Short-range ultrasonic communications in air using capacitive transducers

This thesis describes the development and characterisation of an airborne ultrasonic communication system using electrostatic transducers. Initially, digital modulation schemes, namely OOK, BFSK and BPSK were used for transmission of a digital binary stream across an air gap of 0 – 1.2 m. BPSK was found to be the most favourable solution for the lowest BER performance. BFSK, being the second best among the three was simpler to implement and was more robust against synchronisation errors. Later, to further improve bandwidth efficiency, several M-nary modulations, namely QPSK, MSK and 8PSK were used. The data rate reached 800 kbps when 8PSK was used. An ultrasonic keyboard using BFSK were developed as a simple application. A real-time communication system was also developed using a PXI system and LabVIEW, with which broadband data communication and real-time performance analysis was realised. The channel response of the ultrasonic communication link was modelled, with the characteristic of ultrasonic absorption in air, transducer response, and spatial field response. The latter was found to be crucial factor when the transreceivers were not aligned in parallel. The results recorded at various distances were in good agreement with those predicted using the system model developed. Several equalisation techniques were looked at in this thesis, to combat the selective attenuation response for longer-range propagation, and its effectiveness has been compared to using multicarrier modulation. This thesis has also looked at multipath interference, which is likely to be effect the performance in future applications. It shows that by using spread spectrum techniques, multipath interference can be tackled in an ultrasonic system.EThOS - Electronic Theses Online ServiceGBUnited Kingdo