JP2012175629A - Wireless tag decoding method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wireless tag decoding method with which determination processing is performed surely in accordance with the kind of data.SOLUTION: A wireless tag decoding method includes the steps of receiving a signal from a wireless tag, outputting a reception signal, comparing the reception signal with a threshold value to determine whether the reception is high or low, changing a level of a predetermined ratio with respect to a reception level if determined high, as a new threshold value for determination when reading a fixed pattern from the reception signal, determining a threshold value of a value for which the threshold value has been fixed when decoding a Manchester code from the reception signal, and determining that there are a plurality of wireless tags and a collision has occurred if determined any other than high or low when decoding a Manchester code from the reception signal in the determination.

Description

  The present invention relates to a wireless tag decoding method for reliably decoding a signal from a wireless tag.

  2. Description of the Related Art Conventionally, a wireless tag reader / writer device that reads a signal from a wireless tag and writes data to the wireless tag has been developed and used particularly in the commodity distribution field. In such a wireless tag reader / writer device and system, it is known to determine a magnitude relationship with a threshold value for a received signal that has received a signal from a wireless tag, and to determine a received signal based on the determination result. . However, in such a process for determining a signal from the wireless tag, an error determination may occur, and various methods for avoiding this error determination are known.

JP 2009-253913 A

In a conventional RFID tag decoding method, it is known that when a received signal is decoded, for example, a threshold value is changed according to received power.
However, there is no sufficient description as to what threshold is used for what reason when determining a decoding process according to the type of data, for example, a Manchester encoded signal.
An object of the embodiment of the present invention is to provide a wireless tag decoding method capable of reliably performing a determination process according to the type of data.

One embodiment for solving the problem is:
Receive the signal from the wireless tag and output the received signal,
Comparing the received signal with a threshold to determine high or low;
When reading a fixed pattern from the received signal, change the level of a predetermined ratio with respect to the reception level when determined to be high in the determination, as a new threshold for determination,
When decoding Manchester code from the received signal, the threshold is determined as a fixed value threshold,
In the determination, if it is determined that a Manchester code is decoded other than high or low when a Manchester code is decoded from the received signal, it is determined that there are a plurality of the wireless tags and a collision has occurred. This is a tag decoding method.

  According to the RFID tag decoding method having such characteristics, when reading a fixed pattern in the SOF or EOF area, a predetermined ratio (1/2 or the like) with respect to the reception level when it is determined to be high in the determination On the other hand, when a Manchester code is decoded, the occurrence of a collision can be reliably detected by using a fixed value threshold.

1 is an explanatory diagram illustrating an example of an RFID reader / writer device according to an embodiment of the present invention. Explanatory drawing which similarly shows the collision detection of reception by the Manchester code | symbol in RFID reader-writer apparatus. Explanatory drawing which shows the dullness of the received signal in the same apparatus. Explanatory drawing which shows an example of the reception data format in the same apparatus. Explanatory drawing which shows the encoding pattern of SOF and EOF in the same apparatus. Explanatory drawing which shows an example of the malfunction in reception of the several tag in the apparatus. The flowchart for calculating the variable threshold value in the same apparatus. Explanatory drawing which shows the reception level when two tags respond in the same apparatus. Explanatory drawing which shows the decoding process using two threshold values in the same apparatus. Explanatory drawing which shows the decoding process using the difference of the reception level in the same apparatus.

Embodiments according to the present invention will be described below with reference to the drawings.
As shown in FIG. 1, a wireless tag reader 1 according to an embodiment of the present invention is connected to a host computer 2 which is an example of an external device via a network or the like, and controls a whole operation. CPU 11, FROM 12 connected to CPU 11 for storing application programs and the like, and RAM 13 as a storage area for storing information used in the past.

  Further, the wireless tag reading device 1 is connected to the above-described CPU 11 and communicates with the above-described host computer 2 to acquire character information and the like, and is a reading unit that performs wireless communication with the RFID tag T. The antenna 17 that reads ID information and the like, and the RFID transceiver T 18 that is a reading unit and performs wireless communication via the RFID tag T and the antenna 17 to decode the ID information and the like as will be described in detail below. A power supply circuit 22 for supplying power to the power supply circuit, and a battery 23 for supplying power to the power supply circuit 22.

Next, a specific method of the decoding process performed by the RFID transceiver unit 18 of the above-described RFID tag reader 1 will be described in detail with reference to the drawings. There are the following three types of decoding processing methods in the wireless tag reader 1.
1) Method for determining a threshold when receiving a fixed pattern signal (variable threshold)
2) A method for determining a threshold when decoding a variable pattern signal when it is necessary to receive a plurality of RFID tags T (fixed threshold)
3) A differential decoding method in the case of receiving only one RFID tag T (no threshold is used).

  Further, the RFID transceiver unit 18 performs collision detection using Manchester code. FIG. 2 is an explanatory diagram showing reception collision detection by the Manchester code. The RFID tag standard ISO15693 employs Manchester encoding in order to detect a plurality of responses as collisions. 2 (a) and 2 (b), no collision occurs, but in FIG. 2 (c), when a plurality of tags respond with different data, a reception pattern that does not appear normally is generated. The unit 18 can detect a collision.

  Further, in the RFID transceiver unit 18, it takes time until the reception level falls from high to low due to the characteristics of the reception circuit. As a result, the reception level that should be low increases. FIG. 3 is an explanatory diagram showing the dullness of the received signal. As shown in (a) or (b) of FIG. 3, it can be seen that the higher the high reception level, the more the low reception level is affected.

  Therefore, when the decoding process of Manchester code is performed by high / low determination using a reception level threshold, there is a possibility that a low is erroneously determined as high when a tag with a high reception level is received. In this case, the RFID transceiver unit 18 erroneously determines that a collision has occurred. Also, if the threshold value is increased, a tag with a low reception level cannot be detected.

FIG. 4 shows the format of the received signal received by the RFID transceiver unit 18.
The RFID transceiver unit 18 determines the start and end of received data by detecting SOF (Start Of Frame) and EOF (End Of Frame), which are fixed patterns shown in FIG. After detecting the SOF, the mode is shifted to the data reception mode and the data is decoded. CRC is a calculated value of the data part, and data corruption is detected by checking at the time of reception. FIG. 5 is an explanatory diagram showing the patterns of EOF and SOF shown in FIG.

  The RFID tag T has a unique identification number (UID) (Unique IDentification). A command for processing a plurality of responses by combining this UID and collision detection is an inventory command. When the RFID tag T receives an inventory command from the wireless tag reader 1, the RFID tag T responds by placing the held UID on the data portion of the data format.

Since SOF / EOF is a fixed pattern, all tags respond with the same pattern. Since the UID portion is a number unique to each tag, there is always a place where one bit or more is different when responses of a plurality of tags are compared. Due to this difference, when a plurality of tags respond, it can be detected as a collision.
In the existing system, since the threshold of the reception level is fixed, the threshold is adjusted to the lower reception level and the detection rate is increased as much as possible.

  However, as shown in (a), (b), (c), and (d) of FIG. 6, when a plurality of tags return responses, as shown in (e) of FIG. In addition, there is a problem that the low SOF row looks high.

(Threshold determination method for fixed pattern signal)
In the decoding process of the RFID transceiver unit 18 of the wireless tag reader 1 according to the embodiment of the present invention, a fixed pattern signal (for example, SOF / EOF) is stably decoded regardless of the level of reception. In addition, the determination process is performed with a variable threshold value, as described below, for the purpose of being able to stably decode even when a response of a plurality of tags is received and prevent erroneous detection.

  When receiving a signal of SOF / EOF, that is, a fixed pattern, in the decoding process of the RFID transceiver unit 18, as shown in the flowchart of FIG. 7, the threshold for decoding received data is dynamically set according to the reception level. Change. Thereby, it is possible to stably detect a tag having a low reception level and a tag having a high reception level. In order to distinguish noise from tag responses, a minimum threshold is set so that the threshold does not become too small.

  That is, the RFID transceiver unit 18 first sets the threshold value to the minimum threshold value min (step S11), thereby determining the reception level of the reception signal provided from the antenna 17 (step S12). The threshold value min at this time is a threshold value having such a magnitude that noise is not erroneously determined as “high”. If the reception level does not satisfy the threshold value in step S12, it is determined that the signal is “low”, and then the decoding process is continued without changing the threshold value (step S14).

  However, if the reception level exceeds the threshold in step S12, the RFID transceiver 18 determines that the signal is “high” (step S15). Here, if threshold min <“predetermined ratio of reception level” (for example, “predetermined ratio of reception level” = reception level / 2), threshold = min = reception level / 2, threshold = The reception level is changed to 2 (step S16). The predetermined ratio may be an appropriate value other than 1/2. If threshold value min <reception level / 2 is not satisfied, threshold value = threshold value min remains (step S17). Here, if the reception is completed, the threshold value is also completed. If the reception is continued, the process returns to step S12, and the threshold value is changed again according to the situation (step S18).

  That is, when decoding a fixed pattern signal such as SOF / EOF, the decoding process is reliably performed without causing a determination error as shown in FIG. 6, for example, by variably controlling the threshold value as described above. be able to.

(Threshold determination method for variable pattern signal)
In the decoding process of the RFID transceiver unit 18 of the RFID tag reader 1 according to the embodiment of the present invention, a variable pattern signal, particularly a signal encoded by a Manchester code (for example, a UID in the data part). When receiving multiple tag responses, regardless of the reception level of the signal from each tag, the decision is made with a fixed threshold as described below for the purpose of reliably detecting collisions. The processing is performed.

As shown in the graph of FIG. 8A, the reception levels of the tag A having a higher high level and the tag B having a lower high level are as follows.
“High, High”, “Low, Low”, “High, High”, “Low, Low”, “High, Low”, “Low, High”, “Low, Low”, “High, High”, “Low , High "," high, low "," high, high "
As shown in FIG. 4, it is indicated whether or not the RFID transceiver unit 18 can obtain a correct determination result.

  Here, when the threshold value is controlled by the determination method for changing the threshold value, the transition from the eighth “high, high” to the ninth “low, high” is performed as shown in FIG. In case E1, it can be seen that although the tag B is high, the error is determined to be low because the high level of the tag B is low.

  Therefore, in the present invention, a decoding process with a fixed threshold is performed for the decoding process of the Manchester encoded signal. Assuming that the threshold value is fixed, since all signals having a level exceeding the fixed threshold value are determined to be high, the determination result may be different from the original data. However, in this case, since high is continuously detected, it is determined that collision has occurred. Therefore, although collision does not occur on the original data, it is possible to reliably prevent the collision occurrence from being missed.

  In addition, even if it is determined that a collision has occurred due to an incorrect determination result different from the original data, the response reception from the tag and the decoding process are repeatedly executed by the congestion control, resulting in an incorrect determination result. It is possible to repeat the decoding process until there is no more.

  As the fixed threshold value, a value that is determined to be low with a practically sufficient probability for a signal whose original data is low is used. That is, as the threshold value is higher, collision detection due to an erroneous determination result can be reduced, but the possibility of collision occurrence is increased. On the other hand, as the threshold value is lower, collision detection due to an erroneous determination result increases, but the possibility of a collision being missed can be reduced.

(Collision detection when reading multiple tags)
Next, a method of changing the threshold value by combining the above two threshold value determination methods in correspondence with the data area to be read will be described below with reference to the example of FIG.

  As for the detection of multiple tags, if the tags responding with the congestion control of ISO15693 are sifted, the response will eventually be one by one, so the SOF / EOF detection rate will increase and the collision of the data part will not be missed It becomes important.

  Therefore, for the SOF / EOF which is a fixed pattern, as shown in FIG. 9, it is possible to obtain a determination result that can be followed by congestion control, so that the variable threshold described above in the flowchart of FIG. 7 can be used. Good.

  On the other hand, for the data part, as shown in FIG. 9, various reception levels are mixed due to the combination of data of a plurality of tags, so that a high threshold appears to be low by using a variable threshold value. (E2 in FIG. 9). If high appears to be low, reading may be missed. Therefore, it is preferable to fix the threshold value to the threshold value min so that even if low appears to be high and collision is judged, the reading is not missed. .

(Single tag reading: differential decoding method)
In the case of a UID designation command, there must be a response from only one tag, so there is no need to detect a collision in the data portion. Therefore, when a command is executed from the wireless tag reader 1 by designating the UID of the wireless tag to be read, the data portion is read using the difference in the reception level and Manchester under the following conditions (difference method). It is preferable to perform a decoding process. That is, when the CPU 11 of the wireless tag reading device 1 performs a decoding process by a UID designation command,
1) The threshold value is fixed to the threshold value min, and high / low determination is performed.
2) As shown in FIGS. 10A and 10B, normal patterns [HIGH, LOW] and [LOW, HIGH] are left as they are according to the detection result, and [HIGH, LOW] is represented by sign = 0, [ Low, high] is decoded into code = 1.
3) In the case of an abnormal pattern [High, High] as shown in (c) and (d) of FIG. 10 and an abnormal pattern [Low, Low] as shown in (e) and (f) of FIG. Instead of no data, the reception level is compared and the code 0/1 is determined. That is,
When reception level 1 ≧ reception level 2: Sign = 0
When reception level 1 <reception level 2: Sign = 1.
In this way, reliable determination processing can be performed.

  When a received signal from a single tag is decoded by a UID designation command, SOF / EOF data is decoded using the above-described variable threshold, and the data portion is used using the above-described differential decoding method. A preferred mode is to use a method of decoding.

As described above, it is preferable to make the decoding process different depending on whether the wireless tag to be read is singular or plural.
That is, when it is necessary to detect responses from a plurality of tags, the detection rate is increased by adopting a variable threshold for SOF / EOF of a fixed pattern, and for the data portion of a variable pattern. By using a fixed threshold, reliable collision detection is performed. In this way, by using two types of threshold values, variable and fixed, it is possible to avoid collisions without dropping the detection rate of SOF / EOF.

  If it is known that there is always only a response from one tag, collision detection is not necessary, and therefore minute reception data can be detected by using the reception level difference method described above.

  In addition, this invention is not limited to the said embodiment, A various deformation | transformation is possible in the range which does not deviate from the summary of this invention. In short, the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. Further, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, you may combine suitably the component covering different embodiment.

  DESCRIPTION OF SYMBOLS 1 ... Wireless tag reader / writer apparatus, 2 ... Host computer, 11 ... CPU, 17 ... Antenna, 18 ... RFID transmission / reception part, T ... RFID tag

Claims (1)

Receive the signal from the wireless tag and output the received signal,
Comparing the received signal with a threshold to determine high or low;
When reading a fixed pattern from the received signal, change the level of a predetermined ratio with respect to the reception level when determined to be high in the determination, as a new threshold for determination,
When decoding Manchester code from the received signal, the threshold is determined as a fixed value threshold,
In the determination, if it is determined that a Manchester code is decoded other than high or low when a Manchester code is decoded from the received signal, it is determined that there are a plurality of the wireless tags and a collision has occurred. Tag decoding method.

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