This paper presents methods to achieve high data transmission rates through metallic barriers using ultrasonic signalling techniques. Due to the frequency selective nature of acoustic-electric channels, orthogonal frequency division multiplexing (OFDM) is employed which achieves high spectral efficiency. Multiple parallel channels are used to further increase data rates. Multiple-input multiple-output (MIMO) techniques are used to reduce crosstalk that would otherwise greatly limit performance and achievable data rates. Several crosstalk mitigation techniques are investigated and their theoretical capacity performances are determined for the general case of A transmitters and A receivers (i.e. A×A MIMO). A physical MIMO acoustic-electric channel array is formed using a 40 mm (1.575 in) thick steel barrier with seven pairs of 4 MHz nominal resonant frequency piezoelectric disk transducers, each with 10 mm (0.394 in) diameter. To investigate the effects of crosstalk, the transducers are closely spaced, and each transmitter-receiver pair is coaxially aligned on opposing sides of the metallic barrier. It is shown that, with the use of crosstalk mitigation techniques, the aggregate multichannel capacity performance scales linearly with the number of channels used and approaches 700 Mbps at high average signal-to-noise ratio (SNR) levels. Finally, the use of bit-loading techniques are explored using several levels of rectangular quadrature amplitude modulation (QAM), and the achievable data rates are compared with each other and to the multichannel theoretical capacity performances.
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