Design and Characterization of an MFSK-Based Transmitter/Receiver for Ultrasonic Communication Through Metallic Structures

Some applications require wireless transmission of information to and from devices located inside metal enclosures, e.g., a closed shipping container in transit. However, traditional radio frequency (RF) communication schemes are not capable of transmitting signals through metal enclosures. As an alternative to RF, an ultrasonic communication system based on multitone frequency-shift keying (MFSK) has been developed and evaluated using the steel corner posts of shipping containers as the communication medium. Empirical studies have been performed on the steel channel. The communication system is configurable and consists of two or more modules. The modules are mounted to the metal posts and utilize an inexpensive ultrasonic transducer to send and receive modulated signals through the metal channel. A module also makes use of an inexpensive digital-signal-processing chip for modulating and demodulating MFSK signals. Two case studies for the shipping container application were evaluated: 1) communicating through one container and 2) communicating between stacked containers. Regarding the second case study, a key discovery was that each of the aggregate steel columns formed by a stack of shipping containers can serve as an ultrasonic communication channel that spans all containers in the stack.