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Adaptive Binary Splitting: A RFID Tag Collision

Arbitration Protocol for Tag Identification

Jihoon Myung and Wonjun Lee

Dept of Computer Science and Engineering, Korea University, Seoul, Koreawlee@korea.ac.kr

I.

This work was funded by SK Telecom under Contract Number KU-R0405721 to Korea University.Correspondent Author

Abstract In the RFID system, a reader recognizes tags through

communication over a shared wireless channel. When more than

one tag transmits their IDs at the same time, the tag-to-reader

signals lead to collide and collision disturbs the readers identifi-

cation process. Therefore, tag collision arbitration for passive

RFID tags is significant for fast identification. This paper pre-

sents an Adaptive Binary Splitting (ABS) protocol which is an

improvement on the binary tree protocol. To reduce collisions

and identify tags efficiently, ABS use information which is ob-

tained from the last processes of tag identification. Our perform-

ance evaluation shows that ABS outperforms existing tree basedtag anti-collision protocols.

Keywords RFID, Tag Anti-collision, Tag Identification

INTRODUCTION

Radio frequency identification (RFID) system is an auto-matic identification system. A RFID reader recognizes an ob-

ject through wireless communications with the tag which has aunique ID and information and is attached to the object. Thereader must be able to identify tags as quickly as possible.However, reader-to-tag signals or tag-to-reader signals collide

because readers and tags communicate over the shared wirelesschannel. Collisions make both the communication overhead

and the transmission delay of readers and tags have lost theirusefulness. As a result, either the reader may not recognize allobjects or a tag identification process may suffer from longdelay. Therefore, anti-collision protocols which enable the fastand correct identification regardless of the occurrence of colli-sions are required.

Collisions are divided into reader collisions and tag colli-sions [1]. Reader collisions occur where neighboring readersinterrogate a tag simultaneously and confuse it. Tag collisionsmean that more than one tag tries to respond to a reader at thesame time and make the reader unable to recognize any tags.Reader collisions can be resolved [2][3] because RFID readerscan detect collisions and communicate with one another. Espe-cially, since low-functional passive tags can neither detect col-

lisions nor figure out neighboring tags, tag anti-collision proto-cols are important for identification ability of RFID systems [4].

Tag anti-collision protocols can be grouped into two broadcategories: aloha based protocols and tree based protocols.Aloha based tag anti-collision protocols such as aloha, slottedaloha, and frame slotted aloha [5][7] reduce the possibility ofthe occurrence of tag collisions how tags transmit at the distinct

time. Alohas tags randomly select their transmission time andtags of slotted aloha can try to transmit only at the beginning ofa timeslot which is a certain time period. Frame slotted alohawhich shows the best performance of aloha based protocolsconfigures a frame with continual timeslots. As a tag transmitsits ID only at a timeslot in every frame, the frame slotted alohareduces collisions. Aloha based protocols, however, cannot

perfectly prevent collisions. In addition, they have the seriousproblem that a tag may not be identified for a long time socalled tag starvation problem. On the other hand, the tree based

tag anti-collision protocols such as the binary tree protocol[5][6][9][10] and the query tree protocol [11] do not cause tagstarvation though they have relatively long identification delay[8]. They split a group of colliding tags into two subgroupsuntil the reader receives signals of tags without collisions. Inthe binary tree protocol, tags are required to have functional-ities of managing a counter and a random number generator.The colliding tags are split according to a randomly selectednumber, 0 or 1. The tags which select 0 transmit their IDs im-mediately and the tags which select 1 transmit later. Throughcontinuing to split with a random number, a reader can recog-nize all tags. The query tree protocol is the deterministic taganti-collision protocol. A reader sends a query including a pre-fix and tags of which ID matches the prefix respond. The prefixof the reader has the decision power about splitting the collid-ing tags. The query tree protocol is called the memoryless pro-tocol because tags need not to have additional memory exceptthe ID. Since it, however, uses prefixes, the performance issensitive to the distribution of tags IDs which the reader isgoing to recognize. There are variants of the query tree proto-col in order to reduce the identification delay [12], but theysimply focus on shortening time taken for transmitting the tagsID and cannot reduce the number of collisions.

We propose the Adaptive Binary Splitting (ABS) protocolwhich restrains tag-to-reader signals from colliding. We focusour attention on scheduling tags with less collision. By reduc-ing the number of collisions, a reader can recognize tags fasterand a tag can be recognized with less transmissions. We havestudied the similar approach in the query tree protocol [13].

The essential elements of ABS are the timeslot allocation pro-cedure and the empty timeslot elimination procedure. To iden-tify tags efficiently, the timeslot allocation procedure assignscollision-free timeslots to tags and the empty timeslot elimina-tion procedure removes unnecessary timeslots where any tagsdo not transmit. Tags can quickly reoccupy an exclusive time-slot with schedular information which is obtained from the last

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process of tag identification. The simulation result shows thatour proposed protocol suppresses the occurrence of collisionsand reduces the total delay for identifying all the tags while

preserving low communication overhead. The rest of this paperis organized as follows. Section II describes ABS. In SectionIII and Section IV, the performance analysis including thederivations of several major equations referred to in the analy-sis will be explicated. Finally the conclusions of the paper will

be drawn in Section V.

II.

A.

B.

ADAPTIVE BINARY SPLITTING PROTOCOL

ABS schedules tags transmissions via consecutive commu-nications between a reader and tags. If a tag dwells within thereaders identification range, it is able to communicate with thereader directly. Tags transmit their own ID and then the readerdetects collision. The reader always informs all tags whether ornot the tag-to-reader signals collide. When tag-to-reader signalslead to collision, the colliding tags randomly select a binarynumber, 0 or 1. Based on this selected number, a group of thecolliding tags is split into two subgroups. By continuing thissplit until tags enable to transmit without collision, the readercan recognize all the tags. Since each tag gets an exclusive time

for transmission, ABS can reduce the number of collisions ofthe tag-to-reader signals and identify tags fast. In addition,ABS accomplishes fast re-assigning the collision-free timeslotsto each of the tags even though a set of tags which inhabit inthe vicinity of the reader becomes different.

Description of ABS

We define a frame as the time elapsed for identifying all thetags within the readers transmission range. The reader canadaptively decide the length of a frame with the commandsinforming tags of beginning and terminating the frame. Thereader can change the ending point of a frame at any time. Theframe consists of timeslots which are certain time periods. Ineach timeslot, tags transmit their IDs and the reader receives

the tag-to-reader signals. According to the number of signalstransmitted in a timeslot, we can categorize the timeslots asfollows.

Empty timeslot: No signals are transmitted by tags in atimeslot.

Readable timeslot: Only one tag transmits its ID and issuccessfully recognized by the reader. Since ABS doesnot terminate a frame till all tags are recognized, the

number of readable timeslot equals to the number oftags recognized by the reader.

Figure 1. Three states of the tag

Collisional timeslot: More than one tag transmits andthen the tag-to-reader signals collide. The reader is un-able to recognize any tags.

The reader, in the end of a timeslot, sends a feedback informingall the tags of the type of the current timeslot. After receivingthe feedback, tags operate the timeslot allocation procedure andthe empty timeslot elimination procedure so that a timeslot will

carry only one tags signal. To realize the fast identification,ABS holds down empty and collisional timeslots.

The tag maintains values of a progressed-slot number and anallocated-slot number. The progressed-slot number representsthe number of timeslots passed in a frame and is initializedwith 0 at the beginning of a frame. The progressed-slot num-

bers of all the tags are always equal. To put it concretely, thevalue of the progressed-slot number is not increased in everytimeslot and is only increased by 1 in the readable timeslot, i.e.,when a tag is successfully identified. The allocated-slot numbersignifies the sequence that the tag can access a channel totransmit. In other words, the tags of which the allocated-slotnumber is the same value as the progressed-slot number can tryto transmit at the beginning of the timeslot. As shown in Fig. 1,

the tag has one of three states as follows: Wait state: The tag has the allocated-slot number

greater than the progressed-slot number. It does nottransmit any signal and waits for its turn.

Active state: The tag has the allocated-slot numberequal to the progressed-slot number and tries to trans-mit its own ID.

Sleep state: The tag has the allocated-slot number lessthan the progressed-slot number. Since the tag has al-ready recognized in the ongoing frame, it does nottransmit any signal until the completion of the frame.

As the timeslot allocation procedure and the empty timeslotelimination procedure of ABS change the progressed-slot num-

ber and the allocated-slot number, the tag takes possession ofthe favorable timeslot to transmit. In the collisional timeslot,the colliding tags, i.e., the tags of the active state, add a ran-domly selected binary number (0 or 1) to the allocated-slotnumber. Therefore, the active tags which select 1 convert theirstate into the wait state. The tags in the wait state, when colli-sion occurs, increase the allocated-slot number. The tags in thewait state, when the received feedback points that any signalsare not carried in the current timeslot, decrease the allocated-slot number. The detailed description and the example of two

procedures are given in Section II-C and Section II-D. After all,each of allocated-slot numbers is assigned to only one tag.There exist no ownerless allocated-slot numbers less than anyallocated-slot number which a certain tag owns. The tags pre-serve the allocated-slot number at the beginning of the next

frame and the timeslot allocation procedure and the emptytimeslot elimination procedure re-arrange allocated-slot num-

bers fast. Consequently, tags are fast recognized in next frames.

Frame Termination

The reader determines the end point of a frame with a pro-gressed-slot number and a terminated-slot number. The pro-gressed-slot number of the reader represents the number oftimeslots passed in the ongoing frame like one of the tag. If the

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reader successfully recognizes the tag which transmits the IDalone, the progressed-slot number is increased by 1. The termi-nated-slot number signifies the last timeslot number of theframe. As soon as the progressed-slot number is greater thanthe terminated-slot number, the reader concludes that all tagsare recognized and transmits the command terminating theframe to all the tags.

The frame should have the proper length regarding the num-ber of tags in the identification range of the reader. If the ter-

minated-slot number is too big compared with the number oftags, the empty timeslots is increased and the reader suffersfrom long identification delay. On the other hand, if the termi-nated-slot number is too small, the reader is unable to recog-nize all tags. The number of tags, however, is not correctly

predicted earlier. For this reason, the timeslot allocation proce-dure and the empty timeslot elimination procedure adaptivelychange the terminated-slot number during a frame in order toidentify all the tags and to eliminate the unnecessary emptytimeslots. The reader, in the end of a timeslot, changes the pro-gressed-slot number and the terminated-slot number accordingto the type of the current timeslot. For terminating a frame afteridentifying all the tags, the reader acts as the tag which has thelargest allocated-slot number. At this time, the reader updatesthe terminated-slot number as if the tag updates the allocated-slot number. The reader closes the current frame in its owntimeslot. By changing the values of the progressed-slot numberand the terminated-slot number, the reader can adaptively de-termine the size of the frame.

C.

D.

Timeslot Allocation

Fig. 2(a) describes a tag identification process that the exist-ing tree based anti-collision protocols are recognizing four tags(tag A, B, C, and D) through splitting a group of colliding tagsinto two subgroups. A circle in the figure means a timeslotwhen some tags transmit and a number in a circle correspondsto the number of tags which transmit simultaneously. A num-

ber in a rectangle corresponds to the sequence that tags arerecognized. The reader recognizes four tags after detecting

three collisions. Since the binary tree protocol and the querytree protocol do not exploit information which is obtained fromthe last identification process, they make three collisional time-slots again whenever the reader recognizes tag A, B, C, and D.We are motivated by the observation that the reader may en-able to recognize four tags without collision by using the iden-tification sequence of tags as shown in the dotted line of Figure2(a). The identification sequence is also a good tool for reduc-ing collisions even though new arriving tags, which have not

been recognized by the reader in the last frame, come into theidentification range of the reader. Consider the scenario that tagE arrives at the area where tag A, B, C, and D inhabit. Thereader attempts to recognize five tags. Figure 2(b) shows thatthe increment of the number of tags causes more collisions and

the reader recognizes five tags with four collisions in the exist-ing protocols. The use of the identification sequence, however,enables to make the reader recognize five tags with only onecollision.

If more than one tag has the identical allocated-slot number,the signals of these tags cause collision. The reader cannot rec-ognize any tags due to collision, but can detect the occurrenceof collision. The colliding tags should be allocated differenttimeslots to transmit successfully. Therefore, ABS splits agroup of the colliding tags into two subgroups and creates a

new timeslot for the allocation to one of two subgroups. If the

current timeslot is the collisional timeslot, where the tag-to-reader signals lead to collide, the tags of the active state ran-domly select a binary number, 0 or 1 and then add the selectednumber to their own allocated-slot numbers. Since the pro-gressed-slot number is not changed, the tags which select 0 re-try to transmit at the following timeslot. On the contrary, thetags which select 1 increase their own allocated-slot number by1. These tags get to have the same allocated-slot number asother tags which already have had the allocated-slot numberequal to the progressed-slot number + 1 at the beginning of thetimeslot. For preventing two subgroups from combining, thetags of the wait state add 1 to the allocated-slot number when-ever they receive the feedback pointing the collisional timeslot.When collision occurs, the reader adds 1 to the terminated-slotnumber in order to increase the length of the frame. This opera-tion of splitting tags and creating a timeslot is continued untileach of the timeslots is allocated to only one tag.

a) Recognizing tags (A, B, C and D)

b) Recognizing tags after Tag E comes

Figure 2. Causing collisions

Empty Timeslot Elimination

When a group of colliding tags is split into two subgroups;one subgroup, which contains all the colliding tags, and theother, which contains no tags, the tree based tag anti-collision

protocols cause the empty timeslot. In Fig. 3(a), tag A and tagB transmit at the same time and then they are selected into the

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left subgroup. The timeslot for the right subgroup does notcarry any tag-to-reader signals. In addition to that, ABS whichexploits the identification sequence creates additional emptytimeslots as tags go out of the readers range. Fig. 3(b) showsthe occurrence of the empty timeslot after tag B at Fig. 3(a)moves out of the readers range. The empty timeslots do notmake a reader fail to notice a tag, but extend delay which isrequired to recognize all tags. Therefore, the empty timeslotelimination procedure of ABS re-adjusts the identification se-quence of tags in order to minimize the impact of empty time-slots. If the current timeslot is the empty timeslot, the tags ofthe wait state decrease the allocated-slot number by one. Sincethe progressed-slot number is not changed in the empty time-slot, the decrement of the allocated-slot number gets to pull theschedules of transmissions of the tags which have the allo-cated-slot number greater than the progressed-slot number.When the reader does not receive any tag-to-reader signals inthe current timeslot, it also reduces the terminated-slot number

by 1. ABS can eliminate empty timeslots regardless to the fail-ure of splitting collisions or the disappearance of tags.

E.

III.

Impact of tag movements

As objects which have tags move, the set of tags which thereader has to recognize in the following frame may be differentfrom the set of tags which are recognized in the previous frame.If the set of tags at the next frame is equal to the set of tags atthe last frame, ABS does not make any empty timeslots andany collisional timeslots. Since all the tags maintain the allo-cated-slot number, they can transmit without collision. Thereadable timeslot is the only component to consist a frame andthe tags are recognized with ideal delay. When the tags whichhave not recognized in the last frame come into the vicinity of

the reader, they can use their possessive allocated-slot number(may be set up by other reader) without errors. Only if the allo-cated-slot number of the new arriving tag is greater than theterminated-slot number of the reader, the reader is unable torecognize the new tag. To cope with this problem, the readeroffers its terminated-slot number with the command beginninga frame. The arriving tag which has the allocated-slot numbergreater than the terminated-slot number resets the allocated-slotnumber with 0. When the tags of the old set which has identi-

fied in the previous frame go out of the readers range, theframe has the empty timeslots. However, the empty timeslotelimination procedure removes the empty timeslot and adaptsthe number of reduced timeslots to the length of the frame.Therefore, ABS is not affected by the movement of tags.

a) Empty timeslot by wrong splitting

b) Empty timeslot by tags movement

Figure 3. The occurrence of empty timeslots

IDENTIFICATION DELAY ANALYSIS

We analyze the average identification delay of ABS. The av-erage delay is proportional to the duration of the frame becauseABS terminates the frame as soon as the reader recognizes alltags.

Definition 1:LetAibe the set of tags which are recognizedin the ith frame. The total delay dtotal(Ai)for identifying is

)()( itagreaderitotal ATddAd += (1)

where dreaderis the delay of delivering a readers command, dtagis the delay of delivering a tags message, i.e., ID, and T(Ai)isthe number of timeslots required for identifying all tags of Ai.The total delay is determined by the number of timeslots, T(Ai)

because dreaderand dtagare constant.

Lemma 1:The number of timeslots, T(Ai) is the sum of thenumber of empty timeslots,E(Ai), the number of readable time-slots,R(Ai), and the number of collisional timeslots, C(Ai).

)()()()( iiii ACARAEAT ++= (2)

Proof:The number of tag-to-reader signals in a timeslot can-not be negative. If the number of signals is zero or one, it is theempty timeslot or the readable timeslot respectively. When atimeslot carries two signals or more, it is the collisional time-

slot. Therefore, timeslots of a frame is one of the three.

A. No tag movementAt first, consider the case that any tags do not cross the

readers identification range during two consecutive frames. Inother words, Ai which is the set of recognized tags in the ithframe equals toAi+1which is the set of recognized tags in thei+1th frame.

Lemma 2:Let Cbinary(Ai)be the number of collisional time-slots when the terminal-slot number of the reader and the allo-cated-slot numbers ofAiare initialized with 0 at the beginningof ith frame. Cbinary(Ai)is

=

=1

0

2

11

2

1

2

1

112)(

n

kk

k

n

k

k

ibinary

n

AC

(3)

wheren is the number of tags ofAi, |Ai|.

Proof:When the terminal-slot number and the allocated-slotnumbers are initialized with 0, the operation of ABS is as sameas one of the binary tree protocol. Therefore, timeslots of the

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frame can be represented in a binary tree and the total numberof the collisional timeslots of the frame is

=

=0

),()(

k

binaryibinary knCAC (4)

where Cbinary(n,k) is the number of collisional timeslots in thedepth kof the binary tree. Since the total number of nodes inthe depth kof the binary tree is 2

k, by lemma 1,

( ) ( ) ( )knRknEknC binarybinaryk

binary ,,2, = (5)whereEbinary(n,k)andRbinary(n,k) are the number of empty time-slots and the number of readable timeslots in the depth kof the

binary tree, respectively. The probability that the mth timeslotof the depth k, sk,m (1 m 2

k) is the empty timeslot is theprobability that ntags tries to transmit atsk,1,sk,2, ,sk,m-1,sk,m+1, ,sk,2^k-1, andsk,2^k.Ebinary(n,k) is

n

k

kbinary knE

=

2

112),( (6)

IfAihas n tags,Aiconsists of tag a1, a2, , an-1and an. Theprobability thatsk,mis a readable timeslot is the probability thatonly aj(1 jn) transmits insk,mand tag a1, a2, , aj-1, aj+1, , an-1 and an tries to transmit atsk,1,sk,2, ,sk,m-1,sk,m+1, ,

sk,2^k-1, andsk,2^k.11

2

11

2

11

2

12),(

=

=

n

k

n

kk

kbinary nnknR (7)

By (5), (6) and (7),

=

1

2

11

2

1

2

1112),(

n

kk

n

k

kbinary nknC (8)

By substituting (8) into (4), lemma 2 is derived.Theorem 1:For anyAi,

+=

=

1

0

2

11

2

1

2

111221)(

n

kk

k

n

k

kibinary

n

AT

(9)

where n= |Ai|.

Proof:Every intermediate node of the binary tree is only thecollisional timeslot because only a node of a collisional time-slot has two child nodes. Therefore, the tree is the full binarytree and the total number of nodes of the tree is 2Cbinary(Ai) + 1.By (4),

=

+=

+=

0

),(21

)(21)(

kbinary

ibinaryibinary

knC

ACAT

(10)

From (8), Tbinary(Ai)is proved.

Theorem 2:WhenAi+1is equal toAiand the reader and tagsmaintain their terminated-slot number or allocated-slot num-

bers at the beginning of i+1th frame, the number of timeslotsrequired to identifyAi+1, TABS(Ai+1|Ai)is

111 )()|( +++ == iiiiABS AARAAT (11)

Proof: The timeslot allocation procedure and the emptytimeslot elimination procedure make ntags transmit by follow-ing the identification sequence. There are neither unused allo-cated-slot numbers nor common allocated-slot numbers. Theframe has only readable timeslots.

B. Arriving tagsConsider that the set of tags which the reader needs to rec-

ognize expands. New tags, i.e., tags which are not recognized

in the previous frame, come into the vicinity of the reader fromoutside.

Theorem 3:WhenAiis {a1, a2, , an-1, an} andAi+1is {a1,a2, , an+-1, an+}, TABS(Ai+1|Ai)is

( )

+

+=

=

+

+

kk

kk

kiiABS nAAT

2

11

2

11

2

11122)|(

0

1

1

(12)

Proof: ABS operates the timeslot allocation procedure foran+1, an+2, , an+-1and an+in the i+1th frame. For simplicity,we assume that an+1, an+2, , an+-1 and an+ initialize their

allocated-slot number with 0. Tag a1, a2, , an-1and an, i.e., ntags which have recognized in the ith frame, exploit their allo-cated-slot numbers which are determined in the ith frame.Therefore, the n-1 tags ofAiexcept the tag of which allocated-slot number is 0 transmit in the readable timeslots. The tag ofwhich allocated-slot number is 0 in the ith frame and an+1, an+2, , an+-1and an+has the same allocated-slot number, 0.

)1(1)|( 1 ++=+ binaryiiABS TnAAT (13)

From (9), the number of timeslots for arriving tags is calculated.

C. Leaving tagsNext, consider that some recognized tags go out of the

readers identification range and there are no arriving tags.

Theorem 4:WhenAiis {a1, a2, , an-1, an} andAi+1is Ai-{af(1), af(2), , af(-1), af()} (1 f(x) n), TABS(Ai+1|Ai)is

iiiABSiiABS AAATAAT ==+ )|()|( 1 (14)

Proof:One frame is required in order that ABS operates theempty timeslot elimination for nonexistent tags. Therefore, thenumber of required timeslots of the following frame after sometags go out is equal to one of the last frame.

D. Tag movementsFinally, we derive the average delay when tags move ran-

domly.

Theorem 5:WhenAiis {a1, a2, , an-1, an} andAi+1is {a1,

a2, , an+-1

, an+} - {af(1), af(2), , af(-1), af()} (1

f(x)

n),TABS(Ai+1|Ai)is

( )

+

+=

=

+

+

kk

kk

kiiABS nAAT

2

11

2

11

2

11122)|(

0

1

1

(15)

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Proof: If B1 is {a1, a2, , an-1, an}, B2 is {an+1, an+2, ,an+-1, an+}, andB3is {af(1), af(2), , af(-1), af()}, by theorem 3and theorem 4, the number of timeslots in the i+1th frame is

( 11)|()|(

21

1211321

++=

+=+

BTB

BBBTBBBBT

binary

ABSABS

)

IV.

A.

B.

C.

(16)

By theorem 4, the number of timeslots is not affected by thenumber of leaving tags.

PERFORMANCE EVALUATION

We evaluate the performance of ABS compared to the bi-nary tree protocol and the query tree protocol. To measure effi-ciency of identifying tags in the tree based protocols, we con-sider the following aspects.

Identification delay: This metric is the total delay re-quired to recognize all tags. We measure the delay bythe timeslot because three protocols consist of time-slots of which each has a time period for carrying tag-to-reader signals and a time period for carrying thereader-to-tag signal. The fast identification is the most

significant factor in the tree based anti-collision proto-cols because they do not cause tag starvation problem.

Tags communication overhead: This metric is the av-erage number of bits transmitted by each of the tags. Atag of the tree based protocols transmits with its ownID. The tags communication overhead influences theamount of power consumption. Due to the lack of the

power source of tags, it must be low.

In our simulations, a single reader tries to recognize tags inits identification range. Tags have randomly selected IDs. Thesignals of the tags include only their IDs. The reader transmits

a startFrame signal at the beginning of a frame and an end-Frame signal at the end of a frame. Tags which receivestart-Frame signal can try to transmit their IDs during that frame.The lengths of the readers signals are different according tothe used protocol. In a timeslot of the binary tree protocol andABS, tags transmit first and then the reader responses. Thereader of the binary tree protocol transmits only one bit in orderto indicate whether collision occurs or not. Similarly, the readerof ABS transmits two bits whose values match the empty, read-

able, or collisional timeslot respectively. On the other hand, thereader, in the query tree protocol, transmits first and then tagsresponse it with their IDs. The readers signal contains one

prefix (the maximum length of the prefix is equal to the lengthof tags ID). Therefore, the timeslot period of the query tree

protocol may be, relatively, longer than the period of other pro-tocols. The notations of Table I are used for description of thesimulation result.

No tag movement

First, we simulate the performance with changing the num-ber of tags in the readers range when a set of tags does notbecome different between two consecutive frames. In otherwords, there are neither any leaving tags nor any arriving tags.All the tags are in the vicinity of the reader and enable to di-

rectly communicate with the reader. The reader has recognizedall the tags in the first frame and recognizes the tags in the sec-ond frame again. Fig. 4(a) and Fig. 5(a) show the identificationdelay and the tags communication overhead in the secondframe. Note that all the tags are recognized because three tree

based protocols do not cause tag starvation problem. As thenumber of tags is increased, the delay gets longer and the tagtransmits more. The binary tree protocol imposes a heavycommunication overhead on a tag through the binary tree pro-tocol and the query tree protocol show almost similar identifi-cation delay. This implies that the binary tree protocol hasmore colliding tags in each collisional timeslot than the querytree protocol. ABS degenerates very slowly and takes theshortest time for identifying. In addition to that, the tags of

ABS transmit very small bits because the timeslot allocationprocedure and the empty timeslot elimination procedure regu-late communications between the reader and tags without bothempty and collisional timeslots.

TABLE I.NOTATIONS

Description

BID The number of bits of the tags ID

Nall The total number of tags

Nlast The number of tags recognized in the last frame

Nin The number of tags which arrive in the readers range

after the beginning of the last frame and before the

beginning of the new frame

Nout The number of tags which are recognized in the last

frame and leave before the beginning of the new

frame

Ta The total number of timeslots required for tag identifi-

cation in the new frame

Tr The number of readable timeslots in the new frame (Tr=Nlast+NinNout)

Tc The number of collisional timeslots in the new frame

Te The number of empty timeslots in the new frame

Btx The average number of bits transmitted by a tag in the

new frame

Arriving tags

Fig. 4(b) and Fig. 5(b) are the simulation results when noneof tags recognized in the last frame go out of the readers rangeand the number of new arriving tags varies. After having rec-ognized 500 tags in the first frame, the reader recognizes 500tags and new arriving tags in the second frame. The arrivingtags set their allocated-slot number with 0. As the number ofarriving tags increases, the identification delays of three tree

based protocols are increased by the same incremental ratio.

This results from the same number of empty timeslots and col-lisional timeslots created among new tags. ABS, however, hasless delay and less overhead because of neither the empty time-slots nor the collisional timeslots among tags which have beenrecognized in the first frame.

Leaving tags

Fig. 4(c) and Fig. 5(c) give the identification delay and thetags communication overhead when there are no arriving tagsand the number of tags which go out of the readers range vari-

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a)BID=96,Nall=1000,Nin=0,Nout=0

b)BID=96,Nall=1000,Nlast=500,Nout=0

c)BID=96,Nall=1000,Nlast=500,Nin=0

Figure 4. Identification delay

a)BID=96,Nall=1000,Nin=0,Nout=0

b)BID=96,Nall=1000,Nlast=500,Nout=0

c)BID=96,Nall=1000,Nlast=500,Nin=0

Figure 5. Tags communication overhead

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es. There exist 500 tags in the simulation area. After the readerhas recognized all the tags in the first frame, some leaving tagsdo not transmit any signals since then. As the number of leav-ing tags increases, the identification delay of the binary tree

protocol and the query tree protocol decrease. (Note that thebinary tree protocol and the query tree protocol do not exploitany information from the last frame.) On the other hand, thereader, in ABS, does not know nonexistent tags in the follow-ing frame as soon as some tags have leaved. Since the states of

the timeslots for nonexistent tags change into the empty time-slots, the total number of timeslots is not reduced in the secondframe by the leaving tags. The empty timeslot elimination pro-cedure is operated in the second frame and then the unneces-sary empty timeslots for the leaving tags are removed. Whenabout 300 tags of 500 tags go out, ABS causes larger delaythan other protocols. However, when both the leaving tags andthe arriving tags are considered, ABS has better efficiency even

though the majority of the tags go out.

D.

V.

Tag movements

Next, we vary the number of tags when tags randomly move.As shown Fig. 6, ABS has the best performance of three tree

based protocols. As the number of tags recognized by thereader is increased, the gap of the identification delay betweenABS and other protocols is much larger. The identificationdelay is similar with the result when the tags do not go out of

the readers range although ABS causes a little longer identifi-cation delay when a set of tags decreases rapidly as known inthe previous simulation. The tags communication overhead ofABS is much smaller like other scenarios. The high perform-ance of ABS results from efficiently reducing the number ofthe empty timeslots and the number of the collisional timeslots.ABS is able to eliminate 72% of collisional timeslots and 65%of empty timeslots of the binary tree protocol and the querytree protocol.

a) Identification delay

b) Tags communication overhead

Figure 6.Tags move randomly (Nlast,Nin, andNoutare determined ran-domly andBID=96)

CONCLUSION

In this paper, an adaptive binary splitting collision arbitrationprotocol for passive RFID tags has been proposed and evalu-ated. We develop a novel and enhanced binary tree protocol to

reduce identification delay by exploiting information obtainedin the last process of identification. The timeslot allocationprocedure assigns the collision-free timeslots to each of tagsand the empty timeslot elimination procedure removes unnec-essary timeslots. The simulation results show that the timeslotallocation procedure and the empty timeslot elimination proce-dure of ABS largely diminish the identification delay.

ACKNOWLEDGMENT

This work was supported by grant No. R01-2005-000-10267-0 from Korea Science and Engineering Foundation inMinistry of Science and Technology.

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