Airborne ultrasonic communications using multiple parallel channels

Concerns still exist over the safety of prolonged exposure to radio frequency (RF) wireless transmissions and there are also potential data security issues due to remote signal interception. Airborne ultrasound may be used as an alternative to RF for indoor wireless communication systems for securely transmitting data over parallel channels at short ranges. This thesis describes the design and evaluation of an airborne ultrasonic communication system using capacitive transducers. In this study, basic digital modulation schemes, such as ON-OFF keying (OOK) and binary phase-shift keying (BPSK), were implemented successfully over multiple parallel channels for data transmission over an air gap of up to 10 m with wireless synchronization using ultrasonic means. A simulation model that can precisely predict ultrasonic signals through the air channel was also created to help with signal characterisation. To further improve the bandwidth efficiency and eliminate the need for filtering, quadrature amplitude modulation (QAM) using orthogonal frequency division multiplexing (OFDM) method was investigated. The recorded data rate was 800 kb/s at 0.7 m with no measurable errors using prototype broadband ultrasonic transducers, and 180 kb/s at 6 m using relatively narrowband commercially available transducers. This thesis has also looked at implementing an indoor communication network with ceiling-mounted base stations and a mobile communicator for practical applications. An asynchronous ultrasonic location technique using Gold Code modulated ranging signals was chosen to optimise the modulation schemes, and offer automatic handover between different cell regions on a switch on and off basis as all base stations use the same frequency bands for data transmission