# LBTM: Listen-before-Talk Protocol for Multiclass UHF RFID Networks
## 4 Simulation Framework  
LBTM has been analyzed using the simulator framework described at [30], which is publicly available for download at [31]. The particular setup used in this paper is available in the link provided in the introduction. Figure 1: Listen-before-talk multiclass (LBTM) operation. The upper timeline shows Reader 1, which finds the channel empty and starts interrogation immediately regardless of its priority. If the channel is busy, then the readers have to wait a back-off time, which is higher for lower priority classes. Reader 2 waits a short back-off time (potentially 0 for the highest class), while Reader 3 has to delay its channel access for a longer period. After back-off time, all readers contend in similar conditions, avoiding low-priority class readers’ starvation. The protocol stack implemented is shown in Figure 2, and the main features of the simulator are the following:  
* It uses a detailed and accurate link budget model, considering distance, antenna aiming, multipath propagation, and shadowing effects. The main physical parameters are summarized in Table 1.
* The location of gates, tags, and all the physical elements can be changed, including dynamic motion. * It implements several tag anticollision protocols. In particular, in this work, we use the well-known Dynamic Frame Slotted Aloha (DFSA) approach with perfect tag-count estimation. This protocol adapts the interrogation frame length to the number of tags contending to achieve an optimal throughput (e.g., [32] and references therein). Static FSA is also feasible by setting a correct frame size suitable for the incoming traffic. * It allows multiple traffic patterns, including batch-traffic, continuous-flow traffic, and permanent load. Therefore, it is possible to characterize accurately different real-world situations. * Finally, it is flexible enough to implement any reader anticollision protocol on top of its stack. By default, it uses LBT/LBTM, but it is possible to update it to include other synchronous or asynchronous schemes. * The interrogation process lasts until no more tags respond to the readers' Query packets. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \(\mathbf{P_{T}}\) & \(\mathbf{G_{T}}\),\(\mathbf{G_{R}}\) & \(\mathbf{G_{t}}\) & \(\mathbf{X_{f}}\), \(\mathbf{X_{b}}\) & \(\lambda\) & \(\mathbf{M}\) & \(\tau\) & \(\mathbf{\Theta}\) & \begin{tabular}{c} **Sensitivity** \\ **(dBm)** \\ \end{tabular} \\ \hline \begin{tabular}{c} Transmitter \\ Power \\ \end{tabular} & \begin{tabular}{c} Transceiver \\ Gains \\ \end{tabular} & \begin{tabular}{c} Tag \\ Gain \\ \end{tabular} & \begin{tabular}{c} Polarization \\ Mismatch \\ \end{tabular} & \begin{tabular}{c} Wave- \\ length \\ \end{tabular} & \begin{tabular}{c} Modulation \\ Factor \\ \end{tabular} & \begin{tabular}{c} Power \\ Coefficient \\ \end{tabular} & \
\begin{tabular}{c} On-object \\ Gain \\ \end{tabular} &
\begin{tabular}{c} Receiver \\ Sensitivity \\ \end{tabular} \\ \hline
500 & 5 & 1.62 & 0.5 & 0.35 & 0.25 & 1 & 1.2 & –80 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: 865.7 MHz tag portal example parameters in a linear scale. Full description is in [30]. Figure 2: Simulator protocol stack design. The simulation framework computes the signal to interference and noise ratio (SINR) individually for each tag transmission. It considers the full link budget between the source or the interfering element and the reception point (as an example, Figure 3 shows the received power for two types of setups available in the simulator). The interference can come from other tags or readers, depending on the type of the tag and reader anticollision protocols and on the particular event. However, note that LBT/LBTM avoids simultaneous transmissions except in the case of readers' collisions.