# Tag-to-Tag Interference Suppression Technique Based on Time Division for RFID
## 5 Computer Simulations
### Four Tags with Unique Time Slots  
In this subsection, to illustrate the interference suppression performance of the proposed system, we present simulation results based on four tags with unique time slots in a reader interrogation zone. For this simulation, we considered four RFID tags located at different distances within the reader interrogation zone. Moreover, we employed passive RFID tags (T1, T2, T3 and T4) with a frequency of 900 MHz and a fixed transmitted power for a distance of 2 m with a minimum loss in each bit. After we present the results for only the four tags considered without any other tags, we present other results for the same four tags as well as many other circumjacent tags within the reader interrogation zone.  
Figure 4 shows the variation of the BER with different SNR values. There are twelve curves because the four tags are at different distances (1.8-2.4 m), for three different systems, namely the proposed system, a conventional system, and an anti-collision RFID system that is based on the gold-code sequence. In Figure 4, we observe that the BER curves for the proposed system are much lower than those of the conventional system, and are slightly lower than those of the gold-code system, which means that the performance of the proposed system is better than that of both the conventional and gold-code systems. We also observe that the conventional system does not work properly when there exist interference signals from other tags, and when the distance between a tag and the reader is not close enough. Although the gold-code system has an anti-collision performance that is similar to that of the proposed system, it requires a complicated protocol, which results in an increased cost. If we increase the length of the gold code, the anti-collision performance is improved, but the computational complexity is higher, which results in a long tag-response time.  
Figure 5 illustrates the case where the conventional system works properly in an interference environment. The distances between tags and the reader are 0.6, 1, 2 and 2.5 m, respectively. Moreover, we observe that only the tag at a distance of 0.6 m works properly, but other tags do not work because their BER is too high for all SNRs; this is due to the interference signals from other tags. On the other hand, the proposed system works properly for all tags in a similar case.  
Next, to illustrate the interference suppression performance of the proposed RFID system compared to the conventional system, we present the simulated results for the variation in the FER  
Figure 4: Bit Error Rate (BER) curves of the proposed system, conventional system without anti-collision technique, and anti-collision system based on the gold-code sequence.  
Figure 5: BER curves of the conventional system for four tags with different distances.  
Figure 8: FER curves of the proposed and conventional systems with a frame length of 512 bits (distances of 1.8, 2, 2.2 and 2.4 m).  
Figure 9: FER curves of the proposed and conventional systems with a frame length of 96 bits (distances of 1.5, 1.8, 2.1 and 2.5 m).