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

Abstract

A new multichannel spatial diversity technique has been developed for underwater acoustic communications in very shallow waters. This technique combines a novel synchronization method with maximum-likelihood symbol estimation. It was tested with a multichannel Mills-Cross receiver using various numbers of elements. The FAU general purpose acoustic modem source transmitted messages using four types of frequency-hopped multiple-frequency-chirp-keying (FH-MFSK) modulation: 4 hops at 221 coded bits per second (cps), 2 hops at 442 cps, or no hopping at 1172 cps. These types of modulation allowed for robust data transmission in adverse environment. The modem operated between 16 and 32 kHz at 192-dB source level, at ranges from 1 to 5 km in 40 ft of water. Using only four channels of the Mills-Cross receiver array, messages coded at 1172 cps were received with a frame error rate (FER) of 0% at a range of 3 km. In the same four-channel configuration, messages coded at 221 cps were received with no frame error at 5 km. The experimental results matched the performance predictions from the Crepeaus model. This reliable and computation-efficient method can be implemented on new generations of embedded acoustic modems, such as the four-channel FAU acoustic modem, and can provide significant improvements in communication performance.