# Tag-to-Tag Interference Suppression Technique Based on Time Division for RFID
## 2 RFID Collision Problem
### Related Works on Collision Problem  
RFID is an accurate object identification system that is used commercially to detect and manage information pertaining to various items. However, the large-scale acceptance of RFID for industrial applications is lower than expected, and this is primarily due to several issues including environmental factors, multipath fading [15], as well as the tag-collision problem. The tag-collision problem generally occurs due to a lack of proper co-ordination between the reader and tags. The backscattering response from the tag to the reader can collide with a neighboring tag, which ultimately obscures the reader from properly reading tag IDs. In this paper, we focus on the problem of a single reader in an environment with multiple tags. Further, we focus on the practical application of RFID technology to the construction industry. Typically, on a construction site, there are a large number of tagged objects. Thus, it is extremely important to simultaneously improve the object identification performance and minimize operating costs. A variety of multiple access technologies have been used in recently developed RFID systems such as space-division multiple access (SDMA), frequency-domain multiple access (FDMA), time-division multiple access (TDMA), and code-division multiple access (CDMA), which is known as spread-spectrum [16; 17; 18]. SDMA reuses certain resources (channel capacity) in spatial slots. SDMA protocols separate the channel using electronically controlled directional antennas or multiple readers to identify tags. However, drawbacks of this approach include the high implementation cost, the need to design a complicated antenna system, and its limitation to only a few specialized applications. On the other hand, FDMA uses different frequency ranges for data transmission to and from the transponders, which result in a high cost to implement the reader. This requires a complex receiver at the reader end because a dedicated receiver must be provided for each reception channel. Although CDMA exhibits good performance for multiple tags, it requires that their data be multiplied by a pseudo-random noise (PN) sequence before transmission. On the contrary, the TDMA anti-collision algorithm allocates several timeslots in each frame to solve the collision problem. The TDMA anti-collision technique is the most popular RFID reader-to-reader anti-collision scheme, and it is used in the Colorwave algorithm [19; 20], where each reader chooses a random time slot to transmit data. The TDMA anti-collision method chronologically divides the total available channel capacity among participants. Its procedures are classified as synchronous reader-driven and asynchronous tag-driven. In the reader-driven procedure, at a particular time interval, an individual tag is first selected from a group of tags using specific algorithms, and then a data communication link is made between the selected tag and reader. On the other hand, with the tag-driven procedure, the tag announces itself to the reader by transmitting its IDs in the presence of a reader. The former and latter procedures are also called reader-talk-first (RTF) and tag-talk-first (TTF), respectively. TTF is relatively slow and inflexible compared to RTF [16; 21; 22]. Figure 1: Tag-to-tag collision. For anti-collision algorithms in the RFID system, [PERSON]'s Logic of Hiring Access (ALOHA) protocol and binary tree algorithms, which are based on deterministic and probabilistic approaches, are widely used. Most of the standards employed for ultra-high frequency (UHF) RFID systems refer to binary tree searches for anti-collision algorithms as deterministic schemes because each roof-to-leaf path denotes a unique tag ID, and it is possible to simultaneously retrieve all IDs after all branches are completely searched. On the other hand, the ALOHA anti-collision algorithm is usually referred to as a probabilistic scheme, where each tag ID will have a probability of successful retrieval [23].