Spatial modulation using vertical arrays in an underwater acoustic telemetry channel

Multiple propagation paths between sound sources and receivers in an underwater acoustic channel are a common component of underwater acoustic telemetry. The temporal reverberation that results may adversely affect performance measures such as bit error rate and output signal-to-noise (SNR) ratio. With the use of transducer and hydrophone arrays, however, resolvable paths may also be exploited as additional bandwidth in a manner completely analogous to frequency bandwidth using a parallel channel model. In this discussion, we describe an experiment conducted in Buzzards Bay, MA, that utilized moving, vertical arrays to spatially modulate and demodulate two acoustic communication signals that fully occupied identical time-frequency spaces. Decomposition of the composite shallow water channel into two, parallel communication channels was accomplished solely with spatial degrees of freedom. Performance is compared with a conventional, single signal transmission. In addition, in situ channel impulse response measurements are analyzed to explain the success of the spatial modulation experiment. Performance improvements comparable to those yielded by a 3–6-dB SNR increase are achieved without using additional signal power. We will conclude with a brief discussion regarding the challenges as well as potential applications of the spatial modulation technique.