Abstract:
Provided is a reader for radio frequency identification (RFID) and a RFID system. A reader for RFID that receives a channel signal comprises a frequency oscillator generating an oscillating signal at a frequency that is offset by an offset frequency from a center frequency of the channel signal, a mixer mixing the channel signal with the oscillating signal that is offset, and a filter filtering a mixed frequency signal provided from the mixer. The reader for RFID and the RFID system are improved in reception for the wave identification using a listen before talk (LBT) mode for identifying an electronic tag.

Description:
[0001]    This nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 10-2006-0004789 filed in Republic of Korea on Jan. 17, 2006, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a reader for radio frequency identification (RFID) and a RFID system, and more particularly, to a reader for RFID and a RFID system improved in reception for RFID using a listen before talk (LBT) mode. 
         [0004]    2. Description of the Background Art 
         [0005]    Bar codes have been commonly and widely used to identify books, records or other related items in libraries and consumer items in supermarkets. Although bar codes can be readily implemented at low cost, they usually have a short read distance and are not reusable. Also, when reading, a reader has to be precisely placed on a bar code-printed surface, and if bar codes are damaged, it is impossible to read the bar codes. Thus, an RFID system is developed to overcome these limitations. 
         [0006]    A typical RFID system can read and write wirelessly. Thus, a direct contact or scanning within a visible band is not necessary. Also, many books, records or other items can be identified at the same time in any position or direction. Since electronic tags used in the RFID system have a nearly permanent lifetime, it has been highlighted as an effective tool to establish a ubiquitous system. 
         [0007]    An RFID system identifies identification (ID) information on electronic tags that are adhered to items via wireless telecommunications. Particularly, RFID is a combined technology of using electromagnetism or static electricity coupling within a frequency of an electromagnetic spectrum. 
         [0008]    Due to RFID standardization, decrease in a unit cost and a widened readable distance, an RFID system has been broadly used in various industrial fields. An antenna, a transceiver and a transponder, which is an electronic tag, are common elements of a typical RFID system. 
         [0009]    A typical RFID system will be described with reference to  FIG. 1  herein below. 
         [0010]      FIG. 1  illustrates the configuration of a typical RFID system  100 . 
         [0011]    The RFID system  100  includes a reader  120  for RFID and an electronic tag  110  with a built-in antenna (not shown). 
         [0012]    The reader  120  for RFID continuously sends out electromagnetic waves with a certain frequency. 
         [0013]    The electronic tag  110  is wirelessly supplied with power when displaced within a frequency operation range of the reader  120  and becomes activated. 
         [0014]    The reader  120  for RFID uses a radio frequency to transmit a signal for activating the electronic tag  110 . When the electronic tag  110  is activated, data incorporated within the electronic tag  110  are transmitted to the reader  120  through the antenna. 
         [0015]    The electronic tag  110  activated by the supplied power waits for an instruction from the reader  120 . If a correct instruction is received, data are transmitted to the reader  120  in response to the receipt. 
         [0016]    The reader  120  may transmit data based on an LBT mode or a frequency hopping spread spectrum (FHSS) mode. 
         [0017]    An LBT mode is used to avoid an interference between the readers  120 . Even though the readers  120  are apart several kilometers away from each other, they may affect each other because the reader  120  has a longer read distance than the electronic tag  110 . More specifically, according to the LBT mode, the reader  120  scans a channel that is used prior to the transmission, and transmits data using a channel that is not used to thereby minimize the interference. 
         [0018]    That is, the LBT mode is a method of detecting the channel that is not used during the scanning. 
         [0019]    Also, the reader uses a direct conversion method to minimize the power consumption and the size of the reader. The direct conversion method needs to remove direct current (DC) offset. 
         [0020]    However, when DC offset is removed from the reader  120  using the direct conversion method during execution of the LBT mode, reception around a frequency of 0 Hz decreases abruptly, and thus, continuous waves (CW) may not be identified. 
       SUMMARY OF THE INVENTION 
       [0021]    Accordingly, the present invention is directed to solve at least the problems and disadvantages of the background art. 
         [0022]    The present invention is directed to provide a reader for RFID that can minimize decrease in power of an input frequency so as to allow measuring a level of occupied power of a channel that is used during execution of an LBT mode. 
         [0023]    According to one embodiment of the present invention, there is provided a reader for RFID receiving a channel signal, the reader comprising a frequency oscillator generating an oscillating signal at a frequency that is offset by an offset frequency from a center frequency of the channel signal, a mixer mixing the channel signal with the oscillating signal that is offset, and a filter filtering a mixed frequency signal provided from the mixer. 
         [0024]    Consistent with the embodiment of the present invention, the filter may comprise a direct current cut-off band that cuts off a DC component of the mixed frequency signal and a pass band at which the mixed frequency passes through. 
         [0025]    Consistent with the embodiment of the present invention, the DC cut-off band may include a cut-off frequency less than the offset frequency. 
         [0026]    Consistent with the embodiment of the present invention, the pass band includes a cut-off frequency less than a frequency obtained by adding one half of a band of the channel signal and the offset frequency. 
         [0027]    Consistent with the embodiment of the present invention, the reader for RFID may further comprise a compensator compensating the intensity of a frequency signal filtered at the filter. 
         [0028]    Consistent with the embodiment of the present invention, the compensator comprises an analog-to-digital converter converting the frequency signal filtered at the filter into a digital code, and an intensity indicator mapping the received frequency signal from the analog-to-digital converter with reference to the digital code converted at the analog-to-digital converter. 
         [0029]    Consistent with the embodiment of the present invention, the channel signal may be a signal to execute an LBT (listen before talk) mode. 
         [0030]    Consistent with the embodiment of the present invention, the offset frequency is greater than approximately 10 KHz and less than one half of the channel signal band. 
         [0031]    Consistent with the embodiment of the present invention, the received signal has a band ranging from approximately 908.5 MHz to 914 MHz. 
         [0032]    According to another embodiment of the present invention, there is provided a RFID system comprising a reader for RFID, and an electronic tag. 
         [0033]    Detailed features of other embodiments will be provided in the following section of the detailed description of embodiments and the drawings. 
         [0034]    Various features and advantages of the present invention and many approaches to achieve those advantages will now be described more fully with reference to the accompanying drawings. 
         [0035]    The present invention will be described more fully with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. In the drawings, the same reference numerals in different drawings represent the same element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    The accompanying drawings, which are comprised to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: 
           [0037]      FIG. 1  illustrates the configuration of a typical RFID system; 
           [0038]      FIG. 2  illustrates a reader for RFID according to an embodiment of the present invention; 
           [0039]      FIG. 3  illustrates exemplary spectrums to describe sequential operations of detecting power of a channel signal by the reader for RFID according to the embodiment of the present invention; and 
           [0040]      FIG. 4  illustrates the configuration of a reader for RFID according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0041]    A reader for RFID and RFID system according to various embodiments of the present invention will be described in a more detailed manner with reference to the attached drawings. 
         [0042]      FIG. 2  illustrates the configuration of a reader  200  for RFID according to an embodiment of the present invention. 
         [0043]    The reader  200  includes a frequency oscillator  201 , a mixer  202  and a filter  203 . 
         [0044]    An output terminal of the frequency oscillator  201  is coupled to a second input terminal of the mixer  202 . A channel signal IN is inputted to a first input terminal of the mixer  202 . An output terminal of the mixer  202  is coupled to an input terminal of the filter  203 . A filter frequency is outputted through an output terminal of the filter  203 . 
         [0045]    Operation of the reader  200  will be described hereinafter. 
         [0046]    First, prior to data transmission with an electronic tag (not shown), the reader  200  scans power of a channel that is used and checks which channel is to be used for data transmission. That is, the reader  200  uses an LBT mode to detect a channel that is not used. 
         [0047]    A receipt band of the reader  200  ranging from approximately 908.5 MHz to 914 MHz includes about 27 channels. Each of the channels has a bandwidth of approximately 200 KHz, and a guard band employed to distinguish the individual channels is approximately 50 KHz. 
         [0048]    The reader  200  receives power of the channel signal IN having the center at f 1 . 
         [0049]    Second, the frequency oscillator  201  oscillates an oscillating frequency f 2  as defined in the following equation. 
         [0000]        f   2   =f   1   +Δf   Eq. 1 
         [0050]    Herein, f 1 , f 2  and Δf represent a center frequency of the channel signal IN, an oscillating frequency and an offset frequency, respectively. 
         [0051]    The frequency oscillator  201  oscillates an oscillating frequency f 2  that is offset by the offset frequency Δf from the center frequency f 1 . 
         [0052]    The oscillating frequency f 2  oscillated at the frequency oscillator  201  is supplied to the second input terminal of the mixer  202 . The frequency is to be up converted or down converted at the mixer  202 . 
         [0053]    Third, a frequency obtained by adding the center frequency f 1  and the oscillating frequency f 2  is outputted through the output terminal of the mixer  202 . 
         [0054]    The mixer  202  includes a down-conversion mixer, and outputs a mixed frequency f mix  as defined in the equation below. 
         [0000]        f   mix   =f   1   −f   2   Eq. 2 
         [0055]    Herein, f mix , f 1  and f 2  represent a frequency mixed at the mixer  202 , the center frequency of the channel signal IN and the oscillating frequency, respectively. 
         [0056]    As stated in the equation 1, the oscillating frequency f 2  is an added value of f 1 +Δf. Thus, substituting the equation 1 into the equation 2 gives rise to the equation defined as follows. 
         [0000]        f   mix   =f   1 −( f   1   +Δf )=−Δ f   Eq. 3 
         [0057]    Herein, f mix , f 1 , f 2 , and Δf represent the frequency mixed at the mixer  202 , the center frequency of the channel signal IN, the oscillating frequency, and the offset frequency, respectively. 
         [0058]    The frequency mixed at the mixer  202  is offset as much as the offset frequency Δf. 
         [0059]    The mixed frequency f mix  is inputted to the filter  203 . 
         [0060]    Last, the filter  203  filters the mixed frequency f mix  that is offset as much as the offset frequency Δf. 
         [0061]    The reader  200  is a direct conversion type receiver, and thus, removing DC offset of the received channel signal IN is necessary. The filter  203  removes a DC component of the mixed frequency f mix  by a DC cut-off band f cut-off . 
         [0062]    The filter  203  is designed to filter a specific frequency band f pass  to detect the power of the mixed frequency f mix . In other words, when the above described operations are executed to detect the power of the channel signal IN inputted to the reader  200 , other adjacent channel signals are inputted simultaneously to the reader  200 . As a result, power of the other adjacent channels can be detected concurrently. 
         [0063]    The filter  203  filters the specific frequency band f pass  to correct an error in detecting the power of the channel signal IN. The specific frequency band f pass  passing through the filter  203  is variable. 
         [0064]    Due to the above-described configuration and operation, power of frequency signals adjacent to the mixed frequency f mix  is not detected. Accordingly, the power of the adjacent channels to the target channel is not detected. The power detection is easy since the channel signal to be detected has a small offset. 
         [0065]      FIG. 3  illustrates exemplary spectrums to describe the power detection operations of the channel signal by the reader according to the embodiment of present invention. 
         [0066]    As an initial operation  300 A to detect the power of the channel signal IN by the reader  200 , the reader  200  receives the channel signal IN. 
         [0067]    In operation of  300 B, the reader  200  offsets the center frequency f 1  of the received channel signal IN by the offset frequency Δf. 
         [0068]    In operation of  300 C, the reader  200  removes a DC component of the offset frequency Δf. Because the reader  200  is a direct down-conversion type receiver, the DC component needs to be removed. More specifically, the filter  203  has a DC cut-off band f cut-off  and removes the DC component. 
         [0069]    Since high power is generated around the center frequency f 1  of the channel signal IN, and the center frequency f 1  of the channel signal IN is offset by the offset frequency Δf after the frequency conversion, the power of the channel signal IN can be detected without difficulty when the DC component is removed through the filtering. 
         [0070]    In operation of  300 D, the reader  200  filters a certain band of the channel signal IN to detect the power of the channel signal IN. 
         [0071]    In detail, the reader  200  receives channel signals, each generated at 27 channels having a band ranging from approximately 908.5 MHz to 914 MHz. A guard band is placed between the individual channels. Each of the channel signals may include signals from an electronic tag or signals transmitted from another reader. A bandwidth of each of the channel signals is approximately 200K Hz, and the guard band is approximately 50 KHz. 
         [0072]    Signals of adjacent channels to the target channel may be detected while a frequency oscillated from the frequency oscillator  201  and the channel signal IN inputted to the reader  200  are mixed. 
         [0073]    When the signals of the adjacent channels are detected, the reader  200  may not accurately detect the power of the target channel to be checked by the reader  200 . 
         [0074]    Thus, the reader  200  needs to have a preset pass band that allows filtering of the channel signal IN with the changed center frequency in order to prevent interference from the adjacent channels. 
         [0075]    The pass band is generated variably, and as mentioned above, the pass band plays a role in blocking the interference between the signals from the adjacent channels. 
         [0076]    The cut-off frequency of the DC cuff-off band f cut-off  is less than the offset frequency Δf. The cut-off frequency of the pass band f pass  is less than a frequency obtained by adding the offset frequency Δf and one half of the band at which the channel signal IN is allocated. The offset frequency Δf is greater than approximately 10 KHz but less than one half of the band at which channel signal IN is allocated. 
         [0077]    On the basis of the above sequential operations, the power of the channel signal received at the reader  200  for RFID can be accurately detected. 
         [0078]      FIG. 4  illustrates the configuration of a reader  400  according to another embodiment of the present invention. As illustrated, the reader  400  that is modified from the reader  200  according to the first described embodiment includes a receiver  410  and a compensator  420 . 
         [0079]    Since the receiver  410  can be fully understood based on the detailed description provided in  FIG. 2 , the compensator  420  will be described in detail herein below. 
         [0080]    An output of the receiver  410  is inputted to an input terminal of an analog-to-digital converter  421 . An output of the analog-to-digital converter  421  is inputted to an input terminal of an intensity indicator  422 . 
         [0081]    An output signal of the receiver  410  is converted into a digital code using the analog-to-digital converter  421 . 
         [0082]    The intensity indicator  422  maps the intensity of the received signal using the digital code. In detail, the intensity indicator  422  maps the digital code using an experimentally measured value. 
         [0083]    When the power of the channel signal IN is detected, the power of adjacent channels is not detected. Since channel signal IN to be detected has a small offset, the power of the target channel signal IN can be easily detected. 
         [0084]    Also, the reader  400  can compensate for the power that is lowered as the channel signal IN is filtered through the analog-to-digital conversion. As a result, accurate and fast detection can be achieved. 
         [0085]    According to various embodiment of the present invention, a frequency even with low power that is received at a channel can be easily identified using a reader for RFID that can minimize decrease in the power of a received frequency. 
         [0086]    Also, the reader can be minimized, and the power consumption can be reduced. 
         [0087]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.