# Reader-Tag Commands via Modulation Cutoff Intervals in RFID Systems +
## 2 Background and Motivation  
Due to the simplicity of passive RFID tags, reader-tag communication is designed based on time division multiple access techniques [9; 14; 15]. For instance, the main protocols in anti-collision are TDMA-based, namely, ALOHA- and tree-based protocols [1; 9; 15]. Communicating over other multiple access techniques requires a more sophisticated air interface design at the tags. In TDMA, tags send/receive their data within specific periods (or slots). These slots are initiated by the reader, which sends given data (a command) followed by CW transmission. The tags decode the command and then reply by changing their antenna impedance to backscatter the reader's CW. Consequently, the reader's command may address zero, one, or more than one tag. Ideally, in TDMA, the command should address only one tag. Addressing two or more tags will cause interference at the reader (collision slots); on the other hand, addressing no tag will no replies (empty slots). Thus, both collision and empty slots waste the reader's time and energy. Starting a new command by the reader during empty slots was proposed in [15], and it was called the early ending of empty slots. Nevertheless, starting a new command during collision slots is challenging. Once a tag starts modulating its antenna impedance to backscatter the reader's CW, it is not decoding any command. Therefore, during collision slots, the reader cannot stop two or more tags' backscattering by sending a stop command. At the same time, the reader cannot stop CW transmission to save power, as part of the CW is harvested by the tag during backscattering. Hence, the reader is forced to wait until the tags finish their backscattering to start a new slot. This collision slot dilemma cannot be solved unless the tag is designed to decode commands while backscattering. In [9], MSM was proposed to solve this dilemma and save collision slots' temporal and energy implications. In collision slots, tags start their replies by backscattering a preamble sequence [15]. As the tags are not synchronized at the bit level, the received preamble at the reader will result from multiple interfered backscattering of the CW. In such a case, the reader in MSM halts CW transmission for a specific period. The tags, conversely, can detect CW availability through a simple circuit at the RF frontend, namely, the continuous wave absence detection (CWAD) circuit [13]. Nonetheless, the CWAD circuit has three main limitations that may render MSM ineffective: CWAD placement, initial voltage value dependency, and CWAD passive (current-driven) components. CWAD placement: CWAD was placed after the tag's rectifier to sense the main capacitor voltage drop due to the lack of CW to be harvested, as depicted in Figure 1. Therefore, the absence of CW is detected (i.e., BT flag in Figure 1 is triggered) if the main capacitor voltage drops below a specific voltage. Unfortunately, this detection method leaves a wide chance of having the primary voltage drop to a level that may reset the tag's state. Initial voltage dependency: The initial voltage of the main capacitor before CW cutoff is not uniform for all tags due to different orientations and distances from the reader [9; 15]. Hence, a predefined cutoff period that guarantees all tags to detect CW cutoff simultaneously is not feasible. This non-uniformity complicates protocol design by considering the most extended period that guarantees CW cutoff detection. However, this period is difficult to define for different types of readers, different antenna designs of tags, etc. An example of such a case is depicted in Figure 2, as two tags are sent simultaneously after the reader's command. The two tags have different initial voltages at the main capacitor. The BT flag is triggered by the CWAD circuit if the rectifier's output voltage V\({}_{\text{H}}\) becomes lower than the reference voltage V\({}_{\text{ref}}\) level by a specific threshold denoted by V\({}_{\text{diff}}\). Thus, it is clear that Tag1 will detect the CW much earlier than Tag2.