1. Field of the Invention
The present invention relates to RFID, and more specifically, to a method of recognizing a tag at high speed by preventing collision in an RFID system.
2. Description of the Related Art
Radio Frequency Identification (RFID), which is one field of automatic recognition such as a barcode, a magnetic sensor, an IC-CARD, etc., means a technology of wirelessly recognizing data stored in a microchip of a tag using a very high frequency (VHF) or a long wave.
In principle, an RFID system receives information stored in a tag through an antenna and recognizes and analyzes it by a reader, making it possible to obtain unique information on articles to which the tag is attached. Furthermore, the RFID system is not affected by the environment such as snow, rain, wind, dust, magnetic flux, etc. because it uses a specific frequency and has an advantage in that the recognition can performed even during movement due to high-speed propagation.
Since the RFID can transmit many data at high speed using a wireless channel, it is considered as a technology capable of replacing the currently used barcode in industry fields needing to recognize products for logistics and distribution fields, financial service, etc. As a result, the RFID has been receiving much attention, is becoming more and more used in automatic recognition system, as a technology that can be implemented in a ubiquitous environment.
However, the RFID has issues in reliability such as recognizing data, standardization of technology, improvement of a read rate and identification speed, etc. One of the most common problems to be solved in the current RFID system is that the recognition efficiency is degraded due to collision between tags. Therefore, in order to improve the read rate and the identification speed, a study on anti-collision protocol is needed.
In the RFID system, the basic process for recognizing the tag is that the reader queries the tag for information and the tag receiving the query signal then transmits its own ID to the reader.
However, when a plurality of tags simultaneously respond to the query of one reader, the reader cannot recognize the tags. This is referred to as tag collision. In the case of the tag collision, since the currently used tag or the tag to be used for a large-scale logistics and distribution is an inexpensive manual tag, the usable anti-collision protocol has many limitations such as complexity of computation, absence of battery, cost increase according to a memory size, etc. Therefore, in order to identify the plurality of tags in real time, a method of processing the tag collision phenomenon is a core technology that determines the performance of the RFID standard protocol, which is referred to as an anti-collision method.
The RFID protocol itself has the anti-collision method suitable for each standard protocol.
The RFID standard protocol at 800 MHz to 960 MHz is 18000-6 Type B and Type C established by the International Standardization Organization (ISO/IEC). Type A and Type C adopt an anti-collision method based on slotted ALOHA protocol that is a probabilistic method and Type B adopts an anti-collision method based on a binary tree that is a deterministic method.
The slotted ALOHA protocol is a time division multiple access technology that prevents the collision by dividing one communication channel into timeslots having a predetermined interval and allowing several communication devices to randomly use each timeslot. The slotted ALOHA protocol can be simply implemented, such that it has been widely used for communication systems.
A Framed slotted ALOH (hereinafter, abbreviated to be “FSA”) protocol is one of the most frequently used anti-collision algorithms. The FSA algorithm divides a frame into several timeslots and allows each tag to randomly select one timeslot to transmit its own ID.
FIG. 1 is a flowchart showing a tag recognition process for explaining the anti-collision algorithm in the framed slotted ALOHA (FSA) protocol according to the related art.
The operational principle of the anti-collision algorithm in the FSA protocol according to the related art will be described with reference to FIG. 1. Referring to FIG. 1, if the tag recognition process starts, a frame having a predetermined size starts (step 101) and the reader recognizes the ID of the tag (103). The frame is configured of a plurality of slots and the frame size is equal to the number of slots. The tags receiving the signals respond to the readers and divide the response results of the tags into success 105, collision 107, and idle (or empty) 109. If only one tag responds to one timeslot, the reader recognizes the tag. This is referred to as a success slot. Any tags do not respond to one timeslot, which is referred to an idle slot. More than two tags respond to one timeslot, which is referred to as a collision slot.
A slot counting corresponding to a current frame size ends and then, the current frame ends (111, 113, and 115). After the current frame ends, the anti-collision algorithm according to the related art determines a proper frame size when the number of currently remaining tags is not 0 and starts the frame again (117, 119, and 121). The anti-collision algorithm in the FSA protocol identifies all the tags by repeating the above method.
When the FSA protocol according to the related art seeks an optimal frame size for the number of tags to be currently recognized assume that all the slot sizes are the same. Therefore, when the number of tags is N and access probability is p, throughput THFSA in the FSA protocol according to the related art is represented by the following Equation 1.THFSA(N,p)=N×p×(1−p)N-1  [Equation 1]
The optimal frame size obtained by Equation 1 is the same as the number N of tags. The optimal frame size obtained by the above Equation has a minimum number of slots, but does not have a minimum recognition time.
In addition, as shown in FIG. 7, a phenomenon that the throughput is degraded during the progress of the current frame occurs. In particular, when the frame size is changed into an exponent of 2, similar to a Q algorithm, a throughput inversion phenomenon occurs, such that there is a problem in that throughput loss occurs.