# RF-Access: Barrier-Free Access Control Systems with UHF RFID
## 2 Related Work  
Access control systems need to install flap barriers to help manage pedestrian access and block unauthorized staff at any entrance, which can function properly but require visitors to wait for the opening of the blocking body, leading to low-frequency pedestrian access and even congestion in places with large passenger flow. In recent years, studies have shifted to barrier-free access control. TDHlex [7] designed a customized sensor to detect tailgating intrusion. Specifically, TDHlex uses a non-scanning light source to emit modulated near-infrared light and generate real-time three-dimensional images of the monitoring area by measuring the difference between the lights emitted by the light source and the detection target. Computer vision is another technology for barrier-free access control. For example, HIKVISION [8] produces a binocular intelligent network camera, which is deployed directly above the monitoring area. It adopts binocular stereo vision technology to obtain height information of objects and an intelligent tracking algorithm to analyze the users' behavior trajectory, counting the number of users and detecting access directions. However, vision-based sensing suffers from the impact of environmental factors, including low illumination and non-line-of-sight (the target is blocked by others). Additionally, these solutions cannot figure out the user identity accurately.  
In comparison to the above sensing technologies, RFID offers an appealing alternative, with the advantages of unique identification, non-line-of-sight communication, and high reading rates, which makes it the most widely used in the field of access control [9; 10]. In recent years, some RFID studies have shifted to study barrier-free access control. [PERSON] et al. [11] deploy a group of infrared intrusion detectors to detect the user's pass in and out, which, however, allows only a single-channel pass at a time. [PERSON] [12] and [PERSON] [13] deploy two sets of infrared switches on both sides of the gates to detect the directions of users but cannot detect an illegal user amongst legal users. [PERSON] et al. [14] demand the time interval between the illegal pass and the tag last seen to be greater than 3 s, which is too ideal for practical use. Additionally, the existing solutions all use the HF RFID as the authentication approach, which suffers from a short communication range. Namely, each user needs to stop at the entrance to swipe his/her identity card for authentication, which still has the problem of low-frequency pedestrian access. There are a few commercial RFID readers (antennas) that provide us with the function of moving direction estimation, e.g., Impinj xSpan [15]. They use an antenna-array design to perform beamforming, which is able to dynamically scan the moving tags but suffers from the limitations of the large device size and the high system cost. For example, an Impinj xSpan reader costs nearly nearly 3000 [16], which is much higher than the retail price of an RFID-based access control system. Instead, our design uses the existing RFID reader and antennas embedded in the existing access control system, with no need for any extra hardware augmentation.