Multi-channel data communication through thick metallic barriers

This paper explores the use of multiple communication channels to transmit data at high rates, without physical penetrations, through thick metallic barriers using ultrasound. Two parallel acoustic-electric channels are formed in close proximity utilizing two pairs of coaxially aligned piezoelectric transducers mounted on, and acoustically coupled to, opposing sides of a metal wall. Each channel employs orthogonal frequency division multiplexing (OFDM) which can achieve a high spectral efficiency in frequency selective channels. The two-transmitter-two-receiver MIMO configuration is studied and analytical expressions of channel capacity are determined for the raw channels as well as for several co-channel interference cancellation techniques and they are verified using a Monte-Carlo simulation. It is shown that excessive crosstalk between the channels can lead to marginal increases or decreases in capacity over the single channel when no interference suppression is used. Several interference cancellation structures, including zero forcing, eigenmode transmission, and minimum mean-square error (MMSE) are investigated to mitigate this effect. It is shown that their aggregate capacity may approximately double that of either channel used independently. It is also determined that, in a relatively static MIMO acoustic-electric channel, a minimal complexity adaptive approach such as the Least Mean Squares (LMS) algorithm may be used while achieving a similar capacity to more complex structures.