# Tag-to-Tag Interference Suppression Technique Based on Time Division for RFID
## 4 Anti-Collision for RFID Based on TD  
In this section, we propose an efficient and simple anti-collision RFID system that is specifically for smart green construction technology. The goal of this construction technique is to reduce the construction cost and to enhance the speed of construction by reusing and recycling the materials used in a previous building. In order to improve the traditional construction technology, the smart green construction generally employs a construction management system that is based on a sensing technique such as RFID. In this case, RFID provides the vital roles for managing a variety of construction materials, monitoring the status of new or reused materials, and classifying the generation of each construction materials. On the construction site, the tracking and storing of materials' information using RFID remain challenging because a large number of materials are used in narrow spaces. In this case, we need to employ a low-cost RFID system utilizing a simple protocol, unlike the ALOHA system or gold-code based system, and it should have a good anti-collision performance. In addition, the RFID tag does not need to adaptively allocate time slots in a data frame, as is the case with ALOHA, because construction materials may not move for a relatively long time.  
In this paper, we propose a simple and efficient RFID system that is based on the TD method. The proposed RFID tag transmits data using the specific time slot, and this time slot is fixed. Before tags are attached to construction materials, the tags are grouped according to their time slots (note that the number of groups may be determined according to the size of the building or the amount of construction materials). When we attach tags with specific time slots to the construction materials, they should be managed to avoid that tags that are in the same group are located in a neighbor. For example, if the tag of the first group is attached to a specific construction material, tags belonging to other groups should be attached to the neighboring materials. This process of managing the locations of RFID tags is simple because construction materials are generally on the same site for relatively long periods of time until the building is demolished and the materials reused in another building.  
Although we focus on an application of the proposed RFID system on the construction site in this paper, it might be applied to other areas, where objects are placed in the same site for relatively long periods, such as the storage, library, and industrial sites with various devices installed.  
The proposed anti-collision RFID algorithm focuses on the case involving a single handheld reader and multiple tags. In addition to efficiently reducing the probability of collisions due to other tags by assigning tags to previously allocated time slots to transmit data, it also employs a simple RFID protocol for one-way communication. This simple RFID protocol may reduce the implement cost, because the RFID tag transmits data using the fixed specific time slot, without the complicated mathematical operation process, unlike the anti-collision RFID system based on the gold-code or ALOHA. Note that the gold-code anti-collision RFID tag requires a significant amount of numerical operations including the multiplication and addition, and the ALOHA technique requires an adaptive resource process for allocating time slots within a data frame.