Contents I. Introduction
Ultrasound may be used for short-range wireless communication in air as an alternative to radio frequency (RF) based solutions. It has several advantages over RF in that ultrasonic transmissions are unregulated and interference free to most electronic devices. Ultrasonic signals in air also provide good privacy as they are difficult to intercept through solid barriers, thus increasing network security in an indoor environment. For example, a commercial ultrasonic system Shopkick [1] has been successfully used for a few years. Exclusive deals carried by ultrasonic signals sent from beacons installed in the partner stores are detected by customers' mobile phone microphones. Therefore, member users can only acquire electronic vouchers by physically walking into the stores. However, due to hardware restrictions of current mobile phone microphones, only token signals containing limited information can be transmitted. Previous work has achieved ultrasonic communication via single-channel modulation schemes including on-off keying (OOK), binary frequency shift keying (BFSK) and binary phase shift keying (BPSK) using prototype broad-band air-coupled ultrasonic transducers to produce data transfer rates of up to 83 kbps with a bandwidth efficiency of 0.41 bps/Hz over limited ranges [2]. A later study implemented quadrature modulation method using similar airborne ultrasonic transducers and achieved a bit rate of 200 kbps with an improved spectral efficiency of 0.57 bps/Hz over a distance of 1.2 m in air [3].
