KR100736228B1 - Symbol detection method for rfid system - Google Patents

Abstract

A method for detecting a symbol in an RFID system is provided to increase reception reliability of an RFID reader by using zero crossing instead of the number of peak of a signal. A DC offset is removed from a tag response signal transmitted to the reader from the RFID tag(501). A moving average is performed to reduce effect of noise included in the DC offset-removed signal(502). A phase offset is compensated from the averaged signal(503). The symbol is detected from the phase offset-removed signal by using a predetermined algorithm(504). The algorithm determines whether the symbol is 0 or 1 depending on a count of a zero-crossing by counting whether the phase offset-removed signal crosses 0. If the phase offset-removed signal crosses zero twice, the symbol is determined as 0. If the phase offset-removed signal crosses zero by four times, the symbol is determined as 1.

Description

Symbol detection method in RFID system

1 is a block diagram of a general RFID system.

2 is a flowchart of a conventional symbol detection method.

3 is an explanatory view of the principle of a conventional symbol detection method.

4 is a diagram illustrating a problem in a conventional symbol detection method.

5 is a flowchart of a symbol detection method according to the present invention;

6 is an explanatory view of the principle of a symbol detection method according to the present invention;

The present invention relates to a symbol synchronization demodulation technique for a signal transmission method used in an RFID system.

RFID (Radio Frequency IDentification), which is a technical field of the present invention, is a 'contactless or radio frequency recognition technology' for identifying entities using RF signals for each frequency band, and is a core technology of a ubiquitous sensor network (USN). Field. RFID technology was first developed and used by the United Kingdom during World War II for the Identification of Friend or Foe system (IFF system), but it was difficult to be widely used due to expensive tag cost. . Afterwards, R & D for commercialization began in the 1960s, and some applications began in logistics management automation from the late 60's to the 1970's.In the mid 90's, international standardization was Is being discussed. The RFID system is a radio frequency system consisting of a semiconductor tag, an antenna, and a reader, and integrates the use of electromagnetic or electrostatic coupling within the radio frequency of the electromagnetic spectrum portion. The semiconductor chip stores information on tagged products, and the antenna transmits this information wirelessly for several tens to tens of meters, and the reader receives this signal and decrypts the product information. Therefore, all tagged goods can be automatically checked or tracked anytime, anywhere.

As shown in FIG. 1, an RFID system includes a tag having an IC chip and an antenna and attached to an equipment or an object to exchange data according to a predetermined protocol, an antenna for receiving information transmitted from the tag, and predetermined data. A reader that includes a processing unit, and a host computer (server) that exchanges data by wired or wireless connection with the reader, collects, organizes, processes information, transmits it to a network or central processing system, and controls each subordinate device. It is composed of

In an RFID system, a message exchanged between a tag and a reader is defined as a symbol, which is a protocol for delivering a message by cycles of a signal transmitted for a certain time duration. For example, a message is exchanged between a tag and a reader by defining 0 as being transmitted when a signal of one cycle is transmitted during a certain time, and 1 when a signal of two cycles is transmitted during a certain time. Can be.

2 shows a symbol detection method used in a conventional RFID system, that is, a processing method for detecting a symbol from a signal transmitted from a tag to a reader. First, DC offset compensation 101 is performed to remove DC components included in the received signal. In the next step, phase offset compensation 102 is performed. Next, symbol detection 103 is performed by a peak position that detects a symbol through a positive number of picks.

3 is a waveform diagram for explaining the principle of conventional symbol detection. The CLASS 0 and CLASS 1 standards, one of the standard of RFID system, define the tag response signal as follows.-In case of symbol '0', the sine wave appears one-cycle during the unit period and symbol ' In the case of 1 ', the sine wave appears two-cycles. Accordingly, in the case of symbol '0', a positive peak appears once during one symbol duration (201), and in the case of symbol '1', two positive peaks appear (202). Using this feature, the peak point of the signal is detected during one symbol period from the response signal of the tag, and the symbol is detected through the number. The detection of positive peaks is possible by comparing the magnitude of each sample signal, based on the transition point at which the sample signal magnitude is increased or decreased.

4 shows an example of an error that may occur in a method of detecting a symbol based on the number of existing peak points. Since the received tag response signal may be distorted due to noise, symbol period variation, or the like, a plurality of peaks may be detected during one symbol period as shown in FIG. 4.

Therefore, there is a problem that it is difficult to overcome the error caused by the waveform irregularity in the prior art of detecting a symbol through the number of positive peaks. In addition to such a problem, in order to detect the peak value, the load of the detection means is relatively large because the front and rear values must be repeatedly compared for each waveform section.

The present invention relates to a method capable of accurate symbol detection from a signal transmitted from a tag in an RFID system. RFID tags are primarily aimed at low cost implementations, so there is a need to sacrifice the operating accuracy of internal PLLs. However, RFID readers must be able to receive reliable signals despite various changes. The present invention has been developed for this purpose.

In a typical wireless mobile communication system, a fixed frequency offset occurs due to an oscillator mismatch between a transmitter and a receiver. However, in the RFID system, since the tag response signal of the IC is generated by being coupled with the reader signal, the frequency offset does not occur according to the use of the same oscillator. However, a time independent fixed phase offset may occur in the signal generation process and the signal transmission process in the tag chip. Therefore, the signal transmitted from the tag is not kept constant in amplitude. In addition, since a signal may be distorted due to a change in noise or a symbol interval, a study on a symbol detection algorithm through accurate synchronization is required. The method of the present invention can solve the problems of the conventional method by detecting the symbol relatively accurately without requiring a complicated configuration compared to the prior art.

The symbol detection method according to the present invention is based on zero-crossing (hereinafter, referred to as "zero crossing"). In order to achieve the above object, the symbol detection method of the RFID according to the present invention, removing the DC offset from the tag response signal transmitted from the tag to the reader, the effect of noise on the signal from which the DC offset is removed by the step Performing a moving average to reduce, Compensating a phase offset from the signal performing the moving average, Detecting a symbol from the signal is compensated for the phase offset using a predetermined algorithm.

Here, the predetermined algorithm of the symbol detection step counts the number of zero-crossings in which the phase offset-compensated signal is positive to negative and negative to positive, so that the symbol is zero according to the number of zero crossings. It is characterized by determining whether or not. For example, the symbol detecting step may be set so that the zero point of the signal whose phase offset is compensated is determined as symbol '0' when crossing two times and symbol '1' when crossing four times.

In addition, the reader employing the symbol detection method according to the present invention includes a means for removing a DC offset from a tag response signal transmitted from a tag, and a moving average for reducing the influence of noise on the signal from which the DC offset is removed by the means. means for performing an average), means for compensating a phase offset from the signal on which the moving average is performed, and means for detecting a symbol from the signal for which the phase offset is compensated.

Here, the symbol detecting means counts the number of zero-crossings in which the phase offset-compensated signal is positive to negative and negative to positive to determine whether the symbol is zero or one according to the number of zero crossings. The apparatus further includes a symbol determining unit. For example, the symbol determination unit may be set to determine the symbol 0 when the zero crossing of the signal output from the phase offset compensating unit is performed twice, and the symbol 1 when the zero crossing is performed four times.

Hereinafter, an embodiment of a symbol detection method of the present invention will be described with reference to the drawings.

 The configuration is as shown in FIG. The symbol detection method first removes a DC offset generated from the received signal. The next step is to perform a moving average to reduce the effect of noise on the result of the signal with the DC offset removed. In the next step, phase offset compensation is performed on a signal on which a moving average is performed. The next step is to apply the zero crossing based algorithm to the result of the signal that compensates for the phase offset. The detection of the zero crossing point is based on the point where the sign of the sample signal is converted from positive to negative or negative to positive, and the two zero crossings are determined as the symbol '0' and the four zero crossings as the symbol '1'. .

According to the present invention, there is an additional advantage that the detection processing logic is very simple since it only needs to check the intersection point between the time axis and the waveform compared with the conventional peak value comparison method.

Description of operation of the invention

는 동상성분(Inphase)

와 Q성분(Quadrature)

에 대해 정해진 샘플 구간(N) 동안 평균값을 계산하여, 그 값을 제거함으로써 수학식 (1)과 같이 DC 옵셋 보상(501)을 수행한다. FIG. 5 illustrates a signal processing process and a symbol detection method of a zero crossing based RFID tag response signal proposed by the present invention. Tag response signal

Is Inphase

And Q components (Quadrature)

The DC offset compensation 501 is performed as shown in Equation (1) by calculating an average value during the sample interval N determined for the P2 and removing the average value.

In order to reduce the influence of noise on the signal that has undergone this process, a moving average is performed for a predetermined sample interval M as shown in Equation 2 below (502).

)과 차(

)의 연산을 수학식 (3)과 같이 수행하고 절대값이 큰 쪽(

또는

)을 수학식 (4)와 같이 선택함으로써 위상옵셋의 영향을 제거한다. In the phase offset compensation step 503, the sum of the two signals in consideration of the change in amplitude of the in-phase component signal and the Q component signal (

) And tea (

) Operation is performed as in Equation (3), and the larger absolute value (

or

) Is removed as shown in Equation (4) to remove the influence of the phase offset.

The symbol is detected by counting the number of times the signal crosses zero for one period of the symbol (504).

6 illustrates an example of detecting a symbol based on ideal waveforms for symbols '0' 601 and '1' 602 of a tag response signal according to the present invention. In the ideal case, the sine wave appears once during the unit symbol interval L when the symbol '0', whereas the sine wave appears twice when the symbol '1'. Therefore, if the symbol '0' crosses the zero point twice (intersection of the sine wave and the x-axis), if the symbol '1' it crosses the zero point four times. This feature can be used to detect a symbol from a received signal. The process of finding the zero point during the symbol period may be performed using Equation (5), and the determination of the symbol may be performed by Equation (6).

Although the preferred embodiments of the present invention have been illustrated and described above, the present invention is not limited to the specific embodiments described above, and the present invention is not limited to the specific embodiments of the present invention without departing from the spirit of the present invention as claimed in the claims. Anyone skilled in the art can make various modifications, as well as such modifications are within the scope of the claims.

In the conventional symbol detection method using the peak point, the signal detection performance is remarkably degraded when distortion of the received signal occurs. However, according to the present invention, even if the signal distortion occurs, the symbol detection method does not depend on the number of peak points of the signal but depends on the number of zero crossings, so that the symbol detection performance superior to the conventional technology can be obtained. The reception reliability can be improved. In addition, there is an additional advantage that the detection processing logic is very simple since it only needs to check the intersection point between the time axis and the waveform compared to the conventional iterative peak value comparison method.

Claims (2)

Reader that contains an IC chip and an antenna and is attached to a device or object, and includes a tag for exchanging data according to a predetermined protocol, an antenna for receiving information transmitted from the tag, and a predetermined data processing unit. In the method for detecting a symbol exchanged between Removing the DC offset from the tag response signal sent from the tag to the reader, Performing a moving average to reduce the influence of noise on the signal from which the DC offset is removed by the above step, Compensating for a phase offset from the signal on which the moving average is performed; Detecting a symbol from the signal whose phase offset is compensated using a predetermined algorithm, The predetermined algorithm of the symbol detection step counts the number of zero-crossings in which the phase offset-compensated signal is positive to negative and negative to positive so that the symbol is '0' according to the number of zero crossings. A symbol detection method of RFID, characterized in that it is determined whether it is 1 '. The method of claim 1, wherein in the symbol detection step, The zero point of the signal whose phase offset is compensated for is determined as a symbol '0' when crossing two times, and a symbol '1' when crossing four times.

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