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This paper represents some basic idea about Radio Frequency Identification and tells you a main problem in RFID which is known as "Reader Collision Problem". ". In certain application, two or more readers operate in the same frequency range, where signal interference occurs between them. So in the case of communication between the reader and tag, the tag gets confused with the reader to whom it has to respond. Therefore, the interference between the readers is called as Reader collision problem. After that it gives a solution regarding this particular problem.
With the speedy development of RFID technology, many applications now require several readers to operate in close to each other. If this happens, the signal from one reader may interface with the others. Suppose when two or more readers communicate with one tag, the tag will be failed to give a respond to any readers. And because of this collision occurs. The interference between readers is called Reader Collision Problem in RFID technology.
RFID system consist of Reader, Tag, Antenna and Middleware. Reader is one which emits RF signal activating the tag so that it can read and write data in it. So reader acts as a transmitter/receiver module connected through the antenna which acts as an interrogator between reader and tag. Tag is one which consists of microchip circuitry identified by the Unique identification number called Electronic Product code(EPC). Middleware is the application that interfaces the reader to the back-end application and it interprets and filters as per the business rule to pass on to the back end system. Reader initiates the communication with the tag in which the tags are powered by two ways. One is the Active tag where it generates its own power to communicate with the reader and another is the Passive tag where the power is generated from the readers signal. Due to the own power generation, Active tags are used more than Passive tags. The antenna is the physical part of the RFID reader that generates the RF signal to communicate with the tags. Antenna can be internal or external to the reader.
The RFID device works like a barcode or a magnetic strip on the back of a credit card. The RFID device must be scanned to retrieve the identifying information. The only difference between RFID devices with the other devices is that the RFID device does not need to be positioned precisely relative to the scanner.
When the coverage area of one reader overlaps with other, collision occurs in RFID. This problem can be divided into two parts.
- When two or more readers may overlap and interfere.
- When two or more reader can communicate at the same time.
If two or more readers communicate with same tag, they carry the same information. Also if the same tags can be read by one reader and shared with others, the reading efficiency will be increased. Based on this fact, we proposed one scheme know as a Center Operating Device (COD-RFID) to avoid the reader collision problem in RFID system. This solution is categorized in two methods.
In this method, the resources used are frequency and time which is allocated among the readers properly so as to avoid the collision during simultaneous signal transmission. So this method is used to decrease the reader collision problem effectively. Although this method reduced collision problem effectively, this requires the system to establish and maintain information over the whole network which is a waste of time and energy.
In this method, the range for the readers to read is adapted dynamically to decrease the overlapped region between two adjacent readers. This method increases the space re-used ratio. And also it needs a node to calculate the distance between every two readers and adjust their reading ranges. This makes more complex and increase in cost of the system.
In this system, COD converts "Multiple points to multiple points" in to two "Multiple points to one point" classical collision problems. Also according to this scheme several readers could be multiplexed by COD and the same tag information could be stored and shared among adjacent readers.
Center Operating Devices is located in the center of the RFID system. It communicates with both readers and tags at the same time. That means COD controls the whole process of RFID system.
In COD-RFID, the system provides two kinds of working methods, which are based on different information handling procedures carried out in COD.
In the first method, COD multiplexes several reader's messages and sends out the message to read tags and tags respond to each reader separately.
In the second method, the COD reads and stores the tag's information, sharing it with all requesting readers. At the same time COD updates the tags information periodically to keep the data from being out of date.
In the 'Reader to COD' block there is fight between multiple readers and they get transmitted to COD block. Then the information requested is given to the 'MULTIPLEXING IN COD' block, where the request information is modulated and multiplexed into a single reader's query. Next is the 'COD to Tag' block where it sends the multiplexed query is send to read tags. In the 'FEEDBACK FROM TAG' block, tags receive the query and feed back their identification information to COD. Finally in the 'COD TO READER' block, COD selects the tag response information by testing the original requests from readers and sends it separately to each reader. The process of reading is completed when readers get their information.
The working method of COD-RFID system is divided into two independent processes. One is the reading communication carried out between readers and COD. The other method is the updating process going on between COD to tags.
READING COMMUNICATION PROCESS:
"READER TO COD" block is one where multiple readers fight to send their request to COD. Then "COD SEARCH" block will search their required request for the receiver into the storage module for the corresponding tags ID requested by the reader. Then this information is transferred to the COD FEEDBACK. Thus the reading process gets done completely. There is a problem that each time when time runs the information inside tags gets changed. So it is necessary to update the information.
The updating process is done with a timer. So here we use COD timer block to update the tag information periodically. The updated query is then passed to the Tag Response block to send to the read tags in its coverage area. So again the information is stored in the COD storage. Module up to date.
Simulation is done using OPNET modeler to verify the improvement in reading efficiency of COD RFID when compared to present Reader collision problem called PULSE-RFID.PULSE is one which is an effective method to increase channel throughput and efficiency of the system by reducing the collision between readers.
And in simulations, we would like to use the time slots consumed by all readers to finish reading all tags in its coverage as the metric to evaluate the reading efficiency of RFID system with RCP.
When the time slot used is less, and then the reading efficiency will be more. The simulation result for both PULSE and COD RFID is shown in the figures. As shown in fig, the simulation results show that timeslots needed for reading all tags in reader's area as a function of the number of readers. The efficiency for reading will be higher in case of COD when compared to the PULSE which is shown in the figure.
When there are 25 readers in an area and 50 tags in every reader's coverage, COD consumes one half time slot as that of PULSE protocol. Similarly when the tags number reaches 100, then COD's time slot is one third of PULSE. So when we multiplex reader's request and sharing tags information, then the efficiency of reading will be increased in COD's. Also the COD scheme1 uses more time slot compared to second scheme. This is because, COD uses tags based on the request of the reader. Also the speed and frequency is higher than updating process.
By introducing the COD, a complex collision problem is solved. Furthermore, reader's requests could be multiplexed at COD and tag's information could be shared. Moreover in COD, the air interface is not changes and its used in current RFID system. The reading efficiency and speed of COD is highly increased in current COD RFID on comparing to PULSE protocol. Therefore, finally COD RFID is an effective solution for Reader collision Problem.