Power-Efficient ELF Wireless Communications Using Electro-Mechanical Transmitters

Ultra-miniaturized and highly power-efficient ELF (0.3-3 kHz) transmitters are desirable for underground and undersea wireless communications. This paper proposes the use of rotating permanently polarized dipoles (magnets or electrets) for such applications. A power-efficient data modulation scheme based on continuous-frequency FSK (CF-FSK) is also proposed for such electro-mechanical transmitters. Theoretical analysis and simulations show that 7-ary CF-FSK is optimal in the sense that it minimizes average mechanical torque on the mechatronic antenna for a given bit rate. Preliminary experimental results from a prototype ELF magnetic-field based transmitter based on a high-strength rare-earth (NdFeB) magnet and small brushless DC (BLDC) motor are presented. The results highlight local environment challenges such as power line interference that affect the choice of data encoding and modulation schemes. Reliable non-line-of-sight (NLOS) communication through concrete barriers at 100 Hz and ~0.25 bit/sec is demonstrated at distances up to ~5 m. The proposed mechatronic antenna and modulation scheme can be scaled up in both frequency and transmit power for more challenging applications in surface-to-undersea, surface-to-cave, and other conductive environments that require ELF channels.