# User Visit Certification and Visit Trace System Using Inaudible Frequency
## 2 Previous Work  
In this section, we review existing studies that use inaudible high frequencies in the audible frequency domain. Recent inaudible high frequency studies have used a range of frequencies from 18 kHz to 22 kHz, which can be output from ordinary speakers and are rarely heard by people, as recognized by microphones in smart devices. Studies that have used trigger signals by selecting two frequencies from the range of audible frequencies are [PERSON]'s museum smart guide system, [PERSON]'s user authentication method between smart devices and PCs, and [PERSON]'s public transportation drop-off notification application and spatial density analysis.  
[PERSON] used two frequencies, 20 kHz and 22 kHz, to generate signals with frequency shift keying (FSK) technology and proposed a trigger signal applied with a hamming code to compensate for errors in signal transmission [11]. [PERSON] proposed generating two high-frequency combinations in the range of 15.8 kHz to 20 kHz to transmit 4-bit signals [12]. The authentication signal would then send 2 bytes for 8 s. [PERSON]'s public transportation drop-off notification application also uses [PERSON]'s proposed method, which sends two combinations in the range of 18 kHz to 22 kHz, along with trigger signals when station announcements are made through speakers [13]. This method generates a trigger signal for three seconds, during which time the smart device recognizes the current location of public transportation from the signal and notifies the user of the station to get off at. [PERSON] also proposed a spatial user density analysis system that can identify how many people are gathered in a particular space using the same signal [14]. However, this pair of high-frequency approaches has the disadvantage of having less transferred data value compared to the time required for delivery. In addition, this method had the problem of requiring a gap of more than 600 Hz to avoid interference between frequencies.  
To complement this, [PERSON] proposed a method for transferring near-field data between smart devices using various frequencies ranging from 18 kHz to 22 kHz, rather than only two frequencies, using on-off keying and cyclic redundancy check (CRC) techniques [15]. This method complements the 600 Hz interval of interference between frequencies feared in conventional methods. This method predefines the data frequency protocol to send more data in a short time frame and seeks to find the optimal transmission value by varying the frequency interval between 25 Hz and 100 Hz. To prevent errors in data transmission due to interference between frequencies, this method applies CRC techniques, which can transmit 40-bit data, including 8 bits of CRC data, within 1 s. [PERSON] proposed an invisible data transmission system using dual frequencies that are resistant to ambient noise [16]. [PERSON]'s method uses the Mary frequency shift keying modulation scheme to improve data transmission accuracy [17]. This method was transmitted using two frequencies with 200 Hz intervals in the range of 16 kHz to 22 kHz, showing 90% accuracy within 12 m.