A new multi-channel spatial diversity technique for long range acoustic communications in shallow water

Underwater acoustic communication is limited to very low rates without the use of techniques that mitigate the influence of the acoustic channel. A new multichannel spatial diversity communication scheme has been developed at Florida Atlantic University (FAU). This technique combines a novel synchronization method with a maximum-likelihood symbol estimation. The technique is tested with the FAU Dual Purpose Acoustic Modem (DPAM) which uses one transducer for data transmission and 4 hydrophones for data reception. The DPAM transmits messages using 4 types of Frequency-Hoped Multiple-Frequency-Shift-Keying (FH-MFSK) modulation: 4 hops at 221 coded bits per second (cps), 2 hops at 442 cps, or no hoping at 886 cps, or 1182 cps. The bit sequence modulated can be encoded using 3 different Error Coding Codes (ECC): a convolutional code Viterbi 7/sup 1/2/, a Reed Solomon block code, or a combination of both codes. These types of modulation and encoding techniques allow for robust data transmission in adverse environment. The DPAM operates at ranges from 1 to 3 km in 50 feet of water using all 4 transducers. Such a performance can be confirmed using the model of Crepeau.