A study has been undertaken of ultrasonic communications methods in air, using a quadrature modulation method. Simulations were first performed in order to establish the likely performance of Quadrature Phase Shift Keying (QPSK) over the limited bandwidth available in an ultrasonic system. QPSK modulation was then implemented within an experimental communication system, using capacitive ultrasonic sources and receivers. The results show that such a system is feasible in principle for communications over distances of several metres, using frequencies in the 200-400 kHz range. Data rate is typically at 200 kbps.
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