# User Visit Certification and Visit Trace System Using Inaudible Frequency
## 3 User Visit Certification and Visit Tracing System Using Inaudible Frequency
If the visit approval is rejected, the server sends an approval error message to the application, which collects the trigger signals from the surrounding area again and then performs the action of requesting reapproval. To generate trigger signals for visit certification, we used the predefined protocol proposed by [PERSON]. Bit data using inaudible high frequencies are defined as shown in Figure 3.  
In Figure 3, to avoid interference between frequencies, a guard bandwidth (GB) is placed on either side of the frequency mapped to a bit, and the bit range uses twice that. The value representing the bit is central, so the bit is extracted according to the on-off keying technique. In other words, if a GB is used at 50 Hz in the range of 18 kHz to 19 kHz, the bit range will be 100 kHz, as shown in Figure 4, and the  
Figure 2: The detailed process flow of user visit certification at the certification server. positions will be 18.05 kHz, 18.15 kHz, 18.25 kHz, \(\ldots\), 19.75 kHz, 19.85 kHz, and 19.95 kHz, with a total of 10 values available. As shown in Figure 4, the predefined protocol can be represented using Equation (1). \[P_{df}=B_{r}(f\alpha) \tag{1}\]  
In Equation (1), \(\mathbf{B_{r}}\) is the bit range and \(f\alpha\) refers to the frequency area. When using a GB of 50 Hz, a bit range of 100 Hz, and a frequency area of 18 kHz to 19 kHz, as shown in Figure 4 (\(\mathbf{P_{df}}\)**= 100 Hz (18-19 kHz)), the data can be 10 bits. In other words, the PC connected to the speaker in Figure 1 encodes the trigger signal data, including the CRC, using the predefined protocol with \(\mathbf{P_{df}}\) and then outputs the generated signal. Next, the microphone of the smart device collects ambient sounds, and the proposed application recognizes the data through FFT analysis. The application performs decoding of the data received by modifying the on-off keying technique from the analyzed FFT values. If the signal contains a frequency mapped to each bit, the value of the bit is set to 1, and if the frequency does not exist, the value of the bit is set to 0. For example, if the application receives FFT analysis results with DFP defined as \(\mathbf{P_{df}}\)**= 100 Hz (18-19 kHz)**, as shown in Figure 5, valid peaks can be determined to be in the range of 18.0 kHz to 18.1 kHz, 18.1 kHz to 18.2 kHz, 18.5 kHz to 18.6 kHz, and 18.8 kHz to 18.9 kHz. The mapped bits with center values of 18.05 kHz, 18.15 kHz, 18.55 kHz, and 18.85 kHz, which are the first, second, fifth, and ninth bits of this domain frequency, respectively, have  
Figure 4: An example of a predefined protocol using a frequency area of between 18 kHz and 19 kHz. Figure 5: An example of decoding data from the FFT result using \(\mathbf{P_{df}}\)**= 100 Hz (18 kHz-19 kHz)**. Figure 3: Definition of guard bandwidth and bit range for the predefined protocol. a value of 1. Therefore, the decoded bit value in Figure 5 is 1100010010 as a 1-bit data point. However, in Figure 5, a small peak (18.75 kHz) between 18.7 kHz and 18.8 kHz may be recognized as a valid value, depending on the threshold for extracting the peak. Therefore, to avoid these errors, this study used CRC techniques to detect errors. CRC ensures an error detection rate of at least **(1 - 2-\(\mathbf{m}\)) \(\times\) 100%** for all situations when used for \(\mathbf{m}\) bit error detection, and the \(\mathbf{m}\) bit of the CRC is added to the back of the actual data, not the data that are separately transmitted, and sent together. Thus, if \(\mathbf{m}\) = 3, the last three bits of 1100010010 are used as added data for error detection, and the actual data you want to transmit are 1100010, the previous seven bits. Therefore, the proposed method uses 8 bits as \(\mathbf{n}\) bits to transmit a trigger signal through speakers for visit certification to multi-use facilities, and the proposed application decodes the frequency values as bit data based on DFP and then detects the data from the trigger signal through CRC review.