Improving bit rate of long-range underwater communication with orthogonal M-ary/DS spread spectrum

Spread spectrum is suitable for long-range or low-SNR underwater acoustic communication. But the low bandwidth efficiency of spread spectrum and limited underwater channel bandwidth cause low bit rates. So we propose the method of orthogonal M-ary/DS spread spectrum to improve bandwidth efficiency and bit rates; this method is a combination of orthogonal direct sequence (DS) spread spectrum and parallel M-ary spread spectrum. We introduce orthogonal M-ary/DS spread spectrum method and RAKE receiver. Experimental results in a lake show that, by use of Gold sequences of 63 and 127 lengths, the orthogonal M-ary/DS spread spectrum system achieved error-free communication of 480 symbols at 25 km and 0 dB SNR with RAKE receiver; the bit rates were up to 381.0 bits/s and 220.5 bits/s. In comparison with direct sequence spread spectrum using same sequences, the bandwidth efficiency of the new method increases by about an order of magnitude.