A noncoherent low-frequency ultrasonic (LFU) communication system is proposed for near-field communication using commercial off-the-shelf (COTS) speakers and microphones. Since the LFU communication channel is known to be a frequencyselective characteristic, the proposed system is basically designed by differential phase-shift keying (DPSK) modulation with forward error correction. In addition, automatic gain control of the carrier frequency band over the LFU communication channel is proposed.

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... hen, in order to optimize the symbol length of the proposed LFU communication system under a realistic aerial acoustic channel, a propagation model of the LFU communication channel is proposed by incorporating aerial acoustic attenuation. The performance of the proposed LFU communication system is demonstrated on two different tasks: bit error rate (BER) measurement and successful transmission rate (STR) comparison with Google Tone for various distances between the transmitter and the receiver. Consequently, the proposed method can operate without a bit error at a distance of 8 m under various noise conditions with sound pressure level of 80 dB. Moreover, the proposed method achieves higher STR than Google Tone on a task of URL transmission using two laptops. NFC and BLE have been successfully applied to shortrange communication-based services, including electronic payment or location-based advertisement via smartphones. However, compatibility issues exist in NFC and BLE, since they require corresponding hardware transceiver modules, which are rarely embedded in most devices except recent smartphones or tablets. For this reason, alternative shortrange communication technology that does not rely on a hardware transceiver is in demand for the improved connectivity between various devices. Considering the improved connectivity of short-range wireless communication, acoustic communication can be a suitable alternative to the conventional systems, since acoustic communication transmits and receives data through a speaker and a microphone, which are typically implemented or easily attached to most digital devices. Acoustic communication can be separated into underwater and aerial acoustic communication. First, underwater acoustic communication has been developed for autonomous underwater vehicles, such as submarines or underwater sensors, in order to monitor marine ecosystems or underwater military action [3, 4] . Since underwater acoustic communication delivers sound waves through water, it requires a special form of microphone and speaker that operate underwater (i.e., hydrophones and underwater speakers). In contrast to underwater acoustic communication, aerial acoustic communication utilizes airborne sound waves to transmit or receive data [5] [6] [7] [8] . The operation frequency for aerial acoustic communication can be included in an audible range (50 Hz-18 kHz) [6-8] or within a low-frequency ultrasonic (LFU)