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IEEE COMMUNICATIONS LETTERS, VOL. 11, NO. 1, JANUARY 2007 85

Query Tree-Based Reservation for Efficient RFID Tag Anti-Collision

Ji Hwan Choi, Student Member, IEEE, Dongwook Lee, and Hyuckjae Lee, Member, IEEE

Abstract Since the tree based RFID tag anti-collision protocol

achieves 100% read rate, and the slotted ALOHA based taganti-collision protocol allows simple implementation and goodperformance in small amount of tags, we consider how to takethe advantages of each algorithm. This paper presents query treebased reservation for efficient RFID tag anti-collision. Accordingto the simulation results, the proposed protocol requires lesstime consumption for tag identification than the present tag anti-collision protocols implemented in EPC Class 0, Class 1, andClass 1 Gen. 2.

Index Terms Anti-collision protocols, distributed reservation-based protocols, tag collisions, RFID.

I. INTRODUCTION

RFID systems are coming the most promising technolo-gies used for contactless object identification. One ofthe research areas in RFID systems is a tag anti-collision

protocol; how to reduce identification time with a given

number of tags in the field of an RFID reader [1]. For

fast tag identification, anti-collision protocols, which reduce

the frequency of collisions occurred and identify tags inde-

pendent of occurring collisions, are required. Moreover, tag

anti-collision protocols are more important than reader anti-

collision protocols because of the incompetence of tags in

low-cost passive RFID systems.

There are two types of tag anti-collision protocols for RFIDsystems: deterministic methods and probabilistic methods [1].

The deterministic methods are tree based protocols [1,2,3,6].

The tree based protocols keep splitting the group of colliding

tags into two subgroups until all tags are identified. The

probabilistic methods are based on slotted ALOHA [4]. The

slotted ALOHA based protocols reduce the probability of

occurring tag collisions how tags respond at the different time.

This paper considers how to improve the performance of

the RFID tag anti-collision protocol by applying the char-

acteristics of EPC Class 1 Gen. 2 protocol to the query

tree algorithm. The crux of the proposed query tree based

reservation is that the RFID systems do not have to use thewhole length of tag IDs for resolving each tag in the field of

a reader. Thus, the tags generate reservation sequences, which

are 16-bit random numbers (RN16s), for assigning slots to

transmit their IDs, and the reader identifies tags with the query

tree algorithm using the RN16s. During the identification

process, the reader calls a tag with the received RN16 for

collecting the tag ID when a RN16 is identified by the reader.

Manuscript received September 12, 2006. The associate editor coordinatingthe review of this letter and approving it for publication was Dr. FabrizioGranelli. This work is supported by the RFID/USN standardization project ofETRI under the information communication standardization program of MIC

(Ministry of Information and Communication), Korea.J. H. Choi, D. Lee, and H. Lee are with the School of Engineering,Information and Communications University (ICU), Daejeon, Korea (e-mail:{jihwanchoi, dwook, hjlee}@icu.ac.kr).

Digital Object Identifier 10.1109/LCOMM.2007.061471.

Hence, we call the proposed protocol as 16-bit random number

aided query tree algorithm (RN16QTA). According to the

simulation results, RN16QTA achieves less time consumption

for the tag identification than the present tag anti-collision

protocols implemented in EPC Class 0, Class 1, and Class 1

Gen. 2.

I I . QUERYT RE E-BASEDR ESERVATION(RN16QTA)

Since the slotted ALOHA based algorithms cannot guaran-

tee the100%read rate, and experience significant performancedegradation in the large amount of tags, this paper considers

the performance enhancement of the tree based algorithm. Forenhancing the performance of the tree based algorithm, we

apply the characteristics of EPC Class 1 Gen. 2 protocol to

the query tree algorithm as follows:

1) RN16 Generation: All tags in the field of a reader gen-

erate temporary IDs, 16-bit random numbers (RN16s),

for giving the uniqueness to themselves [4]. This part is

performed exactly like in standard EPC Class 1 Gen. 2.

Probability of a single RN16:

0.8/216 < P(RN16 = j) < 1.25/216, where jis any possible number generated by the random

number generator (RNG). Probability of simultaneously identical sequences:

For a tag population up to 1,000 tags, the probability

of existing tags having the same RN16 is less than

0.1%. Probability of predicting a RN16:

A RN16 shall not be predictable with a probability

greater than 0.025%.

2) Applying RN16s to the Query Tree Algorithm (QTA

[3])

Request: The reader sends n-length inquiring bits

(prefix) to tags. Response: Tags send their RN16s from n+ 1th bit

to the end when the first n bits of the tag IDs are

the same as the prefix.

Decision: Depending on whether collisions have

occurred or not, the reader decides on proceeding

procedure with the following conditions.

If collisions occur, the reader saves two new

prefixes at the last input first output (LIFO).

Two new prefixes: the connection of the prefix

and either 0 or 1

If a collision occurs when the tags respond withonly the last bits in the RN16s to the readers

inquiring, the reader assumes that there are two

tags because of the uniqueness of RN16s.

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86 IEEE COMMUNICATIONS LETTERS, VOL. 11, NO. 1, JANUARY 2007

Initialization

A reade rs Req uest

(n-bit) Prefix=LIFO (end, :)

LIFO=LIFO(1:end-1, :)

For all tags

TmpID(1:n)=Prefix

Tags Response from all activated tags

Res=TmpID(n+1: end)

Is there any Response?

1LIFO=

0

Inquiring bits

Answer bits

LIFO

LIFO= Prefix 1

Prefix 0

Length(LIFO) =0

End

No

Y es

Y es

No

No: Inactivate tags

Yes: Activate tags

Does a collision occur?

Detect_ID

The received IDDetect_ID=

Y es

No

Temporary IDs

Each tag generates

its own 16 bit random number (RN16)

ACK signal

The reader send the ACK signal to make

the tag having the received RN16 send its tag ID

Detect_RN16=[Prefix Res]

Tags ID Response

The tag, which responds to the readers

inquiring a moment ago, sends its tag ID

:The process in the reader

:The process in the tag

[ ]

Fig. 1. The flow chart of query tree-based reservation.

If there is no collision, the reader identifies a

RN16 from the tags.

Do these steps until a RN16 is identified.

3) Sending an ACK command when a RN16 is identified

If the reader identifies a RN16, the reader calls the

tag having the RN16 with an ACK command. Thereare two types of the ACK command.

The reader sends a two-bit ACK code only for

indicating that the signal is an ACK command.

The reader sends the ACK code + the received

RN16 for improving the security of the RFID

systems.

Only the tag sending the whole RN16 is activated

by the ACK command of the reader, and responds

with its tag ID to the ACK command.

4) Do (2) and (3) until the LIFO is null.

The performance of the RFID tag anti-collision protocols

means how fast the identification process in the field of

a reader is. In the tree based algorithms, the performance

depends on the tree depth, which is equal to the length of the

tag IDs. Here, we propose the query tree based reservation

called RN16QTA for efficient RFID tag anti-collision as

shown in Fig.1. RN16QTA does the identification process with

temporary IDs shortening the tree depth. After identifying a

temporary ID, the reader calls a tag with the temporary ID for

collecting the tag ID. With RN16QTA, consequently, we can

reduce the identification time by the shortened tree depth.

III. SIMULATIONS ANDR ESULTS

This simulation is intended for the performance compari-

son between the present tree based RFID tag anti-collision

protocols and RN16QTA in the algorithm point, and the per-

formance comparison between RN16QTA and DFS-ALOHA

in EPC Class 1 Gen. 2 protocol in the implementation point.

One important point of the observation is whether the

simulated protocol is implementable to the low-cost passive

RFID systems or not. QTA is already applied to the RFID

systems with EPC Class 1 protocol. On the other hand,

collision tracking tree algorithm (CTTA [3]) is not adopted toany RFID systems yet because tags cannot support CTTA with

transmitting and receiving simultaneously. Since RN16QTA is

the protocol applying the characteristics of DFS-ALOHA in

EPC Class 1 Gen. 2 protocol to the query tree algorithm,

the algorithm is easily implementable for the low-cost passive

RFID systems.

A. Simulation environment

The simulation condition is as follows. There is only one

reader. In the field of the reader, the number of tags increases

from 2 to 512. The length of the tag IDs is 96 bits. Both tag-to-reader data rate and reader-to-tag data rate are chosen as

80 kbps. The reason is that the middle speed in EPC Class

1 Gen. 2 proposed by EPCglobal to ISO/IEC 18000-6 C is

equal to the chosen data rate [4,5]. There is some iteration

overhead because of propagation delay from the channel and

latency from the signal processing. For the comparison of the

algorithm point, the iteration overhead is not considered.

For the comparison of the implementation point, we set

a simple EPC Class 1 Gen. 2 architecture without consid-

ering both the multi-session commands for the multi-reader

environment and the probability of receiving an invalid tag

ID. To calculate the average number of identified tags persecond, the RFID systems choose 8 bits for request and 3

bits for detecting collision, no collision, or no response for

substituting the iteration overhead. For solving error detecting

problem, moreover, 16-bit CRC coding is required.

B. Results

1) The algorithm point: Above all else, Fig.2 shows the

average inquiring bits and response bits for one-tag identifi-

cation. In the RN16QTA, both the average inquiring bits and

response bits for one-tag identification are between the case

of QTA and the case of CTTA. The reason is that RN16QTAreduces the overhead from both the average inquiring bits and

response bits by using the temporary IDs, but generates the

overhead caused by collecting the tag IDs. The former one

affects RN16QTA to get better performance than QTA, and the

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CHOI et al.: QUERY TREE-BASED RESERVATION FOR EFFICIENT RFID TAG ANTI-COLLISION 87

101

102

0

50

100

150

200

250

The number of tags

Theaveragerequiredb

itsforone-tagidentification

Inquiring - QTA

Inquiring - CTTA

Inquiring - R N16QTA

Response

Response

Response

- QTA

- CTTA

- RN16QTA

Fig. 2. The average inquiring and response bits for one-tag identification.

101

102

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

The number of tags

Theaveragerequirediterationsforone-tagiden

tification

QT A

CTTA

RN16QTA

Fig. 3. The average required iterations for one-tag identification.

latter one influences RN16QTA to get worse performance than

CTTA. Next, Fig.3 indicates the average required iterations for

one-tag identification. RN16QTA requires one more iteration

than QTA because of the step collecting the real tag IDs,

but the overhead from each iteration is 1/6 of the others.

Finally, Fig.4 shows the average number of identified tags

per second in each algorithm. In conformity with the simula-

tions, RN16QTA achieves faster identification than QTA, and

approaches to the performance of CTTA.

2) The implementation point: Fig.5 indicates the averagenumber of identified tags per second in each protocol. De-

pending on the default frame size (16 slots or 64 slots) in EPC

Class 1 Gen. 2 protocol, the tendency of the performance is

different in the small amount of tags. However, the perfor-

mance of DFS-ALOHA in EPC Class 1 Gen. 2 protocol is

always less than that of the proposed RN16QTA as shown

in Fig.5. Consequently, the proposed protocol, RN16QTA,

accomplishes faster tag identification than the implemented

protocols, QTA in EPC Class 1 protocol and DFS-ALOHA in

EPC Class 1 Gen. 2 protocol.

IV. CONCLUSION

This paper proposes the query tree based reservation, which

reduces the tree depth on the identification process for decreas-

ing the identification time. The query tree based reservation

101

102

0

100

200

300

400

500

600

700

800

900

1000

The number of tags

Theaveragenumberofidentifiedtagspersecond

QT A

CTTA

RN16QTA

Fig. 4. The average number of identified tags per second.

101

1020

50

100

150

200

250

300

350

400

450

500

The number of tags

Theaveragenumberofidentifiedtagspersecond

Gen.2 :16s lots

Gen.2 :64s lots

RN16QTA

Fig. 5. The average number of identified tags per second.

applies 16-bit random numbers to the query tree algorithm,

which is for assigning slots to transmit tag IDs, instead of the

real tag IDs. According to the simulation results, the query tree

based reservation achieves less time consumption for the tag

identification than the implemented tag anti-collision protocols

in EPC Class 0, Class 1, and Class 1 Gen. 2.

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[3] F. Zhou, D. Jin, C. Huang, and M. Hao, Optimize the power consumptionof passive electronic tags for anti-collision schemes, in Proc. Fifth Inter.Conf. on ASIC 2003, vol. 2, pp. 1213-1217.

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