A novel spread spectrum OFDM underwater acoustic communication

Spread spectrum methods are used in underwater acoustic communication (UAC) systems to provide robust acoustic links or long distance communication abilities. However, the bandwidths of ocean channels limit the data rate of spread spectrum UAC. The method using quasi-orthogonal sequences with phase shift keying and parallel transmission to increase the data rate of spread spectrum UAC can be found in Refs.1and 2. In this paper, we apply OFDM to realize parallel transmission of spread spectrum signal. Firstly, spread spectrum OFDM (SS-OFDM) is introduced into UAC. Then, a novel SS-OFDM method named MSS-OFDM based on M-ary quasi-orthogonal sequences is proposed in this paper. The processing gain of SS-OFDM is 10 * log 10 (L), where L is the length of PN sequence used in SS-OFDM. The bit error rate (BER) performance of SS-OFDM and MSS-OFDM compare to OFDM is analyzed and simulated in AWGN channel. Computer simulation results using multi-path channel model based on lake test data show that MSS-OFDM has double anti-multipath abilities, which can significantly improve the data rate of spread spectrum UAC with a robust performance, even without cycle prefix. Water tank experiments were conducted to check this novel method. Simulation and test results show that MSS-OFDM can provide robust and moderate high-speed spread spectrum UWA communication.