# An Admission-Control-Based Dynamic Query Tree Protocol for Fast Moving RFID Tag Identification
## 3 System Model
### A Fast Moving RFID Tag Identification System  
Among massive RFID applications, conveyor belt systems with RFID readers and tags are popular in various industries. In these applications, objects with RFID tags attached are placed on a conveyor belt. One or more RFID readers are usually mounted in a fixed position above (or beside) the conveyor belt. As the conveyor belt quickly moves, tags continuously move into and out of the reader's reading range. As is demonstrated in Figure 1, a general conveyor belt system is considered. Assume that each object has one tag attached that has a 96-bit unique identifier. The objects are denoted by their attached tags hereafter. In the model, tags are placed on the conveyor belt in series, and the distance \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Static Situation**} & \multicolumn{2}{c}{**Moving Situation**} & \\ \cline{2-5}  & **ALOHA-Based** & **Tree-Based** & **Frame Adjustment** & **Tree Enhancement** & **Scheduled** \\ \hline \multirow{4}{*}{features} & tags reply to the & tags are & \multirow{2}{*}{\begin{tabular}{c} continuously \\ split into smaller \\ groups \\ \end{tabular} } & frame length in each & tree splitting process & \multirow{2}{*}{\begin{tabular}{c} using \\ rate-adaptive or \\ schedule \\ reading round \\ \end{tabular} } \\  & & & reading round is & & \\  & selected slots & & dynamically adjusted & & \\ \hline \multirow{4}{*}{advantages} & easy to implement, & tags are & \multirow{2}{*}{\begin{tabular}{c} taking advantage of \\ tag number \\ significantly \\ with good \\ performance \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} with adaptive \\ splitting or bit \\ estimation in \\ preceding rounds, \\ slot utilization is \\ improved \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} identification \\ tracking, the \\ identification process \\ is accelerated in each \\ is accelerated in each \\ round \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} identification \\ process is \\ scheduled with \\ the information of \\ ongoing rounds \\ ongoing rounds \\ \end{tabular} } \\ \hline \multicolumn{1}{c}{efficiency} & low & \(\rightarrow\) & high & low & \(\rightarrow\) & high \\ \hline complexity & simple & \(\rightarrow\) & complex & simple & \(\rightarrow\) & complex \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of different RFID tag identification protocols in static and moving situations. between two adjacent tags is \(d\) m. The moving speed of the conveyor belt is \(v\) m/s. For simplicity, a single reader that is mounted above the conveyor belt is considered, and its coverage range is \(L\) m.  
Tags first enter from the left side of the reader's reading range, then pass through a limited signal area and finally leave from the right side of the reading range. Affected by the moving speed and limited reading range, there may be situations where some tags pass through the reader's coverage area without been recognized. We define this type of tags as lost tags, and the tag lost ratio \(\alpha\) is defined as the ratio of the number of tags passing through the reader's coverage area without being identified to the total number of tags passing through the reader's reading range within a certain time. The identification efficiency is defined as the average number of tags successfully identified per second. The goal of this work is to increase tag identification efficiency while minimizing the tag lost ratio in fast-moving RFID systems. Compared with traditional static RFID systems, tag identification in mobile RFID systems is more challenging. The main reasons are twofold, as in the following. * _Limited participating time_: RFID tags stay in the reader's reading range for a limited time, especially in high mobility situations. They enter from the left side of the reading range and leave from the right side. Tags will receive commands sent by the reader only when they are within the reader's reading range.