Burst mode hybrid spread spectrum technology for covert acoustic communication

To overcome the problem of underwater transmitted platform being detected by energy detector, a burst hybrid spread spectrum scheme which transmits extremely short signal for covert underwater acoustic communication is proposed in this paper. A non-data-aided self-synchronization algorithm is achieved using a time-frequency 2-dimensional search algorithm, carrier frequency offset and symbol offset forms a two-dimensional phase space. In the two-dimensional phase space, optimal carrier frequency and chip phase are found at the same time. An orthogonal-hybrid spread spectrum communication scheme with direct sequence spread spectrum (DSSS)-BPSK modulation, Cyclic Shift Keying (CSK) modulation and DSSS-MFSK modulation is proposed which promotes the bandwidth efficiency. The non-coherent maximum likelihood receiver is adopted to counteract the fast selected and frequency selected channel fading. The performance of the system over AWGN channel and underwater acoustic multipath channel is analyzed through the theory analysis and numerical simulation. The data from the shallow water experiment near South Sea, China, is used to demonstrate the system performance. The results from the experiment showed that the data rate can achieve up to 317 bps and bit error rate (BER) under 10^-3 in the distance of 5 km over the bandwidth of 4 kHz.