CN108446577B - Multi-label anti-collision method in high-frequency/ultrahigh-frequency RFID (radio frequency identification) system - Google Patents

Multi-label anti-collision method in high-frequency/ultrahigh-frequency RFID (radio frequency identification) system Download PDF

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CN108446577B
CN108446577B CN201810165233.5A CN201810165233A CN108446577B CN 108446577 B CN108446577 B CN 108446577B CN 201810165233 A CN201810165233 A CN 201810165233A CN 108446577 B CN108446577 B CN 108446577B
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田川
李鑫
高茂生
尹祖伟
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Beijing Hc Innovation Tech Co ltd
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    • G06K7/00—Methods or arrangements for sensing record carriers, e.g. for reading patterns
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    • G06K7/10029—Methods or arrangements for sensing record carriers, e.g. for reading patterns by electromagnetic radiation, e.g. optical sensing; by corpuscular radiation sensing by radiation using wavelengths larger than 0.1 mm, e.g. radio-waves or microwaves resolving collision on the communication channels between simultaneously or concurrently interrogated record carriers. the collision being resolved in the time domain, e.g. using binary tree search or RFID responses allocated to a random time slot

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Abstract

The invention provides a multi-label anti-collision method in a high-frequency/ultrahigh-frequency RFID (radio frequency identification) system, which comprises the following steps: in the RFID identification system, an RFID reader-writer sends a request instruction to an RFID, and the request instruction provides an initial frame length L and a random number used for selecting a time slot to an RFID label; each RFID tag randomly selects a time slot to send information to an RFID reader according to a received instruction; the RFID reader sets an initial uniform distribution letter of the number of the RFID tags; the RFID reader updates the uniform distribution function of the number of the RFID tags according to the received information; adjusting the length of the next frame according to the updated uniform distribution function of the number of the RFID tags and the number of the identified RFID tags; if the tag collision is determined to occur, the process returns to step S4, and if the tag collision does not occur, the tag recognition is determined to be completed. The invention is based on the prediction algorithm of continuous idle time slots, namely the Hash function algorithm, and has certain superiority in improving the throughput rate, transmission overhead and time delay of the algorithm.

Description

Multi-label anti-collision method in high-frequency/ultrahigh-frequency RFID (radio frequency identification) system
Technical Field
The invention relates to the technical field of RFID (radio frequency identification) tags, in particular to a multi-tag anti-collision method in a high-frequency/ultrahigh-frequency RFID identification system.
Background
RFID tags inherently have limitations and constraints such as: the communication broadband of the RFID system cannot be expanded infinitely, and the characteristic of message transmission is limited; the number of tags within the read range is unknown; the low cost tags have limited memory and read-write capability; the label does not have a carrier monitoring function, and communication between the labels cannot be carried out. Because of the limitations of the conditions and the cost, the importance of the research of the anti-collision algorithm is promoted, the collision problem solved by using the collision algorithm is mainly divided into two categories, namely a tree-based deterministic algorithm and an ALOHA-based probabilistic algorithm, but the tree-based deterministic algorithm needs to identify all tags in the query area, so that the complexity is high, the time delay is long, the ALOHA-based probabilistic algorithm can seriously aggravate the operation burden of a reader-writer under the condition of a large number of tags, and the method is not suitable for the big data environment in the internet of things.
In the practical application of the RFID, the electronic tags are in a moving dynamic and working environment, new tags can enter or exceed a read-write working range at any time in the communication process, and a plurality of tags are communicated with a reader-writer in a competition mode, so that collision is generated, and a collision prevention algorithm is to complete smooth communication of the tags.
The electronic tag needs to transmit data to the reader-writer accurately, and needs to meet the maximum rate C in the communication process. The maximum rate is determined by the relationship between the bandwidth B of the tag antenna and the read/write antenna:
C＝B·log2(1+S/N)
(S/N signal-to-noise ratio), therefore, the research of anti-collision algorithm in practical operation is to ensure the success of realizing multiple access, and becomes an important problem for the research of RFID system.
RFID tags inherently have limitations and constraints such as: the communication broadband of the RFID system cannot be expanded infinitely, and the characteristic of message transmission is limited; the number of tags within the read range is unknown; the low cost tags have limited memory and read-write capability; the label does not have a carrier monitoring function, and communication between the labels cannot be carried out. Because of the limitations of these conditions and the cost, it is difficult to research the subject, and the current solution of collision problem using collision algorithm is mainly classified into two categories:
the first type: tree-based deterministic algorithm: tree-based deterministic algorithms have requirements on the chip and are therefore not suitable for a generalized customization, but research is possible. The method comprises a query tree algorithm, a binary tree algorithm, a search tree algorithm, a collision tree algorithm and the like, but the certainty algorithm based on the tree needs to identify all labels in a query area, and is high in complexity and long in time delay.
The second type: the Aloha probabilistic algorithm is not easily affected by the number of bits of the ID (identity) of the tag, and is more suitable for the current Internet of things big data information acquisition application. Typical Aloha algorithms include Pure Aloha (PA) algorithm, Slotted Aloha (SA) algorithm, Frame Slotted Aloha (FSAF) algorithm, and Dynamic Frame Slotted Aloha (DFSA) algorithm, in the PA algorithm, a tag randomly transmits its ID to a reader/writer, and then waits for a response, if other tags are also transmitting during the transmission of information, a partial collision or a complete collision may be caused, and the system throughput is low; to avoid partial collisions, the SA algorithm divides time into a plurality of discrete time slots, causing tags to transmit IDs at the beginning of each discrete time slot simultaneously; the FSA algorithm divides a plurality of time slots into a frame and each label only responds once in each frame on the basis of the SA algorithm, and if collision occurs in the current frame, one time slot is reselected in the next frame, so that the frequent collision of the labels is avoided; the DFSA algorithm is an improvement on the FAS algorithm, making the frame length as equal as possible to the number of tags to be read. In consideration of the influence of a large number of collision time slots and idle time slots on the system efficiency, researchers also propose a DFSA algorithm with different lengths, and the number of invalid collision time slots and idle time slots is reduced through a time slot internal mechanism, so that the system performance is obviously improved, such as the Q algorithm of EPCC1G 2. The Q algorithm adjusts the Q value through a prediction mechanism in the time slot, so that the purpose of adjusting the frame length is achieved, more collision time slots and idle time slots still exist, the Q value and the parameter C need to be calculated after each time slot is ended, the operation burden of a reader-writer is seriously aggravated under the condition of a large number of labels, and the method is not suitable for a large data environment in the Internet of things.
Disclosure of Invention
The object of the present invention is to solve at least one of the technical drawbacks mentioned.
Therefore, the invention aims to provide a multi-label anti-collision method in a high frequency/ultrahigh frequency RFID identification system.
In order to achieve the above object, an embodiment of the present invention provides a method for preventing collision of multiple tags in a high frequency/ultrahigh frequency RFID identification system, including the following steps:
step S1, in the RFID identification system, the RFID reader sends a request instruction to the RFID, the request instruction provides the initial frame length L and the random number used to select the time slot to the RFID label;
step S2, each RFID label randomly selects a time slot to send information to the RFID reader according to the received instruction;
step S3, the RFID reader sets an initial uniform distribution function of the number of the RFID tags;
step S4, the RFID reader updates the uniform distribution function of the number of the RFID labels according to the received information;
step S5, adjusting the length of the next frame according to the updated uniform distribution function of the number of RFID labels and the number of the identified RFID labels;
in step S6, if it is determined that there is a tag collision, the process returns to step S4, and if there is no collision, it is determined that tag recognition is complete.
Further, the acquiring information by the RFID reader includes: the respective number of slots in the last frame in the 3 cases of success, idle and collision,
if the time slot efficiency of a plurality of information frames is N, the number of successful, idle and collision time slots in each information frame when reading N labels is respectively
Figure BDA0001584244750000031
Figure BDA0001584244750000032
ak＝N-a0-a1。
Further, a hash function H is selected as the uniform distribution function of the number of the RFID labels, and the number of the labels is estimated by utilizing mathematical expectation according to the uniform distribution function H so as to remove algorithm errors caused by the randomness of the labels.
Further, in step S2, assuming that a frame includes N slots and N tags to be detected are provided, the frame length is represented by L, and each tag has equal probability
Figure BDA0001584244750000033
Randomly selecting a certain time slot in a frame to transmit data, and obtaining the probability that k labels transmit data simultaneously in one time slot
Figure BDA0001584244750000034
RFID system throughput is defined by the above equation
Figure BDA0001584244750000035
Further, in step S2, after receiving the random number SN from the RFID reader, the RFID tag stores the time slot number in the random number SN, and sends the time slot number to the RFID reader, where SN is a binary number of "0" or "1".
Further, the RFID reader starts a new time slot after receiving SN, judges whether the value of the random number SN of the tag is 0, if so, the RFID tag sends an identification symbol, otherwise, the value of the random number SN of the tag is reduced by one, and the identification symbol is not sent;
and after the RFID label sends the identification symbol, judging whether collision conflict occurs or not, successfully identifying, if so, immediately entering a dormant state by the RFID label, and otherwise, waiting for the RFID reader to restart a frame.
Further, after the RFID reader starts a new time slot after receiving the SN, it further determines whether the frame is finished, if so, it determines whether the RFID reader does not receive any tag signal, if not, the identification is finished, otherwise, it waits for the RFID reader to restart a frame.
According to the multi-label anti-collision method in the high-frequency/ultrahigh-frequency RFID system, the algorithm is a Hash function algorithm which is a prediction algorithm of continuous idle time slots, the algorithm is simple and uniform, and a constructed library is provided, so that invalid time slots can be quickly skipped. Under the large-scale label, the high throughput rate can be kept to be stabilized at 63%, and compared with the previous algorithm, the algorithm provided by the invention has certain superiority in improving the throughput rate, transmission overhead and delay of the algorithm.
The key technology for realizing simultaneous reading of a plurality of field tags in the radio frequency identification system is to find an anti-collision algorithm to indicate that the problem of collision of data sent by the tags is solved. On the basis of a frame time slot ALOHA algorithm in an RFID system, the invention not only can fully utilize the information collected by a reader-writer in the previous frame, but also can reserve the information of the previous frame of the label as the prior information, estimate the number of the labels according to an H function and dynamically adjust the frame length.
The invention solves the problem that the dynamic time slot ALOHA algorithm increases too much in the number of labels or the required time slot increases slowly in the large number of labels. The invention can set the frame length time slot number as 1.6 times of the label, control the label identification rate at 500us, and when the frame length is equal to the label number, the average identification time of each label is about 600 us. This is the algorithm which has considerable popularization value.
Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
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The above and/or additional aspects and advantages of the present invention will become apparent and readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:
fig. 1 is a flowchart of a multi-tag anti-collision method in a high frequency/ultra high frequency RFID identification system according to an embodiment of the present invention;
FIG. 2 is a flow chart of a frame slotted ALOHA algorithm according to an embodiment of the present invention;
FIG. 3 is a flow chart of a hash function core algorithm according to an embodiment of the present invention;
FIG. 4 is a schematic diagram of the throughput of a reader in an RFID system according to an embodiment of the present invention;
FIG. 5 shows system test efficiency of a system using a hash function under 256 labels according to an embodiment of the present invention;
FIG. 6 shows system test efficiency of a system using a hash function under an inner label 512 according to an embodiment of the present invention;
fig. 7 is a diagram illustrating system test efficiency of a system employing a hash function under a 512-time tag according to an embodiment of the present invention.
Detailed Description
Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the drawings are illustrative and intended to be illustrative of the invention and are not to be construed as limiting the invention.
The embodiment of the invention provides a multi-label anti-collision method in a high-frequency/ultrahigh-frequency RFID (radio frequency identification device) identification system, which is a Hash function algorithm based on a prediction algorithm of continuous idle time slots, wherein the algorithm is simple and uniform and has a structural library, so that invalid time slots can be skipped quickly.
Taking the DFSA algorithm as an example to prove the delay and efficiency problems of the system.
In the anti-collision system of the DFSA algorithm, the frame length is L, the number of the labels to be identified is n, and the probabilities that a certain time slot in a frame is an idle time slot, a readable time slot and a collision time slot are respectively
Figure BDA0001584244750000051
Figure BDA0001584244750000052
Figure BDA0001584244750000053
Thus, the average number of idle, readable and collision slots in a frame is
Figure BDA0001584244750000054
Figure BDA0001584244750000055
ak(L,n)＝Lpk (6)
Defining system time delay
Figure BDA0001584244750000056
Defining system efficiency
Figure BDA0001584244750000061
In the RFID system, the information from the tag to the reader is not continuous, so that the information is difficult to abstract as a random process, but the tag generates a random number in the frame length range according to the frame length information in the received reader instruction, and the random number is evenly distributed in the frame length range. From this, the number of possible tags in the time slot in which the collision occurred is estimated as follows:
if n labels are in the effective area of the reader-writer, the probability of selecting k labels in a frame to select a time slot i is
Figure BDA0001584244750000062
If collision occurs in the time slot i, the number of the labels in the selected time slot i is not less than 2, so that the distribution function P can be deducedk{ k tag selection slots i have collisions } -, i
Figure BDA0001584244750000063
The number of labels in slot i averages as
Figure BDA0001584244750000064
Then obtained by the formula (10)
Figure BDA0001584244750000065
If the maximum system efficiency S is to be obtained, the system is to be operated
dS/dn＝0 (13)
From formula (5), formula (8), formula (13)
Figure BDA0001584244750000066
Thereby having
Figure BDA0001584244750000067
According to the Meglalin expansion type
eχ≈1+χ (16)
Order to
Figure BDA0001584244750000071
Then there is
Figure BDA0001584244750000072
And further represented by formula (15), formula (17) has
N＝n+1≈n(n＞＞1) (18)
As can be seen from the above derivation, if a certain time slot is empty, the number of tags in the time slot is selected to be zero; if the information is correctly received, the number of the labels in the time slot is 1; if there is a collision, the number of tags in the timeslot is estimated to be b. The system efficiency is higher as the number of frame slots and the number of tags are closer together.
Whereby the number of unidentified tags can be estimated as
N0＝bC2 (19)
Wherein, C2The number of time slots for which a collision occurs, and thus the length of the next frame, can be determined based on the estimated number of tags.
In the RFID dynamic frame slotted ALOHA, if the number of slots of one frame is too long compared to the number of tags, the number of empty slots is too large, and conversely, the number of collision slots is too large, both of which result in a decrease in tag identification efficiency. Therefore, the frame length and the number of labels should satisfy a certain proportional relationship. From the perspective of the maximum channel utilization of the system, several relation equations of the number of labels and the frame length are derived, and these documents assume that the time occupied by the empty time slot, the conflict time slot and the readable time slot are the same, or the time occupied by the conflict time slot and the readable time slot are the same, however, in order to improve the channel utilization, the time of these time slots is not necessarily set to be the same. In the ISO18000-6C and EPC C1 Gen2 standards, the time occupied by the collision slot, the empty slot, and the readable slot is different. In such a standard system, the tag first sends a 16-bit random code RN16 to the reader, and if the reader does not receive any information, the time slot is terminated in advance; if the data is correctly received, the tag sends 64 bits of EPC data; if a collision occurs, the EPC is no longer transmitted. Thus, the system has the shortest time to empty, followed by conflicting slots, and the longest time to read.
As known through analysis, the maximum systematicness can be obtained only by reading the size of the frame with the closest tag number in each identification period, and therefore the invention provides a specific frame length adjustment scheme.
Because of the randomness of label selection, the probability of collision is increased when the number of labels is increased too much, if the following functions are realized by a scientific time slot distribution method when the time slots are less than the labels, most labels can be matched with the only time slots, so that the probability of collision is greatly reduced, the read-write identification time is shortened, the identification rate is increased, and a reader-writer can count the collision occurring in the period when the collision occursAmount of clearance CkReadable number of slots C1And an empty time slot C0According to the statistical theory and the three parameters, the number C of the read tags is calculatedc。
As previously mentioned, to reduce the optimal situation of collisions, the frame length should be substantially equal to the tag parameters that occur in the reading and recognition ranges, and the tags can transmit information evenly distributed in different time slots.
The invention uses the construction method of the hash function to distribute the time slot, and the selection standard of the function is as follows: the requirement is simple, and the Hash function is simple and quick to calculate; the requirement of uniformity means that the hash function can allocate the labels to the time slots with equal probability, so as to promote the minimization of collision.
The hash function h (ID)/[ L ], where ID denotes a unique identification code of the tag, w is a positive integer sent to the tag and operation by the reader/writer, and L denotes a frame length.
In a general linear table, a tree, the relative position of a record in a structure is random, that is, there is no definite relationship with a key of the record, and therefore, a series of comparisons with the key are required when looking up the record in the structure. This type of search method is based on "comparison", and the efficiency of the search depends on the number of comparisons performed during the search. It is desirable to be able to directly find the desired record, and therefore a certain correspondence f must be established between the storage location of the record and its key, so that each key corresponds to a unique storage location in the structure.
The hash function is an ideal situation, in practical application, the number of tags in the range of a reader-writer is unknown, and the problem of collision of the RFID is to be solved, so that the number of tags is to be predicted, and the current prediction methods include the following methods:
the worst prediction (also called the best prediction) is that at least more than two labels collide in one collision time slot, and the collision labels can be predicted to be 2 xN; generally, predicting (also called poisson probability), that is, storing poisson distribution with λ ═ 1 in each time slot selected by each tag in each information frame, so that the number of tags which are averagely responded to each collision time slot in the information frame is 2.39, and the number of tags which are responded to in collision time slots can be predicted to be 2.39 × N; and if the identification is finished, the identification of the label is proved to be finished. Fig. 3 is a flowchart of a hash function core algorithm according to an embodiment of the present invention.
It can be seen from the analysis that, in order to obtain better system performance, good results can be obtained as long as the frame length closest to the number of tags is obtained in each identification period.
In practical situations, because the distance between each tag and the reader-writer is different, the message sent by the short-distance tag may cover the message sent by the long-distance tag, i.e. a time slot conflict, and the reader-writer may also correctly identify the information of the short-distance tag, and also because the influence of other environmental noises is that only one tag responds in the time slot, the reader-writer may not read. In two extreme cases and the effects of noise. If the time slot efficiency of a plurality of information frames is N, the number of successful, idle and conflict time slots in each information frame when reading N labels is respectively
Figure BDA0001584244750000091
Figure BDA0001584244750000092
ak＝N-a0-a1
Therefore, the throughput rate of the reader-writer in the RFID system is also called the identification rate (the ratio of the number of successfully identified tags in the time of one information frame length)
Figure BDA0001584244750000093
The maximum throughput rate may be derived for N:
Figure BDA0001584244750000094
fig. 4 is a diagram illustrating throughput of a reader/writer in an RFID system according to an embodiment of the present invention.
In the common literature, idle time slots are made, occupied time of collision time slots and occupied time of effective time slots are the same or the time of collision time slots and the time of effective time slots are the same, and a relational expression of a plurality of labels and frame lengths is deduced, so that the maximum utilization rate is obtained.
By obtaining an average estimate e of the time slot of the collision algorithm and the proportion P of the time slot of the collision algorithm to the time slot of the collision0Proportion of free time slots PCThe number of identified tags is estimated, and when the frame lengths are different, the value difference between the number of tags and the actual tag is estimated.
The signal-to-noise utilization is expressed by:
Figure BDA0001584244750000095
wherein t is0Slot of gap, t1Time slot of collision, ckAn active time slot.
And the frame length has a linear relation with n, namely: l ═ Kn(n is the number of labels, L is the frame length)
When the number of tags tends to ∞
Figure BDA0001584244750000101
As shown in fig. 1, the multi-tag anti-collision method in the high frequency/ultra high frequency RFID identification system according to the embodiment of the present invention includes the following steps:
in the RFID identification system, the RFID reader/writer transmits a request instruction, which provides the initial frame length L and a random number for selecting a time slot to the RFID tag, to the RFID tag at step S1.
And step S2, each RFID tag randomly selects a time slot to send information to the RFID reader according to the received instruction.
Setting a frame containing N time slots and N labels to be detected, using L to approximate the frame length, each label having equal probability
Figure BDA0001584244750000102
Randomly selecting a certain time slot in a frame to transmit data, and obtaining the probability that k labels transmit data simultaneously in one time slot
Figure BDA0001584244750000103
RFID system throughput is defined by the above equation
Figure BDA0001584244750000104
Specifically, in the frame time slot ALOHA algorithm, a frame is divided into a plurality of time slots of the same size, the time slots are synchronized by a reader, and at the initial time of each frame, the reader sends a REQUEST command which can provide the frame length and a random number for selecting the time slot to a tag. Assuming that a frame contains N time slots and N labels to be detected, for the sake of discussion, the length of the frame is represented by L approximation, and each label has equal probability
Figure BDA0001584244750000105
The probability that k labels transmit data simultaneously in one time slot can be obtained by randomly selecting a certain time slot in one frame to transmit data
Figure BDA0001584244750000106
RFID system throughput is defined by the above equation
Figure BDA0001584244750000107
From the above equation, when the system throughput is T highest and L > 1, N ≈ N, where T ═ 36.8% >, i.e. when the number of tags is equal to the frame length, the system throughput reaches the maximum.
The randomness of the time slot selected by the tag causes the information collected by the reader to have certain randomness, and the estimation of the number of the tags cannot be only a determined value but a probability uniform function. A more reasonable way to estimate the number of labels is to use a hash function.
By using
Figure BDA0001584244750000111
The probability distribution function is also a feasible scheme, and the hash function has a structure table and can be uniformly distributed, so that the algorithm can be simpler and quicker by selecting the hash function.
H＝P(n)，
Wherein P (n) is a distribution function of the number of tags. If p (n) can be obtained, the algorithm error due to the randomness of the labels can be more effectively removed by estimating the number of labels using the mathematical expectation.
Step S3, the RFID reader sets an initial uniform distribution function of the number of RFID tags.
And step S4, the RFID reader updates the uniform distribution function of the number of the RFID tags according to the received information.
In step S4, the RFID reader acquiring information includes: the respective number of slots in the last frame in the 3 cases of success, idle and collision,
if the time slot efficiency of a plurality of information frames is N, the number of successful, idle and collision time slots in each information frame when reading N labels is respectively
Figure BDA0001584244750000112
Figure BDA0001584244750000113
ak＝N-a0-a1。
Step S5, adjusting the length of the next frame according to the updated uniform distribution function of the number of RFID tags and the number of RFID tags that have been identified.
The Hash function H is selected as the uniform distribution function of the number of the RFID labels, and the number of the labels is estimated by utilizing mathematical expectation according to the uniform distribution function H so as to remove algorithm errors caused by the randomness of the labels.
In step S6, if it is determined that there is a tag collision, the process returns to step S4, and if there is no collision, it is determined that tag recognition is complete.
As shown in fig. 2, after receiving the random number SN from the RFID reader, the RFID tag stores the time slot number in the random number SN, where SN is a binary number of "0" or "1", and transmits the time slot number to the RFID reader.
After receiving SN, the RFID reader starts a new time slot, judges whether the value of the random number SN of the label is 0, if so, the RFID label sends an identification symbol, otherwise, the value of the random number SN of the label is reduced by one, and the identification symbol is not sent;
and after the RFID label sends the identification symbol, judging whether collision conflict occurs or not, successfully identifying, if so, immediately entering a dormant state by the RFID label, and otherwise, waiting for the RFID reader to restart a frame.
And after the RFID reader-writer starts a new time slot after receiving the SN, further judging whether the frame is finished, if so, judging whether the RFID reader-writer does not receive any label signal, if not, finishing the identification, and otherwise, waiting for the RFID reader-writer to restart the frame.
In summary, through the time slot state model analysis of the hash function, the relationship between different frame lengths and the continuous collision time slots and the continuous idle time slots.
Relationship of different frame lengths to successive collisions: still taking the frame length as L and the total number of labels as N, the test shows that,
when L is 0.75N, the probability of collision is 0.05, that is, a small probability event, that is, a general occurrence event can be considered;
when L is 0.5N, the probability of collision occurring is 0.21, and therefore it is reasonable to predict that L will double.
Relation of different frame lengths to probability of consecutive idle time slots:
when L is 1.5N, the probability of occurrence of the free slot distribution is 0.06,
the probability of idle time slot distribution is 0.13 when L is 2N, so that the hash function proves that the label number N is half of the frame length L reasonably.
TABLE 1 optimal frames for different tag counts
Number of tags N Frame length L
6-11 8
12-22 16
23-44 64
43-88 128
89-177 256
178-154 256
A new hash function test method is provided according to the relation test label of the average label number e in the existing collision time slot and the occupation ratio of the collision time slot, when the time used by the idle time slot is less than the collision time slot, the label number read each time is different for the same label number and frame length, the invention tests in different frame lengths, and when the frame length is 8-125, the 7 labels for sample analysis are compared between 100 and 200, and the error in the new method can be seen to be greatly reduced.
TABLE 2 error Rate comparison of different algorithms
Figure BDA0001584244750000121
Figure BDA0001584244750000131
The system efficiency is as follows:
it is estimated that static test is performed under 256 labels, and it can be seen that the system is relatively stable around 0.34 after the hash function is used, as shown in fig. 5.
TABLE 3 comparison of tag numbers for different algorithms
Figure BDA0001584244750000132
When the number of the labels is within 512, the DFSA algorithm and the Hash algorithm have good and bad respectively, and after the number of the labels is increased to 400 at the high end, the Hash algorithm shows the advantages, as shown in FIG. 6.
To further compare the hash function and IDFSA to DFSA algorithms, we will work with a systematic probability of 0.35 for a tag number of 512, but the DFSA case will be worsened, as shown in fig. 7.
According to the multi-label anti-collision method in the high-frequency/ultrahigh-frequency RFID system, the algorithm is a Hash function algorithm which is a prediction algorithm of continuous idle time slots, the algorithm is simple and uniform, and a constructed library is provided, so that invalid time slots can be quickly skipped. Under the large-scale label, the high throughput rate can be kept to be stabilized at 63%, and compared with the previous algorithm, the algorithm provided by the invention has certain superiority in improving the throughput rate, transmission overhead and delay of the algorithm.
The key technology for realizing simultaneous reading of a plurality of field tags in the radio frequency identification system is to find an anti-collision algorithm to indicate that the problem of collision of data sent by the tags is solved. On the basis of a frame time slot ALOHA algorithm in an RFID system, the invention not only can fully utilize the information collected by a reader-writer in the previous frame, but also can reserve the information of the previous frame of the label as the prior information, estimate the number of the labels according to an H function and dynamically adjust the frame length.
The invention solves the problem that the dynamic time slot ALOHA algorithm increases too much in the number of labels or the required time slot increases slowly in the large number of labels. The invention can set the frame length time slot number as 1.6 times of the label, control the label identification rate at 500us, and when the frame length is equal to the label number, the average identification time of each label is about 600 us. This is the algorithm which has considerable popularization value.
In the description herein, references to the description of the term "one embodiment," "some embodiments," "an example," "a specific example," or "some examples," etc., mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.
Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention, and that variations, modifications, substitutions and alterations can be made in the above embodiments by those of ordinary skill in the art without departing from the principle and spirit of the present invention. The scope of the invention is defined by the appended claims and equivalents thereof.

Claims (5)

1. A multi-label anti-collision method in a high frequency/ultrahigh frequency RFID identification system is characterized by comprising the following steps:
step S1, in the RFID identification system, the RFID reader sends a request instruction to the RFID, the request instruction provides the initial frame length L and the random number used to select the time slot to the RFID label;
step S2, each RFID label randomly selects a time slot to send information to the RFID reader according to the received instruction; assuming that a frame contains N time slots and N labels to be detected, the length of the frame is approximately represented by L, and each label has equal probability
Figure FDA0003490970570000011
Randomly selecting a certain time slot in a frame to transmit data, and obtaining the probability that k labels transmit data simultaneously in one time slot
Figure FDA0003490970570000012
RFID system throughput is defined by the above equation
Figure FDA0003490970570000013
From the above equation, when the system throughput rate T is the highest and L > 1, N is approximately equal to N, and T is 36.8% at this time, that is, when the number of tags is equal to the frame length, the system throughput rate reaches the maximum;
adopting a hash function for estimating the number of the labels;
by using
Figure FDA0003490970570000014
The probability distribution function, and the hash function has a structure table and can be uniformly distributed, so the algorithm can be simpler and quicker by selecting the hash function;
H＝P(n)，
wherein P (n) is a distribution function of the number of the labels; if P (n) can be obtained, the algorithm error caused by the randomness of the labels can be effectively removed by estimating the number of the labels by using mathematical expectation;
step S3, the RFID reader sets an initial uniform distribution function of the number of the RFID tags;
step S4, the RFID reader updates the uniform distribution function of the number of the RFID labels according to the received information; wherein the RFID reader acquiring information comprises: the respective number of slots in the last frame in the 3 cases of success, idle and collision,
if the time slot efficiency of a plurality of information frames is N, the number of successful, idle and collision time slots in each information frame when reading N labels is respectively
Figure FDA0003490970570000021
Figure FDA0003490970570000022
ak＝N-a0-a1；
Step S5, adjusting the length of the next frame according to the updated uniform distribution function of the number of RFID labels and the number of the identified RFID labels;
in step S6, if it is determined that there is a tag collision, the process returns to step S4, and if there is no collision, it is determined that tag recognition is complete.
2. The method according to claim 1, wherein the hash function H is selected as the uniform distribution function of the number of RFID tags, and the number of tags is estimated by using mathematical expectation according to the uniform distribution function H to remove the algorithm error caused by the randomness of the tags.
3. The method as claimed in claim 1, wherein in the step S2, the RFID tag stores a time slot number in a random number SN after receiving the random number SN from the RFID reader, and transmits the time slot number to the RFID reader, wherein SN is a binary number of "0" or "1".
4. The multi-tag anti-collision method in the high frequency/ultra high frequency RFID identification system according to claim 3, wherein the RFID reader starts a new time slot after receiving SN, judges whether the value of the tag random number SN is 0, if so, the RFID tag sends the identification symbol, otherwise, the value of the tag random number SN is reduced by one, and the identification symbol is not sent;
and after the RFID label sends the identification symbol, judging whether collision conflict occurs or not, successfully identifying, if so, immediately entering a dormant state by the RFID label, and otherwise, waiting for the RFID reader to restart a frame.
5. The method according to claim 4, wherein the RFID reader starts a new time slot after receiving the SN, and further determines whether the frame is finished, if yes, determines whether the RFID reader does not receive any tag signal, if not, the identification is finished, otherwise, waits for the RFID reader to restart a frame.
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