CN101359361A - Data anticollision algorithm in RFID system - Google Patents

Abstract

The invention discloses a data anti-collision algorithm in the RFID system. The invention introduces the Markov chain for modeling and proposes a reservation ALOHA anti-collision algorithm based on the code division multiple access (CDMA). The invention includes the steps: 1) the channel modeling includes that the channel is supposed to be a frequency flat type channel and includes a down channel and N upper code division channels. 2) The algorithm modeling includes that: based on the channel modeling, the Markov chain modeling process is adopted; the channel is determined if the channel is possibly in the four discrete work states, including the idle state, collision state, reservation request sending state and the sending state; the inter-conversion relations of the four work states are determined; 3) the algorithm testing. The experiment indicates that under the same channel condition, the system throughput of the CDMA-R-ALOHA anti-collision algorithm proposed by the invention is more than the system throughput of the ALOHA algorithm.

Description

Data anticollision algorithm in a kind of rfid system

Technical field

The invention belongs to the RFID communications field, relate to the data anticollision algorithm in a kind of rfid system, be specifically related to a kind of employing Markov Process Modeling, adopt code division multiplexing CDMA multichannel to divide, utilize reservation ALOHA mode to realize the algorithm of many labels radio frequency identification system data anticollision.

Background technology

In REID, the anticollision technology is signal identification and one of gordian technique of handling.When radio-frequency recognition system is worked, in reader (Reader) reach, may have a plurality of labels (Tag) and exist simultaneously, and simultaneously reader be sent data, reply conflict thereby produce.Multiple access collision-proof method commonly used has following several, space division multiple access (SDMA), time division multiple access (TDMA) (TDMA), frequency division multiple access (FDMA) and CDMA (CDMA).The collision-proof method of present existing radio-frequency recognition system, most of access way that adopts time division multiple access (TDMA), promptly label transmits information at different time slots to reader.On the concrete implementation, the ALOHA method is arranged, and (sieve is breathed out algorithm, explains: Aloha agreement or claim Aloha technology, Aloha net is the earliest a radio computer communication network in the world.It is the name of a project of nineteen sixty-eight Hawaii, America university.Succeeding in developing a kind of packet switched computer network that uses wireless broadcast technologies the beginning of the seventies, also is the most basic the earliest wireless data communication protocol.Being named Aloha, is that Hawaiians represent the greeting of giving one's regards, and the purpose of this project is the communication issue that will solve between the Hawaiian Islands.The Aloha network can make a plurality of users that are dispersed in each island use central computer by radio channel, thereby realizes a bit arriving the data communication of multiple spot.ALOHA is used for handling the multi-user to single channel, at the data link layer media access control sublayer, the channel allocation resolution protocol), the binary search method, but it is very strict that this multi-access mode based on the time-division requires for channel, and along with the expansion of number of labels, performance is with rapid deterioration.Have a strong impact on the function of data communication and system.

Summary of the invention

Technical matters to be solved by this invention is to overcome the deficiencies in the prior art, and the data anticollision algorithm in a kind of rfid system is provided, and this method combines CDMA technology with the ALOHA technology, and the handling capacity of system is significantly improved.

Technical solution of the present invention is as follows:

Data anticollision algorithm in a kind of rfid system, described rfid system comprises reader, label and channel; It is characterized in that described channel comprises a downlink broadcast channel and a plurality of up Code Channel; The time shaft of described up Code Channel is divided into a plurality of time slots, and all labels are shared described time slot and up channel; Rfid system is in following a certain state at each time slot:

1) idle condition: the up Code Channel free time, do not have data packet transmission;

2) conflict situation: have two or more labels to send the reserve requests frame to same up Code Channel; The competition conflict takes place, the reserve requests failure;

3) send the reserve requests state: a certain sign indicating number divides up channel to have only a label to send the reserve requests frame, the reserve requests success;

4) expression transmit status: label sends packet at this time slot;

Transformational relation between each state is as follows:

1) if last time slot rfid system is in idle condition, in current time slots, a) neither one label busy channel, rfid system keeps idle condition; B) have only a label busy channel, rfid system changes subscription state over to by idle condition; C) two or more label busy channels are arranged, rfid system changes conflict situation over to by idle condition;

2) if last time slot rfid system is in conflict situation, in current time slots, a) neither one label busy channel, rfid system changes idle condition over to by conflict situation; B) have only a label busy channel, rfid system changes subscription state over to by conflict situation; C) two or more label busy channels are arranged, rfid system keeps conflict situation;

3) if last time slot rfid system is in subscription state, in current time slots, rfid system changes the data transmit status over to;

4) if last time slot rfid system is in the data transmit status, and data are sent completely, in current time slots, and a) neither one label busy channel, rfid system changes idle condition over to by conflict situation; B) have only a label busy channel, rfid system changes subscription state over to by conflict situation; C) two or more label busy channels are arranged, rfid system keeps conflict situation.

Data anticollision algorithm in the described rfid system may further comprise the steps:

1) reader sends activation instruction to label by the downlink broadcast channel; Described activation instruction comprises channel information and frame synchronization information;

2) label sends the reservation instruction to reader;

3) if conflict, reader sends the instruction of mourning in silence to label, and label withdraws from current time slots, waits for next time slot communication; Return step 1); If do not conflict, reader sends communication instruction to the label that proposes reservation; Enter next step;

4) label and reader data communication;

5) reader sends exit instruction, the communication of time slot after this label is no longer participated in to the label that finishes of communicating by letter; Return step 1).

Beneficial effect:

Compared with prior art, advantage of the present invention just is:

Adopt the method for CDMA, not only can effectively suppress upstream noise, and can effectively utilize upstream bandwidth, be very suitable for many labels and communicate.Consider the superior function of CDMA, be particularly suitable for the uplink communication in the rfid system.CDMA technology is combined with ALOHA, the handling capacity of system is significantly improved.

This patent proposes a kind of reservation ALOHA (CDMA-R-ALOHA) anti-collision algorithm based on CDMA, can carry out many labels and read, and improves the handling capacity under more number of tags.

The basic thought of ALOHA anti-collision algorithm is that each label enters a scope and sends Frame immediately, replys if the stipulated time is received reader, represent to send successfully, otherwise expression clashes label retransmission data frame.The present invention has added the mode that the CDMA multichannel is preengage in above-mentioned traditional ALOHA algorithm.By building up a four condition discrete time markov (Markov) chain model, obtained the expression formula of normalized rfid system handling capacity, provided the condition of system stability, experiment shows: along with the increase of Code Channel number, the handling capacity of label increases, and the range of stability of the handling capacity of label is also increasing.When the probability G that label data successfully sends increased, the handling capacity Sn of rfid system increased performance gradually and improves gradually, and when G=2, throughput of system reaches maximal value, and maximum system throughput is 0.538.Under identical channel condition, adopt the CDMA-R-ALOHA anti-collision algorithm of the throughput of system of ALOHA algorithm less than the present invention's proposition.

Description of drawings

Fig. 1 is four attitude Markov models of algorithm;

Fig. 2 is the influence of Code Channel number N to the rfid system tag throughput;

Fig. 3 is the influence of transmission success probability g to the rfid system tag throughput;

Fig. 4 is the influence (N=2) of each algorithm to the rfid system tag throughput;

Fig. 5 is the related data transmission flow process figure of this method.

Embodiment

Below with reference to accompanying drawing and specific implementation process the present invention is described in further details:

Embodiment 1:

The specific implementation step of algorithm is as follows:

1), according to the actual application environment of rfid system, channel is carried out modeling: supposing the system comprises a reader, the label of some, and label enters the probability that a certain time slot sends packet and obeys Poisson distribution, and keeping out of the way does not influence the Poisson characteristic that label arrives when retransmitting; Channel is reliably, and power control also is desirable, ignores capture effect.A downlink broadcast channel and N up Code Channel.(N=2 ^{Code length}-1, in theory can be infinitely great, in fact, depend on practical application and manufacturing process.) time shaft of up channel is divided into time slot one by one, all labels are shared these time slots and up channel.And reader has which time slot and up Code Channel free time, by label selection time slot and Code Channel at random by downlink broadcast channel notification label.

2), introduce Markov chain and set up algorithm model, in a certain time slot, system only may be in (as Fig. 1) in the following one of four states:

Idle condition (S):, the up Code Channel free time, do not have data packet transmission at this time slot.

Conflict situation (C): this time slot has two or more labels to send the reserve requests frame to same up Code Channel.The competition conflict takes place, the reserve requests failure.

Send reserve requests state (R): this time slot, a certain sign indicating number divide up channel to have only a label to send the reserve requests frame, the reserve requests success.

Transmit status (D): label sends packet at this time slot.

3), utilize Markov model, determine in the algorithm the mutual transfer process (seeing accompanying drawing 1) between four kinds of states.

Each state exchange relation is as follows:

(a). if last time slot, system is in idle condition S, current time slots, (1) neither one label busy channel, system has only a label busy channel by continuous idle state S. (2), and system changes subscription state R over to by idle condition S, and (3) have two or more labels, busy channel, system changes conflict situation C over to by idle condition S;

(b). if last time slot, system is in conflict situation, current time slots, (1) neither one label busy channel, system changes idle condition over to by conflict situation. and (2) have only a label busy channel, and system changes subscription state R over to by conflict situation S, and (3) have two or more labels, busy channel, system is by lasting conflict situation C;

(c). if last time slot, system is in reservation, and current time slots, system change the data transmit status over to;

(d). if last time slot, system is in data transmit status D, and data are sent completely, current time slots, (1) neither one label busy channel, system changes idle condition over to by conflict situation. and (2) have only a label busy channel, system changes subscription state R over to by conflict situation S, (3) two or more labels are arranged, busy channel, system is by lasting conflict situation C.

By analyzing, this Markov chain is a limited state as can be known, and irreducible, it travels through, and has equilibrium solution.Make its state vector for (S, C, R, D), probability of stability vector is ∏=(π ₀, π ₁, π ₂, π ₃), establish p ₀Be that a certain time slot does not have frame to arrive probability, p ₁Be the probability that a certain time slot has only a frame to arrive, analysis has:

p ₀＝e ^-G，p ₁＝Ge ^-G

Wherein G is an offered load, equals in the time slot number of the average frame that sends altogether.

Transition matrix M

$M=\left(\begin{array}{cccc}{m}_0& m_1& {\mathrm{<figure-callout\; id="2"\; label="m"\; filenames="A20081014323300131.png"\; state="\{\{state\}\}">m</figure-callout>}}_2& 0\\ m_0& m_1& {\mathrm{<figure-callout\; id="2"\; label="m"\; filenames="A20081014323300131.png"\; state="\{\{state\}\}">m</figure-callout>}}_2& 0\\ 0& 0& 0& 1\\ {\mathrm{<figure-callout\; id="0"\; label="gm"\; filenames="A20081014323300122.png,A20081014323300132.png"\; state="\{\{state\}\}">gm</figure-callout>}}_0& {\mathrm{<figure-callout\; id="1"\; label="gm"\; filenames="A20081014323300122.png,A20081014323300131.png"\; state="\{\{state\}\}">gm</figure-callout>}}_1& {\mathrm{<figure-callout\; id="2"\; label="gm"\; filenames="A20081014323300131.png"\; state="\{\{state\}\}">gm</figure-callout>}}_2& {\mathrm{<figure-callout\; id="3"\; label="gm"\; filenames="A20081014323300122.png,A20081014323300131.png"\; state="\{\{state\}\}">gm</figure-callout>}}_3\end{array}\right)$

Wherein, g is that Frame sends the probability that finishes, m in a time slot ₀=p ₀, m ₁=1-p ₀-p ₁, m ₂=p ₁, m ₃=1-g.

According to system of equations $\left\{\begin{array}{c}\mathrm{\&Pi;}=\mathrm{\&Pi;M}\\ \underset{j}{\mathrm{\&Sigma;}}{\mathrm{\&pi;}}_j=1\end{array}\right.$ Can obtain probability of stability π ₂So system at the average S of the label of the successful despreading of a certain time slot energy (being the normalization handling capacity of system under the condition that sign indicating number divides) is:

$S={\mathrm{\&pi;}}_2=\frac{{\mathrm{gm}}_2}{g+{\mathrm{<figure-callout\; id="2"\; label="m"\; filenames="A20081014323300131.png"\; state="\{\{state\}\}">m</figure-callout>}}_2},$ M wherein ₂=Ge ^-G

4), the performance of algorithm under the varying environment is analyzed, suppose that the Code Channel number represents that with N total load is represented with G in the system, the successful transmission probability of label data represents with g, then:

When having a Code Channel (N=1) in the system, the average S of label, $s=\frac{{\mathrm{gm}}_2}{g{+\mathrm{<figure-callout\; id="2"\; label="m"\; filenames="A20081014323300131.png"\; state="\{\{state\}\}">m</figure-callout>}}_2},$ M wherein ₂=Ge ^-G

When having a plurality of Code Channels (N＞1) in the system, then as can be known by decomposition theorem at random:

The handling capacity of single Code Channel is: $s_0=\frac{{\mathrm{gm}}_2}{g+{\mathrm{<figure-callout\; id="2"\; label="m"\; filenames="A20081014323300131.png"\; state="\{\{state\}\}">m</figure-callout>}}_2},$ Wherein ${\mathrm{<figure-callout\; id="2"\; label="m"\; filenames="A20081014323300131.png"\; state="\{\{state\}\}">m</figure-callout>}}_2=\frac{G}{N}{e}^{-\frac{G}{N}},$

The total handling capacity of system is: $s_N=N\frac{{\mathrm{gm}}_2}{g+{\mathrm{<figure-callout\; id="2"\; label="m"\; filenames="A20081014323300131.png"\; state="\{\{state\}\}">m</figure-callout>}}_2},$ Wherein ${\mathrm{<figure-callout\; id="2"\; label="m"\; filenames="A20081014323300131.png"\; state="\{\{state\}\}">m</figure-callout>}}_2=\frac{G}{N}{e}^{-\frac{G}{N}}$

5), the stability analysis of system.When G=N, the tag throughput s of rfid system _NGet maximal value, $\max \left(s_N\right)=\frac{\mathrm{<figure-callout\; id="1"\; label="Ng\; ge\; +"\; filenames="A20081014323300122.png,A20081014323300131.png"\; state="\{\{state\}\}">Ng</figure-callout>}}{\mathrm{ge}+}.$ When G＜N, the handling capacity S of system _NIncrease along with the increase of system load G.System will become unstable when G＞N, and handling capacity becomes downtrending.Therefore in theory, when N gets higher value, can satisfy the stable condition of system, G＜N.But when practical application, number of tags may be far longer than channel number, thereby has different labels and select same Code Channel, thereby competes conflict.

6), analysis influences bigger parameter to algorithm performance, and provides best realization state:

Work as ceteris paribus, when the Code Channel number N increased, the handling capacity of label increased, and the range of stability of the handling capacity of label is also increasing.When G=N, it is maximum that the handling capacity of label reaches.

Work as ceteris paribus, when the probability g that label data successfully transmits increased, the throughput performance of rfid system was improved gradually.

The present invention has added the mode of CDMA multichannel reservation in traditional ALOHA algorithm.By building up a four condition discrete time Markov chain model, obtained the expression formula of normalized rfid system handling capacity, provided the condition of system stability, experiment shows: along with the increase of Code Channel number, the handling capacity of label increases, and the range of stability of the handling capacity of label is also increasing.When the probability G that label data successfully sends increased, the handling capacity Sn of rfid system increased performance gradually and improves gradually, and when G=2, throughput of system reaches maximal value, and maximum system throughput is 0.538.Under identical channel condition, adopt the CDMA-R-ALOHA anti-collision algorithm of the throughput of system of ALOHA algorithm less than the present invention's proposition.

The present invention has added the method for CDMA multichannel reservation in traditional ALOHA algorithm, be modeled as the Markov chain of a four condition discrete time, and its concrete implementation procedure is as follows:

1), Channel Modeling.Suppose that channel is a frequency-flat type channel, and channel comprises a downlink broadcast channel and N up Code Channel.The time shaft of up channel is divided into time slot one by one, and all labels are shared these time slots and up channel.

2), label sends the DATA DISTRIBUTION situation.The situation that comprises a reader and a plurality of labels in the taking into account system, label enter the form that data that a certain time slot sends present Poisson distribution, and label is kept out of the way the form that does not influence its Poisson distribution after the repeating transmission.

3), algorithm modeling.The prerequisite that algorithm is realized is based on above channel model.Adopt the markov modeling process, determine that channel may be in four kinds of discrete duties: idle condition; Conflict situation; Send the reserve requests state; Transmit status.

4), determine the mutual transformational relation between four kinds of duties, referring to accompanying drawing 1, the initiating terminal of arrow is represented the channel status of last time slot among the figure, the sensing end of arrow is represented the residing state of next time slot channel.

The implication of alphabetical representative is as follows among the figure:

S represents idle condition: at this time slot, the up Code Channel free time, do not have data packet transmission.

C represents conflict situation: this time slot has two or more labels to sending the reserve requests frame with up Code Channel.The competition conflict takes place, the reserve requests failure.

R represents to send reserve requests: this time slot, a certain sign indicating number divide up channel to have only a label to send the reserve requests frame, the reserve requests success.

D represents transmit status: label sends packet at this time slot.

5), the test of algorithm.Suppose that N represents the subchannel number of the up channel of system, G represents charge capacity total in the system, and its size is determined jointly by the label number of system and the probability g that successfully transmits data.During test these data are specialized processing.

A), test N is to the influence of rfid system handling capacity, hypothesis g=0.5 during test is that handling capacity under the different value condition is tested to N respectively, respectively N is got the testing throughput that carried out system at 1,3,5 o'clock respectively here;

B), test g is to the influence of rfid system handling capacity, hypothesis N=2 increases g gradually during test, checks the change in throughput trend map;

C), under the test same channel environment, ALOHA algorithm, based on the time slot A LOHA algorithm (CDMA-S-ALOHA) of CDMA and based on the influence of the reservation ALOHA algorithm (CDMA-R-ALOHA) of CDMA to throughput of system.Get N during test and equal 2, g equals 0.5.

6), test of heuristics result's analysis.

A), number of channel N to the test result of throughput of system as shown in Figure 2.As can be seen from the figure along with the increase of Code Channel number, the handling capacity of label increases, and the range of stability of the handling capacity of label is also increasing.When G=N, it is maximum that the handling capacity of label reaches.

B), the probability g that successfully sends of label data influences test result as shown in Figure 3 to throughput of system.As can be seen from the figure, when g increased, the throughput performance of rfid system was improved gradually, and when G increased, throughput of system Sn increased gradually simultaneously.When G=2, throughput of system reaches maximal value, and maximum system throughput is 0.538.

C), under identical channel condition, algorithms of different is to the test result of the influence of throughput of system as shown in Figure 4.Adopt the ALOHA algorithm and adopt throughput of system based on the CDMA-R-ALOHA anti-collision algorithm all less than the CDMA-S-ALOHA anti-collision algorithm.This is because taken the part Code Channel based on the data transmission of CDMA-R-ALOHA anti-collision algorithm, and the Code Channel number that the application frame can be used is less than based on the CDMA-S-ALOHA anti-collision algorithm.But be greater than handling capacity based on the system of ALOHA algorithm based on the handling capacity of CDMA-R-ALOHA algorithmic system.

In addition, the related data transmission flow process figure of this method as shown in Figure 5.

Claims (2)

1. the data anticollision algorithm in the rfid system, described rfid system comprises reader, label and channel; It is characterized in that described channel comprises a downlink broadcast channel and a plurality of up Code Channel; The time shaft of described up Code Channel is divided into a plurality of time slots, and all labels are shared described time slot and up channel; Rfid system is in following a certain state at each time slot:

1) idle condition: the up Code Channel free time, do not have data packet transmission;

2) conflict situation: have two or more labels to send the reserve requests frame to same up Code Channel; The competition conflict takes place, the reserve requests failure;

3) send the reserve requests state: a certain sign indicating number divides up channel to have only a label to send the reserve requests frame, the reserve requests success;

4) expression transmit status: label sends packet at this time slot;

Transformational relation between each state is as follows:

1) if last time slot rfid system is in idle condition, in current time slots, a) neither one label busy channel, rfid system keeps idle condition; B) have only a label busy channel, rfid system changes subscription state over to by idle condition; C) two or more label busy channels are arranged, rfid system changes conflict situation over to by idle condition;

2) if last time slot rfid system is in conflict situation, in current time slots, a) neither one label busy channel, rfid system changes idle condition over to by conflict situation; B) have only a label busy channel, rfid system changes subscription state over to by conflict situation; C) two or more label busy channels are arranged, rfid system keeps conflict situation;

3) if last time slot rfid system is in subscription state, in current time slots, rfid system changes the data transmit status over to;

4) if last time slot rfid system is in the data transmit status, and data are sent completely, in current time slots, and a) neither one label busy channel, rfid system changes idle condition over to by conflict situation; B) have only a label busy channel, rfid system changes subscription state over to by conflict situation; C) two or more label busy channels are arranged, rfid system keeps conflict situation.

2. the data anticollision algorithm in the rfid system according to claim 1 is characterized in that, may further comprise the steps:

1) reader sends activation instruction to label by the downlink broadcast channel; Described activation instruction comprises channel information and frame synchronization information;

2) label sends the reservation instruction to reader;

3) if conflict, reader sends the instruction of mourning in silence to label, and label withdraws from current time slots, waits for next time slot communication; Return step 1); If do not conflict, reader sends communication instruction to the label that proposes reservation; Enter next step;

4) label and reader data communication;

5) reader sends exit instruction, the communication of time slot after this label is no longer participated in to the label that finishes of communicating by letter; Return step 1).

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