Full-Duplex Airborne Ultrasonic Data Communication Using a Pilot-Aided QAM-OFDM Modulation Scheme

Orthogonal frequency division multiplexing (OFDM) has been extensively used in a variety of broadband digital wireless communications applications because of its high bandwidth utilization efficiency and effective immunity to multipath distortion. This paper has investigated quadrature amplitude modulation and OFDM methods in air-coupled ultrasonic communication, using broadband capacitive ultrasonic transducers with high-k dielectric layers. OFDM phase noise was discussed and corrected using a pilot-aided estimation algorithm. The overall system data rate achieved was up to 400 kb/s with a spectral efficiency of 2 b/s/Hz. An ultrasonic propagation model for signal prediction considered atmospheric absorption of sound in air, beam divergence, and transducer frequency response. The simulations were compared with the experimental results, and good agreement was found between the two. Two-way communication through air was also implemented successfully by applying three-way handshaking initialization and an adaptive modulation scheme with variable data rates depending on the transmission distance, estimated using received signal strength indication measurement. It was shown that the error-free transmission range could be extended up to 2.5 m using different system transfer rates from 400 kb/s down to 100 kb/s. In full-duplex transmission mode, the overall error-free system data rate achieved was 0.8 Mb/s up to 1.5 m.