Indoor wireless communication using airborne ultrasound and OFDM methods

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 techniques such as Bluesniping. Airborne ultrasound may be used as an alternative to RF for indoor wireless communication systems for securely transmitting data over short ranges, as signals are difficult to intercept from outside the room. A pair of off-the-shelf air-coupled capacitive ultrasonic transducers with a centre frequency of 50 kHz was used in the implementation of an indoor airborne wireless ultrasonic communication system. Binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK) and quadrature amplitude modulation (QAM) based orthogonal frequency division multiplexing (OFDM) modulation methods were successfully implemented using multiple orthogonal subchannels. The maximum system data rate of up to 180 kb/s was achieved using 16-QAM modulation with ultrasonic channels from 55 kHz to 99 kHz, over an error-free line-of-sight transmission distance of 6 m. The spectral efficiency achieved was 4 b/s/Hz. The transmission range with no measurable errors could be extended to 9 m and 11 m using QPSK and BPSK modulation schemes, respectively. The achieved data rates for the QPSK and BPSK schemes were 90 kb/s and 45 kb/s using the same bandwidth.