JP2006238381A - System and identification method for wireless tag, and identification program thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a system and a method for a wireless tag for efficiently detecting all wireless tags without omission in the case that data transmission reception is carried out with many unspecified wireless tags where the information of the number of which is not obtained in advance. <P>SOLUTION: In the wireless tag system, a read unit transmits an instruction packet for collecting tag IDs, receives each response packet including its tag ID from each wireless tag by each slot for a tag ID read period (frame) comprising a plurality of slots, and identifies many unspecified wireless tags, the read unit includes: a slot confirmation section that discriminates whether each slot includes a successful packet whose tag ID is successfully read, or a mixed slot wherein a plurality of response packets are intermingled, or a free packet from which no response packet is received; a tag estimate section for calculating the number of estimated wireless tags depending on at least the number of successful packets and the number of mixed slots; and a frame update section for obtaining the number of slots by multiplying a prescribed coefficient with the estimated numbers. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a wireless tag system using a slot aloha system, and more particularly to a wireless tag system in which a predetermined data reader transmits and receives data to and from an unspecified number of portable wireless devices.

In recent years, wireless tag systems that use RFID (Radio Frequency IDentification) tags provided with predetermined identification information and perform article management by wireless communication have been widely used.
The wireless tag system has a small number of unique identification information (ID number or the like) given to an RFID, and is composed of a large number of inexpensive RFID tags and receivers (data readers).
As a result, the RFID tag system attaches an RFID tag to an article to be managed (for example, embeds), tracks the position of the article, monitors and records the movement history and the environmental condition each time, and It is possible to control an object that moves away from a distance that does not enter. (For example, Patent Document 1)
JP 2004-334678 A

However, in the wireless tag system disclosed in Patent Document 1, when a data reader transmits a read command, response signals (ID packets) are transmitted simultaneously from an unspecified number of RFID tags existing within the reach of radio waves. Is done.
At this time, the data reader cannot accurately receive the ID packet from each RFID tag due to the collision of the ID packets from the plurality of RFID tags.

That is, since the data reader uses the same radio frequency band set in advance for data transmission from each RFID tag, the RFID tag that could not be detected once cannot be detected indefinitely.
For example, when the data reader receives ID packets from a plurality of RFID tags, the data reader can detect the one from the RFID tag having the strongest signal level among a plurality of colliding ID packets. .

Therefore, the data reader can receive ID packets from all RFID tags within the detectable range and detect some of the received RFID tags in a multiplex communication method in which the detection process is probabilistic. I have a probability that I can't.
For this reason, when a number of RFID tags are in the detectable range, the data reader needs to repeatedly send an inquiry signal in order to detect all the RFID tags in this range. In addition, since all RFID tags transmit ID packets in the same frequency band, the frequency with which RFID tags compete in this frequency band increases.
The present invention has been made in view of such circumstances, and when performing data transmission / reception with an unspecified number of RFID tags for which a number of information has not been obtained in advance, all RFID tags are efficiently transmitted without omission. It is an object to provide a wireless tag system and method for detection.

  In the wireless tag system of the present invention, the reader transmits an instruction packet for collecting the tag ID, and the tag ID reading period (frame) is set to the tag ID reading period composed of a plurality of slots, and the tag ID is read from each wireless tag for each slot. A wireless tag system that receives a response packet including a tag ID and identifies an unspecified number of wireless tags, and for each slot, the reader is a successful packet that successfully reads the tag ID, A slot confirmation unit that determines whether the response packet is a mixed slot or a response packet that is not received, and a tag that calculates the estimated number of wireless tags from at least the number of successful packets and the number of mixed slots An estimator and a frame updater that obtains the number of slots by multiplying the estimated number by a predetermined coefficient.

ここで、αは信頼度の確率で、０．９９。 The wireless tag system of the present invention is characterized in that the predetermined coefficient is obtained by a simulation using at least the following formula (corresponding to the first embodiment).

Where α is the probability of reliability and is 0.99.

Ｎ(i)；スロット数、タグＩＤを収集する前記読取り期間の数
前記式において、フレームサイズがＮ(i)で、ｎ(i)個のアクティブ型の無線タグ、１つのスロットにｑ個の無線タグが存在する確率はＰ_ｑ(i）であり、すべてのタグを読出す読取り期間の数（収集ラウンド数）がＲ_ａ＿ｆで，アクティブ型の無線タグを逃す確率がαであり、ｑ＝１の場合、成功しているＩＤタグ識別の確率はｐ_１(i)である。 The wireless tag system of the present invention is characterized in that the predetermined coefficient is obtained by simulation using at least the following formula (corresponding to the second embodiment).

N (i): number of slots, number of reading periods for collecting tag IDs In the above equation, the frame size is N (i), n (i) active wireless tags, q per slot The probability that a wireless tag exists is P _q (i), the number of reading periods (number of collection rounds) for reading all tags is _{Ra_f} , the probability of missing an active wireless tag is α, and q = In the case of 1, the probability of successful ID tag identification is p ₁ (i).

In the wireless tag system of the present invention, the tag estimation unit calculates the estimated number n _est (i) of the wireless tag according to an expression of n _est (i) = s (the number of successful slots) +2.39 g (the number of mixed slots). It is calculated and obtained.

  In the wireless tag identification method of the present invention, the reader transmits a command packet for collecting the tag ID, the tag ID reading period is set to the tag ID reading period composed of a plurality of slots, and the tag ID from each wireless tag for each slot. Is a wireless tag identification method for identifying an unspecified number of wireless tags, and the identification process of the reader is a successful packet in which the slot confirmation unit has successfully read the tag ID for each slot. Confirmation process for determining whether the response packet is a mixed slot in which a plurality of response packets are mixed or an empty packet in which the response packet is not received, and the tag estimation unit has at least the number of successful packets and the number of mixed slots A tag estimation process for calculating the estimated number of wireless tags from the frame, and a frame update unit that multiplies the estimated number by a predetermined coefficient to obtain the slot number. And having a chromatography beam update process.

  The wireless tag identification program according to the present invention transmits a command packet for collecting a tag ID by a reader, the tag ID reading period is a tag ID reading period composed of a plurality of slots, and the tag ID from each wireless tag for each slot. Is a program that causes a computer to execute a wireless tag identification process for identifying an unspecified number of wireless tags, and in the reader identification process, the slot confirmation unit sets a tag ID for each slot. Slot confirmation processing for determining whether the packet is a successful packet that has been successfully read, a mixed slot in which multiple response packets are mixed, or an empty packet in which the response packet is not received, and the tag estimation unit includes at least a success packet Tag estimation processing for calculating the estimated number of wireless tags from the number of mixed slots and the number of mixed slots; After multiplying a predetermined coefficient and having a frame update process for obtaining the number of the slots.

  The recording medium of the present invention is a computer-readable recording medium in which the above-described RFID tag identification program is recorded.

As described above, according to the present invention, the number of tags is estimated by an equation using a numerical value obtained from the simulation result, the necessary frame size is calculated from the number of tags, and the estimated number of tags is predetermined. By changing the frame size when the threshold value is exceeded, the tag ID can be efficiently read, that is, identified in a short time.
In addition, according to the present invention, it is possible to efficiently identify the tag ID in a short time. In particular, in the active wireless tag, the power consumption is reduced, so that the lifetime can be extended.

  That is, according to the present invention, in a wireless tag system using a passive or active wireless tag, a passive wireless tag having no prior information on the number of tags can be read out by using an ALOHA frame. The reading of the tag ID can be completed in a short time, and the reliability of the reading result (identification result) can be guaranteed, and the overall reading time and power consumption and the level of reliability of the reading result can be mutually adjusted. Yes (using formulas (2) and (3) described later).

  In the wireless tag system of the present invention, the reader transmits an instruction packet for collecting the tag ID, and the tag ID reading period (frame) is set to the tag ID reading period composed of a plurality of slots, and the tag ID is read from each wireless tag for each slot. A wireless tag system that receives a response packet including a tag ID and identifies an unspecified number of wireless tags, and a reader is a successful packet that has successfully read the tag ID for each slot. A slot check unit that determines whether the packet is a mixed slot or an empty packet in which a response packet is not received, and tag estimation that calculates the estimated number of wireless tags from at least the number of successful packets and the number of mixed slots And a frame updating unit that obtains the number of slots by multiplying the estimated number by a predetermined coefficient.

<First Embodiment>
Passive collision prevention can be deterministic or stochastic. In the deterministic method, RFID tags (hereinafter referred to as wireless tags) are fixed to avoid line contention (the wireless communication from the tag to the receiver uses the same frequency, so that multiple tags share each other). Respond to the query in a planned manner using standard selection criteria.
This requires the transmission of a significant amount of data from the RFID tag to the tag reader. Moreover, since it is a query (using data of a predetermined radio frequency) that follows a predetermined path (data transmission), a missed tag cannot be detected indefinitely. For example, for effective capture, receiver reception can correctly return the strongest signal from two collision packets.

  Therefore, there is still a probability that the receiver will miss some tags. With a probabilistic multiplex technique, any RFID tag within the tag reader's transmission distance responds randomly to the query from the receiver. Propagation contention (the wireless communication from tag to receiver uses the same frequency, so many RFID tags are within the transmission distance of the tag reader, so repeated queries are required to read all the RFID tags. The frequency at which multiple tags interact with each other) may be high. As a result, a long read time is required. Furthermore, since the number of tags within the inquiry range of the receiver is usually not known in advance, the reliability in the final read result is difficult to be guaranteed. The first embodiment of the present invention solves this problem.

Therefore, in the first embodiment of the present invention, a mechanism is proposed for recognizing a large number of passive radio tags without knowing the number of tags in advance by using the ALOHA frame in the inquiry range of the reader. To do.
Thereby, according to the first embodiment, the reading procedure can be optimized between the overall reading time and the power consumption.
The reader first broadcasts the initial frame length and optionally receives a response from the tag. Once the number of empty slots, mixed slots, and successful slots is obtained, the receiver estimates the total number of tags and updates the length of the frame taking into account the read time and power consumption requirements. To do. The procedure is repeated until the receiver gets a final estimate of the tag number. As a result, an optimized frame size and number of read loops are obtained. Finally, the reader performs the reading procedure described on the left until the defined reliability reaches a certain level.

Hereinafter, a wireless tag system using a passive wireless tag according to a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing a configuration example of the embodiment.
In this figure, the reader 1 has a slot confirmation unit 11, a tag estimation unit 12, a frame update unit 13, a transmission unit 14, a reception unit 15, a control unit 16 and a power supply unit 17.
The wireless tag 2 includes a transmission unit 21 that transmits data, a reception unit 22 that receives data, and a control unit 23 that controls transmission / reception of data.

The control unit 16 generates a command packet of the collection command having the configuration shown in FIG. 2 and outputs it to each wireless tag 2 via the transmission unit 14.
As shown in FIG. 2, the command packet includes, for example, a receiver preamble that notifies transmission of a packet, a slot size that indicates a time width of a slot that receives a response from the wireless tag 2, and the number of slots in the frame. And a CRC (Cyclic Redundancy Check) code which is an additional code for guaranteeing the reliability of the communication packet. Here, the result of dividing the frame size by the slot size is the slot size.

In the wireless tag 2, the command packet received by the receiving unit 22 is input, the control unit 23 detects the collection command, generates a response packet of the tag response shown in FIG. A message is transmitted to the reader 1 at a predetermined timing, that is, at any slot in the frame.
In the case of the passive tag of the first embodiment, the power supply unit 24 generates power by electromagnetic induction or radio waves from the reader 1 and supplies power to each circuit in the tag.
As shown in FIG. 3, this response packet is, for example, a tag preamble that notifies the transmission of the packet, tag ID data that is its own identification information, and an additional code for guaranteeing the reliability of the communication packet. CRC code.

The slot confirmation unit 11 performs a tag ID detection process on the received data input by the receiving unit 14 in each slot, and according to the detection result, each slot of the frame is an empty slot in which no response packet exists, or two or more A mixed slot where response packets from a plurality of wireless tags 2 exist, and a good slot where only response packets from one wireless tag exist are detected.
The tag estimation unit 12 estimates the number of tags in the detectable range based on the detection result of the slot confirmation unit 11.
The frame update unit 13 calculates a frame size corresponding to the number of tags and outputs the frame size to the control unit 16.
The transmission unit 14 transmits data including a command packet to the tag 2 under the control of the control unit 16, and the reception unit 15 receives data including a response packet from the tag 2.

Next, before explaining the operation, processing performed by the tag estimation unit 12 and the frame update unit 13 in the first embodiment of the reader 1 passive type wireless tag 2 and the reader 1 will be described.
Since all the wireless tags 2 are in the dormant state before the collection, the reader 1 sends a start command for starting them, and then sends a command packet (collection command).
Then, before transmitting a command packet in a new collection round, the reader 1 transmits a pause command to the wireless tag 2 whose tag ID has been identified, and transmits a command packet in the next collection round. The wireless tag 2 is permitted to transmit once every frame, that is, in any slot.

The wireless tag 2 generates a response packet in the slot to be transmitted in accordance with the start timing, and terminates transmission in this slot.
As described above, the command packet transmitted by the reader 1 includes the frame size (the number of slots in the frame) and the slot interval. The slot interval is sequentially set corresponding to the frequency band used for packet transmission / reception. Further, the frame size is obtained based on the number of bits used for reading identification in the tag ID of the wireless tag 2. For example, when the upper 2 bits of the tag ID are used, the frame size is “2” which is the square of 2.

Thereby, each wireless tag 2 calculates the number of bits used for identification (number of identification bits) from the frame size by the control unit 23 (determining the square root of the frame size), and starts at the end timing of the instruction packet. In the frame to be processed, the slot interval is used to sequentially calculate the number of the slot, that is, the slot number of each slot.
Next, in the wireless tag 2, the control unit 23 extracts a bit string corresponding to the number of identification bits from the upper bits of its tag ID, and uses the number indicated by the bit string as a response slot number. The wireless tag 2 transmits a response packet at the timing of the slot in which the response slot number matches the slot number calculated sequentially.

Next, the operation of the wireless tag system according to the first embodiment of the present invention will be described with reference to FIGS. FIG. 4 is a conceptual diagram showing a procedure for detecting tag IDs of an unspecified number of passive RFIDs using the ALOHA method. FIG. 5 is a flowchart illustrating an operation example of the wireless tag system according to the first embodiment.
As shown in FIG. 4, each wireless tag 2 is set so that a response packet is transmitted once in one frame, that is, a response packet is transmitted in any slot in the frame. That is, the wireless tag 2 ends the transmission of the response packet between the start and end of any slot in the frame.

Next, an example of the reading operation of the tag ID from each wireless tag 2 by the reader 1 in the flowchart shown in FIG. 5 will be described.
In step S1, the control unit 16 collects tag IDs for an unspecified number of wireless tags 2, reads each condition set in advance by a user, etc., generates a command packet based on the initial setting value, This command packet is transmitted to a specific number of wireless tags 2.
At this time, the wireless tag 2 is permitted to transmit once every frame, that is, in any slot.

That is, the wireless tag 2 generates a response packet in the slot to be transmitted in accordance with the start timing, and terminates transmission in this slot.
Here, the command packet transmitted by the reader 1 includes a frame size (the number of slots in the frame) and a slot interval. The slot interval is sequentially set corresponding to the frequency band used for packet transmission / reception. Further, the frame size is obtained based on the number of bits used for reading identification in the tag ID of the wireless tag 2. For example, when the upper 2 bits of the tag ID are used, the frame size is “2” which is the square of 2.

Thereby, each wireless tag 2 calculates the number of bits used for identification (number of identification bits) from the frame size by the control unit 23 (determining the square root of the frame size), and starts at the end timing of the instruction packet. In the frame to be processed, the slot interval is used to sequentially calculate the number of the slot, that is, the slot number of each slot.
Next, in the wireless tag 2, the control unit 23 extracts a bit string corresponding to the number of identification bits from the upper bits of its tag ID, and uses the number indicated by the bit string as a response slot number. The wireless tag 2 transmits a response packet at the timing of the slot in which the response slot number matches the slot number calculated sequentially.
At this time, the control unit 16, for example, tag set estimation value In _set (i) = 0 indicating the estimated number of unspecified many wireless tags 2, frame size (number of slots) N = 16, overall collection round R _p — _f = 4, collection round counter i = 1, frame collection round counter j = 1.

Next, in step S <b> 2, the reader 1 receives a response packet from each wireless tag 2.
Then, the slot confirmation unit 11 receives a response packet corresponding to the slot number from each slot of the frame starting from the end timing of the command packet transmitted by itself, or from a plurality of wireless tags 2. It is detected whether the response packet is a slot in which the response packet collides or a slot from which no response packet exists. As described above, the passive tag only transmits a response packet to a command packet at a slot number based on its own tag ID, and a fixed frame size cannot eliminate a slot that causes a collision. For this reason, in the first embodiment of the present invention, an appropriate frame size corresponding to the number of tags is obtained and newly transmitted to the wireless tag 2 by a command packet.

Next, in step S <b> 3, the tag estimation unit 12 performs the following processing to estimate the number of wireless tags 2 that are unspecified and the number is not clear. The collection round used here indicates the number of repeated frames, that is, the number of repeated tag ID readings.
First, a description will be given of a simulation performed in advance in order to obtain an expression used by the tag estimation unit 12 for estimating the number of tags. The simulation result is mounted on the reader 1 and the processes of the tag estimation unit 12 and the frame update unit 13 are executed.
In the i-th round, in the frame size of N (i) slot, the read tag is n (i), and the probability when q slots collide in one slot is expressed by the following equation (1): The binomial distribution is as follows.

In the equation (1), when q = 1, p _{1 (i)} is a probability of successful tag ID reading.
Thus, the number of successful tag ID reads in a frame with N (i) slots is N (i) * p _{1 (i)} .
In addition, the probability that the tag ID of the wireless tag 2 cannot be read after R _{p_f frames} (the number of collection rounds set in advance) is obtained by (2) below.

Therefore, since the number of collection rounds R _{p_f} is at least a certain level α of the reliability of reading the tag ID, the relationship between the tag set (number of tags) n of the passive wireless tag 2 and the fixed frame size N is It is obtained by the following equation (3).

In addition, the _minimum number of collection round repetitions R _{p_t_min} for reading all unspecified number of wireless tags 2 is obtained by the following equation (4).

According to the above equation (4), the optimum frame size N _{min — p} can be _obtained together with the smallest number of collection round repetitions R _{p — t — min} .
Then, the above equation (4) is expressed as follows: tag set n, frame size N, overall readout time, number of tag responses (number of collection rounds at which readout of tag IDs from all wireless tags 2 is completed), The relationship with the level α with the reliability of reading the tag ID is shown.
In addition, the expression (4) is changed in a timely manner by changing numerical values such as a frame size, a reading time, and a reliability.

For example, the optimal frame size can be obtained by the above equation (4), and the frame size that achieves the shortest reading time is N _min — _p = 1.4 * n (slot), and the required number of collection times R _{p_t_min} is less than 7, and at this time, a certain level α = 0.99 is satisfied.
The tag ID reading process in successive collection rounds starts with a rough estimation of the updated frame size with the tag set and initial frame size in the next round.
For example, in the i-th round, the slot confirmation unit 11 observes the mixed slot as g slot, the successful slot as s slot, and the empty slot as e slot.
Here, for the observed slots, the posterior probability distribution of k tags that select the same slot is obtained by the following equation (5).

  Assuming that an observation result comes when the number of slots and tags is equal, the number of tags competing for the same slot is calculated by the following equation (6).

When simulation (numerical calculation) is performed using the above-described equations (1) to (6), FIGS. 6 (a), 6 (b), and 6 (c) are obtained. The probability α of reliability is “0.99”.
FIG. 6A is a graph generated based on the equation (3), and a collection round in which tag ID responses from all the wireless tag 2 of the number indicated by each curve on the vertical axis can be obtained without collision. It indicates the number R _{p_f} the horizontal axis represents the frame size N (number of slots). Here, α = 0.99 in the equation (3), and P1 is calculated by giving the tag set n and the frame size N at q = 1 from the equation (1). The unit of the overall reading time on the vertical axis in FIG. 6A is a slot, and this one slot is a tag response transmission time. As will be described later, the time width of one slot used for the simulation parameters in the first embodiment in FIGS. 7 and 8 is “5 ms”.
FIG. 6B is a graph generated by using a part of the equation (4), where the vertical axis indicates the overall readout time (unit: msec), and the horizontal axis indicates the frame size (number of slots). ing. That is, since “R _{p_t} = N * R _{p_f} ” and R _{p_f} is obtained in FIG. _6A , the vertical axis R _{p_f} in FIG. _6A is simply _combined with the horizontal frame size N. Then, it is obtained by multiplying it by N times to obtain the vertical axis R _{p_f in} FIG. Here, Rp_t_min in the equation (4) is the minimum value of each curve in FIG. 6B, which is plotted by rhombuses.
6 (a) and 6 (b), the tag set of each curve is a step of 290, 270, 250, 230, 210, 190, 170, with the number of parameter wireless tags in units of 20. 150, 130, 110, 90, 70, 50, 30, 10 (pieces).
In FIG. 6C, the left vertical axis indicates the number of optimal collection rounds (the least successful tag ID collection), corresponding to the solid line graph, and the right vertical axis indicates the optimal frame size. The horizontal axis indicates the tag set (the number of wireless tags 2) corresponding to the broken line graph.

As can be seen from FIG. 6 (b), as the frame size increases, the overall readout time decreases rapidly and decreases to the point marked with “◇ diamond”, which is the minimum value, It gradually increases from this point.
In FIG. 6B, when attention is paid to the graph of the number of any of the wireless tags 2, there are two corresponding values for the frame size on the horizontal axis for the same reading time on the vertical axis. That is, it can be considered that the two frame sizes similarly have the requirement to delay the collection of the overall tag ID.

Here, in the two frame sizes, the point located on the left side with respect to the minimum value (the smaller frame size) is located in the area where the ALOHA slot fluctuates, while the point located on the right side of the minimum value. (The larger frame size) is in a stable region, and it is considered that the expected response is hardly obtained.
Further, in FIG. 6C, the broken line graph indicates the optimum frame size (right vertical axis) with respect to the number of wireless tags, and the solid line corresponds to the number of wireless tags and the overall passive. The minimum number of collection rounds (left vertical axis) repeated to reach the minimum tag read time for the type is shown. The solid line in FIG. 6C is a curve in which the points marked with “◇ diamond” in FIG. 6A are plotted, and the broken line in FIG. 6C is the point marked with “◇ diamond” in FIG. 6B. Is a plotted curve.

  Therefore, the estimated tag set of the wireless tag 2 in the i-th collection round is calculated by the following equation (7).

The tag estimation unit 12 stores the above equation (7), and inputs the number of slots s and the mixed slot g in which the tag ID detected by the slot confirmation unit 11 has been successfully detected. Estimate the tag set for the second collection round).
Also, the frame update unit 13 generates the next collection round according to the following expression (8) generated by first approximating the broken line in FIG. 6C from the current tag set obtained by the tag estimation unit 12. Calculate the frame size. By using the equation (8), it is possible to set the collection of the tag ID of α = 0.99 in the (i + 1) th collection round in the shortest readout time.

Next, in step S4, the frame update unit 13 determines whether or not to change the frame size according to the threshold value γ = 1.15 set inside in order to suppress random fluctuation during tag ID collection. Judgment is made. When the next frame size N (i + 1) to be used next is calculated from the tag set estimation n _set (i) according to the equation (8), the coefficient “1.4” of the equation (8) is calculated as shown in FIG. The broken line graph of c) was used as the primary approximation, and the error derived from this primary approximation was “1.15”, which was set as the threshold value γ. If this threshold value γ, that is, the error of the first-order approximation is exceeded, it will deviate from the broken-line graph of FIG. When the threshold value γ is exceeded, the frame size N (i + 1) is updated and approximated to the broken line graph of FIG.
At this time, the frame updating unit 13 updates the frame size only when the fluctuation of the tag number estimation is greater than or equal to the threshold γ, that is, when n _est (i) ≧ γ * n _est (i−1). Here, n _est (i-1) is the tag estimation value of the immediately preceding collection round.

The frame update unit 13 then multiplies the tag set estimated value n _est (i−1) of the immediately previous collection round by γ to obtain the tag set estimated value n _{est of} the current collection round obtained by the tag estimating unit 12. If (i) is less than or equal to, the process proceeds to step S5, and if the value obtained by multiplying _nest (i-1) by γ exceeds _nest (i), the process proceeds to step S7.
That is, when the final tag set estimate is obtained, the same frame size must remain within the collection round R _{p_f in} order to achieve a predetermined level of reliability α = 0.99.
For this reason, whenever changing to a new frame size, it is necessary to reset (initialize) the counter of the tag ID collection round.

Next, in step S5, the frame updating unit 12 sets j = 1, sets n (i) to a value of n (i + 1), increments i (increases by one), and advances the process to step S6. .
In step S6, the frame control unit 12 sets the frame size to N (i) obtained by the equation (8), and changes the total number of collection rounds R _{p_f} to the number obtained by the equation (3). To do.
Thereby, the control part 16 produces | generates a command packet with a new frame size, transmits a command packet via the transmission part 14, and returns a process to step S2.

In step S7, the frame update unit 12 increments the collection round counter j (increases by one) and advances the process to step S8.
Next, in step S8, the frame update unit 12 detects whether or not the value of the collection round counter j exceeds R _{p_f} +2.
At this time, if the frame update unit 12 detects that the value of the collection round counter j is less than R _{p_f} +2, the process returns to step S2, while the value of the collection round counter j exceeds R _{p_f} +2 If this is detected, the process is terminated.

In step S8, the meaning of “2” in “R _{p — f} +2” _depends on the processing convenience described below. Since the initial value j = 1 and the process of “j = j + 1” is performed in step S7, j = 2. On the other hand, since the overall collection round is defined as _{Rp_f} = 4 in the initial setting, the frame update unit 12 detects that _{Rp_f} is less than 2 in step S8, and returns the process to step S2. Thereafter, if the wireless tag 2 does not exist, the process returns to step S2 three times in the same manner, and the collection round is performed a total of four times, exits the loop and ends normally. The number of rounds of collection is matched to the point that tag set estimation described later is obtained after four rounds of collection.

The process of the flowchart of FIG. 5 described above is executed by the following algorithm.
N = 16; //
i = 0; // counter of collection round given a frame size
do {
i ++;
[g, s] = perform_read_cycle (N);
n _est (i) = estimate_tag_set (N, g, s); // as per (7)
if n _est (i)> γ * n _est (i-1) {// start a new frame size and reset
N = dynamic_frame_size (n _est (i)); // as per (8) expression
i = 0;}
} while (i <R _{p_f} ); // read all tags with confidence level
}

FIGS. 7 and 8 show the simulation results of reading the tag ID in the dynamic frame size update algorithm according to the first embodiment of the present invention. The simulation parameters are 52 m (milliseconds) for the transmission time of the instruction packet (reader → wireless tag) and 5 milliseconds for the transmission time of the tag response (wireless tag → reader).
In FIG. 7, the horizontal axis represents the tag set (unit: number) of the wireless tags, and the vertical axis represents the overall readout time (unit: second). In FIG. 8, the horizontal axis represents the tag set (unit: number) of the wireless tags, and the vertical axis represents the finally obtained frame size (slot number) necessary for obtaining the read reliability. Here, if the number of wireless tags 2 is large, the frame size is updated in the process of reading the frame size. The finally obtained frame size indicates the frame size when the reading of the tag IDs of the entire wireless tag 2 is finally completed.

FIG. 7 shows the overall readout time of the passive wireless tag 2 corresponding to the number of tag sets. That is, when the number of wireless tags is known in advance, “opt_ana” indicates a theoretically optimal numerical value. “PTD_rnd” is a result of updating the frame size by dynamically estimating the tag set using the frame size algorithm according to the first embodiment of the present invention. Similarly to “PTD_rnd”, “PTD_bin” indicates expected (estimation) in a state where the frame size is set to (16, 32, 64, 128, 256).
Here, “PTD_rnd” is used when updating the frame size N (i + 1) calculated according to the equation (8) from the estimated value n _set (i) of the tag set. In “PTD_bin”, when the frame size N (i + 1) is updated (increased), it is assigned in the order of 16, 32, 54, 128, 256. However, in the response from the wireless tag 2, a certain collection round ends before using all the slots of the allocated frame size. This situation is the final frame size graph of FIG. Looking at FIG. 8 in detail, it can be seen that the graph of “PTD_bin” is close to the graph of “PTD_rnd” around the frame sizes of 32, 64 and 128.

As a result, by using the frame size change algorithm of the present invention, the tag ID identification process when there are a large number of wireless tags can be performed at the same time as the identification process when the number of wireless tags is already known ( For example, reliable reading can be ensured for the number of unspecified number of wireless tags to be identified and processed within several times.
An estimate of a good tag set that is close to the actual number of unspecified radio tags is obtained after the fourth collection round. That is, as an initial value, the overall collection round R _{p_f} is _set to “4” and simulation is performed, and the result of FIG. 8 is obtained. That is, a result close to the theoretical value in which the number of wireless tags is known can be obtained from the fourth collection round in the wireless tag identification processing of the present invention. On the other hand, when the collection round R _{p_f} is less than “4”, the number of wireless tags obtained from the equation (3) cannot be guaranteed.
And as shown in FIG. 8, when we limit the frame size to be 16, 32, 64, 128, or 256, the overall readout time does not increase much. That is, “PTD_rnd” in FIG. 8 is the overall reading time when the frame size is changed according to the equation (8). On the other hand, “PTD_bin” is the overall readout time when the frame size is limited to 16, 32,. In comparison between “PTD_rnd” and “PTD_bin”, “PTD_bin” exceeds the overall reading time than “PTD_rnd”. However, even if the increase in the reading time from “PTD_rnd” to “PTD_bin” is large, it remains at about 20 to 30%.

<Second Embodiment>
RFID collision prevention may be deterministic or stochastic. The deterministic method is mainly used in passive RFID systems, which require the transmission of a significant amount of data from the RFID tag to the tag reader. In addition, since it is a query (using data of a predetermined radio frequency) that follows a predetermined path (data transmission), a missed tag cannot be detected indefinitely. For example, for effective capture, receiver reception can correctly return the strongest signal from two collision packets. Therefore, there is still a probability that the receiver will miss some tags. With a probabilistic multiplex technique, any RFID tag within the tag reader's transmission distance responds randomly to the query from the receiver.

  When many RFID tags are within the transmission distance of a tag reader, the contention for propagation (the wireless communication from the tag to the receiver uses the same frequency, so repeated queries are required to read all RFID tags) The frequency at which multiple tags interact with each other) may be high. As a result, longer readout times and higher battery consumption are required. These will shorten the lifetime of active tags. Furthermore, since the number of tags within the receiver's query range is usually not known in advance, it is difficult to coordinate between the minimum readout time of the system and the least battery energy usage. The second embodiment of the present invention solves the above-described problem.

The second embodiment of the present invention provides a mechanism for recognizing a large number of active wireless tags without knowing the number of tags in advance by using an ALOHA frame within the inquiry range of the reader. suggest.
According to this proposal, the reading procedure can be optimized between the overall reading time and the power consumption. The read procedure starts with a fixed initial frame length. Once the number of empty slots, mixed slots, and successful slots is obtained, the reader estimates the total number of tags and updates the frame length taking into account the read time and power requirements. . The procedure is repeated until all tags have been read.

Hereinafter, a wireless tag system using an active wireless tag according to a second embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing a configuration example of the embodiment.
In this figure, the reader 1 has a slot confirmation unit 11, a tag estimation unit 12, a frame update unit 13, a transmission unit 14, a reception unit 15, a control unit 16 and a power supply unit 17.
The wireless tag 2 includes a transmission unit 21 that transmits data, a reception unit 22 that receives data, a control unit 23 that controls transmission / reception of data, and a power supply unit 24 that supplies power to each internal circuit. Yes.

The control unit 16 generates a command packet of the collection command having the configuration shown in FIG. 2 and outputs it to each wireless tag 2 via the transmission unit 14.
As shown in FIG. 2, the command packet includes, for example, a receiver preamble that notifies transmission of a packet, a slot size that indicates a time width of a slot that receives a response from the wireless tag 2, and the number of slots in the frame. And a CRC (Cyclic Redundancy Check) code which is an additional code for guaranteeing the reliability of the communication packet. Here, the result of dividing the frame size by the slot size is the slot size.

In the wireless tag 2, the command packet received by the receiving unit 22 is input, the control unit 23 detects the collection command, generates a response packet of the tag response shown in FIG. A message is transmitted to the reader 1 at a predetermined timing, that is, at any slot in the frame.
In the case of the active tag of the second embodiment, the power supply unit 24 is a battery and supplies power to each circuit in the tag.
As shown in FIG. 3, this response packet is, for example, a tag preamble that notifies the transmission of the packet, tag ID data that is its own identification information, and an additional code for guaranteeing the reliability of the communication packet. CRC code.

The slot confirmation unit 11 performs a tag ID detection process on the received data input by the receiving unit 14 in each slot, and according to the detection result, each slot of the frame is an empty slot in which no response packet exists, or two or more A mixed slot where response packets from a plurality of wireless tags 2 exist, and a good slot where only response packets from one wireless tag exist are detected.
The tag estimation unit 12 estimates the number of tags in the detectable range based on the detection result of the slot confirmation unit 11.
The frame update unit 13 calculates a frame size corresponding to the number of tags and outputs the frame size to the control unit 16.
The transmission unit 14 transmits data including a command packet to the tag 2 under the control of the control unit 16, and the reception unit 15 receives data including a response packet from the tag 2.

Next, before explaining the operation, processing performed by the tag estimation unit 12 and the frame update unit 13 in the first embodiment of the reader 1 passive type wireless tag 2 and the reader 1 will be described.
The wireless tag 2 is permitted to transmit once every frame, that is, in any slot. The wireless tag 2 generates a response packet in the slot to be transmitted in accordance with the start timing, and terminates transmission in this slot.
As described above, the command packet transmitted by the reader 1 includes the frame size (the number of slots in the frame) and the slot interval. The slot interval is sequentially set corresponding to the frequency band used for packet transmission / reception. Further, the reading air 1 instructs each wireless tag 2 to specify any two bits of the bit string of the tag ID and respond in the slot designated by the two bits. For example, it is designated as the upper 2 bits of the tag ID. This number of bits is determined by the frame size.

Accordingly, each wireless tag 2 calculates a number indicated by 2 bits indicated by the control unit 23 in the bit string of the tag ID. For example, if 2 bits are “11”, it is the third slot. Further, the wireless tag 2 sequentially calculates the slot number, that is, the slot number of each slot, using the slot interval in the frame started at the end timing of the instruction packet.
Next, the wireless tag 2 uses the number calculated from the 2 bits indicated by the control unit 23 in the bit string of its own tag ID as the response slot number. The wireless tag 2 transmits a response packet at the timing of the slot in which the response slot number matches the slot number calculated sequentially. The reader 1 receives the response packet and transmits a pause command to the wireless tag 2 whose tag ID has been identified. Since the active tag is operated by a battery, no extra operation is performed to extend the life of the active tag.

Next, the operation of the wireless tag system according to the first embodiment of the present invention will be described with reference to FIGS. FIG. 9 is a conceptual diagram showing a procedure for detecting tag IDs of an unspecified number of passive RFIDs using the ALOHA method. FIG. 10 is a flowchart illustrating an operation example of the wireless tag system according to the first embodiment.
As shown in FIG. 9, each wireless tag 2 is set so that a response packet is transmitted once in one frame, that is, a response packet is transmitted in any slot in the frame. That is, the wireless tag 2 terminates the transmission of the response packet between the start and end of any slot in the frame, and transmits a pause command to the wireless tag 2 whose tag ID has been identified. The completed wireless tag is shifted from the activated state to a dormant state in order to reduce power consumption.

Next, in the flowchart shown in FIG. 10, an example of a tag ID reading operation from each wireless tag 2 by the reader 1 will be described.
In step S <b> 11, the control unit 16 reads each condition set in advance by a user or the like and performs initial setting in order to collect tag IDs of an unspecified number of wireless tags 2.
At this time, the control unit 16, for example, tag set estimated value In _set (i) = 0 indicating the estimated number of unspecified many wireless tags 2, frame size (number of slots) N = 20, collection round counter i = Set to 1.

Next, in step S <b> 12, the control unit 16 sends an activation command to an unspecified number of wireless tags 2.
As a result, all the wireless tags 2 that have received the activation command transmitted by the reader 1 shift from the dormant state to the activated state.
In step S <b> 13, the control unit 16 generates a command packet based on the initial setting value, and transmits this command packet to an unspecified number of wireless tags 2.
At this time, the wireless tag 2 is permitted to transmit once every frame, that is, in any slot.

That is, the wireless tag 2 generates a response packet in the slot to be transmitted in accordance with the start timing, and terminates transmission in this slot.
Here, the command packet transmitted by the reader 1 includes a frame size (the number of slots in the frame) and a slot interval. The slot interval is sequentially set corresponding to the frequency band used for packet transmission / reception.
Each wireless tag 2 uses the control unit 23 to obtain the number indicated by a plurality of designated bits in the bit string indicated by the instruction slot in its own tag ID, and calculates the number of the slot that transmits its own response packet. .

Thereby, each wireless tag 2 sequentially calculates the slot number, that is, the slot number of each slot using the slot interval in the frame started at the end timing of the instruction packet by the control unit 23. .
Next, the wireless tag 2 takes a plurality of bits at a position designated from the bit string of its own tag ID by the control unit 23 and obtains a value indicated by the plurality of bits as a response slot number. The wireless tag 2 transmits a response packet at the timing of the slot in which the response slot number matches the slot number calculated sequentially.

Next, in step S <b> 14, the slot confirmation unit 11 of the reader 11 determines whether a response packet from the wireless tag 2 is received in each slot in the frame.
That is, the slot confirming unit 11 determines whether the slot successfully received the response packet corresponding to the slot number from each slot of the frame started from the end timing of the instruction packet transmitted by itself. It is detected whether it is a mixed slot in which the response packet from is colliding or an empty slot in which there is no response packet from any slot.

Next, in step S15, when the slot confirmation unit 11 detects that all the slots of the frame are empty slots (no response from the wireless tag), the process proceeds to step S21, and any of the slots in the frame is detected. When it is detected that is a successful slot or a mixed slot (a response from any wireless tag is received), the process proceeds to step S16.
In step S <b> 16, the control unit 16 stores the tag ID acquired by the slot confirmation unit 11 in the successful slot, and transmits a pause command to the wireless tag 2 with the tag ID.
As a result, the wireless tag 2 identified by the reader 1 shifts from an activated state to a suspended state that consumes less power.

Next, in step S <b> 17, the tag estimation unit 12 performs the following process to estimate the number of wireless tags 2 that are unspecified and whose number is not clear.
First, a description will be given of a simulation performed in advance in order to obtain an expression used by the tag estimation unit 12 for estimating the number of tags. The simulation result is mounted on the reader 1 and the processes of the tag estimation unit 12 and the frame update unit 13 are executed.
In the i-th round, in the frame size of N (i) slot, the read tag is n (i), and the probability when q slots collide in one slot is the expression (1) described above. The binomial distribution is as follows.

In the equation (1), when q = 1, p _{1 (i)} is a probability of successful tag ID reading.
Since the wireless tag that has been successfully identified is in the dormant state, the tag set in the next (i + 1) -th collection round with respect to the i-th collection round is as shown in the following formula (9): , Equal to or less than the i th tag set.

Further, the number of collection rounds R a — _f for identifying the tag IDs of all the wireless tags 2 is _calculated in the above equation (2). Here, the probability α is a probability of losing sight of the wireless tag, and R a — _f is calculated based on the value of α. Here, the number of successful tag ID reads in a frame having N (i) slots is N (i) * p _{1 (i)} .
Then, the average time for identifying the tag ID of the shortest overall wireless tag 2 is calculated by the following equation (10).

In addition, the above expression (10) is expressed as follows: tag set n, frame size N, overall reading time, number of tag responses (number of collection rounds at which reading of tag IDs from all wireless tags 2 is completed), The relationship with the level α with the reliability of reading the tag ID is shown.
Further, the expression (10) is changed in a timely manner by changing numerical values such as the frame size, the readout time, and the reliability.

For example, the frame size that achieves the shortest active tag read time is N _min — a = 0.65 * n (where N is the slot size and n is the tag set) and the corresponding average active type Tag response, that is, the round number _{Ra_t_min} for which identification of the tag ID is completed is approximately “3” (a numerical value obtained from FIG. 11 (c) described later) (0.65 used here is FIG. 11 (c)). ) Is a linear approximation of the broken line).
However, when the shortest read time is achieved, the rate of drop of the packet response (location marked with diamond “” ”in FIG. 11A) is the overall average read time (diamond in FIG. 11B). Compared to the rate of increase in the section marked “◇”), it has changed significantly. That is, a frame size that slightly increases the reading time (a frame size longer than the above) is selected rather than a frame size that provides the shortest reading time. This frame size selection process can greatly reduce the packet response and reduce the power consumption of the wireless tag, which is effective for an active wireless tag.

This means that a large amount of battery power can be saved in the overall readout time with a small amount of power consumption due to the short transmission time. When N is a slot unit and n is the number of tags, and a frame size of N = n is used, as shown in FIGS. 11A and 11B, 290 active wireless tags 2 as tag sets are used. Is given, the overall tag read time increases by 12.4% and the tag response can be reduced by up to 33.3%. That is, from the graph of FIG. 11B, the overall readout time of 290 wireless tags is 530 slot unit time at a position where the horizontal axis frame size is about 188 slots (= 0.65 × 290). In FIG. 11B, the overall read time is 595 slot unit time in a frame size of N = n, that is, a frame size “290” on the horizontal axis. In other words, assuming that the 530 slot unit time is 100% of the reference, the increase is 12.4%. Next, from the graph of FIG. 11A, at the position where the horizontal axis frame size is about 188 slots (similar to FIG. 11B), the tag response is 3 collection rounds. Similar to FIG. 11B above, in the frame size “290” on the horizontal axis, the tag response is reduced to two collection rounds, so the overall collection time is 1/3 (33.3%). Reduction.
The tag ID reading process in successive collection rounds starts with a rough estimation of the updated frame size with the tag set and initial frame size in the next round.

For example, in the i-th round, the slot confirmation unit 11 observes the mixed slot as g slot, the successful successful slot as s slot, and the empty slot as e slot.
Here, for the observed slots, the posterior probability distribution of k tags that select the same slot is obtained by the above equation (5).
Assuming that an observation result comes when the number of slots and tags is equal, the number of tags competing for the same slot is calculated by the above equation (6).

When simulation (numerical calculation) is performed using the above-described equations (1), (2), (5), (9), and (10), FIGS. 11 (a), (b), and (c) are obtained. . The probability α of reliability is “0.99”. That is, FIG. 11A is a graph showing the results calculated based on the equations (2) and (9). The frame size N is set on the horizontal axis, and the horizontal axis is set with R _f in the equation (2) as the tag response _{Ra_f} . As parameters, the number of wireless tags n is 10, 30, 50,. It should be noted that α = 0.99, and P1 can be obtained from equation (1) when q = 1 and n (i) and N (i) are given. Here, n (i) is obtained by mathematical induction (substituting numerical values in order of i = 1, 2,...) By the equation (9). N (i) is the horizontal axis N in FIG. Furthermore, the rhombus “ _{プ ロ ッ ト”} is plotted at the position of the frame size N satisfying the equation (4) and the tag response _{Ra_f} . Moreover, FIG.11 (b) is the graph produced | generated using a part of (10) Formula. That is, R a — _t corresponds to the expression shown in “Expression 3 (claim) and Expression 3 (expression of means for solving the problem)” in parentheses “min ()” on the right side of Expression (10). Here, since R _{a_f} has already been obtained in FIG. 6 (a), the value of the horizontal axis R _{a_f} in FIG. 6 (a) is substituted into the “Equation 3” to obtain R _{a_t} . The vertical axis of (b) may be used. Here, R a — _{t — min} in the equation (10) is the minimum value of each curve graph in FIG. 6B, and this minimum value is plotted by a diamond “◇”.
Further, the unit of the overall reading time on the vertical axis in FIG. 11A is a slot, and this one slot is the transmission time of the tag response. As will be described later, one slot used for the simulation parameters in the second embodiment in FIGS. 12 and 13 is 5.364 seconds.
In FIG. 11B, the vertical axis represents the overall read time (unit: msec), and the horizontal axis represents the frame size (number of slots).

11 (a) and 11 (b), the tag set of each curve is 290, 270, 250, 230, 210, 190, 170, 150, 130, 110, 90 in steps of 20 units. , 70, 50, 30, 10 (pieces), and frame sizes from 10 to 300 are possible.
In FIG. 11C, the left vertical axis indicates the number of optimal collection rounds (the least successful tag ID collection), corresponding to the solid line graph, and the right vertical axis indicates the optimal frame size. The horizontal axis indicates the tag set (the number of wireless tags 2) corresponding to the broken line graph.

As can be seen with reference to FIG. 11 (b), as the frame size increases, the overall readout time decreases rapidly and decreases to the point marked with “◇ diamond”, which is the minimum value, It gradually increases from this point.
In FIG. 11B, when attention is paid to the graph of the number of any of the wireless tags 2, there are two corresponding values for the frame size on the horizontal axis while the reading time on the vertical axis is the same value. That is, it can be considered that the two frame sizes similarly have the requirement to delay the collection of the overall tag ID.

Here, in the two frame sizes, the point located on the right side of the minimum value (the one with the larger frame size) is in a stable region, and it is considered that the expected response is hardly obtained.
As described above, the frame size for achieving the shortest tag ID identification time is N _min — a = 0.65 * n, which corresponds to the minimum value in FIG.
The number of rounds for which the corresponding averaged wireless tag 2 is optimally responded is about “3”, which corresponds to the average value of the solid line values in FIG.

  Further, in FIG. 11C, the broken line graph indicates the optimum frame size (right vertical axis) with respect to the number of wireless tags, and the solid line corresponds to the number of wireless tags and the overall passive. The minimum number of collection rounds (left vertical axis) repeated to reach the minimum tag read time for the type is shown. The solid line in FIG. 11 (c) is a curve obtained by plotting the points marked with “◇ rhombus” in FIG. 11 (a), and the broken line in FIG. 11 (c) is the point marked with “◇ rhombus” in FIG. 11 (b). Is a curve plotted.

Therefore, the estimated tag set of the wireless tag 2 in the i-th collection round is calculated by the above equation (7).
The tag estimation unit 12 stores the above equation (7), and inputs the number of slots s and the mixed slot g in which the tag ID detected by the slot confirmation unit 11 has been successfully detected. Estimate the tag set for the second collection round).
Further, the frame update unit 13 calculates the frame size of the next collection round from the current tag set obtained by the tag estimation unit 12 by the following equation (11).
As described above, when the broken line in FIG. 11C is first-order approximated, β = 0.65 in the equation (11) is obtained. This β = 0.65 shortens the overall readout time of the wireless tag. A larger value (0.65 ≦ β), but if a larger frame size is selected within the range of β <2, unnecessary collision of response packets from the wireless tag is avoided according to the size. In this case, the energy consumption can be suppressed because no re-response is performed.

A constant β is (logically) associated with the tag set, and depending on the requirements from the application, it may be appropriate to select a frame size in the range [0.65, 2]. That is, in an application using a wireless tag, the requirements differ depending on the application. For example, in a case where a wireless tag or a reader moves, it is necessary to finish reading response packets from all wireless tags in a shorter time. is there. For this reason, β = 0.65 is selected. On the other hand, in the case of checking the presence / absence of the wireless tag over a long period of time without much movement, on the other hand, in order to suppress the consumption of the battery of the wireless tag, that is, to save energy, β is “ Select a value close to 2 ”.
A beta less than 0.65 leads to a tag response that means more energy consumption with a shorter readout time, while a beta response greater than 2 also means a tag response that means more energy consumption. Leads to.
In order to suppress random fluctuations during collection, the threshold γ = 1.15 is defined in the same manner as in the first embodiment.

Next, in step S18, in order to suppress random fluctuation during tag ID collection, the frame update unit 13 changes the frame size according to the above-described threshold γ = 1.15. Determine whether or not.
At this time, the frame updating unit 13 updates the frame size only when the fluctuation of the tag number estimation is greater than or equal to the threshold γ, that is, when n _est (i) ≧ γ * n _est (i−1). Here, n _est (i-1) is the tag estimation value of the immediately preceding collection round.

The frame update unit 13 then multiplies the tag set estimated value n _est (i−1) of the immediately previous collection round by γ to obtain the tag set estimated value n _{est of} the current collection round obtained by the tag estimating unit 12. If (i) is less than or equal to, the process proceeds to step S19, and if the value obtained by multiplying _nest (i-1) by γ exceeds _nest (i), the process returns to step S13.
That is, when the final tag set estimate is obtained, it must remain the same frame size within the collection round R _{p_f in} order to achieve a predetermined level of confidence.
For this reason, the counter for the collection round of the tag ID is updated in step S19.

Next, in step S19, the frame updating unit 13 uses the tag set n _est (i−1) obtained in the current (n−1) th collection round as the newly obtained nth (current) collection round. To the tag set n _est (i), the collection round counter i is incremented, and the process proceeds to step S20.
In step S20, the frame update unit 13 replaces the currently used frame size with the newly obtained frame size N (i) by the nth designated tag set n _est (i), Return to S13.

The process of the flowchart of FIG. 10 described above is executed by the following frame size update algorithm.
Dynamic_identification {
N = 20; //
i = 0; // counter of collection round given a frame size
do {
i ++;
[g, s] = perform_read_cycle (N);
n _est (i) = estimate_tag_set (N, g, s); // as per (7)
if n _est (i)> γ * n _est (i-1) {// start a new frame size and reset
N = dynamic_frame_size (n _est (i)); // as per (11) expression
}
} no tag response
}

FIG. 12 shows a simulation result using a frame size update algorithm that dynamically changes the number of reading slots used in the tag ID reading process of the active type wireless tag in the second embodiment described above according to the estimated number of wireless tags. And shown in FIG.
FIGS. 12 and 13 each show the correspondence between the tag set and the entire readout time and the identification of the tag ID in the shortest tag set and the shortest in the update algorithm according to the second embodiment when β = 1. The correspondence of the number of collection rounds is shown.
In FIG. 12, the horizontal axis represents the tag set (unit: number) of the wireless tags, and the vertical axis represents the overall readout time (unit: second). In FIG. 13, the horizontal axis represents a tag set (unit: number) of wireless tags, and the vertical axis represents the number of collection rounds in which tag IDs can be identified in the shortest time.

  In both FIG. 12 and FIG. 13, if the number of wireless tags (tag set) is known in advance, the curve of “opt_ana” indicates the theoretical optimum. The curves “ATD_12” and “ATD_20” are for identifying tag IDs of an unspecified number of wireless tags, and each of the active wireless tag tag sets (a set; the number of wireless tags) is dynamic. (The frame size update algorithm according to the second embodiment of the present invention described above) is performed. Each of the curves “ATD — 12” and “ATD — 20” are simulation results using “12” and “20” as the initial frame sizes.

In this simulation, the data transmission time between the reader 1 and the wireless tag 2 was set as follows. In tag ID collection, command packet transmission from reader 1 to wireless tag 2 “4.224 m (millisecond)”, tag response from wireless tag 2 to reader 1 (response packet transmission) “5.364 msec”, The pause packet transmission from the reader 1 to the wireless tag 2 is “4.512 milliseconds”.
These simulation parameters are taken from the ISO / IEC 18000-7 draft. The draft ISO / IEC 18000-7 recommends an initial frame size with a fixed number of 12 slots.

Referring to FIG. 12, there is almost no difference between the theoretical value and the algorithm according to the present invention in the overall tag reading time.
That is, by using the frame size change algorithm of the present invention, the tag ID identification process when there are a large number of wireless tags is equivalent to the identification process when the number of wireless tags is already known, that is, It can be performed in a time close to the theoretical estimated value, and the identification process can be speeded up.

Furthermore, referring to FIG. 13, by using 20 as the initial frame size, a tag response of 10% or more can be saved.
According to the frame update algorithm according to the second embodiment of the present invention, a numerical value averaged as approximately “4.25” is obtained as the tag response, and the second number increases as the number of unspecified number of wireless tags increases. It is considered that the effect of the embodiment is more effective.
Further, according to the second embodiment of the present invention, as shown in FIG. 12, the tag IDs of 120 wireless tags are identified within “2.64 seconds”, and the number of wireless tags is unspecified at the start. It is also possible.

  The program for realizing the functions of the slot confirmation unit 11, the tag estimation unit 12, the frame update unit 13, and the control unit 16 of the reader 1 in FIG. 1 is recorded on a computer-readable recording medium. The number of unspecified radio tags may be estimated and the tag frame size may be updated by causing the computer system to read and execute the program recorded in (1). Here, the “computer system” includes an OS and hardware such as peripheral devices. The “computer system” includes a WWW system provided with a homepage providing environment (or display environment). The “computer-readable recording medium” refers to a portable medium such as a flexible disk, a magneto-optical disk, a ROM, and a CD-ROM, and a storage device such as a hard disk built in the computer system. Further, the “computer-readable recording medium” refers to a volatile memory (RAM) in a computer system that becomes a server or a client when a program is transmitted via a network such as the Internet or a communication line such as a telephone line. In addition, those holding programs for a certain period of time are also included.

  The program may be transmitted from a computer system storing the program in a storage device or the like to another computer system via a transmission medium or by a transmission wave in the transmission medium. Here, the “transmission medium” for transmitting the program refers to a medium having a function of transmitting information, such as a network (communication network) such as the Internet or a communication line (communication line) such as a telephone line. The program may be for realizing a part of the functions described above. Furthermore, what can implement | achieve the function mentioned above in combination with the program already recorded on the computer system, and what is called a difference file (difference program) may be sufficient.

It is a block diagram which shows the structural example of the wireless tag system by the 1st and 2nd embodiment of this invention. FIG. 2 is a conceptual diagram illustrating a configuration example of a command packet that the reader 1 of FIG. 1 transmits to a wireless tag 2. FIG. 2 is a conceptual diagram illustrating a configuration example of a response packet that the wireless tag 2 of FIG. 1 transmits to a reader 1. It is a conceptual diagram explaining the tag ID collection procedure from the radio | wireless tag of the passive radio | wireless tag using the ALOHA frame in 1st Embodiment. It is a flowchart explaining the tag ID collection procedure from the radio | wireless tag of the passive radio | wireless tag using the ALOHA frame in 1st Embodiment. It is the simulation result which calculated | required the read time of the response packet corresponding to frame size and a tag set in collection of tag ID from the passive radio | wireless tag by a reader. 6 is a graph showing a comparison between the optimum value theoretically obtained for the correspondence between the tag set and the overall readout time from the wireless tag and the calculation result by the frame size changing algorithm according to the first embodiment of the present invention. . It is a graph which shows the comparison with the optimal value which theoretically calculated | required the correspondence of a tag set and a final frame size, and the calculation result by the frame size change algorithm by the 1st Embodiment of this invention. 1 shows a configuration of an embodiment of the present invention. It is a conceptual diagram explaining the tag ID collection procedure from the radio | wireless tag of the active type radio | wireless tag using the ALOHA frame in 2nd Embodiment. It is a flowchart explaining the tag ID collection procedure from the radio | wireless tag of the active type radio | wireless tag using the ALOHA frame in 2nd Embodiment. It is the simulation result which calculated | required the read time of the response packet corresponding to a frame size and a tag set in collection of tag ID from the active type | mold radio | wireless tag by a reader. It is a graph which shows the comparison with the optimal value calculated | required theoretically, and the calculation result by the frame size change algorithm by the 2nd Embodiment of this invention about the response | compatibility with the tag set and the whole reading time from a wireless tag. . It is a graph which shows the comparison of the optimal value calculated | required theoretically, and the calculation result by the frame size change algorithm by the 2nd Embodiment of this invention about the response | compatibility with the tag set and the frequency | count of the optimized collection round.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Reader 2 ... Wireless tag 11 ... Slot confirmation part 12 ... Tag estimation part 13 ... Frame update part 14 ... Transmission part 15 ... Reception part 16 ... Control part 21 ... Transmission part 22 ... Reception part 23 ... Control part 24 ... Power supply Part

Claims (7)

The reader transmits a command packet for collecting the tag ID, receives the response packet including the tag ID from each wireless tag for each slot during the tag ID reading period of the tag ID consisting of a plurality of slots, A wireless tag system for identifying a specific number of wireless tags;
The reader is
A slot confirmation unit that determines whether each slot is a successful packet successfully read the tag ID, a mixed slot in which a plurality of response packets are mixed, or an empty packet in which no response packet is received;
A tag estimation unit for calculating an estimated number of wireless tags from at least the number of successful packets and the number of mixed slots;
A wireless tag system comprising: a frame update unit that obtains the number of slots by multiplying the estimated number by a predetermined coefficient. 2. The wireless tag system according to claim 1, wherein the predetermined coefficient is obtained using at least the following equation.

2. The wireless tag system according to claim 1, wherein the predetermined coefficient is obtained using at least the following equation.

N (i): number of slots, number of reading periods for collecting tag IDs In the above equation, the frame size is N (i), n (i) active wireless tags, q per slot The probability that a wireless tag exists is P _q (i), the number of reading periods for reading all tags is _{Ra_f} , the probability of missing an active wireless tag is α, and if q = 1, the success The ID tag identification probability is p ₁ (i). The tag estimation unit obtains the estimated number n _est (i) of the wireless tag by calculating using an equation of n _est (i) = s (number of successful slots) +2.39 g (number of mixed slots). The wireless tag system according to any one of claims 1 to 3. The reader transmits a command packet for collecting the tag ID, receives the response packet including the tag ID from each wireless tag for each slot during the tag ID reading period of the tag ID consisting of a plurality of slots, A wireless tag identification method for identifying a specific number of wireless tags,
The identification process of the reader is
Slot for which the slot confirmation unit determines, for each slot, whether it is a successful packet that has succeeded in reading the tag ID, a mixed slot in which a plurality of response packets are mixed, or an empty packet in which no response packet is received. Confirmation process,
A tag estimation process in which a tag estimation unit calculates an estimated number of wireless tags from at least the number of successful packets and the number of mixed slots;
And a frame updating process in which a frame updating unit multiplies the estimated number by a predetermined coefficient to obtain the slot number. The reader transmits a command packet for collecting the tag ID, receives the response packet including the tag ID from each wireless tag for each slot during the tag ID reading period of the tag ID consisting of a plurality of slots, A program that causes a computer to execute wireless tag identification processing for identifying a specific number of wireless tags,
In the identification process of the reader,
Slot for which the slot confirmation unit determines, for each slot, whether it is a successful packet that has succeeded in reading the tag ID, a mixed slot in which a plurality of response packets are mixed, or an empty packet in which no response packet is received. Confirmation process,
Tag estimation processing in which the tag estimation unit calculates an estimated number of wireless tags from at least the number of successful packets and the number of mixed slots;
And a frame update process in which a frame update unit multiplies the estimated number by a predetermined coefficient to obtain the slot number. A computer-readable recording medium on which the wireless tag identification program according to claim 6 is recorded.

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