OCSID: Orthogonal Accessing Control without Spectrum Spreading for Massive RFID Network

In radio-frequency identification (RFID)-based sensors networks, each sensor is integrated with a tag and sensors may co-exist in an area of interest. Before sending data packets, the sensors send their IDs for channel reservations. However, ID collisions happen frequently at the reservation stage which leads to significant time delays, especially for massive and dense networks. In this article, by employing group theory, we show that for $B=2^{k}$ , where $k$ is a positive integer, the set $\{-1,+1\}^{B}$ and the Hadamard product $\circ $ forms a group $\{\{-1,+1\}^{B}, \circ \}$ that can be divided into $(2^{B}/B)$ disjoint subsets, each of which has $B$ binary vectors that are mutually orthogonal. Based on this finding, we propose orthogonal coset identification (OCSID) and its generalization, query tree (QT)-OCSID, that can recover ID information from collisions, and thus considerably improve the efficiency at the reservation stage, particularly when the network is large and/or dense. In an ideal case, it can recover/decode $B$ tags for each query. The fundamental difference between the proposed OCSID schemes and code-division multiple access-based schemes is that, OCSID achieves orthogonal design for ID information recovery by exploiting the inherent orthogonal structure of the binary vector set, instead of spreading the spectrum. Hence, it requires a narrower frequency band, lower circuit complexity, and lower synchronization precision, which are much more preferred by hardware limited devices.