# A Filter-Based and Parallel Unknown Tag Identification Protocol in Open RFID Systems
## 2 Related Work  
Many recent studies on RFID technology focus on functional applications, including information collection [30; 31], cardinality estimation [32; 33], tag grouping [34; 35], searching a wanted tag set [36; 37], and missing tag identification [38; 39]. Unknown tag identification, as a research branch, is practically important because it helps identify the unknown tags that are misplaced or moved in the reader's interrogation area. To solve this problem, two categories of protocols have been designed: the probabilistic protocol [21; 22; 23] and the deterministic protocol [24; 25; 26; 27; 28].  
The probabilistic protocol can identify unknown tags rapidly with an expected accuracy. [PERSON] et al. designed an unknown tag detection scheme for detecting unknown tags with an expected confidence level [21]. [PERSON] et al. proposed the _white paper_ (WP) protocol [22], which constructs a composite message data structure consisting of all useful informative data from some independent detection synopses. This structure helps improve the unknown tag detection efficiency and decrease the failure probability. Although these probabilistic protocols can judge unknown tags in a fast way, they cannot identify the information of all unknown tags' IDs. Compared with the probabilistic protocol for unknown tag identification, the deterministic protocol can collect all information of unknown tags' IDs. For example, [PERSON] et al. firstly formulated the problem of unknown tag identification and presented an unknown tag identification protocol called _continuous scanning_ (CU), which can collect a specified fraction of intact unknown tags [24]. However, CU only achieves incomplete unknown tag identification caused by randomized algorithms. [PERSON] et al. proposed the BUIP, SUIP, and MUIP protocols [25], which adopt indicator vectors to distinguish known tags from unknown tags and deactivate the known tags. Although they can identify the intact unknown tag set with higher efficiency, more time is needed. [PERSON] et al. proposed the FUTI and IFUTI protocols [27], which investigate the interactive filters to accelerate the process of unknown tag identification. [PERSON] et al. proposed the unknown tag identification protocol based on a coded filtering vector [26], which utilizes the coded filtering vector technique to separate unknown tags from known tags and assign each singleton slot a status code to reduce known-unknown collisions, thus improving the identification performance. [PERSON] et al. proposed the _physical-layer unknown tag identification_ (PUTI) protocol [28], which identifies the unknown tags by aggregating physical layer signals. Although these deterministic protocols can identify unknown tags, they are time-consuming and suffer from a high false positive rate.  
This paper proposes a filter-based and parallel unknown tag identification protocol called FPUI. The FPUI thoroughly separates known tags from unknown tags and deactivates all known tags by adopting an RSQF-based fingerprint filter, thus improving the slot utilization and the accuracy of the protocol. Meanwhile, the FPUI adopts a parallel identification method to collect multiple unknown tag IDs in parallel, thus increasing the efficiency of the protocol.