation consists of a single bit sequence simultaneously modulated onto multiple carriers. As all bands carry the same symbol stream, they can be adaptively combined with a multi-channel equalizer. A multiband equalizer with K feedforward filters is thus devised for joint equalization and despreading of K frequency bands. The idea is tested on acoustic data collected during sea experiments in the Baltic Sea. Eight carriers were used to divide the available bandwidth up into K = 8 binary phase-shift keyed bands of 460 Hz each. Covert operation at low SNR is enabled by the spread-spectrum gain delivered by the adaptive combiner, a rate-1/3 turbo coding scheme, and through the use of periodic training. Results are shown for three experiments over ranges of 8, 28, and 52 km, using a prototype acoustic modem as the transmitter and a hydrophone at the receiving end. Both the modem and the hydrophone were lowered into the Baltic sound channel from surface ships. At an effective data rate of 75 bit/s, the user message is correctly recovered at SNRs down to -12 dB under various multipath conditions. A comparison is made with two other modulations, direct-sequence spread spectrum and orthogonal frequency-division multiplexing. These signals were broadcast during the same experiments, in the same frequency band, and at the same data rate. The proposed multicarrier scheme compares favorably with the other modulations.