# User Visit Certification and Visit Trace System Using Inaudible Frequency
## 1 Introduction  
Recently, due to COVID-19, many untact (non-face-to-face) service technologies have been provided at various commercial facilities, such as restaurants, performances, shops, department stores, and companies. These technologies require people's visit records during check-ins using a quick response (QR) code. Electronic access registration services using a phone can be found anywhere users enter. QR codes have large squares of three vertices and small squares close to each vertex, and they can store up to 7089 digits and 4296 ASCII characters [1]. Initially, QR codes were used to provide URL addresses and simple text information for website links. However, their range of use has gradually expanded, and QR codes are now used as electronic payment methods, such as Ali Pay, Zero Pay, and Kakao Pay, and they are also used for mobile tickets, such as express buses, trains, and airlines [2]. In June 2020, due to COVID-19, QR codes were also used in the KI-PASS (an electronic access list), which would be prepared when entering and exiting multi-use facilities. Therefore, visits are currently restricted to access through QR code recognition methods or call-in to caller phone numbers when attempting to enter most facility buildings [3, 4, 5]. There are two methods of certifying electronic entry lists using QR codes. The first method installs QR code recognition applications on the user's smart device and recognizes the QR code placed in the visiting facility. The application manages visitor information by sending user information along with recognized data to a designated server [6]. This method requires the user's smart device to have a visiting certification application with QR detection functionality, and the user's information must be registered in the application. This method does not require the installation of smart devices for QR code recognition in electronic entry lists at access facilities, and it also has the advantage of only printing QR codes containing facility information and placing them in facilities. The second method is to issue an authentication QR code from the Naver or Kakao application (in Republic of Korea) using a user's smart device, then to allow the user to recognize the code through the camera of the smart device installed in the facility [7]. The smart device installed for visit certification does not store the data of the recognized code in the smart device itself but rather sends it to a designated server to manage visitors' information. This method requires the establishment of smart devices, applications, and servers for electronic entry lists at the access facilities because visitors' smart devices have user information. However, both of these methods require specific actions from users who recognize QR code on smart devices. For example, in stores where only QR codes are printed, users should recognize the QR code by running a designated visit certification application. If a smart device for visit certification is installed at the entrance of the store, the authentication QR code with user information should be generated from the smart device, and the authentication QR code should be recognized by the installed smart device. In other words, users may be confused about which of the two methods should be used, because both methods are not unified. There is also a disadvantage: when several people visit together, they often have to line up at the entrance in turn to certify their visit. Call check-in with caller telephone number access is a method of calling a designated contact at the point of visit and certifying the visit. In Republic of Korea, the user can call a designated phone number starting with 070 or 080 for the store they are visiting and enter the store after confirmation of certification [8; 9]. The certification center where the user dials, the user's contact number, the phone reception time, and the designated number are recorded on the server to create an electronic entry list.