KR20090044348A - Method for detecting symbol in rfid system - Google Patents

Abstract

The present invention provides a method for detecting a symbol in an RFID system. To this end, the present invention removes the DC offset from the tag response signal transmitted from the tag to the RFID reader, and compensates the phase offset from the signal from which the DC offset is removed. In addition, the present invention performs a moving average to reduce the influence of noise on the signal compensated for the phase offset, and then checks the slope of the waveform of the signal on which the moving average is performed and detects the symbol according to the number of times of change of the slope. Therefore, the present invention has an advantage in that the symbol can be detected more accurately by increasing the reception reliability of the RFID reader.

RFID, Symbol Detection

Description

 Method for detecting symbols in RFID system {METHOD FOR DETECTING SYMBOL IN RFID SYSTEM}

The present invention relates to radio frequency identification (RFID), and more particularly to a method for detecting a symbol in an RFID system.

RFID is a technical field that is the core of the ubiquitous sensor network (USN) as a contactless or radio frequency recognition technology for identifying entities using RF signals for each frequency band. RFID was first developed and used by the UK during World War II for the Identification of Friend or Foe system (IFF system), but it is widely used due to the high cost of tags. It was difficult. Afterwards, R & D for commercialization began in the 1960s, and it was partially applied to logistics manager assimilation from the late 60's to the 70's.In the mid 90's, the international standardization organization (ISO) Is being discussed.

An RFID system is a radio frequency system that integrates the use of electromagnetic or electrostatic coupling within the radio frequency of an electromagnetic spectrum portion, and is composed of a semiconductor chip, an antenna, and a reader. Information on a tagged product is stored on a semiconductor chip, and the antenna transmits this information wirelessly to several tens of meters, and the reader receives this signal to decode the product information. Therefore, all tagged products can be automatically checked or tracked anytime, anywhere through the RFID system.

Hereinafter, a control flowchart for detecting a symbol in a conventional RFID system will be described with reference to FIG. 1.

In an RFID system, a symbol is a message exchanged between a tag and a reader. In a conventional RFID system, symbols are detected from a reader through the process of FIG. In step 100, the RFID system determines whether a tag response signal has been received by the reader. When the tag response signal is received, the RFID system proceeds to step 102 and removes a DC component generated from the received signal. In step 104, the RFID system performs phase offset compensation on the signal from which the DC offset is removed, and then counts the number of positive peaks in step 106 to detect a symbol. A symbol detection process is performed according to a peak position.

Hereinafter, a waveform diagram of symbols '0' and '1' according to a conventional ideal reception signal will be described with reference to FIG. 2.

When the tag response signal is ideally received by the reader, a sine wave appears once during a unit period in the case of symbol '0', as shown in FIGS. 2 (a) and 2 (b), respectively, and in the case of symbol '1' Appears twice. Accordingly, in the case of symbol '0', a positive peak appears once in 201 during one symbol duration, and in the case of symbol '1', a positive peak appears twice in two (202, 203). Using this feature, the RFID system detects peak points for one symbol period from the tag response signal, and detects symbols according to the number of detected peak points. In this case, the detection of the positive peak is performed by comparing the magnitudes between the respective sample signals, and is based on the conversion point at which the magnitude of the sample signal is increased or decreased.

Meanwhile, in the symbol detection method based on the number of peak points in the related art, an error may occur as shown in FIG. 3. This is because the received tag response signal may be distorted due to a change in noise and symbol period. Therefore, in this case, as shown in FIG. 3, a problem occurs in that a plurality of peaks are detected during one symbol period. However, in the conventional RFID system that detects a symbol using a positive number of peaks, there is a limit in overcoming the symbol error problem described above.

Because RFID tags are intended for low cost implementations, they may lack accuracy, such as internal PLLs. However, RFID readers must be able to receive reliable signals under various changes.

In a typical wireless mobile communication system, a fixed frequency offset occurs due to an oscillator mismatch between transmission and reception terminals. However, in the RFID system, since the tag response signal of the integrated circuit (IC) is generated by being coupled with the reader signal, frequency offset does not occur according to the use of the same oscillator. However, a time-dependent fixed phase offset may occur in a signal generation process and a signal transmission process in a tag chip, but a signal transmitted from a tag may not have a constant amplitude. In addition, since a signal may be distorted due to a change in noise or a symbol interval, a research on a symbol detection algorithm through accurate synchronization is required.

The present invention provides a method for detecting a symbol in an RFID system for enabling accurate detection of a symbol in a signal transmitted from a tag in an RFID system.

The present invention for achieving the above-mentioned tag is a tag (Tag) for embedding IC chip and antenna and attached to the equipment or object to exchange data according to a predetermined protocol, antenna for receiving information transmitted from the tag and predetermined data A method for detecting a symbol exchanged between an RFID reader including a processing unit, the method comprising: removing a DC offset from a tag response signal sent from a tag to a reader, and a phase offset from the signal from which the DC offset is removed Compensating the signal, performing a moving average to reduce the influence of noise on the signal compensated for the phase offset, and checking a waveform slope of the signal on which the moving average is performed during the symbol period. And detecting a symbol according to the number of changes.

The present invention has the advantage that can effectively detect the symbol even if the signal received in the RFID system is distorted. In addition, the present invention has the effect of improving the reception reliability of the RFID reader and reduce the influence of noise.

DETAILED DESCRIPTION Hereinafter, detailed descriptions of preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the following description of the present invention, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

The present invention provides a method and RFID system for detecting symbols. To this end, the present invention removes the DC offset from the tag response signal transmitted from the tag to the RFID reader, and compensates the phase offset from the signal from which the DC offset is removed. In addition, the present invention performs a moving average for reducing the influence of noise on the signal compensated for the phase offset, and then checks the slope of the waveform of the signal on which the moving average is performed to detect a symbol according to the number of times of change of the slope. Therefore, the present invention has an advantage in that the symbol can be detected more accurately by increasing the reception reliability of the RFID reader.

Prior to the detailed description of the present invention, the slope characteristics of each waveform corresponding to the symbol '0' and '1' according to the ideal received signal will be briefly described with reference to FIG.

If the tag response signal transmitted from the tag to the RFID reader is ideal, that is, a symbol '0' in which a sine wave appears once during a unit period as defined in the RFID system standard is shown in Fig. 4 (a). As shown, the slope change appears three times. In the case of the symbol '1' in which the sine wave appears twice during the unit period, the gradient change occurs five times as shown in FIG. 4 (b). However, when the symbol interval of the received signal received by the reader is changed due to the inaccuracy of the tag internal circuit which is a low-cost passive device as shown in FIGS. 5A and 5B, accurate symbol detection becomes difficult. Detection measures are required. Accordingly, the present invention proposes the following RFID system.

The RFID system for symbol detection according to the present invention comprises a tag, a reader and an antenna. First, a tag consists of an IC chip and an antenna to transmit a response signal for a radio signal. The reader then sends a radio signal to the tag and receives a response signal from the tag. The reader first checks the point at which the magnitude of the waveform slope is converted to detect a symbol from the tag response signal. The reader then calculates the number of changes in the waveform slope based on the checked point, and detects the symbol according to the calculated number. As such, the RFID reader of the present invention performs symbol detection based on the number of gradient changes in the waveform of the received signal.

Hereinafter, a block diagram of an RFID reader according to an embodiment of the present invention will be described with reference to FIG. 6.

Referring to FIG. 6, an RFID reader (hereinafter referred to as a 'reader') includes a wireless unit 600, a controller 602, a DC offset compensator 604, a phase offset compensator 606, and a moving average unit 608. And a symbol detector 610.

The wireless unit 600 receives a signal transmitted from a tag through an antenna. The controller 602 controls the overall operation of the reader, and in particular, the DC offset compensator 604, the phase offset compensator 606, the moving average part 608, and the symbol detector 610 according to an embodiment of the present invention. The control detects a symbol based on the slope in the waveform of the received signal.

The DC offset compensator 604 performs DC offset compensation to remove the DC offset from the received signal. The phase offset compensation unit 606 performs phase offset compensation on the result of the signal from which the DC offset is removed, and the moving average unit 608 moves the phase offset compensated signal to reduce the influence of noise. Perform a moving average.

The symbol detector 610 detects a symbol by using a slope-based algorithm under the control of the controller 602. That is, the symbol detection unit 610 detects a symbol according to the number of changes of the slope by checking the slope of the symbol period in the waveform of the signal on which the moving average is performed. In detail, the symbol detector 610 extracts sample signals at predetermined intervals in a symbol period and compares the magnitude of the slope for each sample signal. In addition, the symbol detector 610 regards the specific point as the gradient change point when the portion where the magnitude of the gradient is increased or decreased based on the specific point appears over a certain period. The symbol detector 610 calculates the number of inclination changes in consideration of the periodic change of the response signal based on the inclination change point. The symbol detector 610 determines the symbol '0' when the calculated number of gradient changes is 2, and determines the symbol '1' when the number is 4. In addition, when the number of inclination changes is three, the symbol detector 610 determines a symbol by using the number of samples between the inclination change points.

As described above, the RFID system processes the tag response signal through the reader configured as described above and detects the symbol based on the number of change of the slope.

Hereinafter, a control flowchart for detecting a symbol in an RFID reader according to an embodiment of the present invention will be described in detail with reference to FIG. 7.

에서 동위상(Inphase) 신호인

과 구적(Quadrature) 신호인

을 추출하여 각각 정해진 샘플 구간인 M동안에 평균값을 산출한다. 그리고 옵셋 보상부(604)는 태그 응답 신호

에서 산출된 각 평균값을 제거하여 하기 수학식 1과 같은 DC 옵셋 제거 과정을 수행한다. Referring to FIG. 7, the controller 602 first determines whether a tag response signal is received in operation 700. When the tag response signal is received, the controller 602 proceeds to step 702 to remove the DC offset from the received signal. This process is performed by the DC offset compensator 604 under the control of the controller 602. Specifically, the DC offset compensator 6604 is a tag response signal.

In phase signal at

And quadrature signals,

Is extracted to calculate an average value during each of the predetermined sample interval M. And the offset compensation unit 604 is a tag response signal

The DC offset removal process as shown in Equation 1 is performed by removing each average value calculated by Equation 1 below.

After performing the DC offset removal process, the controller 602 proceeds to step 704 to perform phase offset compensation on the signal from which the DC offset is removed. That is, the controller 602 controls the phase offset compensator 606 to perform phase offset compensation on the result of the signal from which the DC offset is removed as described above. Here, the phase offset compensation means that the quadrature signal compensates for the reduction of the amplitude of the in-phase signal due to the phase offset.

과 두 신호의 차인

를 다음의 수학식 2를 통해 산출한다. 그리고 위상 옵셋 보상부(606)는, 수학식 3을 통해

와

중 절대값이 큰 쪽을 선택하여 수신된 신호에서 위상 옵셋으로 인한 진폭 감소 영향을 보상해준다. The phase offset compensator 606 takes the sum of the two signals in consideration of the amplitude change of the in-phase signal and the quadrature signal.

Difference between two signals

Is calculated through the following equation (2). The phase offset compensator 606 uses Equation 3 below.

Wow

The larger absolute value is selected to compensate for the effect of amplitude reduction due to phase offset in the received signal.

If it is determined in step 706 that all of the above processes are completed, the control unit 602 proceeds to step 708 to perform a moving average to reduce the influence of noise. In this case, the moving average unit 608 controlled by the controller 602 performs a moving average on the phase offset compensated signal at a predetermined sample interval N to reduce the influence of noise. According to an exemplary embodiment of the present invention, the moving average is performed through Equation 4 below, and the waveform of the signal according to the execution of the moving average is shown in FIGS. 8A and 8B.

When the moving average is performed, the controller 602 checks the number of change of the slope of the signal during the symbol period in step 710. The controller 602 detects a symbol corresponding to the number of times checked in step 712.

In detail, the controller 602 controls the symbol detector 610 to check the number of gradient changes during the symbol period L of the signal. Subsequently, the controller 602 determines the symbol '0' when the number of inclination changes checked by the symbol detector 610 is two, and determines the symbol '1' when the fourth. When the number of times of changing the tilt is three, the controller 602 determines a symbol by using the number of samples between the points of changing the tilt, which will be described in detail later.

Hereinafter, the symbol detection process of the above-described symbol detection unit 610 will be described in detail with reference to the following equations.

에 입력한다. 아래 수학식의 설명에 있어서,

은 기울기의 증가 또는 감소를 확인하기 위한 변수이며,

은 기울기의 증가 또는 감소의 지속 구간을 확인하기 위한 변수이다. 그리고

는 극대점 또는 극소점을 통과와 같이 부호 변화의 지점을 확인하기 위한 변수이다. 본 발명의 실시 예에 따라

,

,

는 순서대로 연산되며, 각각의 초기값은 0으로 설정된다. The symbol detector 610 compares the current sample with the previous sample, as shown in Equation 5 below, to increase +1 when the value of the current sample is increased and -1 when it is decreased.

Type in In the description of the following equation,

Is a variable for checking the increase or decrease of the slope.

Is a variable for checking the duration of the increase or decrease of the slope. And

Is a variable for identifying the point of sign change, such as passing through a local maximum or local minimum. According to an embodiment of the present invention

,

,

Are computed in order, and each initial value is set to zero.

과

을 비교하여 동일한 증가 또는 감소를 보여 같다고 판단되는 경우,

을 누적한다. 그리고 심볼 검출부(610)는 전술한 두 값이 다르다고 판단되는 경우에는 누적된

을 임계값

과 비교한다. 심볼 검출부(610)는 비교 결과

이 임계값

보다 작은 경우

을 유지하고, 클 경우에는

을 새롭게 누적한다. 이와 같은 과정은 다음 수학식 6을 통해 수행된다. The symbol detector 610 is then

and

If we compare the same and show the same increase or decrease,

Accumulate. The symbol detector 610 may accumulate when it is determined that the above two values are different.

Threshold

Compare with The symbol detector 610 compares the result

This threshold

Less than

And if large,

Accumulate newly. This process is performed through the following equation (6).

과

을 비교하여 부호가 다른 경우, 기울기 변환 지점에 대한 확인값 1을 수학식 7에 따라 기울기의 부호 변화 지점을 체크하기 위한 변수인

에 누적한다. The symbol detector 610 is

and

If the signs are different from each other, the value 1 for the gradient conversion point is a variable for checking the sign change point of the slope according to Equation 7.

Accumulate in.

Subsequently, the symbol detection unit 610 determines that the gradient transformation point is 2 for the symbol '0' during the sample interval L and the symbol '1' for the fourth using the Equation 8 below.

In addition, when the gradient change occurs three times during the symbol interval due to the change in the symbol interval as shown in FIG. 9, the symbol detector 610 considers the number of samples between the gradient change points in accordance with the embodiment of the present invention. To judge.

10, the number of samples between the gradient change points is larger in the case of symbol '0' in FIG. 10A than in the case of symbol '1' in FIG. Using this feature, the symbol detector 610 counts the number of samples between the first and third slope change points and compares the number of samples with a predetermined reference number W. The symbol detector 610 determines the symbol as '0' or '1' according to the comparison result. In the embodiment of the present invention, the reference sample number is set to W = L / 2 with respect to the sample number L of one ideal symbol. When the number of counted samples is larger than W, it is determined as a symbol '0', otherwise it is determined as a symbol '1'. Meanwhile, the predetermined reference number of samples may be calculated by various methods according to an embodiment of the present invention.

As described above, the present invention can easily perform symbol synchronization even when signal distortion occurs using the number of gradient changes of the received signal, and has an advantage of more accurately detecting a symbol.

1 is a diagram illustrating a signal processing process and a symbol detection method in a conventional RFID system;

2 is a waveform diagram of symbols '0' and '1' according to a conventional ideal received signal;

3 is a diagram illustrating an error occurring in a method of detecting a symbol based on the number of conventional peak points;

4 is a view showing slopes of waveforms corresponding to symbols '0' and '1' according to an ideal received signal;

5 is a diagram illustrating an example of waveforms for symbols '0' and '1' when a symbol section of a received signal is changed;

6 is a block diagram of an RFID reader according to an embodiment of the present invention;

7 is a control flowchart for detecting a symbol in an RFID reader according to an embodiment of the present invention;

8 is a waveform diagram of a signal resulting from performing a moving average according to an embodiment of the present invention;

9 is a view illustrating an example in which a gradient change occurs three times during a symbol section due to a change in the symbol section according to an embodiment of the present invention;

FIG. 10 is a diagram illustrating the number of samples between a first and a third slope change point in case of symbol '0' and symbol '1' according to an exemplary embodiment of the present invention. FIG.

Claims (3)

An RFID reader having a built-in 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 a predetermined data processing unit ( In the method for detecting a symbol exchanged between Removing the DC offset from the tag response signal transmitted from the tag to the reader; Compensating for the phase offset from the signal from which the DC offset is removed; Performing a moving average to reduce the influence of noise on the signal compensated for the phase offset; And detecting a symbol according to the number of changes of the slope by checking a waveform slope of a signal on which a moving average has been performed during a symbol period. The method of claim 1, The detecting of the symbol is performed by determining the symbol '0' when the number of change of the gradient is 2, and determining the symbol '1' when the number of change of the gradient is 4 by the symbol. Method for detecting. The method of claim 2, Counting the number of samples between gradient change points when the number of gradient changes is three; Comparing the counted number of samples with a predetermined number of samples; And determining the symbol as '0' or '1' according to the comparison result.

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