# Reader-Tag Commands via Modulation Cutoff Intervals in RFID Systems +
###### Abstract  
Radio frequency identification (RFID) technology facilitates a myriad of applications. In such applications, an efficient reader-tag interrogation process is crucial. Nevertheless, throughout reader-tag communication, significant amounts of time and power are consumed on inseccapable simultaneous tag replies (i.e., collisions) due to the lack of carrier sensing at the tags. This paper proposes the modulation cutoff intervals (MCI) process as a novel reader-tag interaction given the lack of carrier sensing constraints in passive RFID tags. MCI is facilitated through a simple digital baseband modulation termination (DBMT) circuit at the tag. DBMT detects the continuous-wave cutoff by the reader. In addition, DBMT provides different flags based on the duration of the continuous-wave cutoff. Given this capability at the tag, the reader cuts off its continuous-wave transmission for predefined intervals to indicate different commands to the interrogated tag(s). The MCI process is applied to tag interrogation (or anti-collision) and tag-counting protocols. The MCI process effect was evaluated by the two protocols under high and low tag populations. The performance of such protocols was significantly enhanced with precise synchronization within time slots with more than 50% and more than 55.6% enhancement on time and power performance of anti-collision and counting protocols, respectively. Through the MCI process, fast and power-efficient tag identification is achieved in inventory systems with low and high tag mobility; alternatively, in addition to the rapid and power efficient interaction with tags, anonymous tag counting is conducted by the proposed process. RFID; anti-collision; continuous wave (CW); counting protocols; passive tags; envelope detection; collision slots +
Footnote †: This paper is an extended version of our paper published in the proceedings of from the International Conference on Communications, Signal Processing and Their Applications (ICCSPA 20), Sharjah, United Arab Emirates, 16-18 March 2021.
reader-dependent half-duplex link. The main drawback of such a link is having a deaf tag when it starts data backscattering, preventing the reader from silencing the tag if the received data is not needed or corrupted [9; 11]. For example, if two or more tags reply simultaneously (i.e., during collision slots in interrogation protocols), sending a command from the reader to indicate the collision is worthless and will not be decoded by the backscattering tags. Formerly, the modulation silencing mechanism (MSM) has been proposed to terminate the tag's backscattering if a collision slot is detected in anti-collision protocols [12; 13]. In addition, MSM allows for virtual media sensing by the tag. In MSM, if the reader wants a tag to stop its backscattering, it turns off CW transmission, and an analog-based circuit equips the tag to detect the drop in the voltage at the main capacitor due to the lack of CW energy harvesting. In MSM, a capacitor voltage detection circuit is placed at the RF frontend of the tag. In this paper, we improve the modulation silencing process to facilitate three objectives:  
* Replacing the CW detection circuit from the RF frontend to the digital part of the tag's IC, namely, the digital baseband modulation termination (DBMT) circuit. This objective eases the fabrication process of the tag's IC and reduces the power consumption of the previously current-driven and \"power-hungry\" operational amplifiers. * Robustness to distance/orientation of the tags to the reader. The DMBT circuit is placed after the envelope detector module in the tag's IC. This placement eliminates the possibility of resetting the far and/or poorly oriented tags due to the low harvested energy to start with [9; 10; 11]. * Enabling not only the detection of CW cutoffs but also the periods of such cutoffs. This objective utilizes the DBMT circuit to provide multiple flags, and each flag indicates a specific period of the CW cutoff.