Proc. SPIE. 9825, Sensors, and Command, Control, Communications, and Intelligence (C3I) Technologies for Homeland Security, Defense, and Law Enforcement Applications XV
KEYWORDS: Signal to noise ratio, Ferroelectric materials, Digital signal processing, Quadrature amplitude modulation, Statistical analysis, Modulation, Receivers, Transducers, Data communications, Acoustics
Acoustic-electric channels can be used to send data through metallic barriers, enabling communications where electromagnetic signals are ineffective. This paper considers an acoustic-electric channel that is formed by mounting piezoelectric transducers on metallic barriers that are separated by a thin water layer. The transducers are coupled to the barriers using epoxy and the barriers are positioned to axially-align the PZTs, maximizing energy transfer efficiency. The electrical signals are converted by the transmitting transducers into acoustic waves, which propagate through the elastic walls and water medium to the receiving transducers. The reverberation of the acoustic signals in these channels can produce multipath distortion with a significant delay spread that introduces inter-symbol interference (ISI) into the received signal. While the multipath effects can be severe, the channel does not change rapidly which makes equalization easier. Here we implement a 16-QAM system on this channel, including a method for obtaining accurate carrier frequency offset (CFO) estimates in the presence of the quasi-static multipath propagation. A raised-power approach is considered but found to suffer from excessive data noise resulting from the ISI. An alternative approach that utilizes a pilot tone burst at the start of a data packet is used for CFO estimation and found to be effective. The autocorrelation method is used to estimate the frequency of the received burst. A real-time prototype of the 16 QAM system that uses a Texas Instruments MSP430 microcontroller-based transmitter and a personal computer-based receiver is presented along with performance results.
Proc. SPIE. 8753, Wireless Sensing, Localization, and Processing VIII
KEYWORDS: Signal to noise ratio, Quadrature amplitude modulation, Data modeling, Modulation, Signal attenuation, Interfaces, Data transmission, Transducers, Orthogonal frequency division multiplexing, Acoustics
Recent research has shown that acoustic waves can be used to transmit data and power through metallic barriers. In this paper, we extend this work to consider the case where the channel consists of multiple layers of different materials. In particular, a steel-water-steel type of interface i.e., a layer of water sandwiched between two steel walls, is investigated. A pair of 1 MHz resonant (25.4 mm diameter) piezoelectric transducers are co-axially aligned and mounted on the dry side of each steel wall to form the channel. This channel is acoustic-electric in nature and is modeled as cascade of layers and interfaces in MATLAB. Each layer (single material) and interface is interpreted as transmission line in the acoustic domain. Experimental channels are implemented and the measured channel characteristics are compared to those obtained using the model. The power transfer efficiency and channel capacity are determined using the measured channel data. To maximize the capacity and reduce interference, it is assumed that data transmission is performed using orthogonal frequency division multiplexing
(OFDM). The width of water column is varied and its effect on the power transfer efficiency and data capacity are shown. Results indicate that a channel formed by two steel walls of 15.97 mm and 10.92 mm thickness separated by 88.3 mm water column is capable of supporting data rates of several megabits/sec and of transferring power with more than 30 percent efficiency.