# Reader-Tag Commands via Modulation Cutoff Intervals in RFID Systems +
## 4 Modulation Cutoff Intervals Process
### Anti-Collision Protocols  
The MCI process is utilized in anti-collision protocols to indicate the four commands from the reader to the surrounding tags; namely, collision, no reply (empty), no acknowledgment (NACK), and acknowledgment (ACK) commands. During or after the tag(s) replies, the reader terminate its CW transmission for four possible intervals, \(T_{1}\), \(T_{2}\), \(T_{3}\), and \(T_{4}\), where \(T_{1}<T_{2}<T_{3}<T_{4}\). The intervals are interpreted as follows:  
In Figure 7, MCI process command decoding circuit is shown for \(T_{1}\), \(T_{2}\), \(T_{3}\) and \(T_{4}\) of 4, 6, 8, and 10 clock cycles, respectively. Note that two clock cycles indicate the transmission of one symbol in PIE encoding, which is denoted by \(T_{\text{pri}}\) in the EPC standard documentation [16]. That is, \(T_{1}\), \(T_{2}\), \(T_{3}\) and \(T_{4}\) correspond to 2, 3, 4, and 5 symbol durations after MT assertion, respectively. The circuit is simply a ripple up-counter that is connected to the DBMT circuit. The counter is preceded by a frequency divider (the highlighted DFF in Figure 7) to output the number of symbols at which MT flag is asserted (i.e., high). If the count is two (i.e., \(Q_{a}=0\), \(Q_{b}=1\)) and CW is resumed, the MT flag will no longer be high. The leftmost AND gate will assert \(T_{1}\) flag, and a Collision command is assumed. Alternatively, \(T_{2}\) flag is asserted if the count is three \(Q_{a}=Q_{b}=1\), and CW is resumed, indicating an empty slot command. \(T_{3}\) flag is asserted after CW resuming and the count is four (i.e., \(Q_{a}=Q_{b}=0\)_and_\(Q_{c}=1\)), indicating a NACK command. Finally, if the count is five and the CW is resumed, \(T_{4}\) will be asserted, and an ACK command is assumed. Following the CW transmission resumption, the control signal EN\({}_{2}\) is pulsed to reset the counter for the next slot. Note that the assertion of the MT flag requires three clock cycles (1.5 symbol duration); therefore, \(T_{1}-T_{4}\) are asserted after 7, 9, 11, and 13 clock cycles, respectively.  
An example is illustrated in Figure 8 of the above commands. In the top waveform, the reader detects a collision (corrupted data that does not follow PIE encoding), stops the CW for 7 clock cycles (3.5 \(T_{\text{pri}}\)), and then resumes its CW transmission. The received reply after that CW resumption was from a single tag, but for some reason, it has an erroneous CRC; the reader then negatively acknowledges the transmission by stopping the CW for 11 clock cycles (5.5 \(T_{\text{pri}}\)). In the middle waveform in Figure 8, the data are received from a single tag after a collision slot, and the CRC was correct. Hence, the reader acknowledges the tag's  
Figure 7: MCI process in anti-collision protocols with PIE encoding.  
transmission by stopping the CW for 13 clock cycles (6.5 \(T_{\text{pri}}\)). In the bottom waveform of Figure 8, no reply is detected from any tag after the collision (since in standardized anti-collision protocols, tags decide randomly to reply or not to reply). Therefore, the reader cuts off the CW for nine clock cycles to indicate an empty (no reply) slot and the beginning of a new slot.  
MCI provides prompt feedback to the tags on the status of the ongoing transmission during the time slot and eliminates the need for guard periods between tag transmission and reader commands. Therefore, with MCI, collision and empty slots are significantly shortened compared to traditional collision and empty slots resolution. The above values of the cutoff periods are selected for illustrating the process; however, the general rule is to select the cutoff periods that are long enough to trigger the proper command at the tags but not too long to cause a significant power drop.